U.S. patent application number 11/147727 was filed with the patent office on 2005-10-13 for method for a group of services to operate in two modes simultaneously.
This patent application is currently assigned to Kayak Interactive Corporation. Invention is credited to Ricciardi, Aleta.
Application Number | 20050228857 11/147727 |
Document ID | / |
Family ID | 25474701 |
Filed Date | 2005-10-13 |
United States Patent
Application |
20050228857 |
Kind Code |
A1 |
Ricciardi, Aleta |
October 13, 2005 |
Method for a group of services to operate in two modes
simultaneously
Abstract
The invention is a method of handling groups of services where
the makeup and structure of the groups can be determined and
changed while the application is running. In one embodiment a group
of services is grouped as a coordinator cohort group with respect
to one client and, at the same time, as a peer group with respect
to another client. This is accomplished by registering with a
lookup service a coordinator cohort group proxy and a peer group
proxy. Clients may download from the lookup service either group
proxy in order to use the group of services in the desired
mode.
Inventors: |
Ricciardi, Aleta; (West
Windsor, NJ) |
Correspondence
Address: |
STUART RUDOLER LLC
ATTN: DOCKET CLERK
2 BALA PLAZA, SUTIE 300
BALA CYNWYD
PA
19004
US
|
Assignee: |
Kayak Interactive
Corporation
Princeton
NJ
|
Family ID: |
25474701 |
Appl. No.: |
11/147727 |
Filed: |
June 8, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11147727 |
Jun 8, 2005 |
|
|
|
09940367 |
Aug 28, 2001 |
|
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Current U.S.
Class: |
709/200 |
Current CPC
Class: |
H04L 67/10 20130101;
H04L 69/329 20130101; G06F 9/465 20130101; G06F 9/5055 20130101;
H04L 29/06 20130101 |
Class at
Publication: |
709/200 |
International
Class: |
G06F 015/16 |
Claims
What is claimed is:
1. A computer implemented method of grouping services in a
distributed computing application comprising the steps of:
initiating a plurality of services arranged into a group, with each
service having its own service proxy and grouping agent; each
grouping agent registering its associated service with a group
service including providing the group service with the service
proxy of the service it is registering; the group service bundling
a first group logic shell for a first group mode with at least one
of the service proxies to form a first group proxy for the group;
the group service bundling a second group logic shell for a second
group mode with at least one of the service proxies to form a
second group proxy for the group; the group service providing the
first group proxy to a first client and the second group proxy to a
second client; the first client using the first group proxy to call
the group in the first group mode; and the second client using the
second group proxy to call the group in the second group mode.
2. The method of claim 1 wherein the first group mode and second
group mode are selected from the set comprising peer and
coordinator cohort.
3. The method of claim 1 wherein the first client and the second
client are the same client.
4. The method of claim 1 wherein the group acts simultaneously in
the first group mode and second group mode.
5. A computer readable medium containing instructions for
controlling a computer system to perform a method of grouping
services in a distributed computing application comprising the
steps of: initiating a plurality of services arranged into a group,
with each service having its own service proxy and grouping agent;
each grouping agent registering its associated service with a group
service including providing the group service with the service
proxy of the service it is registering; the group service bundling
a first group logic shell for a first group mode with at least one
of the service proxies to form a first group proxy for the group;
the group service bundling a second group logic shell for a second
group mode with at least one of the service proxies to form a
second group proxy for the group; the group service providing the
first group proxy to a first client and the second group proxy to a
second client; the first client using the first group proxy to call
the group in the first group mode; and the second client using the
second group proxy to call the group in the second group mode.
6. The computer readable medium of claim 5 wherein the first group
mode and second group mode are selected from the set comprising
peer and coordinator cohort.
7. The computer readable medium of claim 5 wherein the first client
and the second client are the same client.
8. The computer readable medium of claim 5 wherein the group acts
simultaneously in the first group mode and second group mode.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a divisional application of U.S.
application Ser. No. 09/940,367 filed Aug. 28, 2001, incorporated
herein by reference.
BACKGROUND OF INVENTION
[0002] A distributed system is a collection of autonomous computing
entities, hardware or software, connected by some communication
medium. While often the computing entities are geographically
dispersed, in some instances they might be separate processors in a
multi-processor computer or even separate software routines
executing in logically isolated memory space on the same computer.
A computing entity need not be a traditional computer, but more
generally can be any computing device, ranging from a large
mainframe to a refrigerator or a cell phone. A distributed
application is an application that executes on a distributed system
and one in which parts of the application execute on distinct
autonomous computing entities.
[0003] Whenever a distinct component of a distributed application
requests something (e.g., a data value, a computation) of another
component, the former is called a client and the latter is called a
service. It is worth noting that the terms service and client are
not exclusionary in that an item can be both a client and a
service. For example, a routine that calculates the time between
two events may be a client and of a clock service; if the clock
service then calls a routine that converts to Daylight Savings
Time, the clock becomes a client and the Daylight Savings Time
converter is its service.
[0004] FIG. 1 shows a typical distributed application of the
existing art. There are two clients 2, 4 and four services 10, 12,
14, 16 that the clients 2, 4 might need. Each service has a service
proxy 10a, 12a, 14a, 16a which is a module of mobile code that can
be used by clients to invoke that service. A service proxy 10a,
12a, 14a, 16a contains the code needed by a client 2, 4 to interact
with a service. For instance if a service is a digital camera on a
robotic arm, the interfaces might include Initialize( ), Zoom( ),
Rotate( ) and Get_Picture( ). The service proxy 10a, 12a, 14a, 16a
may also provide the expected return values for the service, which
might include error codes as well.
[0005] Mobile code generally refers to a computer program that can
be written on one platform and executed on numerous others,
irrespective of differences in hardware, operating system, file
system, and many other details of the execution environment. In
addition to independence from the physical characteristics of the
execution environment, a mobile program may move from one computer
to another in the middle of its execution.
[0006] Mobile code may be pre-compiled, or compiled when it arrives
at the execution platform. In the first case, numerous versions of
the program must be written and compiled, then matched across
run-time environments; this is mobile code in the letter, but not
the spirit, of the definition. In addition, the same pre-compiled
program cannot move from one platform to a different one during its
execution. In the second, the program text may be distributed along
with configuration scripts describing what to do in each execution
environment. This distributes and delays the specificity of the
pre-compiled option. The more interesting, and far more common
approach exploits a standard virtual machine, which finesses all
the issues of platform heterogeneity. The virtual machine is a
program that itself mitigates the machine dependencies and
idiosyncrasies, taking the raw program text and compiling it into
binary executable.
[0007] In addition to clients 2, 4 and general services 10, 12, 14,
16, all distributed applications need some mechanism for clients 2,
4 to find services. Often such knowledge is assumed a priori, but
many distributed applications use a look-up service 20. The look-up
service 20 is a service with which the other services are
registered or advertised to be available to for use by clients. In
a simple system, where there is no attempt to coordinate replicas
of services, each new service registers with the look-up service 20
(in the case of replicas, the onus falls on the client to resolve
conflicts and ambiguity). When a service 10, 12, 14, 16 registers,
it provides information telling clients 2, 4 how to find it.
