coherence-command-pattern

The Command Pattern Project Provides a distributed implementation of the classic Command Pattern.

Overview

This is an example implementation of the Command Pattern (Wikipedia) built using Oracle Coherence.

The Command Pattern advocates that;

1. An action to be performed zero or more times at some point in the future should be represented as an object called a Command.
2. All necessary parameters required to perform the said action, should be encapsulated as attributes with in the said Command object.
3. The Command object must provide a mechanism to perform the action (typically represented an execute method defined on the said Command object)

This implementation of the Command Pattern additionally advocates that;

1. A Command may only be executed with in the scope of a target object, called a Context.
2. To execute a Command it must be submitted to a Context.
3. Once submitted , Commands are executed asynchronously. That is, the submitter of the Command does not block or wait for the Command to be executed after it has been submitted.
4. Commands are executed one-at-a-time, in the order in which they arrive at their Context.
5. A Command may not return a value to the application that submitted the said Command. If you require a Command that returns a value after execution, you need a Functor (see the Functor Pattern)

Revision History

Version 2.8.5 (changes since 2.8.3)

Version 2.8.5 is a patch release to upgrade dependencies to Coherence Common. It also adds the ability to specify the number of threads provided for executing commands, using the command line argument com.oracle.coherence.commandpattern.threadpoolsize. The default size is 5.

Version 2.8.4 was an internal unreleased version.

Version 2.8.3 (changes since 2.8.2)

Version 2.8.3 is a patch release to upgrade dependencies to Coherence Common.

Version 2.8.2 (changes since 2.8.1)

Version 2.8.2 is a patch release to upgrade dependencies to Coherence Common.

Version 2.8.1 (changes since 2.8.0)

Version 2.8.1 is a patch release to upgrade dependencies to Coherence Common and support the out and coming Coherence release.

Version 2.8.0 (changes since 2.7.3)

The most significant upgrade for this release is the adoption of Coherence 3.7. Overtime this will allow the pattern to take advantage of the lastest features, like "lite transactions" to ensure atomicity of multi-cache updates, without the overhead of "heavy" transactions.

Legend | = Fixed | = Breaking Change / Deprecation | = Improved | = New

• Upgraded to use the latest Coherence Common and Coherence 3.7 releases.

• Removed the separate pof and non-pof cache configurations. Cache configuration files now use the system-wide defined serializer. This dramatically simplifies serialization configuration and halves the number of cache configuration files.

• Explicitly specifies xsds in configuration files to strictly enforce document validation.

Step 1: The Initial Environment

• One Client Application
• Two Coherence Cache Servers
• All part of the same Coherence Cluster ^*

^* The Client Application may be external to the Coherence Cluster when using Coherence *Extend

Step 2: Context Creation

• The client application creates an instance of a Context

Step 3: Context Registration

• The client application registers the created Context using a ContextsManager ^*
• The ContextsManager places the Context into a "primary partition" of the "contexts" Distributed Cache.
• Coherence ensures a backup of the Context is made to a "backup partition" (on separate JVMs and different machines where possible)

^* Typically an instance of the DefaultContextsManager

Step 4: Establishing the CommandExecutor (automatic)

• When a Context is registered, an internal event establishes an appropriate CommandExecutor and necessary infrastructure.

Step 5: Creating a Command

• The client application creates an instance of a Command

Step 6: Submitting a Command for execution

• The client application uses a CommandSubmitter to submit a Command for execution with an identified Context ^*
• The submitted Command is placed into the "commands" Distributed Cache (and automatically backed up)
• The submitted Command is then automatically queued (FIFO) and scheduled for asynchronous execution by the CommandExecutor for the Context

^* An individual Command instance may be submitted any number of times for execution to one or more Contexts. There is no need to create new instances of the same Command if it needs to be submitted for execution multiple times.

Step 7: Numerous Commands Submitted

• Multiple Commands may be submitted for execution at once.
• Commands are queued for execution (FIFO) in order of arrival at the Context

Step 8: Commands are executed by the CommandExecutor (Asynchronously)

• When Commands are queued for execution, an internal event notifies the CommandExecutor to start executing the Commands
• For efficiency, the CommandExecutor may execute Commands in batches (but remaining in order).

Step 9: Commands Automatically Cleaned-up when Executed.

• Once a Command has been executed, it is removed from the "commands" cache (as is the backup of the Command).