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In this chapter, we consider multiway communications - communications to or from several interfaces - and notably parallel communications, which are common in Grid computing.
Our objective is to simplify the design of distributed Grid applications with multiway interactions.
The driving idea is to manage the semantics and behavior of collective communications at the level of the interfaces.
Grid computing uses the resources of many separate computers connected by a network (usually the Internet) to solve large-scale computation problems. Because of the number of available computers, it is fundamental to provide tools for facilitating communications to and from these computers. Moreover, Grids may contain clusters of computers, where local parallel computations can be very efficiently performed - this is part of the solution for solving large-scale computation problems - , which means that programming models for Grid computing should include parallel programming facilities. We address this issue, in the context of a component model for Grid computing, by introducing collective interfaces.
The component model that we use, Fractal, proposes two kinds of cardinalities for interfaces, singleton or collection, which result in one-to-one bindings between client and server interfaces. It is possible though to introduce binding components, which act as brokers and may handle different communication paradigms. Using these intermediate binding components, it is therefore possible to achieve one-to-n, n-to-one or n-to-n communications between components. It is not possible however for an interface to express a collective behavior: explicit binding components are needed in this case.
We propose the addition of new cardinalities in the specification of Fractal interfaces, namely multicast and gathercast. Multicast and gathercast interfaces give the possibility to manage a group of interfaces as a single entity (which is not the case with a collection interface, where the user can only manipulate individual members of the collection), and they expose the collective nature of a given interface. Moreover, specific semantics for multiway invocations can be configured, providing users with flexible communications to or from gathercast and multicast interfaces. Lastly, avoiding the use of explicit intermediate binding components simplifies the programming model and type compatibility is automatically verified.
The role and use of multicast and gathercast interfaces are complementary. Multicast interfaces are used for parallel invocations, whereas gathercast interfaces are used for synchronization and gathering purposes.
Note that in our implementation of collective interfaces, new features of the Java language introduced in Java 5 are extensively used, notably annotations and generics.
A multicast interface transforms a single invocation into a list of invocations
A multicast interface is an abstraction for 1-to-n communications. When a single invocation is transformed into a set of invocations, these invocations are forwarded to a set of connected server interfaces. A multicast interface is unique and it exists at runtime (it is not lazily created). The semantics of the propagation of the invocation and of the distribution of the invocation parameters are customizable (through annotations), and the result of an invocation on a multicast interface - if there is a result - is always a list of results.
Invocations forwarded to the connected server interfaces occur in parallel, which is one of the main reasons for defining this kind of interface: it enables parallel invocations, with automatic distribution of invocation parameters.
A multicast invocation leads to the invocation services offered by one or several connected server interfaces, with possibly distinct parameters for each server interface.
If some of the parameters of a given method of a multicast interface are lists of values, these values can be distributed in various ways through method invocations to the server interfaces connected to the multicast interface. The default behavior - namely broadcast - is to send the same parameters to each of the connected server interfaces. In the case some parameters are lists of values, copies of the lists are sent to each receiver. However, similar to what SPMD programming offers, it may be adequate to strip some of the parameters so that the bound components will work on different data. In MPI for instance, this can be explicitly specified by stripping a data buffer and using the scatter primitive.
The following figure illustrates such distribution mechanisms: broadcast (a.) and scatter (b.)
Invocations occur in parallel and the distribution of parameters is automatic.
3 modes of distribution of parameters are provided by default, and define distribution policies for lists of parameters:
BROADCAST, which copies a list of parameters and sends a copy to each connected server interface.
ParamDispatchMode.BROADCAST
ONE-TO-ONE, which sends the ith parameter to the connected server interface of index i. This implies that the number of elements in the annotated list is equal to the number of connected server interfaces.
ParamDispatchMode.ONE_TO_ONE
ROUND-ROBIN, which distributes each element of the list parameter in a round-robin fashion to the connected server interfaces.
ParamDispatchMode.ROUND_ROBIN
It is also possible to define a custom distribution by specifying the distribution algorithm in a class which implements the org.objectweb.proactive.core.component.type.annotations.multicast.ParamDispatch interface.
@ParamDispatchMetadata(mode =ParamDispatchMode.CUSTOM, customMode = CustomParametersDispatch.class
))
If the invoked method returns a value, then the invocation on
the multicast interface returns an ordered collection of result
values: a parameterized list, or List<T>.
This implies that, for the multicast interface, the signature of the
invoked method has to explicitly specify
List<T> as a return type. This also implies
that each method of the interface returns either nothing, or a list.
Valid return types for methods of multicast interfaces are illustrated
as follows:
public List<Something> foo(); public void bar();
Note that our implementation of collective interfaces extensively uses new features of the Java language introduced in Java 5, such as generics and annotations.
The distribution of parameters in our framework is specified in the definition of the multicast interface, using annotations.
Elements of a multicast interface which can be annotated are: interface, methods and parameters. The different distribution modes are explained in the next section. The examples in this section all specify broadcast as the distribution mode.
A distribution mode declared at the level of the interface defines the distribution mode for all parameters of all methods of this interface, but may be overriden by a distribution mode declared at the level of a method or of a parameter.
The annotation for declaring distribution policies at level of
an interface is
@org.objectweb.proactive.core.component.type.annotations.multicast.ClassDispatchMetadata
and is used as follows:
@ClassDispatchMetadata(mode=@ParamDispatchMetadata(mode=ParamDispatchMode.BROADCAST)) interface MyMulticastItf { public void foo(List<T> parameters); }
A distribution mode declared at the level of a method defines the distribution mode for all parameters of this method, but may be overriden at the level of each individual parameter.