Commonly, this is a physical location such as an IP address and
port number, but in the most modern systems this can be as powerful
as giving the look-up service 20 a service proxy 10a, 12a, 14a,
16a, which is actual mobile code that clients 2, 4 can execute and
use to invoke that service 10, 12, 14, 16. In this way, the service
proxy 10a, 12a, 14a, 16a contains not just location information,
but information for how to use the service 10, 12, 14, 16. While
just as necessary for the client 2, 4 as location information, this
has previously been assumed as a priori knowledge. When a client 2,
4 wishes to work with a service 10, 12, 14, 16 it finds it through
the look-up service 20, downloads the service proxy 10a, 12a, 14a,
16a for that service 10, 12, 14, 16 from the look-up service 20,
then uses the service proxy 10a, 12a, 14a, 16a to invoke the
service 10,12,14,16. The look-up service 20 may also have
attributes of the services 10, 12, 14, 16, such as whether it is a
grouped service, what type of group it is, what its cost to use is,
how accurate it is, how reliable it is, or how long it takes to
execute. In such cases the clients 2, 4 can use the attributes to
decide which of a number of services 10, 12, 14, 16 it wishes to
use.
[0008] Each of the foregoing has access to a communication network
22 so that it is capable of communicating with at least some of the
other members in the distributed computing application. The
communication network 22 may be wireless, a local area network, an
internal computer bus, a wide area network such as the Internet, a
corporate intranet or extranet, a virtual private network, any
other communication medium or any combination of the foregoing.
[0009] In the prior art example shown in FIG. 1, one client 2 is a
traffic monitoring program that notifies a user when and where
traffic has occurred and the other client 4 is an automated toll
collection program. The services are a clock 10, a road sensor 12
that monitors traffic flow on a highway, a toll booth sensor 14
that detects an ID device in each car that passes through the toll,
and a credit card charge program 16. When each service 10, 12, 14,
16 becomes available to the application it registers with the
look-up service 20 and provides the look-up service with its
service proxy 10a, 12a, 14a, 16a.
[0010] When the traffic monitoring client 2 begins, it queries the
look-up service to see if a clock is available and what sensors are
available. The look-up service 20 responds by providing the client
2 with the clock proxy 10a, the road sensor proxy 12a and the toll
booth sensor proxy 14a. The traffic monitoring client 2 uses the
service proxies 10a, 12a, 14a to invoke the clock 10 and the
sensors 12, 14, and then to monitor traffic at various times of the
day.
[0011] Similarly when the toll collector client 4 begins, it
queries the look-up service 20 to see if a toll booth sensor 14 and
a credit card charge service 16 are available. The look-up service
20 responds by providing the client 4 with the toll booth sensor
proxy 14a and the credit card charge proxy 16a. The toll collector
client 4 uses the service proxies 14a, 16a, to invoke the toll
booth sensor 14 and the credit card charge program 16, and then to
identify cars that pass through the toll booth and charge their
credit cards for the toll.
[0012] A known feature of distributed applications is that services
may be grouped. For instance there may be several services capable
of performing the traffic sensor functionality. These can be
grouped to form a logical notion of traffic sensor that is separate
from the particular implementation of the sensors. This may be done
for redundancy purposes in case one of the services fails, to
provide parallel processing for computationally intensive tasks, to
provide extra capacity for peak loads, as well as for many other
reasons. Services in a group may communicate with each other to
coordinate their activities and states. For instance in the example
shown in FIG. 1 it may be advantageous to group the two sensors 12,
14.
[0013] There are two primary types of group structures: the
coordinator cohort (CC) group and the peer group. In a CC group
there is one distinguished member of the group, the coordinator,
that processes requests from clients. The coordinator periodically
updates the other services in the group, the cohorts, with
information about its current state and completed requests, so that
if the coordinator fails, the cohort selected to replace it will be
as current as possible. The more frequent the updates, the more
tightly coupled the states are between group members, and so the
more likely the transition will occur without being visible to
existing clients of the group. On the other hand, more frequent
updates require additional computational capacity and communication
bandwidth.
[0014] In a peer group, all of the members of the group process
requests from a client, which itself requires some logic to decide
how to use the multiple results returned from the group members.
For example, if three thermometers exist in peer group, and a
client requests the temperature it will receive three answers. Many
options exist for using the multiple results, such as taking the
first to respond, taking the average value of all the responses, or
taking the highest value. A peer group is more robust and
fault-tolerant than a CC group because each of the group members
should always be in the correct state, and because the likelihood
of the representative member (which is all members in a peer group,
but only the coordinator in a CC group) being unavailable is
drastically lower. However, a peer group also requires more
resources, both bandwidth and computational, than a CC group
because all of the group members are working and responding to each
client request.
[0015] Another technique known in the existing art is leasing. A
lease is an important concept throughout distributed computing,
generally used between a client and service as a way for the
service to indicate its availability to the client for a length of
time. At the end of the lease, if the lease is not renewed, there
is no guarantee of availability. In a simple example, a service may
register with a look-up service and be granted a lease for five
minutes. This means that the look-up service will make itself
available to the service (i.e., list it) for five minutes. If a
camera grants a lease to a client for two minutes, then that client
will be able to position, zoom, and take pictures for two minutes.
There are a wide variety of ways to handle lease negotiation,
renewal and termination which are well known to those skilled in
the art of distributed computing and all such methods are meant to
be incorporated within the scope of the disclosed invention. A
detailed explanation of leases can be found in, Jim Waldo, The Jini
Specification, 2nd Edition, chapter LE (2001), which is
incorporated herein by reference.
[0016] One useful aspect of leases is that they can be used for
simple failure detection. If the expectation is that a client will
continue to request lease renewal from a service, but then does not
renew its lease, the service may assume that the client has failed,
or is otherwise unavailable. This allows the service to more
efficiently manage its own resources, by releasing any that were
dedicated to expired clients. Such a use of leasing is described in
U.S. Pat. No. 5,832,529 to Wollrath et al.
[0017] This is especially important because components only rarely
plan and announce their failure and are not able to predict network
outages. It is far more common that failures and outages are
unexpected, and that the consequence is an inability to announce
anything. In these cases, a client will not renew its lease so that
eventually, the granting service will reallocate its resources. The
shorter the lease period, the sooner a failure can be detected. The
tradeoff is that both client and service spend proportionately more
time and resources dealing with leasing and that timing anomalies
may have implications for correctness.
[0018] Some benefits of distributed computing and mobile code can
immediately be seen from this example. First, the clients 2, 4 in
FIG. 1 do not need to know ahead of time which sensors 12, 14 are
available, or even how many. They simply query the look-up service
20, which provides this information along with the necessary mobile
code 12a, 14a to call the sensors. Similarly, the clients 2, 4 do
not care which clock 10 is available, as long as any clock 10 is
available. Again, this is because through the use of mobile code, a
client 2, 4 is provided with the necessary service proxy 10a to
invoke and work with the clock 10. Also, the failure or
unavailability of a single sensor 12, 14 or other service is not
likely to cause the entire application to stop running. Further,
the processing load is distributed among a number of computing
devices. Also, the various computing entities need not use the same
operating system, so long as they conform to a common interface
standard.
