Active Record, Sidekiq, pools and threads

Appaloosa Store
Feb 6 · 5 min read

At Appaloosa, we use Sidekiq a lot. In fact, we use it for all the background jobs we need. We also use a database (yes, this sounds crazy, I know.) with Active Record as ORM. Sometimes, Sidekiq jobs need to do stuff in our database, this looks like a normal situation, until…

The recurring error…

could not obtain a connection from the pool within 5.000 seconds (waited 5.002 seconds); all pooled connections were in use (ActiveRecord::ConnectionTimeoutError)

And it pops many times, every day, at the same hour. Something like late at night, so yes, let’s try to understand and fix it.

But how can this happen?


Remember the error? could not obtain a connection from the pool. It must have something to do with that pool configuration!

Ok let’s check it. The official documentation tells us that:

Active Record database connections are managed by ActiveRecord::ConnectionAdapters::ConnectionPool which ensures that a connection pool synchronizes the amount of thread access to a limited number of database connections.


If you try to use more connections than are available, Active Record will block you and wait for a connection from the pool. If it cannot get a connection, a timeout error similar to that given below will be thrown.

Here it is! This is what is happening:

That’s what must happen here!

Ok, then, what about the other side? How is Sidekiq managing its threads to prevent this error? The official wiki is pretty straightforward:

You can tune the amount of concurrency in your sidekiq process. By default, one sidekiq process creates 10 threads.

So, it creates as many threads as we mentioned on our config/sidekiq.yml

concurrency must be the key!

There is also a little note on the wiki:

Note that ActiveRecord has a connection pool which needs to be properly configured in config/database.yml to work well with heavy concurrency. Set the pool setting to something close or equal to the number of threads

THAT’S IT! If we have Sidekiq concurrency value equal to Active Record pool value we can’t have more threads than the accepted by Active Record, and no more errors. Voilà! Let’s check those values on our project, something should not be right.



So… is everything ok?

Let’s double check everything, somewhere one of those value has to be overridden.

Of course, the answer was NOPE.

Let’s investigate 🕵🏻‍♀️

We analyzed 40 lines that occured in less than 10 seconds. The result is a beautiful file named sidekiq logs sherlock with many background colors and others for the text. And we found something interesting…

Can we reproduce it?

Now we can create a worker:

This is a simple worker, doing nothing but calling a service which is going to do some stuff. For this service, we tried to reproduce the code we have, but this code is calling some other gem. To avoid complexity, we try to simplify it but keeping the main behavior.

Try to run your worker and look at the result…

ruby-2.5.3/gems/activerecord- `block in wait_poll’: could not obtain a connection from the pool within 5.000 seconds (waited 5.010 seconds); all pooled connections were in use (ActiveRecord::ConnectionTimeoutError)

We are not sure that is exactly mimicking our behavior as the code uses transitive dependencies that are creating threads. But it looks like this and we ran into the same issue.

And it makes sense. Remember, we said that for each process Active Record assigns x connections depending on the pool value. Each connection is made by one thread. If our Sidekiq concurrency value is equal to the pool value, we should not have any problem, unless, something is creating a new thread and calling the database. That’s the case on our example above with the MyService class!

The why!

In fact, it’s possible to do async job here but we need to be sure that we are not dealing with database, or at least, it’s not creating too much connections. In our case, we are inside a deep loop, which leads to too many database calls. 💥

If you don’t want to have some headaches, be careful on what you put inside your workers 🙂

Photo by Ayo Ogunseinde on Unsplash

Thank you for reading! 🙏

This article was written by Mélanie Araujo Martins of Appaloosa’s dev team.

Appaloosa Store Engineering

The devs behind Appaloosa Store

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Appaloosa Store Engineering

The devs behind Appaloosa Store