This is a requirements document for EJB proxies. Please direct questions and comments to this thread: Proxy Requirements
Please do not remove any sections. If a requirement is to be removed, please strike it out so that the numbering remains consistent.
- Invocation Processor
- Invocation Dispatcher
- Invocation Target
- Client Invocation Context
- Server Invocation Context
- Invocation Components
- Remote Proxies
- EJB Proxies
The Invocation object shall consist of the reflection Method which was invoked, as well as the parameters of the method invocation. In addition, it shall provide an Attachment mechanism by which contextual information may be associated with the Invocation (for example, security and transactional information).
The serialized format of the invocation shall consist of no more or less than a means to identify the target method, the arguments passed to the target method, and the set of attachments which are associated with the invocation.
A mechanism shall be provided by which each Invocation may be preprocessed in one or more steps before it is dispatched to its final target. Such processors should have the opportunity to add, remove, or modify Attachments on the Invocation. Processors should also have the opportunity to "short-circuit" an invocation and reply directly, either with a successful return or an exception of some sort.
The Invocation Dispatcher represents the path to the target of the Invocation and shall have the ability to execute an Invocation. For Remote proxies this typically will entail transmitting the Invocation across the network in some form, to be received and forwarded to another Invocation Dispatcher to be executed directly, or passed through another set of Invocation Processors which may or may not correspond to the sending side's Processor set before handing it to a Dispatcher to be executed.
Since the Invocation Disptacher is solely responsible for the execution of the method, it can and should impose whatever policy is appropriate for the management of any target objects which may exist (such as a registration policy, or instance pooling, or session management). The Dispatcher need not correspond to one single target object instance, though that may be the common usage outside of EJB. In particular, a target object instance may be acquired from a pool, or by way of a session ID which may be attached to the Invocation.
Default (Simple) Implementations
A Dispatcher implementation shall be provided which calls the Invocation method on a given fixed Object.
A Dispatcher implementation shall be provided which passes an Invocation through an Invocation Processor to a delegate Invocation Dispatcher.
The target of an Invocation is an object upon which the Invocation is ultimately intended to be executed. Since there is not always a one-to-one correspondance between a remote proxy and a target instance, the task of passing an invocation to a target is always fulfilled by an Invocation Dispatcher.
Client Invocation Context
A Client Invocation Context is an environment which is aware of how to forward Invocations from an Exported Dispatcher to its corresponding Server Invocation Context(s).
Local Invocation Processor Chain
Each Client Invocation Context has a local Invocation Processor chain which is used for any Invocations which are sent through an exported Dispatcher. The purpose of this Processor chain is to associate any local context - such as transactional or security information - with the outbound Invocation, to be consumed the the Processor chain on the receiving side.
Remote Invocation Forwarder Registry
The Client Invocation Context shall maintain a registry of Remote Invocation Forwarders which are responsible for forwarding Invocations to the appropriate destination Server Invocation Context. The registry is keyed by a Server Invocation Context Identifier. This registry may be maintained statically or by some dynamic detection mechanism.
Remote Invocation Forwarders
A Remote Invocation Forwarder is responsible for implementing the transport layer for forwarding an Invocation to a specific Dispatcher in a specific Server Invocation Context. A Forwarder may forward to exactly one Server Invocation Context, or it may have a clustering policy of some sort.
Forwarders may have a specific policy for dealing with connection or networking failures, changes in networking topology, or any other transport-specific concerns.
In a standalone environment, a global Client Invocation Context shall be made available in order to support simple, rapid bootstraping of a working remote invocation environment. In a container (application server) environment, the Client Invocation Context may be attached to a deployment and made available in a container-specific manner.
Server Invocation Context
A Server Invocation Context is an environment which can receive and act upon incoming remote Invocations. In addition, a Server Invocation Context has an associated Client Invocation Context which is responsible for handling outbound Invocations as well as Invocations which are handled locally by the Server Invocation Context.
In order for a Dispatcher to be sendable to, and usable by, Client Invocation Contexts (i.e. Serializable), it must be exported via the Server Invocation Context which "owns" it. This registration may optionally include an Invocation Processor which should be applied to any Invocation before it is passed to the corresponding local Dispatcher.
Any Invocation which is sent to an exported Dispatcher shall always be processed by the current Client Invocation Context. When a Server Invocation Context is active, the Server Invocation Context's associated Client Invocation Context shall be used to process Invocations. The local Invocation Processor chain associated with the Client Invocation Context shall always be used to process Invocations, even if the Invocation is forwarded to the currently active Server Invocation Context.
The Server Invocation Context shall maintain a registry of exported Invocation Dispatchers. The exported Dispatchers are registered by name; thus, a name must be provided when initially exporting a Dispatcher instance.
A Server Invocation Context is generally a container (application server)-related entity. As such, the availability of a Server invocation Context may be attached to a deployment and made available in a container-specific manner.
A Remote Proxy is a proxy object whose proxy invocation handler executes Invocations through an exported Invocation Dispatcher.
Properties of Remote Proxies
It is possible that more than one remote proxy can ultimately resolve into invocation of the same target object. Because of the variety of ways in which the destination of an Invocation is ultimately resolved, there is no way to meaningfully express equality between two remote proxies.
A remote proxy is always Serializable, since its constituent Dispatcher is exported. Since an exported Dispatcher is at the heart of a remote proxy, a Client Invocation Context must be present to deserialize a remote proxy instance.
An EJB Proxy is a remote proxy which utilizes a Dispatcher that may associate a session ID (in the case of stateful session beans, for example) or other EJB-specific information with an Invocation. This Dispatcher will ultimately delegate to an exported Invocation Dispatcher.
Properties of EJB Proxies
EJB proxies are generally similar to general remote proxies in behavior; where they differ is listed below.
An EJB proxy is serializable like a remote proxy; however it may have an additional serializable Dispatcher or Dispachers associated with it (in order to supply additional information to the Invocation such as session ID).
Relationship to EJB Environment
These invocation concepts have a direct mapping to concepts in an EJB environment.
- A named EJB within a module is equivalent to a single Invocation Dispatcher. Sessions are mapped by a session ID property on an Invocation.
- An EJB module within an application server is identified by an application name and module name. This corresponds to a single Server Invocation Context, which contains entries for each EJB within the module as its registered, named Dispatchers.