EventBus

EventBus is a MediaWiki extension that produces changes to an [event intake service].

This extension is part of a larger effort to create a general purpose event system, to reliably propagate state changes from one part of the infrastructure, to another. Since this approach relies upon MediaWiki Hooks and deferred (async) PHP function calls, it is not atomic (delivery of an event could fail after MediaWiki has committed the corresponding change), and so does not provide the reliability we're aiming for. Therefore, this extension should be considered an interim solution, until the requisite changes can be made to MediaWiki core. See this task for more information.

This EventBus extension is really meant to be just a Wikimedia Event Platform producer client library for MediaWiki PHP, but over time it has also become a place where the logic for constructing events themselves is placed.

EventBus was originally intended just for the propagation of changes to core MediaWiki entities (pages, revisions, users, etc.). Until a better place becomes available, or MediaWiki core provides us with a default way to do this, event construction logic for MediaWiki core entities should remain here in this EventBus extension.

However, other usages of EventBus as a library to produce events should live in other extensions that use the EventBus extension as a dependency.

Configuration

Event Services definition

EventBus supports configuration of multiple event intake service endpoints via the $wgEventServices main config array. It expects entries keyed by event service name pointing at arrays of event service config. E.g.

$wgEventServices = [
    'eventgate-analytics' => [
        'url' => 'http://locahost:8085/v1/events',
        'timeout' => 5,
    ],
    'eventgate-main' => [
        'url' => 'http://localhost:8192/v1/events',
    ],
];

EventBus instances should be created via either the static EventBusFactory::getInstance or EventBusFactory::getInstanceForStream methods.

EventBusFactory::getInstance takes one of the configured event service names from the EventServices main config.

EventBusFactory::getInstanceForStream takes a stream name and looks up the event service name configured for that stream in $wgEventStreams config array, via the EventStreamConfig extension (see below).

Stream specific settings for EventBus

EventBus also supports some per stream configuration. These settings are defined in the producers.mediawiki_eventbus stream config setting for a stream.

Destination Event Intake servive

The event service name setting (and other settings for EventBus) for a stream is defined in the producers.mediawiki_eventbus.event_service_name stream config setting. E.g. for a stream named 'my_stream':

$wgEventStreams = [
    'my_stream' => [
        'producers' => [
            // EventBus specific settings go here:
            'mediawiki_eventbus' => [
                // Key of the event service in EventServices
                'event_service_name' => 'eventgate-main'
            ],
        ],
    ],
];

NOTE: Until T321557 - EventBus' stream config destination_event_service setting should move into producers.mediawiki_eventbus specific settings is complete, a legacy top level destination_event_service setting to look up the event service name in $wgEventServices is still supported.

Per stream configuration of the event intake service to use via EventStreamConfig is optional. The default behavior is to produce all streams to the service name specified by $wgEventServiceDefault. You must set $wgEventServiceDefault to an entry in $wgEventServices to be used in case a stream's producers.mediawiki_eventbus.event_service_name setting is not provided. E.g.

$wgEventServiceDefault = 'eventgate-main';

Enabling/disabling EventBus stream producing

If EventStreamConfig is not enabled ($wgEventStreams is undefined), then all streams are considered 'enabled' and will be produced to the $wgEventServiceDefault.

If EventStreamConfig is enabled, then all declared streams are enabled by default and will be produced by EventBus instances to the destination event intake service as defined above.

You can disable a stream by removing its entry in $wgEventStreams, or by by setting producers.mediawiki_eventbus.enabled = false , e.g.

$wgEventStreams = [
    'my_stream' => [
        'producers' => [
            // EventBus specific settings go here:
            'mediawiki_eventbus' => [
                // Key of the event service in EventServices
                'event_service_name' => 'eventgate-main',
                // Disable this stream
                'enabled' => false,
            ],
        ],
    ],
];

In either case, EventBus will not produce events to a stream that is not enabled.

EnableEventBus

$wgEnableEventBus is an extension wide config parameter specifies which types of events the extension will produce. Possible options are TYPE_NONE, TYPE_EVENT, TYPE_JOB, TYPE_PURGE or TYPE_ALL. Specifying multiple types using | as a delimiter is supported. Example: TYPE_JOB|TYPE_PURGE

This setting can be used to globally restrict the 'types' of events the extension can produce. E.g. if you want to fully disable producing real events, perhaps in testing environments, you can set $wgEnableEventBus = "TYPE_NONE".

EventBusStreamNamesMap config

StreamNameMapper adds a config EventBusStreamNamesMap that maps from the default names of streams to the actual stream name that will be produced. This allows wikis with special considerations, such as access restricted wikis in the same cluster as public wikis, to produce their events to a separate stream such that the public streams will not leak private details. This additionally allows developers to vary the stream name used for e.g. 'mediawiki.page_change.v1' in testing and staging environments. Perhaps you want to produce page change events to a release candidate stream before promoting it to 'production'. The mapper will translate 'mediawiki.page_change.v1', hardcoded in the hook handler, to the stream name to produce. It will be looked up in config from EventBusStreamNamesMap['mediawiki.page_change.v1'], and default to the provided name.

Any HookHandlers that produce events should inject the 'EventBus.StreamNameMapper' service from the service container and use it to support configuring the stream name that they produce to.

EventBus HookHandlers and generated events

As noted above, this EventBus extension should probably be just a producer library. At the very least, events that are not about carrying MediaWiki core state to external systems shouldn't be here.

For the time being though, this EventBus extension contains several MediaWiki hook handlers that actually produce events.

EventBusHooks and EventFactory are deprecated in favor of HookHandler classes and specific Serializers that serialize from MediaWiki classes to event objects.

Each single stream that is produced by this extension should have its own HookHandler class that is solely responsible for producing events to that stream.

EventBus RCFeed

This extension also provides an FormattedRCFeed and RCFeedFormatter implementation That will allow RCFeed configuration to post to the EventBus service in the mediawiki.recentchange topic. To use, add the following to your LocalSettings.php:

use MediaWiki\Extension\EventBus\Adapters\RCFeed\EventBusRCFeedEngine;
use MediaWiki\Extension\EventBus\Adapters\RCFeed\EventBusRCFeedFormatter;

$wgRCFeeds['eventgate-main'] = array(
    'class'            => EventBusRCFeedEngine::class,
    'formatter'        => EventBusRCFeedFormatter::class,
    // This should be the name of an event service entry
    // defined in $wgEventServices.
    'eventServiceName' => 'eventgate-main',
);

References

• Wikimedia Foundation's Event Platform
• Reliable publish / subscribe event bus
• Integrate event production into MediaWiki

License

EventBus is licensed under the GNU General Public License 2.0 or any later version. You may obtain a copy of this license at http://www.gnu.org/copyleft/gpl.html.