Implementing the Command Pattern

by jmorris 19. August 2009 22:38

Command Pattern Background

One of the most ubiquitous software patterns in existence is the Command Pattern:  “Encapsulate a request as an object, thereby allowing for the parameterization of clients with different requests, queue or log requests, and support undoable operations” – GOF. It is generally used in situations where the all of the information necessary for a future call can be ‘built up” sequentially and finally executed at a point in time in the future – i.e. each action is state-full.

A common scenario is an object used to build up a database request taking in several different parameters such as the database name and location, the stored procedure name, any data that needs to be passed to the stored procedure and possibly a transaction object. Once the command has been constructed and all required parts are defined, it is executed, which delegates an action to call the stored procedure on the database and return a result set or perform some operation on an object, with the provided parameters. In the C# world, we are talking about System.Data.DbCommand and its concrete classes such as System.Data.SqlClient.SqlCommand or any other database specific implementation of DbCommand (Oracle, MySql, etc.).

Here is the basic UML diagram of the Command Pattern:

The Command Pattern typically utilizes the following actors:
  1. Client – creates the command object providing the relevant information (parameters) needed to fulfill its obligations
  2. Invoker – initiates the action on the command so that it’s obligations are fulfilled, making the decision of which of the commands actions should be called
  3. Receiver – performs the command object’s obligations, whatever they might be – i.e. makes database call or some other action

The Command Pattern Implemented

The scenario is this: a service exists which takes as input a series of inputs (a query) and returns as output, the results of those inputs as an xml stream of attributes and values. Caching at the service level and several operations are also supported: filtering, sorting, paging, etc.

Each request varies by parameters and is encapsulated as a query object defined by the client. This query object is simply a data structure that contains the fields accepted optionally be the service. The query object is executed by a command object created by the client, who delegates control to the receiver encapsulated by it’s invoking one of its action methods.

The following diagram illustrates the implementation:

 

 

The Command implementation is very simple. It contains an ICommandReceiver reference and IQueryRequest reference which stores a reference to the current query being executed. The command delegates control to the receiver, which performs its work and returns back an IQueryResponse object containing the results of the query.

 

The ContentReceiver class implements ICommandReceiver interface and simply makes the call to the remote service via a WCF client. This happens to be a very simplistic implementation in that much of the heavy lifting (i.e. caching, query generation, etc.) occurs in the service. In this case, we are simply translating a client query into a SOAP request to access the services resources.


The client uses an IQueryRequest object to build up a set of parameters which are consumed by the service endpoint via the command object. The results are returned to the client with an IQueryResponse object. How the results are used is dependent upon the client.




Finally, a simple unit test illustrating the usage:

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Comments (1) -

Jack
Jack
5/8/2011 5:50:33 AM #

A command is a message. Messages are one-way almost by definition. That is one of the primary wins of a command. The executor does not know what execute() does, such that these things can be queued, transmitted, undone etc.

If you start returning a value from Execute then that all breaks. What you have is no longer a command. Its a curried function - A function that has parameters baked in. It is no longer a message because the 'sender' needs to know stuff about its internals.

Jeff Morris

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