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Active Record Connection Management in Rails 5.1 or lower

Active Record Connection Management in Rails 5.1 or lower

This guide is for Rails 5.1 or lower. Starting in 5.2, all these issues have been made irrelevant.


In general, Active Record will automagically manage database connections in an efficient manner. See Configuring a Database and Database pooling for information about the basics of optimally configuring your application to use available resources.

Sometimes, you will need to manage Active Record connections and/or connection pools yourself. These situations are:

  • Configuring a multi-process web server
  • Spawning processes

In previous version of rails, you needed to think about managing connections when spawning threads, but this is no longer the case (I'm not sure in which version this was changed).


Configuring a multi-process web server

Each process in a multi-process web server needs its own database connection pool. If the server is also multi-threaded, the threads within each process will automatically share that processes' connection pool.

Here is how to configure a server to set up its pools while booting your app:

Before forking

Before the server forks, you need to disconnect the pool so that the forked processes don't use the same connections as the parent process. Here's how to do that in a puma config:

before_fork do
  ActiveRecord::Base.connection_pool.disconnect!
end

After forking

After a process forks and has no connections, it needs to establish connections. Here's how to do that in a puma config:

c.on_worker_boot do
  ActiveRecord::Base.establish_connection
end

Spawning processes

When spawning processes, you must do a small amount of manual database pool management.

Let's say you you have a script that you are going to run as a worker using rails runner, my_worker.rb. In this script, you are going to start two long-running processes. Here's how you would go about doing that:

ActiveRecord::Base.connection_pool.disconnect!
thing1_pid = Process.fork do
  ActiveRecord::Base.establish_connection

  Thing1.new.run
end

thing2_pid = Process.fork do
  ActiveRecord::Base.establish_connection

  Thing2.new.run
end

# optional: If you want to do other DB operations in the parent thread,
# you will need to now reconnect like this:
ActiveRecord::Base.establish_connection

Process.wait thing1_pid
Process.wait thing2_pid

Note that each of the forked processes will have their own connection pool with the number of connections configured in your app. So if database.yml specified a connection pool of 5, then running rails runner my_worker.rb will use up to 10 connections.

The relationship between parent and child pools

Now, one might imagine that, in the above example (which is exemplary of typical code), because...

  1. A completely new pool of connections is established in each child process.
  2. No DB operations are done in the children before the child pools are established.
  3. No DB operations are done in the parent before the child pools are established.

...then maybe the disconnect in the parent isn't necessary, since no resources are attempted to be shared between processes at the same time?

However, this is not the case. There is no mechanism in Rails for "forgetting" the old pool without destroying it. The parent pool must always be disconnected in the parent process first.

Spawning threads

Rails will automatically allow threads to share connections from a connection pool. As long as your app is configured to have at least as many connections as there are threads running at the same time, you won't have to worry about managing connections.

You can experiment with this behavior in the console:

500.times{ Thread.new{print User.count}.join }
# succeeds

500.times{ Thread.new{print User.count; sleep 1} }
# after a few successful threads, raises "could not obtain a connection from the pool"

Further Reading

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