> For the complete documentation index, see [llms.txt](https://eric-zhang-seattle.gitbook.io/mess-around/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://eric-zhang-seattle.gitbook.io/mess-around/scenarios/overview-1/redis_customized.md).

# Redis-based customized delay time

* [Redis based delay queue](#redis-based-delay-queue)
  * [Flowchart](#flowchart)
  * [Redis ZSet](#redis-zset)
  * [Timer](#timer)
  * [Redis List](#redis-list)
  * [Dispatcher](#dispatcher)
  * [Job pool: Redis HashTable](#job-pool-redis-hashtable)
* [References](#references)

## Redis based delay queue

### Flowchart

![Delay Queue Components](/files/PoAy1oVXxBAxJITyfPhy)

### Redis ZSet

* Delay queue: A list of ordered queues which store all delayed/reserved jobs (only stores job Id)

```
redis cli> ZADD delayqueue <future_timestamp> "messsage"
redis cli> MULTI
redis cli> ZRANGEBYSCORE delayqueue 0 <current_timestamp>
redis cli> ZREMRANGEBYSCORE delayqueue 0 <current_timestamp>
redis cli> EXEC
```

### Timer

* Scan delay bucket and put expired jobs into ready queue

### Redis List

* Ready queue: A list of ordered queues which store jobs in Ready state.
  * Topic: The same category of job collections

### Dispatcher

* It will poll the delay queue and move items to the corresponding topic within ready queues if the tasks are ready.
* Worker: Workers use BLPOP on the ready queue and process the message. Once done, the response could be put in a response queue and send to consumer.

### Job pool: Redis HashTable

* JobPool: Store all metadata about jobs
  * Stores as key value pairs. Key is job id and value is job struct.
  * Job struct contains the following:
    1. topic: job category. Needed because each category will has its own callback function.
    2. id: job unique identifier
    3. delayTime: time to delay before executing the task
    4. ttr: timeout duration for this job to be executed
    5. body: job content
    6. callback: http url for calling a specific function

## References

* [基于 Redis 自研延迟队列](https://hacpai.com/article/1565796946371)