Real world example of application of the command pattern

Question

Command pattern can be used to implement Transactional behavior (and Undo).
But I could not find an example of these by googling. I could only find some trivial examples of a lamp that is switched on or off.
Where can I find a coding example (preferably in Java)of this/these behaviors implemented using the Command Pattern?

Singlebreasted

Search for any usage of Runnable or Callable. – Summers

Answer 1

In one of our projects, we have the following requirement:

1. Create a record in DB.
2. Call a service to update a related record.
3. Call another service to log a ticket.

To perform this in a transactional manner, each operation is implemented as a command with undo operation. At the end of each step, the command is pushed onto a stack. If the operation fails at some step, then we pop the commands from the stack and call undo operation on each of the command popped out. The undo operation of each step is defined in that command implementation to reverse the earlier command.execute().

Hope this helps.

Avraham

Not sure I follow.A transaction is rolledbacked.Is not composed of undo operations.How did you implement the atomic part with the command pattern? – Singlebreasted

In a database level transaction, each operation is a command with undo and redo operations. Upon rollback, the DBMS will call the undo operation to revert the changes made to the copy of the DB it held. We are simulating the same thing in a distributed transaction that spans across systems. Does this make sense now? – Avraham

revert the changes made to the copy of the DB it held.So the command applies first to a copy of the data and not the actual data?I thought that it applied directly to data that is why you need undo.Your description is somewhat clear but I need a little more low level details if possible to see the big picture – Singlebreasted

No, the DBMS will make a copy of the data so that others will see the data without your changes (transaction isolation levels are about this only). When you commit, then the copy will be updated to the actual location so that everyone else gets to see your changes. The redo operation on a command is used during failure recovery in conjunction with checkpoints (DBMS concept worth reading once) – Avraham

So in your example of your implementation which parts of data did you copy to emulate the transactional behavior?The whole data? – Singlebreasted

In a distributed transaction, the modifications are done to the actual copies and hence you need a specific implementation of what an undo would mean. In our case, the undo operation for the service call (step2) is to issue another update that reverts the state of the record to previous values and that for the command that logs a ticket is to cancel it. – Avraham

Answer 2

public final class Ping implements Callable<Boolean> {

  private final InetAddress peer;

  public Ping(final InetAddress peer) {
    this.peer = peer;
  }

  public Boolean call() {
    /* do the ping */
    ...
  }
}
...
final Future<Boolean> result
    = executorService.submit(new Ping(InetAddress.getByName("google.com")));
System.out.println("google.com is " + (result.get() ? "UP" : "DOWN"));

Hematology

@Singlebreasted the Wikipedia article on the pattern mentions explicitly the following... "A typical, general-purpose thread pool class might have a public addTask method that adds a work item to an internal queue of tasks waiting to be done. ... The items in the queue are command objects. Typically these objects implement a common interface such as java.lang.Runnable that allows the thread pool to execute the command even though the thread pool class itself was written without any knowledge of the specific tasks for which it would be used." – Hematology

@Singlebreasted this is such an example in Java except using Callable as opposed to Runnable in conjunction with an ExecutorService. A similar such example can be seen in the discussion regarding the Wikipedia article itself here. – Hematology

But I am asking for an example of specific application of the pattern.You have provided a gereral example of the pattern though – Singlebreasted

@Singlebreasted in general any Runnable will fulfill the command pattern. e.g. class ExitTask implements Runnable { public void run() { System.exit(0); } } – Hematology

Perhaps I am misreading your answer but my question in the OP is, how to use the command pattern (with Java example) to implement transactional behavior. – Singlebreasted

@Singlebreasted indeed I was. I thought you were requesting a general Java example of the command pattern :-p – Hematology

Answer 3

Command Patterns are used in a lot of places.

1. Of course what you see everywhere is a very trivial example of GUI Implementation, switches. It is also used extensively is game development. With this pattern the user can configure his buttons on screen as well.
2. It is used in Networking as well, if a command has to be passed to the other end.
3. When the programmers want to store all the commands executed by the user, e.g. sometimes a game lets you replay the whole level.
4. It is used to implement callbacks.

Here is a site which provides as example of command pattern used for callback. http://www.javaworld.com/article/2077569/core-java/java-tip-68--learn-how-to-implement-the-command-pattern-in-java.html?page=2

5. Here's another link which shows command pattern with database. The code is in C#. http://www.codeproject.com/Articles/154606/Command-Pattern-at-Work-in-a-Database-Application

Tyronetyrosinase

The code in the codeproject link is in C++, not C#. – Luciferous