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