 | This documentation applies to the Command Pattern 2.3.0. The latest Command Pattern documentation is available is here. |
The Command Pattern
This is an example implementation of the Command Pattern (Wikipedia) design pattern build on top of Coherence.
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 object.
- 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;
- 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)
Dependencies
This project (like other Coherence Incubator projects) uses Apache Ivy for dependency specification and management. While a standard ivy.xml definition file ships with the source and documentation distribution, the following diagram visually indicates the current dependencies.
|
Outline
Rationale
The Command Pattern provides a useful alternative to Oracle Coherence EntryProcessors with the advantage that Commands are executed asynchronously, instead of synchronously as is the case with EntryProcessors.
Additionally this implementation of the Command Pattern provides essential infrastructure for several other Incubator projects (namely the Store and Forward Messaging Pattern) to permit guaranteed, in-order, asynchronous processing of Commands. |
What's New?
The following changes have been made since the Command Pattern 2.2.0 release.
- Migrated to use Apache Ivy for dependency management, building and publishing artifacts
- Added generic exception handler to the CommandExecutor.execute() method so that the underlying Java
ExecutorServices don't just ignore internal exceptions (and not log them).
This can help debug serialization problems caused by defective Commands.
- Added the ability to cancel the execution of a Command after it has been submitted, but before it has been executed with the new method;
CommandSubmitter.cancelCommand(Identifier commandIdentifier);
- A CommandSubmitter will now return the Identifier that was allocated to the submitted Command
when calling one of the submitCommand(...) methods.
- Added support for PriorityCommands through the addition of the PriorityCommand marker interface.
- Added the ability to convert regular commands into priority commands with the PriorityCommandAdapter.
- Fixed the ContextWrapper POF writeExternal method to correctly write out longs as longs instead of Objects
- Added the ability to extract values from a Context using ContextsManager.extractValueFromContext method
- Resolved potential race-condition when concurrently submitting Commands and a Context is in the process of recovering due to partition recovery or balancing.
Structure
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 Command objects. 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 CommandExecutor s, 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 Command s 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 Command s for execution with a CommandSubmitter.
| Be Careful
As Context and Command instances are stored in Coherence cluster members within the Data Grid, they 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)
* 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).
|
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=$PATH_TO/coherence-commandpattern-cacheconfig.xml com.tangosol.net.DefaultCacheServer
Once the cache server is running you can start the CommandPatternExample as a cache client as seen here:
-Dtangosol.coherence.distributed.localstorage=false -Dtangosol.coherence.cacheconfig=$PATH_TO/coherence-commandpattern-cacheconfig.xml CommandPatternExample
 | The SetValueCommand.java, CommandPatternExample.java and commandpattern-cacheconfig.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.
| |
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 (unless using PriorityTasks) |
Order of Submission |
Supports Return Values?
(and re-throwing Exceptions) |
Yes |
No |
Yes |
Yes |
Guaranteed to Execute?
(if Submitter terminates after submission) |
Yes |
Yes |
Yes (submitter blocks) |
Yes (for blocking), No (for non-blocking) |
Guaranteed to Execute?
(if Target JVM terminates during execution) |
Yes |
Yes |
Yes (submitter automatically retries) |
No (retry up to developer) |
| Submitted requests recoverable from disk? |
Yes (when using Cache Store) |
Yes (when using Cache Store) |
No |
No |
| Request may be cancelled? |
Yes |
Yes |
No |
Yes (for non-blocking) |
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