The annotation for declaring distribution policies at level of a
method is
@org.objectweb.proactive.core.component.type.annotations.multicast.MethodDispatchMetadata
and is used as follows:
@MethodDispatchMetadata(mode = @ParamDispatchMetadata(mode =ParamDispatchMode.BROADCAST)) public void foo(List<T> parameters);
The annotation for declaring distribution policies at level of a
parameter is
@org.objectweb.proactive.core.component.type.annotations.multicast.ParamDispatchMetadata
and is used as follows:
public void foo(@ParamDispatchMetadata(mode=ParamDispatchMode.BROADCAST) List<T> parameters);
For each method invoked and returning a result of type
T, a multicast invocation returns an aggregation of
the results: a List<T>.
There is a type conversion, from return type
T in a method of the server interface, to return
type List<T> in the corresponding method of
the multicast interface. The framework transparently handles the type
conversion between return types, which is just an aggregation of
elements of type T into a structure of type
list<T>.
Multicast interfaces manipulate lists of parameters (say,
List<ParamType>), and expect lists of results
(say, List<ResultType>). With respect to a
multicast interface, connected server interfaces, on the contrary, may
work with lists of parameters (List<ParamType),
but also with individual parameters (ParamType) and
return individual results (ResultType).
Therefore, the signatures of methods differ from a
multicast client interface to its connected server
interfaces. This is illustrated in the following figure: in
a. the foo method of the multicast interface returns
a list of elements of type T collected from the
invocations to the server interfaces, and in b. the
bar method distributes elements of type
A to the connected server interfaces.
Figure 31.3. Comparison of signatures of methods between client multicast interfaces and server interfaces.
For a given multicast interface, the type of server interfaces which may be connected to it can be infered by applying the following rules: for a given multicast interface,
the server interface must have the same number of methods
for a given method method foo of the multicast interface, there must be a matching method in the server interface:
named foo
which returns:
void if the method in the multicast method returns void
T if the multicast method returns list<T>
for a given parameter List<T> in the multicast method, there must be a corresponding parameter, either List<T> or T, in the server interface, which matches the distribution mode for this parameter.
The compatibility of interface signatures is verified automatically at binding time, resulting in a documented IllegalBindingException if signatures are incompatible.
A gathercast interface transforms a list of invocations into a single invocation
A gathercast interface is an abstraction for n-to-1 communications. It handles data aggregation for invocation parameters, as well as process coordination. It gathers incoming data, and can also coordinate incoming invocations before continuing the invocation flow, by defining synchronization barriers.
Gathering operations require knowledge of the participants of the collective communication (i.e. the clients of the gathercast interface). Therefore, the binding mechanism, when performing a binding to a gathercast interface, provides references on client interfaces bound to the gathercast interface. This is handled transparently by the framework. As a consequence, bindings to gathercast interfaces are bidirectional links.
Gathercast interfaces aggregate parameters from method invocations from client interfaces into lists of invocations parameters, and they redistribute results to each client interface.
Invocation parameters are simply gathered into lists of parameters. The indexes of the parameters in the list correspond the index of the parameters in the list of connected client interfaces, managed internally by the gathercast interface.
The result of the invocation transformed by the gathercast interface is a list of values. Each result value is therefore indexed and redistributed to the client interface with the same index in the list of client interfaces managed internally by the gathercast interface.
Similarly to the distribution of invocation parameters in multicast interfaces, a redistribution function could be applied to the results of a gathercast invocation, however this feature is not implemented yet.
An invocation from a client interface to a gathercast interface is asynchronous, provided it matches the usual conditions for asynchronous invocations in ProActive, however the gathercast interface only creates and executes a new invocation with gathered parameters when all connected client interfaces have performed an invocation on it.
It is possible to specify a timeout, which corresponds to the maximum amount of time between the moment the first invocation of a client interface is processed by the gathercast interface, and the moment the invocation of the last client interface is processed. Indeed, the gathercast interface will not forward a transformed invocation until all invocations of all client interfaces are processed by this gathercast interface.
Timeouts for gathercast invocations are specified by an annotation on the method subject to the timeout, the value of the timeout is specified in milliseconds:
@org.objectweb.proactive.core.component.type.annotations.gathercast.MethodSynchro(timeout=20)
If a timeout is reached before a gathercast interface could gather
and process all incoming requests, a
org.objectweb.proactive.core.component.exceptions.GathercastTimeoutException
is returned to each client participating in the invocation. This
exception is a runtime exception.
Gathercast interfaces manipulate lists of parameters (say,
List<ParamType>), and return lists of results
(say, List<ResultType>). With respect to a
gathercast interface, connected client interface work with parameters
which can be contained in the lists of parameters of the methods of the
bound gathercast interface (ParamType), and they return results which
can be contained in the lists of results of the methods of the bound
gathercast interface (ResultType).
Therefore, by analogy to the case of multicast interfaces,
the signatures of methods differ from a gathercast server
interface to its connected client interfaces. This is
illustrated in the following figure: the foo method of interfaces which
are client of the gathercast interface exhibit a parameter of type
V, the foo method of the gathercast interface
exhibits a parameter of type List<V>.
Similarly, the foo method of client interfaces return a parameter of
type T, and the foo method of the gathercast
interface returns a parameter of type
List<T>.
The compatibility of interface signatures is verified automatically at binding time, resulting in a documented IllegalBindingException if signatures are incompatible
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