Transients that serve stale data while regenerating the new transients in the background.

Requires support for fastcgi_finish_request, or else transients will regenerate expired data immediately.

Transients are great for storing data that is expensive to regenerate, but we still run in to the problem of needing to regenerate that data synchronously once the transient expires. This library solves that problem by serving stale data once the transient is expired, and processing the regenerate callback after the request has finished, so that end users never see the impact of regenerating transients.

This library is meant to be included with composer. To install, run composer require 10up/async-transients. The library is set up to use composer's autoloader, so make certain you are loading your vendor/autoload.php file.

Usage is similar to standard WordPress transient functions, in that you provide a transient key and an expiration time, but its different in that you must also provide a callback function, as well as any (optional) parameters to pass to the callback function, that should be called to regenerate the transient data if it is expired.

Example Usage:

// Function to regenerate expired transient
function get_data_from_api( $user_id ) {
	// Fake function, that we assume is really time consuming to run
	$my_result = get_api_data_for_user( $user_id );

	\TenUp\AsyncTransients\set_async_transient( 'transient-key-' . $user_id, $my_result, MINUTE_IN_SECONDS );
}

// This would very likely not be hardcoded...
$user_id = 1;

// If the transient is expired get_data_from_api() is called, with $user_id as a parameter
$transient_value = \TenUp\AsyncTransients\get_async_transient( 'transient-key-' . $user_id, 'get_data_from_api', array( $user_id ) );

// Outputs the value stored in the transient
// If the transient is expired, it will still show the last known data, while queueing the transient to be updated behind the scenes.
var_dump( $transient_value );

How does all of this work?

First, when calling get_async_transient, you now have to pass a callback function, and optionally, any parameters to pass to the callback function. The transient is then retrieved, much like how WordPress core would retrieve it, but with a key difference. Instead of returning nothing if the transient is expired, we return the last known value, and add the callback function and params to a queue, to process later. By the end of the request, we have a queue of that contains callback functions for all transients that were accessed, that had expired data.

Next, we hook into the WordPress shutdown action. The shutdown action runs just before PHP shuts down execution. The Transient class hooks into that action, and calls the fastcgi_finish_request function, if it is available. That function flushes all response data to the client, and as far as the browser is concerned, the request is done, however, php is allowed to keep running in the background.

At this point, we iterate over all the callback functions in the queue, which then regenerate any transient data that was accessed, but was expired.

If you identify any errors or have an idea for improving the plugin, please open an issue. We're excited to see what the community thinks of this project, and we would love your input!

Async Transients is free software; you can redistribute it and/or modify it under the terms of the MIT License.