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[Vufind-admins] [VuFind Issues] (VUFIND-924) Use Command Pattern in search system
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From: David M. (JIRA) <dem...@vi...> - 2013-10-25 07:02:00
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<a class="user-hover" rel="dmaus" id="email_dmaus" href="https://vufind.org/jira/secure/ViewProfile.jspa?name=dmaus" style="color:#326ca6;">David Maus</a>
created <img src="https://vufind.org/jira/images/icons/issuetypes/improvement.png" height="16" width="16" border="0" align="absmiddle" alt="Improvement"> <a style='color:#326ca6;text-decoration:none;' href='https://vufind.org/jira/browse/VUFIND-924'>VUFIND-924</a>
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<a style='color:#326ca6;text-decoration:none;' href='https://vufind.org/jira/browse/VUFIND-924'><strong>Use Command Pattern in search system</strong></a>
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<strong style="font-weight:normal;color:#505050;">Issue Type:</strong>
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<img src="https://vufind.org/jira/images/icons/issuetypes/improvement.png" height="16" width="16" border="0" align="absmiddle" alt="Improvement"> Improvement
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<strong style="font-weight:normal;color:#505050;">Assignee:</strong>
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<a class="user-hover" rel="dmaus" id="email_dmaus" href="https://vufind.org/jira/secure/ViewProfile.jspa?name=dmaus" style="color:#326ca6;">David Maus</a>
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<strong style="font-weight:normal;color:#505050;">Components:</strong>
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Search </td>
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<strong style="font-weight:normal;color:#505050;">Created:</strong>
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25/Oct/13 3:00 AM
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<strong style="font-weight:normal;color:#505050;">Description:</strong>
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Limitations of the current implementation
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<br/>
In order to initiate a backend operation via the search system, the operation in question must be known to the search system service. The service resolves the backend name and delegates the call; thus it must know not only the requested operation but also its formal parameters.
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The Command Pattern [19 provides a solution to this be encapsulating the call to the backend method in a Command that is invoked by the search service. Using the command pattern executing a operation works as follows:
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1. The caller creates a Command object that provides the name of the target backend
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2. The caller passes the command to the service's invoke() method
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3. The service resolves the backend name to a backend instance
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4. The service triggers the .pre event with command and backend instance
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5. The service executes the command
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6. The service triggers the .post event with command and backend instance
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#+BEGIN_SRC php
<br/>
namespace VuFindSearch;
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<br/>
class Service
<br/>
{
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// ...
<br/>
<br/>
/**
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* Invoke command.
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*
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* @param Command\CommandInterface $command
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* @return void
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*/
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public function invoke (Command\CommandInterface $command)
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{
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$backend = $this->resolve($command->getTargetBackendName());
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$this->triggerPre($command, $backend);
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try {
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$command->execute($backend);
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} catch (BackendException $error) {
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$this->triggerError($command, $backend, $error);
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throw $error;
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}
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$this->triggerPost($command, $backend);
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}
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}
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#+END_SRC
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The caller can check if the command was executed by calling the command's isExecuted() method and retrieve the result by calling the command's getResult() method. A command SHOULD throw a LogicException if getResult() is called before the command is executed.
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<br/>
#+BEGIN_SRC php
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namespace VuFindSearch\Command
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<br/>
interface CommandInterface
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{
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/**
<br/>
* Return name of target backend.
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*
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* @return string
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*/
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public function getTargetBackendName ();
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/**
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* Execute command on backend.
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*
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* @param BackendInterface $backend
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* @return mixed
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*/
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public function execute (BackendInterface $backend);
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/**
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* Was the command executed?
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*
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* @return boolean
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*/
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public function isExecuted ();
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/**
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* Return result of executed operation.
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*
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* @throws LogicException Command was not yet executed
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*
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* @return mixed
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*/
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public function getResult ();
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/**
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* Return search parameters.
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*
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* @return ParamBag
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*/
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public function getSearchParameters ();
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}
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#+END_SRC
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<br/>
Listeners
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<br/>
One implication of the command pattern is, that Listeners need to check the command class to decide whether they should activate or not. This is not a problem per-se: The search service provides interfaces for common operations (search, retrieve, retrieveBatch etc.) that listeners can check for.
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[1] <a href="http://en.wikipedia.org/wiki/Command_pattern">http://en.wikipedia.org/wiki/Command_pattern</a>
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<strong style="font-weight:normal;color:#505050;">Fix Versions:</strong>
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2.2 </td>
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<strong style="font-weight:normal;color:#505050;">Project:</strong>
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<a style="color:#326ca6;" href="https://vufind.org/jira/browse/VUFIND">VuFind</a>
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<strong style="font-weight:normal;color:#505050;">Priority:</strong>
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<img src="https://vufind.org/jira/images/icons/priorities/major.png" height="16" width="16" border="0" align="absmiddle" alt="Major"> Major
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<strong style="font-weight:normal;color:#505050;">Reporter:</strong>
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<a class="user-hover" rel="dmaus" id="email_dmaus" href="https://vufind.org/jira/secure/ViewProfile.jspa?name=dmaus" style="color:#326ca6;">David Maus</a>
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