[0019] Jini is one example of a commercially available
specification for a distributed object infrastructure (or
middleware) for more easily writing, executing and managing
object-oriented distributed applications. Jini was developed by Sun
Microsystems and is based on the Java programming language;
consequently, objects in a Jini system are mobile. Jini is
described in Jim Waldo, The Jini Specification, 2nd Edition (2001).
The Common Object Request Broker Architecture (CORBA), developed by
the Object Management Group, and Distributed Component Object
Module (DCOM), developed Microsoft Corporation, are two other
commercially available examples that are well known in the prior
art. Jini, DCOM, CORBA and a number of other distributed computing
specifications are described by Benchiao Jai et al., Effortless
Software Interoperability with Jini Connection Technology, Bell
Labs Technical Journal, April-June 2000, pp. 88-101, which is
hereby incorporated by reference.
[0020] Distributed computing systems with groups can also be found
in the prior art, particularly in the academic literature. For
example, Ozalp Babaoglu et al., Partitionable Group Membership:
Specification and Algorithms, University of Bologna, Department of
Computer Science, Technical Report UBLCS-97-1 describe groups, but
assumes the services in the group are group-aware. Similarly static
group proxies, or software wrappers, for clients have been
described in Alberto Montresor et al., Enhancing Jini with Group
Communication, University of Bologna, Department of Computer
Science, Technical Report UBLCS-2000-16, but these group proxies
cannot be modified during execution of the distributed application
to accommodate changes in group make-up and structure.
[0021] A number of problems can be found in these and other
implementations and putative descriptions of distributed
applications. Chief among these is that, even if some notion of
groups is available within the infrastructure, both services and
clients need to be group-aware; that is they need to contain logic
to interact either within and as part of a group (in the case of
grouped services), or with a group (in the case of clients of a
group of services). This logic is very complex and the skill set
required to write such software is very different from the skills
required to write the underlying client or service. Further, many
existing clients and services exist that do not have group logic,
and even for clients and services that are being newly written it
can be challenging to write this logic as part of the module. Even
if group logic is coded into new clients or services, they become
locked into a particular instance and type of group and in most
cases will need to be rewritten if the group architecture or makeup
changes. Therefore it is desirable to develop a methodology wherein
the group-aware logic for clients and services are provided in
separate code modules. Existing and previously described attempts
at group services have always assumed that both the services to be
grouped and the clients using group services are group-aware. The
assumption of group-awareness prevents existing, or legacy,
software from being able to take advantage of the benefits of
groups (unless they are rewritten) and burdens new applications
with providing the necessary group logic to operate with the
particular implementation of the group service. If wrappers were
considered for grouping legacy services, they were static and
hard-coded, locking the service into a single framework. Moreover,
static wrappers introduce an additional, distinct point in the
computation, with negative performance and, ironically, fault
tolerance implications, since such solutions can never operate in
the same process space. In all frameworks, group structures were
static and therefore did not permit transitions between group
structures.
[0022] All previous frameworks also ignored clients. Further, even
if clients are written to be group-aware, they must be group-aware
in the very particular way that the group of services are
implemented. For example, if a client is capable of delaying its
requests during membership changes to a group of services, until it
receives a signal informing it that the membership change has
completed, then it cannot interact with a system in which groups
send no such signal, but instead expect the client to poll for this
information. Therefore it would be preferable for this logic to be
provided at run time when the groups are established.
[0023] A major problem with current distributed computing
methodologies that support groups is that changes to the group's
membership are invasive; that is, services within a group cannot be
changed without temporarily halting the availability of the
application. Further, in current systems, if a service, whether
grouped or single, is unavailable, the client is burdened with
handling this unavailability; if it does not, the client may wait
indefinitely, take incorrect steps, or even crash. This is true,
even in simple redundant backup systems, where the client must
handle any delays caused by the switch from a primary to a backup
service. Another limitation of current approaches is that group
structure, CC, peer or otherwise, is not modifiable without also
stopping and then restarting the application, again leaving
existing clients in the lurch. Fluid group structure transitions
could be used to increase or decrease quality of service properties
such as load-balancing or fault tolerance, and to simplify peer
group operation when the service code calls for external
interaction.
[0024] Thus, it is desirable to have a distributed application in
which new services can be added, or services in a group
restructured, "on the fly", that is without halting other members
of the application.
[0025] It is therefore an object of this invention to provide a
method for transparently managing and interacting with groups of
services in a distributed application in which groups are dynamic
in their membership, organizational structure, and their members'
individual functionality.
[0026] It is a further object of this invention to provide a method
of handling transitions in a group of services that does not burden
the client.
[0027] It is a further object of this invention to provide a method
for grouping services wherein a group of services can
simultaneously be arranged in multiple group modes.
[0028] It is a further object of this invention to provide a method
of grouping services in which the group-aware logic is provided in
separate code modules from the core functional logic of the clients
and services.
[0029] It is a further object of this invention to provide a method
of grouping services in which the code modules that handle the
group-aware logic are highly reusable from one application to the
next.
[0030] It is a further object of the invention to provide for a
method of grouping services where services can be added or removed,
and groups restructured during operation, yet without interrupting,
execution.
BRIEF DESCRIPTION OF THE INVENTION
[0031] The present invention is a method of handling a wide range
of dynamic groups of services where the makeup of the groups can be
determined and changed while the application is running. This is
mainly accomplished through a group proxy, which is generated at
run time, and which handles interactions with groups of services on
behalf of one or more clients. The group proxy consists of a group
logic shell which contains all the group-aware logic, and a service
proxy for each service in the group which contains the necessary
logic to interact with the particular service. The group proxy,
which is given to a client for all of its interactions with the
group of services, buffers calls from that client to its group when
the group is unavailable because it is in transition. When the
transition is complete the group proxy transmits the stored client
commands to the group. In the preferred embodiment of the
invention, all the group-aware logic for a distributed computing
application is provided in separate code modules, namely the group
proxy, group service and grouping agent, thus relieving clients and
services from providing this logic and maintaining the purity of
the look-up service and other infrastructure services.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] FIG. 1 shows an example of a distributed computing
application of the prior art.
[0033] FIG. 2 shows an example of an improved distributed computing
application of the current invention.
[0034] FIG. 3 shows a generic representation of the current
invention.
[0035] FIG. 4 shows a Foo service joining as the k.sup.th member of
a coordinator cohort group.
[0036] FIG. 5 shows a Foo service joining as the k.sup.th member of
a peer group of Foos.
[0037] FIG. 6 shows a fail-over from Foo-1 to Foo-2 in a
coordinator cohort group
[0038] FIG. 7 shows a peer group of services reorganizing itself as
a coordinator cohort group to call another service.
[0039] FIG. 8 shows a peer group reorganizing itself as a
coordinator cohort group for use by clients.
[0040] FIG. 9 shows an example of a group being in both peer and CC
modes.
