Content Observer (WordPress-Plugin)

Efficiently observe content changes between WordPress instances. One site records what changed - posts, deleted posts, or anything a theme or plugin reports - and the sites observing it learn about those changes and react to them in their own code.

The plugin moves no content by itself. It transports modifications (content id, content type, create/update/delete, time); what an observing site does with them is up to a hook in that site's code.

Use cases

The examples use fictional sites: a City Portal that observes news.example.com, events.example.com and archive.example.com.

• Keep teaser lists fresh. The City Portal shows the latest articles from news.example.com and caches that list. When the newsroom edits or deletes an article, the portal clears exactly that cache instead of waiting for it to expire.

  use Palasthotel\WordPress\ContentObserver\Model\SiteModificationAction;
  
  add_action( 'content_observer_on_modifications', function ( SiteModificationAction $action ) {
    if ( 'news' !== $action->site->slug ) {
        return;
    }
    foreach ( $action->modifications as $mod ) {
        if ( 'post' === $mod->content_type ) {
            delete_transient( 'city_portal_news_teasers' );
            return;
        }
    }
  } );

• Share one navigation. The main site records Modification::build( 'main-navigation', 'wp-menu' )->update() whenever its menu changes, and every site observing it rebuilds its mirrored navigation.

• Central search. A search site observes all the others. On create and update it re-indexes the post from the observed site, on delete it drops it from the index.

• Archive articles. archive.example.com observes news.example.com and stores a copy of every article it hears about on create and update, so the article survives a later delete on the news site. Because the archive is observer and observable at once, the City Portal in turn learns when the archive changes.

Concepts

• Observable: a site whose changes others want to know about.
• Observer: a site that wants to know about another site's changes.
• Modification: content_id, content_type (post by default, any string you choose), type (create, update, delete) and the time of the change. Saving and deleting posts records modifications automatically.

Both sites have the plugin and an API key (Content Observer → Settings). Under Content Observer → Manage the sites are connected; the connection exchanges the keys.

How changes travel

• notify (observable → observer): sends new modifications to every observer, hourly by cron or with wp content-observer notify.
• fetch (observer → observable): asks the observable for its new modifications, with wp content-observer fetch or the Fetch button.
• apply (on the observer): runs the hooks below for every modification since the last run, hourly by cron or with the Apply button.

Both schedules can be disabled under Settings if a system cron runs the WP-CLI commands instead.

Hooks

use Palasthotel\WordPress\ContentObserver\Model\SiteModificationAction;

add_action( 'content_observer_on_modifications', function ( SiteModificationAction $action ) {
    foreach ( $action->modifications as $mod ) {
        // $action->site->url, $mod->content_id, $mod->content_type, $mod->type
    }
} );

Recording your own modifications:

use Palasthotel\WordPress\ContentObserver\Model\Modification;

content_observer_add_modification(
    Modification::build( 'main-navigation', 'wp-menu' )->update()
);

WP-CLI

wp content-observer ping
wp content-observer connect
wp content-observer notify
wp content-observer fetch
wp content-observer modifications

Repository layout

See CONTRIBUTING.md for the local setup and commit conventions.

License

GPL-3.0-or-later. Versions up to 1.2.3 were published under the GPL-2.0 named in their plugin header.