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I was in contact with Heroku support a couple of weeks ago since we experienced some timeout on our production app. Got a detailed explanation how the routing on heroku works by a Heroku engineer, and thought I could share: "I am a bit confused by what you mean by an "available" dyno. Requests get queued at the application level, rather than at the router level. Basically, as soon as a request comes in, it gets fired off randomly to any one of your web dynos. Say your request that takes 2 seconds to be handled by the dyno was dispatched to a dyno that was running a long running request. Eventually, after 29 seconds, it completed serving the response, and started working on the new, faster 2 second request. Now, at this point it had already been waiting in the queue for 29 seconds, so after 1 second, it'll get dropped, and after another 1 second, the dyno will be done processing it, but the router is no longer waiting for the response as it has already returned an H12. That's how a fast request can be dropped. Now, the one long 29 second request could also be a series of not-that-long-but-still-long requests. Say you had 8 requests dispatched to that dyno at the same time, and they all took 4 seconds to process. The last one would have been waiting for 28 second, and so would be dropped before completion and result in an H12." |