DETAILED DESCRIPTION OF THE INVENTION
[0041] This invention is related to the invention described in the
previously filed U.S. patent application, Ser. No. 09/928,028,
Group Proxy and Method for Grouping Services in a Distributed
Computing Application, filed on Aug. 10, 2001, which is hereby
incorporated by reference.
[0042] FIG. 2 shows an example of a distributed computing
application of the current invention. As in FIG. 1 there is a
communication network 22, a look-up service 20, a number of clients
2, 4, and a number of services 10, 12, 14, 16, 18, each of the
latter having a service proxy 10a, 12a, 14a, 16a, 18a. In the
current invention some of the services are grouped. In this example
one group of services is a CC group 50 and the other group is a
peer group 52. To support the group activity each grouped service
is provided with a grouping agent 10b, 12b, 14b, 16b, 18b and there
is a group service 24. In addition to there being proxies for each
service there are also group proxies 40, 42, which act as proxies
for each group.
[0043] The example shown in FIG. 2 provides specific clients
services and groups, but the invention is generic in application
and the example is not meant to limit the invention in any way.
[0044] Overview
[0045] Prior to describing the method for handling transitions it
is helpful to understand the system on which the method is
executed. While the detailed workings of the present embodiment of
the invention will be described below, a general introduction is
provided here using the example shown in FIG. 2. As in FIG. 1, the
example of FIG. 2 is related to traffic monitoring and toll
collection. An additional service, a log service 18, has been added
which copies all information sent to it to some form of
non-volatile memory. The log service 18 is essentially a recorder.
The non-volatile memory might be a magnetic or optical medium, or
even a paper print-out.
[0046] In this embodiment of the invention the road sensor 12 and
the toll booth sensor 14 are grouped together in a CC group 50. As
in FIG. 1 the traffic monitor client 2 makes calls to a clock 10,
which is not grouped, and a sensor. However, in this example the
sensor is grouped. From the point of view of the traffic monitor
client 2, it does not need to know that the sensor is grouped, it
simply calls a sensor service to get road traffic information,
which in this case is a CC group 50. In the example the road sensor
12 is the coordinator and the toll booth sensor 14 is the cohort.
If the road sensor 12 becomes unavailable, due to failure or any
other reason, the toll booth sensor 14 will act as its backup and
become the coordinator. The road sensor 12 might be designated as
coordinator simply because it was the first to register with the
group service 24, is more accurate, is more reliable, is less
expensive or for any other reason.
[0047] The credit card charge service 16 and log service 18 are
also grouped together, in this case as a peer group 52. Because
they are grouped as a peer group, calls by any client to the credit
group service 52 are executed by both the credit card charge
service 16 and the log service 18. This is convenient in that a
permanent record of charges is made by the log service 18 so that
audits can be made to make sure that all credit charges executed by
the credit charge service 16 were properly credited. In the event
the credit card charge service 16 becomes unavailable, instead of
failing, the credit group service 52, through the log service 18,
at least creates a permanent record of charges, which can be
retrieved later and processed.
[0048] Grouping Agent and Group Service
[0049] An improvement of the current invention is the use of
grouping agents 12b, 14b, 16b, 18b, to handle the group-aware logic
for the grouped services 12, 14, 16, 18. It is the grouping agent
that intercept a registration call from a service to the look-up
service 20 and directs the call to the group service 24. It is also
the grouping agents 12b, 14b, 16b, 18b, that handles coordination
between the services in a group. If a service belongs to more than
one group, it might have multiple grouping agents.
[0050] While in a new service being written from scratch the
grouping functions performed by the grouping agent can be written
as an integrated part of the service, it is preferable that the
grouping agent be written as a distinct code module from the core
functions (i.e., addition and subtraction in a calculator). This
allows 1) the grouping agent to be modified without affecting the
core, 2) the core to operate with numerous different (or no)
grouping agents simultaneously, 3) the grouping agent code to be
used with a variety of different services, in most cases, with only
minor modification, and 4) grouping agents to be switched on the
fly. In services that are not group-aware, a grouping agent can be
added to the existing core to make the legacy service
group-aware.
[0051] The invention further provides for a novel group service 24
which performs a variety of functions that facilitate groups in the
application. All of the services that wish to be grouped register
their service proxies with the grouping service 24 instead of the
look-up service 20. More accurately, a service's grouping agent
registers its service proxy with the grouping service. However, for
purposes of simplicity any group related activity described as
taken by a service shall mean that the action is taken either by
the service itself, if it is inherently group-aware, or by its
grouping agent. The group service 24 then registers the appropriate
service proxies with the lookup service 20. The group service 24
also coordinates whether each group will be a CC or peer group, and
each such group's operation, transitions, and interactions. Most
importantly the group service 24 dynamically creates the group
proxies 40,42 for each group by adding the appropriate service
proxy (in the case of a CC group) or proxies (in the case of peer
group) 10a, 12a, 14a, 16a, 18a to the appropriate group logic shell
30, 32, and then the group service 24 registers the group proxies
40, 42 with the look-up service 20 for use by the clients 2, 4. The
group service 24 also coordinates the activities of the group
proxies 40, 42 during fail-overs or other transitions and handles
the updating of group proxies 40, 42 with the look-up service 20
and the various fielded (i.e. already attached to a client) group
proxies 40, 42 when it is necessary to add, delete or switch the
service proxies 10a, 12a, 14, 16a, 18a. The group service 24 also
handles the swapping of group proxies 40, 42 if a group switches
from CC mode to peer mode or vice versa.
[0052] Group Proxy
[0053] The group proxy 40,42 represents another improvement of the
current invention. Its task, as each grouping agent does for its
service, is to handle all the group-aware logic for its client. It
can be thought of as a device driver for a group of services. In
addition, and of particular importance, a group proxy can buffer or
redirect communication to and from a client when the group that
client is calling is in transition. Such a transition may occur due
to a failure of a service in a group, the addition or removal of a
service in a group, changing of coordinators in a CC group, or a
group switching between CC and peer mode. Since the group proxy
provides an easily configurable software layer between the client
and the rest of the distributed application it can also be used to
perform other useful tasks such as copying commands to a test
service, resolving the results of multiple responses from a peer
group of services, or copying communication to a log service.
[0054] The group proxy 40,42 is made up of a group logic shell 30,
32 and one or more service proxies 10a, 12a, 14a, 16a, 18a. The
group logic shell 30, 32 contains all of the necessary group logic
for a client to interact with a group of services. Assuming there
is a defined interface (e.g. syntax) to call a service, the group
logic shell 30, 32 contains this interface to present to clients 2,
4. The group logic shell 30, 32 contains the logic to intercept
client 2, 4 commands to a group 50, 52, store them, and retransmit
the commands at a later time. The group logic shell 30, 32 may also
contain logic to copy or redirect client 2, 4 communication to
other services. However, the group logic shell 30, 32 does not
contain the necessary mechanisms, such as wire protocol
implementation, to interact with the services 10, 12, 14, 16, 18
within a group. These are contained within the service proxies 10a,
12a, 14a, 16a, 18a. The group service 24 bundles the group logic
shell 30, 32 with one or more service proxies 10a, 12a, 14a, 16a,
18a to form a group proxy 40, 42.
