How to tell if a plugin is multisite compatible?

Question

As a WordPress admin newbie, I just went through my first non-automatic WordPress upgrade.

I upgraded five different WordPress installs from 4.1.3 to 4.2.

They all refer to different web sites hosted on the same web host as one main domain and four addon domains.

On each of the websites, I use the following plug-ins:

• Akismet;
• Google Analytics by Yoast;
• Google Adsense;
• Google XML Sitemaps and
• Jetpack by WordPress.com.

In order to streamline further administration, I considering consolidating all five installs into a single multisite install.

So far, I have found it quite hard to find definitive answers on the compatibility of each of those plugins with a multisite installation on the plugin's websites.

Is the plugin's page the right place to search for this?

Is there a central location where this information can be found?

Answer 1

There are two types of multisite compatibility:

1. Passive compatibility: doing nothing multisite specific, just works without breaking anything.
2. Active compatibility: changing or extending multisite specific behavior.

I guess you are out for 1. See my slides from WordCamp Prague 2015 for the second part.

Plugins that do not say anything about multisite should not be activated as network plugins. WooCommerce for example creates some custom tables during the installation. If you activate it network-wide, the subsites don’t get these tables and the sky will fall onto your head.

Unfortunately, most plugins don’t check for their activation type, so they let you do the wrong activation.

related are UX problems like admin pointers or special "About" pages that you have to click away on ever subsite in non-compatible plugins. Yoast’s WP SEO is one example. This will be fixed in that plugin soon, I guess. :)

Other issues depend on what you do with that multisite. If you are building a multilingual website where each site is written in one language and the sites are connected with each other, you want to synchronize the posts when you write content. That means that you call switch_to_blog() on the hook save_post, and save the connected posts too. save_post will be called multiple times during one request now. Many plugins are not aware of such a situation, so they just overwrite the post meta information for the connected posts, thinking that they are still on the first post.

Look out for plugins that are dealing with post meta and lack a check like this:

if ( is_multisite() && ms_is_switched() )
    return FALSE;

These plugins are not compatible.

Similar, albeit harder to specify, are issues when plugins touch user meta fields or rewrite rules.

Some plugins try write content into files without including the site ID in the file name. They are very likely broken too.

Like Tom said: Create a test installation, run every use case you can imagine. You cannot trust the plugin page, and usually there is not enough information anyway.

Answer 2

Unless a plugin states it's compatible, the only reliable way to find out is to create a local multisite installation and test it.

Sometimes there are signs you can look for, but these are always a symptom of bad code, e.g. hardcoding database table names rather than using the prefix from $wpdb, in which case I would recommend steering clear of the plugin even if you don't use multisite