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add contributing docs about IPC

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Hazelnoot 2025-11-14 18:58:39 -05:00
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CONTRIBUTING.Sharkey.md
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@ -501,3 +501,30 @@ following apply:
- It's necessary to use `null` as a data value. - It's necessary to use `null` as a data value.
`QuantumKVCache` does not allow null values, and thus another option should be chosen. `QuantumKVCache` does not allow null values, and thus another option should be chosen.
### Inter-Process Communication
Sharkey can utilize multiple processes for a single server.
When running in this mode, a mechanism to synchronization changes between each process is necessary.
This is accomplished through the use of **Redis IPC**.
#### IPC Options
There are three methods to access this IPC system, all of which are available through Dependency Injection:
* Using the `pub` and `sub` Redis instances to directly establish channels.
This should only be done when necessary, like when implementing very low-level utilities.
* The `publishInternalEvent` method of `GlobalEventService`.
This method will asynchronously publish an event to redis, which will forward it to all connected processes - **including the sending process**.
Due to this and other issues, `publishInternalEvent` should be considered obsolete and avoided in new code.
Instead, consider one of the other options.
* `InternalEventService`, which is the newest and recommended way to handle IPC.
The `emit` method accepts arguments identical to `publishInternalEvent`, which eases migration, while also accepting a configuration object to control event propagation.
Additionally, `InternalEventService` ensures that local event listeners are called *before* notifying other processes, avoiding potential data races and other weirdness.
#### When to use IPC
IPC should be used whenever cacheable data is modified.
By cacheable, we mean any data that could be stored in any of the supported memory caches.
Changes to `MiUser`, `MiUserProfile`, or `MiInstance` entities should **always** be considered cacheable, but these are not the only options.
A major exception is when the local data is cached in a Quantum cache (`QuantumKVCache`).
Quantum caches automatically call `InternalEventService.emit` to synchronize changes, so you only need to `await set()` and the changes will be reflected in other processes' caches too.