Spring is slow with LaunchDarkly enabled

[jeromedalbert]

Config

We are using the 5.0.1 version of this gem.

Here is our LaunchDarkly Spring config, as per the docs:

# config/spring.rb

Spring.after_fork do
  config = LaunchDarkly::Config.new(logger: Logger.new('/dev/null'))
  Rails.configuration.ld_client = LaunchDarkly::LDClient.new(ENV['LD_KEY'], config)
end

Problem

When spring is already loaded, Spring takes 1.5 seconds to boot:

$ time (rails c)
Loading development environment (Rails 5.1.6)
[1] pry(main)>
( rails c; )  0.24s user 0.10s system 22% cpu 1.521 total

1.5 seconds is too long, booting rails should feel instant since spring is already loaded.

Notes

• Removing the LD config from Spring makes Rails boot back to 0.5 seconds, which is ideal. But then the LD client will fail to receive updates.
• Is there any way to make the LD client work with Spring while not impacting Spring boot time? Because at the moment this makes all our Spring commands (console, rspec, rake tasks...) slower by a good 1 second.

[eli-darkly]

If I understand correctly, it sounds like what you want is a client that doesn't actually get initialized until you need to use it. This is somewhat problematic because initialization of the client is not thread-safe, and if you wait until some later point in the application to create the client on demand, you could end up with several threads each trying to call LdClient.new.

Unfortunately, we haven't used Spring with Ruby, so I'm not sure what other options there may be to control the boot process. I can only hope that there is some way to detect if your application is really being started, or if it is just initializing Spring because you're running some other Spring command, and only call LdClient.new in the former case.

[jeromedalbert]

It sounds like what you want is a client that doesn't actually get initialized until you need to use it

We ended up doing just that. This is approximately what we did:

# config/initializers/launchdarkly.rb

$ld_client = nil

# lib/ld_client.rb

class LdClient
  def self.client
    if !$ld_client
      config = ...
      $ld_client = LaunchDarkly::LDClient.new(ENV['LD_KEY'], config)
    end
    $ld_client
  end
end

# config/spring.rb

# No LD config at all

Then we make our code call LdClient.client whenever LaunchDarkly needs to be called. This means that Spring is not slowed down at startup, and the client will be lazily initialized for each of our web server processes. This may not be thread-safe, but we use the Unicorn web server which uses forked processes, not threads. So this works for us for now.