Decreasing Clojure Boot Time on Heroku (Error R10)

Question

My Clojure Compojure application exceeds the boot time limit for binding to a port on Heroku 50% of the time. Then it takes one or two calls to heroku restart to get it going. It is possible I am requiring too many dependencies, but I don't know if I can do this dynamically.

Is there a way to bind faster in Clojure by loading dependencies later? The generally accepted solution in Ruby land is heroku-forward, but this does not work for Clojure.

This issue is related to Heroku Boot Timeout (Error R10).

CircleCI handles my deployment and the uberjar is built up-front.

Answer 1

TL;DR: There's no easy way to decrease Clojure boot time on Heroku. Deploy uberjar instead.

I had the same problem with Clojure + Heroku. I think the problem is mostly not about time to download dependencies because Heroku has them cached so a solution like heroku-forward won't work. Clojure apps require a full Clojure runtime to be loaded first, then the runtime will compile your app's code. That amount of bootstrapping and compiling time results a timeout error. There are two ways to work around this problem:

1. Compile in another machine by using lein uberjar and deploy the jar instead.
2. Don't use Leiningen. Write a small Java just to claim the port first (kind of a ghost), then manually bootstrap Clojure from Java, close the port while starting your app's main function to re-occupy the port. However, this way is kinda weird and becase you claim the port so early, Heroku will bring your (ghost) app to the world eagerly so downtime is pretty high.

Answer 2

I produce an ubjerjar as well, and downtime during upgrades due to the startup time was an issue for me (even if I was not on Heroku).

I noticed that if I aot my code beforehand, it takes longer to produce the uberjar, but it boots faster afterwards. That was enough in my case. With lein, add :aot :all. There is a built-in (aot) task in boot.

You could also try to control the JVM options (heroku seems to support them according to this link). Some options to try would be :

-client -XX:+TieredCompilation -XX:TieredStopAtLevel=1 -Xverify:none