Expanding documentation in queue.go (#26889)

A set of terminology, along with a broader description, can help more
people engage with the Gitea queue system, providing insights and
ensuring its correct use.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/queue/queue.go

@@ -1,27 +1,62 @@
 // Copyright 2023 The Gitea Authors. All rights reserved.
 // SPDX-License-Identifier: MIT
 
-// Package queue implements a specialized queue system for Gitea.
+// Package queue implements a specialized concurrent queue system for Gitea.
 //
-// There are two major kinds of concepts:
+// Terminology:
 //
-// * The "base queue": channel, level, redis:
-//   - They have the same abstraction, the same interface, and they are tested by the same testing code.
-//   - The dummy(immediate) queue is special, it's not a real queue, it's only used as a no-op queue or a testing queue.
+//  1. Item:
+//     - An item can be a simple value, such as an integer, or a more complex structure that has multiple fields.
+//     Usually a item serves as a task or a message. Sets of items will be sent to a queue handler to be processed.
+//     - It's represented as a JSON-marshaled binary slice in the queue
 //
-// * The WorkerPoolQueue: it uses the "base queue" to provide "worker pool" function.
-//   - It calls the "handler" to process the data in the base queue.
-//   - Its "Push" function doesn't block forever,
-//     it will return an error if the queue is full after the timeout.
+//  2. Batch:
+//     - A collection of items that are grouped together for processing. Each worker receives a batch of items.
+//
+//  3. Worker:
+//     - Individual unit of execution designed to process items from the queue. It's a goroutine that calls the Handler.
+//     - Workers will get new items through a channel (WorkerPoolQueue is responsible for the distribution).
+//     - Workers operate in parallel. The default value of max workers is determined by the setting system.
+//
+//  4. Handler (represented by HandlerFuncT type):
+//     - It's the function responsible for processing items. Each active worker will call it.
+//     - If an item or some items are not psuccessfully rocessed, the handler could return them as "unhandled items".
+//     In such scenarios, the queue system ensures these unhandled items are returned to the base queue after a brief delay.
+//     This mechanism is particularly beneficial in cases where the processing entity (like a document indexer) is
+//     temporarily unavailable. It ensures that no item is skipped or lost due to transient failures in the processing
+//     mechanism.
+//
+//  5. Base queue:
+//     - Represents the underlying storage mechanism for the queue. There are several implementations:
+//     - Channel: Uses Go's native channel constructs to manage the queue, suitable for in-memory queuing.
+//     - LevelDB: Especially useful in persistent queues for single instances.
+//     - Redis: Suitable for clusters, where we may have multiple nodes.
+//     - Dummy: This is special, it's not a real queue, it's a immediate no-op queue, which is useful for tests.
+//     - They all have the same abstraction, the same interface, and they are tested by the same testing code.
+//
+// 6. WorkerPoolQueue:
+//   - It's responsible to glue all together, using the "base queue" to provide "worker pool" functionality. It creates
+//     new workers if needed and can flush the queue, running all the items synchronously till it finishes.
+//   - Its "Push" function doesn't block forever, it will return an error if the queue is full after the timeout.
+//
+// 7. Manager:
+//   - The purpose of it is to serve as a centralized manager for multiple WorkerPoolQueue instances. Whenever we want
+//     to create a new queue, flush, or get a specific queue, we could use it.
 //
 // A queue can be "simple" or "unique". A unique queue will try to avoid duplicate items.
 // Unique queue's "Has" function can be used to check whether an item is already in the queue,
-// although it's not 100% reliable due to there is no proper transaction support.
+// although it's not 100% reliable due to the lack of proper transaction support.
 // Simple queue's "Has" function always returns "has=false".
 //
-// The HandlerFuncT function is called by the WorkerPoolQueue to process the data in the base queue.
-// If the handler returns "unhandled" items, they will be re-queued to the base queue after a slight delay,
-// in case the item processor (eg: document indexer) is not available.
+// A WorkerPoolQueue is a generic struct; this means it will work with any type but just for that type.
+// If you want another kind of items to run, you would have to call the manager to create a new WorkerPoolQueue for you
+// with a different handler that works with this new type of item. As an example of this:
+//
+//	 func Init() error {
+//		 itemQueue = queue.CreateSimpleQueue(graceful.GetManager().ShutdownContext(), "queue-name", handler)
+//		 ...
+//	 }
+//	 func handler(items ...*mypkg.QueueItem) []*mypkg.QueueItem { ... }
 package queue
 
 import "code.gitea.io/gitea/modules/util"