Timothy Jacobs

When a network error is encountered, API clients should be able to retry a request without a risk of their request being processed twice. 
WP API Idempotence adds support for clients to include an idempotency key that uniquely identifies that request. If the server detects 
that a request with the same key has already been processed or is currently being processed, the response for the initial request will 
be returned.
For Example:
<pre><code>{
 "title": "My Important Post",
 "content": "This will only go out once!",
 "status": "draft",
 "idempotency_key": "1ced64e9-9537-4b7b-9919-444d9e15e201"
}
</code></pre>
<h4>Configuration</h4>
<ul>
<li>Idempotency key can either be passed in the request header or the request body.</li>
<li>The idempotency key name can be customized.</li>
<li>Change the HTTP methods the idempotency key is supported for. Defaults to <code>POST</code>, <code>PUT</code>, <code>PATCH</code>.</li>
</ul>
A sample request interface is included to demonstrate the selected configuration.
<h4>Developers</h4>
The plugin includes two actions to modify the dependency injection container (DIC) and insert custom services.
The <code>wp_api_idempotence_initialize_container_builder</code> action allows you to modify the Dependency Injection builder itself and 
the <code>wp_api_idempotence_initialize_container</code> action allows you to override dependencies. For example:
<pre><code>add_action( 'wp_api_idempotence_initialize_container', function( $container ) {
 $container->set( '_dataStore', DI\object( 'YourName\CustomDataStore' ) );
} );
</code></pre>
Under the hood, the plugin is made up of a <code>DataStore</code>, <code>RequestHasher</code>, <code>ResponseSerializer</code> and <code>RequestPoller</code>.
The <code>DataStore</code> is primarily responsible for retrieving or storing an idempotent request. By default, requests 
are stored in a custom database table. This could be substituted for a custom driver by implementing the <code>DataStore</code> 
interface. For example a Redis server.
The <code>RequestHasher</code> produces a unique hash for a <code>WP_REST_Request</code> object. This hash is based off of the contents of 
the request, not for the object via <code>spl_object_hash</code> or similar. This can also be substituted by implementing the 
 RequestHasher interface.
The <code>ResponseSerializer</code> converts a <code>WP_REST_Response</code> or <code>WP_Error</code> object back and forth from a string representation. 
The default JSON serializer supports filtering the serialization process using the <code>wp_api_idempotence_serialized_response_data</code> 
and <code>wp_api_idempotence_attach_serialized_response_data</code> filters. See <code>src/ResponseSerializer/Filtered.php</code>. 
The entire serializer can be substituted by implementing the <code>ResponseSerializer</code> interface.
Finally, the <code>RequestPoller</code> class polls the data store for a response if it is determined that an idempotent request 
is currently being processed when another request with the same key arrives. By default, the data store is polled 
every seconds a maximum of 15 times to try and retrieve a response object. If no response is found, an error with code 
 rest_duplicate_idempotency_key will be returned. This can be adjusted by overwriting the <code>poll.sleepSeconds</code> and 
 poll.maxQueries in the DIC. The <code>RequestPoller</code> can also be entirely subsituted by implementing the <code>RequestPoller</code> 
interface.
<h4>Contributing</h4>
This plugin is hosted on <a href="https://github.com/iron-bound-designs/wp-api-idempotence" rel="nofollow ugc">GitHub</a>. Issues and pull requests are welcomed.

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