It has been requested for a long time to be able to have coordination among plugins. The two most commonly requested capabilities are
Force the callback for plugin A to run before / after the callback for plugin B on a specific hook.
Disable a plugin and / or callback for a specific hook for a specific transaction.
I had a version of this ready for production testing, based on the Yahoo! 5.3 fork of Traffic Server. Based on that work there are a number of other features that are implicit in making the primary capabilities possible.
- Modular hook dispatch.
To make this work consistently, it is very desirable to create a class to handle the hook dispatching. This provides a uniform code base to handle callback dispatch.
- Continuation tracking
Each continuation needs to be tracked back to the original plugin (or core) that created it in order to correctly handle when or if the callback is dispatched. This has the ancillary benefit of being able to provide more detailed debugging and failure messages when something goes wrong with a continuation. In theory this may also make it possible to handle plugin reload so that events for continuations associated with an unloaded plugin can be dropped instead of brokenly dispatched.
- Plugin API to probe hooks for callbacks.
If callbacks are going to be controllable from plugins, it is also needful to allow plugins to check the callbackson a hook. This is another plugin developer request that has been around for a while.
- Plugin API to manipulate callbacks on hooks
Once callbacks are accessible via inspection from the previous point, a request for the ability to manipulate them is inevitable.
The full change is large and should be broken up into smaller updates, each of which provides some independent value. The natural progression is basically the feature list from the previous section. In this progression each update depends on the previous one.
Modularize the callback dispatch. Handle the three levels of dispatch (global, session, transaction) in a consistent way.
Track continuations. Each continuation is either from the core or from a plugin. Associate the data for this with each continuation and whenever a continuation is created use the current plugin context to mark it.
Plugin priorities, implementation and configuration. This assigns priorities to plugins and dispatches callbacks in priority order. This must also include the capability to configure at least statically the priorities for the plugins.
Plugin priority API. This enables plugins to manipulate plugin priorities in code. This includes both adjusting priorities and enabling / disabling plugins for a transaction, and setting the priority threshold for a hook.
Callback inspection plugin API. Enable plugins to examine the callbacks associated with a hook.
Callback manipluation plugin API. Enable plugins to manipulate the callbacks on a hook, including changing priorities and enabling / disabling specific callbacks or plugins for a hook.
At one point (Feb 2016) I had a working prototype of plugin priorities based on the YahoO! 5.3.x fork which assigned priorities to plugin callbacks along with an API to manipulate not just the current plugins priorities but that of other plugins. Although useful, to make it work in production it was necessary to add plugin level enable / disable. The code for this is here.