[0055] As shown in FIG. 2, there are separate group logic shells
for a CC group 30 and for peer group 32. In fact, in the current
embodiment there are two group logic shells for each group, one
peer and one CC. Although a large portion of the group logic shell
code is the same from group to group, each group has its own shells
because the group logic shell has to present the identical
interface to the client as any single member of the group would
present. In an alternative embodiment, the group logic shells 30,
32 for each group stored within the group service 24 are identical,
and when a group logic shell initializes it receives the necessary
service interface from the grouping agents, or determines the
appropriate interface using a process known as reflection.
Reflection is well known to those skilled in the art of
object-oriented computing and programming languages, and will not
be elaborated upon here. Since storage space is generally
inexpensive and the executable code for the group logic shells is
not unduly large, in the shown embodiment the group service 24
stores a set of two group shells, peer 32 and CC 30, for each
group.
[0056] In an alternative embodiment, the peer and CC group logic
shells 32, 30 are combined into a single mobile code module and the
group service 24 simply tells the group proxy in which mode to act.
Such an architecture has certain advantages when it is desirable to
transition groups between CC and peer mode during, yet without
interrupting, execution, since it is not necessary to switch group
proxies or logic shells at the clients, and therefore it is easier
to ensure that no client commands are dropped in transition.
[0057] The use of a group logic shell to form a group proxy is an
improvement of the current invention. It makes it possible to
create and reconfigure group proxies on the fly as the application
is running. It enables an architecture where, in most cases, only
service proxies in the group proxy need to be updated as services
are added and deleted from a group, instead of replacing the entire
group proxy. Alternatively, logic shells may be changed, perhaps to
switch between peer and CC modes, without replacing the service
proxies.
[0058] FIG. 2 demonstrates another improvement of the current
invention, namely that the same service can be simultaneously
grouped and ungrouped with respect to different clients. In FIG. 2
the traffic monitor client 2 calls the sensor group 50 which
includes the toll booth sensor 14. Simultaneously, the toll both
sensor 14 is called directly by the toll collector client 4. The
difference is that the toll collector client 4 uses the toll booth
sensor service proxy 14a directly, while the traffic monitor client
2 uses the sensor group proxy 40. As shown the road sensor 12 is
the coordinator of the sensor group 50 so that the sensor group
proxy 40 attached to the traffic monitor client 2 is bundled with
the road sensor service proxy 12a. Although not shown, if the toll
booth sensor 14 becomes the coordinator for the sensor group 50,
the group service 24 would swap the toll booth sensor service proxy
14a for the road sensor service proxy 12a in the sensor group proxy
40 at the traffic monitor client 2. Then both clients 2, 4 could
use the toll both sensor 14 simultaneously, assuming it had enough
processing power and bandwidth to serve both. Such a configuration
may require a more sophisticated grouping agent that is able to
differentiate between calls to the group and calls directly to the
service. In such a scenario it is also beneficial that the client
querying the look-up service be able to establish whether a
particular service is grouped or ungrouped.
[0059] The group service manages the membership and structure of
groups of services, is responsible for registering each group with
the look-up service when its composition and structure are stable,
and de-registering it when these are in transition. By way of an
example, if there are three distinct services that have indicated
(possibly through a grouping agent) a desire to form a particular
group, the group service might determine that the instance with
oldest time stamp be the representative provided to the look-up
service; upon monitoring that instance the group service might
later determine that some other instance (e.g., with the next
oldest time stamp) should replace it and be registered with the
look-up service. The group service also provides group proxies and
is responsible for alerting clients through the group proxies of
transitions within a group. The group service may also determine
into which group structure the services are organized.
[0060] In the present embodiment of the invention it assumed that
all group members expose and implement the same external interface.
This makes all services in a group appear to be identical, even if
they are not exact replicas. For example, a group of calculators
may each perform addition, subtraction, multiplication and
division. Regardless of whether the calculators were true identical
replicas, as long as they implement the same interface they can
easily be grouped in CC or peer group modes. In the likely event
the actual programmer interfaces are not identical, a single
interface must be decided on by the system architect, and the
service proxy can implement the interface and its translation to
the actual programmer interface. Consider that the Calculator group
desires to provide a multiplication function, and consider that
Calc-1 natively provides the interface Mult (float x, float y) and
returns the result of x multiplied by y, while Calc-2 provides the
interface multiply_by(float x, float y, float z) and returns the
result of x multiplied by y in the variable z. The system architect
may decide that the Calculator interface will have syntax
Multiply(float x, float y) and provide the result of x multiplied
by y. Then the service proxy for Calc-1 will implement Multiply(x,
y) as Mult(x, y), while the service proxy for Calc-2 will implement
Multiply (x, y) as multiply_by (x, y, z), having previously
declared its own local variable z, and then return the value z. To
further the example, suppose Calc-3 supports 64-bit precision, but
Multiply (x, y) provides for only 16-bit precision; then the
service proxy for Calc-3 will need to truncate 48 bits. If a member
of the group cannot perform all the functions defined in the common
interface, then the service proxy will need to compensate, either
by completing the functionality, or by returning an exception
(provided exceptions are defined in the common interface). For
instance, suppose Calc-4 provides only for addition. Then its
service proxy could implement Multiply (x, y) as y additions of x
to itself (for example: float result=0.0; for int i=1 to y,
{result=add(x, result)}).
[0061] While in the preferred embodiment, the translations
necessary to provide a common interface are handled by the service
proxies, a similar function can be performed by the grouping agent
for the service. Taking advantage of mobile code, another solution
to this problem is to provide a special dedicated wrapper to the
client or the service to handle this translation. Other solutions
will be obvious to those skilled in the art, and are included
within the scope of this invention. In an alternative embodiment
services that do not present the same interface are grouped
together.
[0062] The invention is not meant to be limited to the particular
application or number of services, groups and clients shown in FIG.
2. FIG. 3 shows a generic implementation of the present invention
in which there are three clients 2, 4, 6 and three different groups
of services 50, 52, 54, although there need not always be an equal
number of clients and groups. In this representation groups are
represented in capital letters and services in small letters. For
each group 50, 52, 54 the group service 24 has a CC group logic
shell 30, 34, 38 (indicated by a subscript "c") and a peer group
logic shell 32, 36, 39 (indicated by a subscript "p"). One point of
this representation is to demonstrate that a client can call
multiple groups, and a single group can be called by multiple
clients, provided that each client 2, 4, 6 has the appropriate
group proxy 40, 42, 44. For instance one client 2 calls all three
groups: A 50, B 52, and C 54. Likewise, one group, C 54, is used by
all three clients 2, 4, 6, and therefore each client has the group
proxy 44 for that group. Also, in this representation there is a
group, group A 50, consisting of only one service, thereby allowing
the client of a single service to obtain some of the benefits of
the group proxy, such as failure masking by buffering. In this
embodiment, as presently shown, group A 50 and group B 52 are peer
groups, and group C 54 is a CC group, although the structure of
each group can be reconfigured.
