Trimming Dependencies: concurrent-ruby


I’ve advocated for fewer dependencies for years now. Every long-running project needs to periodically re-evaluate their usage of each dependency and the value it provides. To that end, I recently considered Sidekiq’s dependencies:

We might be able to remove rack-protection gem if we re-implement the CSRF functionality but none of the other gems are easy to remove (indeed, as I was writing this post, @seuros stepped up and took on issue #4587).

Sidekiq Pro

Sidekiq Pro adds a dependency on concurrent-ruby because Pro uses its ThreadPoolExecutor to limit access to Redis in super_fetch. Can we remove it?

It turns out we can. We can reuse a connection pool as a pool of tokens for accessing Redis. By forcing all Processor threads to get a token before calling Redis, we can limit concurrent access to Redis. After refactoring connection_pool a bit, I was able to replace Concurrent usage with just a few lines of ConnectionPool code.

-        @pool ||=
-          min_threads: 1,
-          max_threads: 2,
-          max_queue: options[:concurrency]
-        )
+        @pool = 2, timeout: 120) { }

@@ -249,12 +244,20 @@ module Sidekiq::Pro
-          future = Concurrent::Future.execute(executor: @pool, &method(:get_job))
-          val = future.value(nil)
-          return val if val
-          raise future.reason if future.rejected?
-        rescue Concurrent::RejectedExecutionError
+          # Since every Processor is a Thread, we want to limit the concurrency
+          # going to Redis. We do this with an artifically limited connection pool
+          # that holds tokens allowing the Thread to call Redis. We need to do
+          # this because Redis does not have a command which is both Reliable
+          # *and* operates on multiple queues.
+          #
+          @pool.with do
+            get_job
+          end
+        rescue ConnectionPool::TimeoutError
+          nil
+        rescue ConnectionPool::PoolShuttingDownError
           # shutting down race condition, #2827, nbd
+          nil

Sidekiq Enterprise

Sidekiq Enterprise adds a dependency on einhorn for the rolling restart feature. We can’t really remove this gem as customers use the einhorn binary.

Enterprise also uses the Concurrent.processor_count API so it can fork a Sidekiq process for each core. It turns out that Ruby already provides this API in Etc.nprocessors, awesome!!! With this change, we can completely remove the concurrent-ruby gem from all Sidekiq variants.

@@ -46,7 +45,7 @@ module Sidekiq
         @signal = signal
         @io = STDOUT
-        @count = Integer(env["SIDEKIQ_COUNT"] || Concurrent.processor_count)
+        @count = Integer(env["SIDEKIQ_COUNT"] || Etc.nprocessors)

Who cares? What’s the effect?

Almost all OSS projects will struggle to get long term maintenance. concurrent-ruby had a maintainer for many years but he eventually moved onto other things. I believe it currently has another maintainer but there’s no guarantee how long that arrangement will last. Without maintenance, OSS projects are a snapshot in time: eventually they will become outdated.

Once I removed all usage of the gem I booted a bare Sidekiq Pro process, had it process a few thousand jobs, and examined the RSS before and after:

Before: 60MB
After: 50MB

10MB shaved off by removing concurrent-ruby!!!


Alas, a reality check: Rails depends on concurrent-ruby so most apps using Sidekiq will pull it in anyways. But this is a great example where stable, well used projects can always improve. Rails depends on many dozens of gems: can you remove one?