把 RabbitMQ 換成 PostgreSQL 的那篇文章…

在 Hacker News 上看到「SQL Maxis: Why We Ditched RabbitMQ and Replaced It with a Postgres Queue (prequel.co)」這篇文章，原文在「SQL Maxis: Why We Ditched RabbitMQ And Replaced It With A Postgres Queue」這邊，裡面在講他們把 RabbitMQ 換成 PostgreSQL 的前因後果。

文章裡面可以吐嘈的點其實蠻多的，而且在 Hacker News 上也有被點出來，像是有人就有提到他們遇到了 bug (或是 feature) 卻不解決 bug，而是決定直接改寫成用 PostgreSQL 來解決，其實很怪：

In summary — their RabbitMQ consumer library and config is broken in that their consumers are fetching additional messages when they shouldn’t. I’ve never seen this in years of dealing with RabbitMQ. This caused a cascading failure in that consumers were unable to grab messages, rightfully, when only one of the messages was manually ack’ed. Fixing this one fetch issue with their consumer would have fixed the entire problem. Switching to pg probably caused them to rewrite their message fetching code, which probably fixed the underlying issue.

另外一個吐嘈的點是量的部份，如果就這樣的量，用 PostgreSQL 降低使用的 tech stack 應該是個不錯的決定 (但另外一個問題就是，當初為什麼要導入 RabbitMQ…)：

>To make all of this run smoothly, we enqueue and dequeue thousands of jobs every day.

If you your needs aren’t that expensive, and you don’t anticipate growing a ton, then it’s probably a smart technical decision to minimize your operational stack. Assuming 10k/jobs a day, thats roughly 7 jobs per minute. Even the most unoptimized database should be able to handle this.

在同一個 thread 下面也有人提到這個量真的很小，甚至直接不講武德提到可以用 Jenkins 解 XD：

Years of being bullshitted have taught me to instantly distrust anyone who is telling me about how many things they do per day. Jobs or customers per day is something to tell you banker, or investors. For tech people it’s per second, per minute, maybe per hour, or self aggrandizement.

A million requests a day sounds really impressive, but it’s 12req/s which is not a lot. I had a project that needed 100 req/s ages ago. That was considered a reasonably complex problem but not world class, and only because C10k was an open problem. Now you could do that with a single 8xlarge. You don’t even need a cluster.

10k tasks a day is 7 per minute. You could do that with Jenkins.

然後意外看到 Simon Willison 提到了一個重點，就是 RabbitMQ 到現在還是不支援 ACID 等級的 job queuing (尤其是 Durability 的部份)，也就是希望 MQ 系統回報成功收到的 task 一定會被處理：

The best thing about using PostgreSQL for a queue is that you can benefit from transactions: only queue a job if the related data is 100% guaranteed to have been written to the database, in such a way that it’s not possible for the queue entry not to be written.

Brandur wrote a great piece about a related pattern here: https://brandur.org/job-drain

He recommends using a transactional “staging” queue in your database which is then written out to your actual queue by a separate process.

這也是當年為什麼用 MySQL 幹類似的事情，要 ACID 的特性來確保內容不會掉。

這也是目前我覺得唯一還需要用 RDBMS 當 queue backend 的地方，但原文公司的想法就很迷，遇到 library bug 後決定換架構，而不是想辦法解 bug，還很開心的寫一篇文章來宣傳…

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