[0063] While this description has principally referred to two types
of group modes, peer and coordinator cohort, hybrids of these
types, and other types of modes are possible, and the invention is
meant to incorporate all such group modes, whether currently
existing or invented hereafter. It has also been assumed herein
that a grouping agent contains all the necessary logic to act in
either CC or peer mode. However in an alternative embodiment, a
service may have separate grouping agents for CC and peer modes.
Likewise, although not optimal for reasons discussed above, a
service could be written to incorporate the grouping agent
functions, without having a separate group agent.
[0064] A group service is not necessary to gain the client-side
benefits of command buffering using a group proxy. As described,
the group service performs both failure detection and group
management. In the absence of true groups, but given a mechanism
for detecting failures, the "group" proxy could buffer requests
upon being notified of a failure. Upon noticing that the service
had been reestablished (for example, by periodically querying the
look-up service) this group proxy would resume normal operation.
This provides for less overall reliability (the existence of a
group of replicas is proportionately more reliable), increased
fail-over time (the length of service unavailability due to a
failure) and increased latency (the length of time to complete a
client's request), but still shields clients from the effects of
service failures or transitions. In the preferred embodiment for
implementing fault tolerance, the distributed system will implement
physical replication of services (though they need not be identical
replicas), and therefore will have a group service.
[0065] It is also possible, in an alternative embodiment, to
combine the group service and lookup services into a single
service. Likewise, in an alternative embodiment, the group logic
shell, instead of being stored in the group service could be
provided by the system designer ahead of time to each client that
will need a particular group, and then the group service simply
provides and updates the appropriate service proxies in those group
logic shells. Such an architecture is less desirable in that it is
less flexible, since it requires prior knowledge for each client,
that it will use a group and which groups a service will be
using.
[0066] The basic methodology for handling transitions within a
group is for the group proxy to buffer commands while the group is
in transition, to update or replace the group proxy so that it can
work with the revised group, and then for the group proxy to
transmit the commands it has buffered.
[0067] The remainder of the discussion will describe the particular
methodology used to handle fail-overs and other transitions within
a group. Both peer groups and CC groups are described. In the
discussions that follows, a generic service will be called a Foo,
which could be any functionality. A Foo could be a clock, a
counter, a display driver, a traffic sensor, or a calculator.
Further a reference to a service taking a particular action being
taken by a service shall mean the service taking that action either
directly, or, in the preferred embodiment, through its grouping
agent.
[0068] Addition and Removal of Services from a Group
[0069] FIG. 4 shows how another instance of a Foo service, Foo-k
14, joins an existing CC Foo group. In order to join a Foo group,
Foo-k 14 (or its grouping agent 14b) queries the look-up service 20
to see if a group service is available 401. The group service 24
has already registered with the look-up service 20 and has given
the look-up service 20 its own proxy (not shown). The look-up
service 20 responds to Foo-k's (or its grouping agent's) request by
providing it with the group service proxy 402. The Foo-k grouping
agent 14b uses the group service proxy to invoke a method
specifying a group name to join (in this case the Foo group),
possibly the group structure it desires to participate in, and
provides the Foo-k service proxy 14a to the group service 24, 403.
Then, since there already is an established coordinator for the Foo
group (assuming it is Foo-1), the group service 24 simply notifies
the grouping agent 10b for the group coordinator 10 that there is a
new member, or multiple new members, of the Foo group 404. The
Foo-1 grouping agent 10b then begins to include the Foo-k grouping
agent 14b in its periodic broadcasts to all the other Foos of its
current group 405. In an alternate embodiment, the grouping agents
would be initially designed to listen for relevant update events,
so that updates can be done without requiring the coordinator to be
aware of its cohorts' identities. Analogously, when a cohort Foo
service, Foo-j fails or is removed from the group, in the current
embodiment, the coordinator must be informed by the group service;
in the anonymous embodiment it would not need to be. Removal of a
Foo service from the Foo group, other than a coordinator, is
similar to adding a Foo service. The Foo-j grouping agent notifies
the group service 24 that Foo-j is leaving the service. The group
service 24 deletes the Foo-j from the proxy list for the Foo group,
and then instructs the coordinator's grouping agent that it no
longer needs to include Foo-j in its periodic state updates. In the
event that a Foo-j leaving is the coordinator, a new group
coordinator must be designated. This process is similar to the
fail-over in a CC group described below.
[0070] FIG. 5 shows how another instance of a Foo service, Foo-k
14, joins an existing peer Foo group. In order to join a Foo group,
Foo-k 14 (or its grouping agent 14a) queries the look-up service 20
to see if a group service 24 is available 501. The group service 24
has already registered with the look-up service 20 and has given
the look-up service 20 its own proxy (not shown). The look-up
service 20 responds to Foo-k's 14 (or its grouping agent's 14b)
request by providing it with the group service proxy 502. The Foo-k
grouping agent 14b uses the group service proxy to invoke a method
specifying a group name to join (in this case the Foo group),
possibly the group structure it desires to participate in, and
provides the Foo-k service proxy 14a to the group service 503.
Continuing, the group service 24 deregisters Foo from the look-up
service 20 so that outdated Foo proxies 10a, 12a are no longer
distributed 504. The group service adds the Foo-k service proxy 14a
to the existing set of proxies for Foo members, adding the Foo-k
service proxy 14a to the peer Foo group logic shell 32, and
re-registers Foo with the look-up service 20, 505. The group
service 24 then distributes Foo-k's service proxy 14a to all
fielded peer Foo group proxies (those already attached to clients),
which add it to the bundle of other Foo member proxies already
within the Foo group logic shell 506. Future client requests are
therefore sent to Foo-k as well as all previous Foo group members.
Steps 505 and 506 can be executed in either order or concurrently.
The group service 24 might also instruct the group proxy for the
clients to buffer commands until they receive the Foo-k proxy 42.
However, in contrast with a CC group transition, there is generally
no need for group proxies of peer groups to await further
information about the peer group transition, so that there is no
need for peer group proxies to buffer client commands.
[0071] To remove Foo-j from a peer Foo group, the group service 24
distributes instructions to the Foo peer group proxies 42 (already
attached to clients 2) to remove the Foo-j service proxy from each
of the Foo peer group logic shells 32. As in steps 504 and 505
above, the group service unregisters then re-registers Foo with the
look-up service, and, as above, the group proxy 42 at the look-up
service 20 and clients 2 can be updated in either order or
concurrently.
[0072] Fail-overs
[0073] FIG. 6 is a description of how the invention handles a
fail-over in a CC group specifically, and transitions within a
group generally. To begin, Foo-1 10 has a lease with the group
service 24, where the group service 24 is the lease grantor and
Foo-1 10 is the lease holder. The group service 24 has in turn
negotiated a lease for the grouped Foo service with the look-up
service 20. Foo-1 10 fails and therefore does not renew its lease
with the group service 24. The group service 24 assumes that Foo-1
10 has not renewed its lease because it has failed. The group
service 24 then cancels the Foo lease with the look-up service 20,
601 thereby temporarily preventing any new client from finding the
Foo group. The group service 24 also announces (whether through
multicast, broadcast, or individual event notification) to the
group proxy 40 using the Foo service that Foo is unavailable 602.
