WP REST API Idempotent Requests

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:

{
    "title": "My Important Post",
    "content": "This will only go out once!",
    "status": "draft",
    "idempotency_key": "1ced64e9-9537-4b7b-9919-444d9e15e201"
}

Configuration

• Idempotency key can either be passed in the request header or the request body.
• The idempotency key name can be customized.
• Change the HTTP methods the idempotency key is supported for. Defaults to POST, PUT, PATCH.

A sample request interface is included to demonstrate the selected configuration.

Developers

The plugin includes two actions to modify the dependency injection container (DIC) and insert custom services.

The wp_api_idempotence_initialize_container_builder action allows you to modify the Dependency Injection builder itself and
the wp_api_idempotence_initialize_container action allows you to override dependencies. For example:

add_action( 'wp_api_idempotence_initialize_container', function( $container ) {
    $container->set( '_dataStore', DI\object( 'YourName\CustomDataStore' ) );
} );

Under the hood, the plugin is made up of a DataStore, RequestHasher, ResponseSerializer and RequestPoller.

The DataStore 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 DataStore
interface. For example a Redis server.

The RequestHasher produces a unique hash for a WP_REST_Request object. This hash is based off of the contents of
the request, not for the object via spl_object_hash or similar. This can also be substituted by implementing the
RequestHasher interface.

The ResponseSerializer converts a WP_REST_Response or WP_Error object back and forth from a string representation.
The default JSON serializer supports filtering the serialization process using the wp_api_idempotence_serialized_response_data
and wp_api_idempotence_attach_serialized_response_data filters. See src/ResponseSerializer/Filtered.php.
The entire serializer can be substituted by implementing the ResponseSerializer interface.

Finally, the RequestPoller 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 poll.sleepSeconds and
poll.maxQueries in the DIC. The RequestPoller can also be entirely subsituted by implementing the RequestPoller
interface.

Contributing

This plugin is hosted on GitHub. Issues and pull requests are welcomed.

Installation Instructions

1. Upload the plugin files to the /wp-content/plugins/wp-api-idempotence directory, or install the plugin through the WordPress plugins screen directly.
2. Activate the plugin through the ‘Plugins’ screen in WordPress
3. Use the Settings -> WP API Idempotence screen to modify the idempotency key location or name
4. Ensure the plugin is working by using the “Sample Requests” section.

1.0

• Initial version.