The Command Pattern

Overview

Provides a distributed implementation of the classic Command Pattern (Wikipedia) built using Oracle Coherence.

Distribution

The Command Pattern implementation is distributed in the jar file called: coherence-commandpattern-11.0.0.jar.

However as the Command Pattern has several other dependencies, we **strongly recommend** that developers adopt and use tools like Apache Maven/Ivy/Gradle to transitively resolve the said dependencies, instead of attempting to do so manually.

To configure your Apache Maven-based application to use the Command Pattern, simply add the following declaration to your application pom.xml file <dependencies> element.

<dependency>
    <groupId>com.oracle.coherence.incubator</groupId>
    <artifactId>coherence-commandpattern</artifactId>
    <version>11.0.0</version>
</dependency>

Details

The Command Pattern advocates that: * An action to be performed zero or more times at some point in the future should be represented as an object called a Command.

• All necessary parameters required to perform the said action, should be encapsulated as attributes with in the said Command.

• The Command must provide a mechanism to perform the action, typically represented by an execute method defined on the said Command.

This implementation of the Command Pattern additionally advocates that: * A Command may only be executed with in the scope of a target object, called a Context

• To execute a Command it must be submitted to a Context

• 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.

• Commands are executed one-at-a-time, in the order in which they arrive at their Context

• 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.)

Design

The following diagram outlines the core classes and interfaces defined in this implementation of the Command Pattern.

The Command interface specifies the minimum requirements for Commands. It defines a single method called execute that is invoked by the internal CommandExecutor when a Command is to be executed. The execute method requires a single parameter, called the ExecutionEnvironment. The ExecutionEnvironment provides valuable information to the execute method, including the Context in which the Command is being executed. To submit Commands for execution by CommandExecutors, you must use an implementation of the CommandSubmitter interface (the DefaultCommandSubmitter class provides the standard implementation).

To define and register a Context about which you may submit Commands for execution, use an implementation of the ContextsManager interface (the DefaultContextsManager class provides the standard implementation). Once a Context is registered, you may use the returned Identifier to submit Commands for execution with a CommandSubmitter.

As Context and Command instances are stored in Coherence cluster members, they both must be Serializable or better still, implement ExternalizableLite or PortableObject.

Command Pattern Usage Walkthrough

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)

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 Command Submission

• 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).

Example

Let's start with a simple example where we use the Command Pattern to setValue(T) on the GenericContext.

First, we'll start by writing a simple SetValueCommand that extends ` AbstractCommand` as seen here:

import java.io.DataInput;
import java.io.DataOutput;
import java.io.IOException;

import com.oracle.coherence.patterns.command.Command;
import com.oracle.coherence.patterns.command.ExecutionEnvironment;
import com.tangosol.io.ExternalizableLite;
import com.tangosol.util.ExternalizableHelper;

@SuppressWarnings("serial")
public class SetValueCommand<T> implements Command<GenericContext<T>>, ExternalizableLite {

  private T value;

  public SetValueCommand() {
  }

  public SetValueCommand(T value) {
    this.value = value;
  }

  public void execute(ExecutionEnvironment<GenericContext<T>> executionEnvironment) {
    GenericContext<T> context = executionEnvironment.getContext();
    context.setValue(value);
    executionEnvironment.setContext(context);
  }

  @SuppressWarnings("unchecked")
  public void readExternal(DataInput in) throws IOException {
    this.value = (T)ExternalizableHelper.readObject(in);
  }

  public void writeExternal(DataOutput out) throws IOException {
    ExternalizableHelper.writeObject(out, value);
  }

  public String toString() {
    return String.format("SetValueCommand{value=%s}", value);
  }
}

Second, let's write a quick test that submits a number (iCount) of SetValueCommand s on "mycontext" as seen here:

import com.oracle.coherence.common.identifiers.Identifier;
import com.oracle.coherence.patterns.command.CommandSubmitter;
import com.oracle.coherence.patterns.command.ContextsManager;
import com.oracle.coherence.patterns.command.DefaultCommandSubmitter;
import com.oracle.coherence.patterns.command.DefaultContextConfiguration;
import com.oracle.coherence.patterns.command.DefaultContextsManager;
import com.oracle.coherence.patterns.command.ContextConfiguration.ManagementStrategy;
import com.tangosol.net.CacheFactory;

public class CommandPatternExample {
  /**
   * @param args
   * @throws InterruptedException
   */
  public static void main(String[] args) throws InterruptedException {