The announcement may also be heard by other interested members of
the distributed application, such a log service that records errors
or a beeper service that notifies a human operator. These decisions
are left to the system designer, but may be implemented the same
way.
[0074] In this example there is a single client 2, but there may be
multiple clients using the Foo group, in which case each client
would have an instance of the Foo group proxy 40 and would be
notified and updated by the group service. Likewise, the Foo group
proxy 40 for each client would buffer that client's commands during
any transitions.
[0075] While in the described embodiment a service detects a
client's unavailability through leasing, any other method of
detecting unavailability can be used. For example, a dedicated
failure detection service may be employed to actively and
interactively monitor the status of all system components. Many
methods for detecting unavailability, whether performed by each
service or by a generic failure detection service, are known to
those skilled in the art, and all such methods, as well as any
others later invented, are included within the scope of this
invention.
[0076] Similarly, while in the described embodiment the group
service announces the notification of the Foo-1 10 failure,
essentially combining the functions of failure detection, failure
announcement and group organization, the system can be designed to
separate these functions; specifically, a failure detection service
could detect and announce failures to clients and to the group
service, or it could pass detections on to an announcement
service.
[0077] Continuing in FIG. 6, upon notification of the
unavailability of Foo, the group proxy 40 begins to buffer commands
to Foo from the client 2 it represents. The group service 24 then
requests 604 that another Foo service, in this case Foo-2 12,
become the coordinator of the group and synchronize its state with
the remaining Foos 605, 606. The state synchronization is handled
by the grouping agent 12b, 14b for each of the services 12, 14.
Depending on the degree of assurance of synchronization required,
this can be done anonymously through event notification (low degree
of assurance) or explicitly through tightly-coupled individual
method invocations (high degree of assurance). Foo-2 12 becomes the
coordinator and then acknowledges the group service 24, 607. The
group service 24 registers Foo-2 12 as the Foo service with the
look-up service 20, 608, preferably by providing the look-up
service 20 with a new Foo group proxy 40a, 608a containing the same
group logic shell 30, but now with the Foo-2 service proxy 12a.
Alternatively, if the look-up service 20 is capable of modifying
registered proxies, the group service 24 can provide the look-up
service 20 with the Foo-2 service proxy 12a to update the Foo group
proxy 40 with (but leaving the existing group logic shell 30 in
place). The group service 24 then distributes the Foo-2 service
proxy 12a to the clients' group proxies (only one shown) 609. The
group proxies 40 delete the Foo-1 service proxy 10a and add the
Foo-2 service proxy 12b, 609a. The group service 24 then announces
(not shown) to all the group proxies that the Foo service is again
available. Note that steps 608 and 609 can be executed in either
order or concurrently. Using the Foo-2 service proxy 12a the group
proxy 40 directs the buffered commands to Foo-2 610. Once all
buffered command have been sent, the client 2 commands can again be
sent directly.
[0078] The handling of a failure of one of the services in a peer
group is relatively trivial. The failure might be detected when a
failed Foo service does not renew its lease with the group service,
or when the client's group proxy detects that a failed Foo did not
provide a response to an invocation and then notifies the group
service 24. The failed Foo's service proxy is simply removed from
the peer group logic shells at the clients 2 and the look-up
service 20 bundle as described above with respect to FIG. 8. In a
peer group configuration, the transition period is much short than
for a CC group, so buffering may not be needed.
[0079] As in the case of the CC groups, while the details of the
peer group have been described with a single client, it is equally
applicable to an application with multiple clients, where each
client has a replica of one or more group proxies. The group
service notifies and updates the group proxies at each of the
clients and each group proxy buffers commands for the client it is
attached to.
[0080] Swapping Services
[0081] In addition to group membership changes and fail-overs the
group proxy can be used to handle other types of transitions. For
instance, it may be desirable to swap one service for another on
the fly, that is without stopping the application or the without
stopping the clients that call those services. This is easily
achieved, for peer or CC groups, with the current architecture. The
group service instructs the relevant grouping agents to begin
buffering clients' commands, and then deregisters the retiring
service's group proxy from the look-up service. The retiring
service's proxy is also removed from the group service. The
replacement service registers with the group service to be a member
of the group. The group service then updates the group proxies at
the clients with the new service proxy, or an entirely new group
proxy. The group service then registers the new group proxy with
the lookup service. The group service then instructs the group
proxies at the clients to again start processing commands, and the
group proxies release their queue of buffered client commands to
the replacement service. Alternatively, the replacement service can
be added to the group of the retiring service first, and then the
retiring service can be removed. On the fly swapping of services
can also easily be accomplished even for a client that is ungrouped
by treating it as a group of one.
[0082] The group proxy can also be used to test a new service that
is being run in parallel with an existing service. The service to
be tested is registered as part of the same group as the prime
service. Commands from the client are transmitted from the group
proxy to the prime service as well as the test service. In a peer
group this is accomplished by broadcasting client commands to all
members of the group. In a CC group it is accomplished by
instructing the grouping agent for the coordinator (assuming the
prime service is the coordinator) to effectively superimpose a peer
group subset containing it and the service to be tested (or its
grouping agent), though still operating in CC mode with the
remainder of the cohorts. In either peer or CC mode, as responses
are returned to the group proxy it compares results, performance,
and so forth, from the service to be tested with the previously
established services. Alternately, the group proxy can pass these
results to a specialized benchmarking or evaluation program to
perform these comparisons, or to a log service for later
evaluation. It is also possible to have the grouping agent for the
test service handle or discard the responses. Many other ways of
performing comparison will be obvious to those skilled in the art
and are incorporated within the scope of this invention.
[0083] It is also possible to test an ungrouped service (and even a
service that is not group-aware) in parallel with a group of
services by having all the group proxies that call the group copy
their commands to the service being tested. Similarly, the group
proxies can be used to copy and redirect client communications for
other purposes such as recording commands to a log service or
copying communication to a display monitored by a human. The latter
use may be particularly useful when debugging new applications.
[0084] Switching Between CC and Peer Modes
[0085] As discussed above, many times a service to one client calls
yet another service to perform a function, and in such instances
the first service becomes a client of the second service. For
services grouped in peer mode, it is often not desirable for the
each service in the group to call the second service. This can
result in overloading the second service, incorrect answers, delay,
or excessive use of bandwidth. Some mechanism is required for the
plurality of members of the peer group to send a single command
when they are acting as a client. More generally, there are times
when a peer group needs to organize itself internally as CC group,
while still appearing to its own clients as a peer group.