    DefaultContextConfiguration contextConfiguration = new DefaultContextConfiguration(ManagementStrategy.DISTRIBUTED);
    ContextsManager contextsManager = DefaultContextsManager.getInstance();

    Identifier contextIdentifier = contextsManager.registerContext("mycontext", new GenericContext<Long>(0L), contextConfiguration);

    CommandSubmitter commandSubmitter = DefaultCommandSubmitter.getInstance();

    commandSubmitter.submitCommand(contextIdentifier, new LoggingCommand("Commenced", 0));

    for (long i = 1; i <= 10000; i++) {
      commandSubmitter.submitCommand(contextIdentifier, new SetValueCommand<Long>(i));
    }

    commandSubmitter.submitCommand(contextIdentifier, new LoggingCommand("Completed", 0));
    CacheFactory.shutdown();
  }
}

To run this test you will need to start a Coherence cache server, in this example we use the DefaultCacheServer that ships with Coherence as seen here:

java -Dtangosol.coherence.cacheconfig=$PATH_TO/coherence-commandpattern-cacheconfig.xml \
     com.tangosol.net.DefaultCacheServer

Once the cache server is running you can start the CommandPatternExample as a cache client as seen here:

java -Dtangosol.coherence.distributed.localstorage=false -Dtangosol.coherence.cacheconfig=$PATH_TO/coherence-commandpattern-cacheconfig.xml CommandPatternExample

The SetValueCommand.java, CommandPatternExample.java and coherence-commandpattern-cache-config.xml files are included in the source code distribution.

Frequently Asked Questions

What is the relationship between the Functor Pattern, Command Pattern, Entry Processors and the Invocation Service for performing work in a cluster?

The following table outlines the main differences.

Description	Functor Pattern	Command Pattern	EntryProcessors	InvocationService
Processing Target	A Context	A Context	A Cache Entry	A Cluster Member
Submission Behavior	Non-Blocking	Non-Blocking	Blocking	Blocking and Non-Blocking
Execution Order	Order of Submission	Order of Submission	Order of Submission^1	Order of Submission
Supports Return Values?	Yes	No	Yes	Yes
Guaranteed to Execute?^2	Yes	Yes	Yes^3	Yes^4 and No^5
Guaranteed to Execute?^6	Yes	Yes	Yes^7	No^8
Recoverable?^9	Yes^10	Yes^10	No	No
Request may be cancelled?	Yes	Yes	No	Yes ^5

1. Unless using PriorityTasks
2. If Submitter terminates after submission
3. Submitter blocks
4. For blocking calls
5. For non-blocking calls
6. If Target JVM terminates during execution
7. Submitter automatically retries
8. Retry up the responsibility of the developer
9. Recoverable from disk
10. When using a Cache Store

What is a ManagementStrategy?

A ManagementStrategy defines how Commands are managed in a Coherence Cluster.
By default Commands are always co-located in the JVM that is hosting the Context associated with the said Commands. This provides instant access to the Commands for a CommandExecutor but has the disadvantage of limiting the number of Commands that may be queued for execution (before a JVM may run out of memory). The alternative is to use the DISTRIBUTED strategy, where the Commands are distributed across the cluster and only retrieved for execution when required. This provides the advantage of enabling massive capacity (scaling to the size of the cluster and beyond when disk storage is configured), but has the disadvantage that retrieving Commands for execution may take longer.

How do I monitor the execution of Commands?

The Command Pattern CommandExecutors are JMX enabled by default. In order to monitor Command execution for a Context, simply enable JMX for Coherence and look for the "CommandExecutors" in your JMX monitoring application (ie: Something like JConsole). All kinds of JMX information is available, including Min, Max and Average Execution time, together with the number of current Commands pending to be executed and those that have been executed.

References and Additional Information

• The Functor Pattern

• Wikipedia - Command Pattern