[0086] The present invention provides a mechanism for accomplishing
such an internal reorganization as shown in FIG. 7. The peer group
50 switches to a CC group whenever its role switches from service
to client; that is, from provider to requester. In effect, one
group member assumes the role of coordinator client, while the
remaining group members become cohort clients. In this case, the
re-organization is purely internal to the peer group; it does not
reflect any desire for the group to be perceived externally by its
clients as a CC group. In FIG. 7 the Foo group 50 periodically
needs to call another service, named Goo 60. When any member of the
peer group is ready to issue such a command, in this example Foo-2
12, to the Goo service 60 it (or its grouping agent 12b) notifies
the other members 10, 14 of the group 50 (or their grouping agents
10b, 14b) 701 which then buffer their own outgoing commands to the
Goo service 60 until told otherwise. The coordinator 12 (or its
grouping agent 12b) then makes the relevant invocation on the Goo
service 60, 702. The Goo service 60 processes Foo-2's command and
returns a response 703. When Foo-2 receives the response it
distributes it to the cohorts 10, 14. The coordinator indicates
that internal operation of the group can now resume in peer mode.
If, during the time when the group 50 is in CC mode, it is
desirable to prevent any of the Foo services 10, 12, 14 from
executing any new commands from clients 4, then grouping agents
10b, 12b, 14b can buffer incoming commands. Alternatively, the Foo
service that is acting as coordinator may, instruct the group proxy
42, either directly or preferably through the group service 24, to
buffer client 4 commands.
[0087] In other cases, the transition between group modes, whether
peer to CC or CC to peer, may be triggered by an external
circumstance. For example, going from CC to peer may be occasioned
by a policy-based need for increased fault tolerance, whereas going
from peer to CC may be occasioned by clients accepting a lesser
quality of service. An example of this might be the traffic
monitoring system described in FIG. 2. During normal times it may
be sufficient to operate in CC mode with the road sensor 12 as the
coordinator and the toll booth sensor 14 as the cohort, especially
if the traffic monitoring client 2 has to pay an extra fee to get
data from the toll booth sensor 14. However, during rush hour the
sensor group 50 might switch to peer mode to ensure greater
accuracy of data and provide higher reliability.
[0088] In an alternative embodiment, the grouping agents can rely
on the group service to assist in directing transitions from peer
to CC or vice versa, but as long as the reorganization is purely
internal the group service would not update the fielded group
proxies or the group proxy being distributed by the lookup
service.
[0089] In cases where the transition is explicit, that is the group
not only reorganizes its structure, but intends to make itself
available to clients in this new structure, the group service would
be involved. In addition to directing the transitions of the
services in the group, the group service would coordinate updating
the fielded instances of the group proxy at clients and the lookup
service.
[0090] A group mode reorganization from peer to CC mode is shown in
FIG. 8 in which the change is recognized externally by clients. To
begin, the group service 24 unregisters the group from the lookup
service 20, 801. The group service 24 then creates a new group
proxy 40, comprised of the appropriate group logic shell (CC 30 or
peer 32) with the appropriate service proxies 10a, 12a, 14a bundled
in it, and distributes the new group proxy 40 to the clients 4 to
replace the old group proxy 42, 802. When the new group proxy 40
arrives at the client 4 it must handshake 803, 804 with the old
group proxy 42 in order to be made aware of any outstanding
requests, or any previously buffered commands from uncompleted
membership transitions. Once this handshake has been completed, the
new group proxy 40 can take over at that client 4 and the old group
proxy 42 can be deleted from that client 4. The group service 24
then updates the lookup service 20 with a new group proxy 805, and
then instructs the new group proxies 42 at the clients 4 to begin
transmitting commands. While in FIG. 8 the transition is from peer
to CC mode, a transition from CC mode to peer is substantially the
same
[0091] In the described embodiment, because there are different
group logic shells for peer and CC modes, the group proxy is
completely replaced when transitioning between modes. In an
alternative embodiment a single group logic shell can contain the
logic for both a CC group and a peer group. With a single group
logic shell, in order to switch modes, the group service only has
to update the service proxies bundled within group logic shell at
the clients and look-up service. Specifically, in a switch from
peer mode to CC mode, the group service would select one service
and instruct it (or its grouping agent) to become the coordinator.
The group service would then announce to all fielded group proxies
that are distributed at clients that they should delete all of the
service proxies except the coordinator's service proxy from the
group logic shell. In a switch from CC mode to peer mode, the group
service would add the necessary service proxies to the multiple
instances of the group logic shells at the clients to form a new
group proxy. In both cases, the group service would still
unregister and reregister the group with the look-up service to
reflect the group's new incarnation. One advantage to this
embodiment is that there is no need to handshake and pass buffered
commands from one group proxy to another, since the same group
proxy remains in place at each client.
[0092] In yet another alternative embodiment the group proxy could
store all the service proxies for the group in whichever mode it
was operating, and then the group service would not have to add and
delete service proxies, but simply tell the group proxies at the
clients which mode to act in and which proxies to use. In this
embodiment service proxies would still be updated as services were
added to and deleted from the group.
[0093] Dual Group Modes
[0094] It is also possible for the same group to simultaneously be
present in both peer and CC modes. The groups may have the
identical set of members or only some members in common. Clients
would determine which group mode they desired, for example based on
price or quality of service offered, and be provided by the group
service with the appropriate group proxy. FIG. 9 shows such a
distributed system in which Group B 52 is used by Client 2 4 in
peer mode, by Client 3 6 in CC mode, and by Client 1 2 in both
modes. In the example, Service b 12 is the coordinator of Group B
52 when it is in CC mode. Each client 2, 4, 6 is provided with the
appropriate group proxy 42, 44 so that it can call Group B 52 in
the desired manner. The look-up service 20 has registered a peer
group proxy 42 and a CC group proxy 44 for Group B 52. It is
therefore necessary that clients that query the look-up service 20
not only be able to find a Group B proxy, but that the lookup
service be able to provide clients with a description of the group
proxy's structure (peer or CC).
[0095] In the embodiment as shown the grouping agent 12b, 14b, 16b
for each service 12, 14, 16 detects whether a command is being sent
to the group 52 as a CC or peer group, and coordinates with the
other grouping agents to appropriately update the states of each of
the services. While it may be desirable for the services to handle
multitasking in order to effectively switch states, this is no
different from any service that can be called asynchronously by two
different clients. Alternatively, the burden of multitasking
between various states can be put onto the grouping agents 12, 14,
16. While in FIG. 9, a single grouping agent 12b, 14b, 16b is shown
for each service 12, 14, 16, in an alternative embodiment each
service could have a peer grouping agent and a CC grouping
agent.
[0096] An interesting aspect of the distributed computing system
shown in FIG. 9 is that Client 1 2 can call Group B 52 in either CC
or peer mode, depending on which group proxy it uses. The client
may be executing two processes, one of which uses Group B in peer
mode and the other uses it in CC mode. Alternatively, the client
may be group-aware and decide based on certain criteria
(reliability, cost, time of day) to execute certain commands in one
mode, and certain commands in another mode.
[0097] It is understood that the invention is not limited to the
disclosed embodiments, but on the contrary, is intended to cover
various modifications and equivalent arrangements included within
the spirit and scope of the appended claims. Without further
elaboration, the foregoing will so fully illustrate the invention,
that others may by current or future knowledge, readily adapt the
same for use under the various conditions of service.
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