Slow Performance in Heroku during Grape::Middleware::Formatter.call

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Hi all,

I am noticing some odd behavior with Grape in Heroku. Requests are coming back extremely slowly and New Relic tells me that Grape::Middleware::Formatter.call requests are the key source of the problem. Unfortunately my version of New Relic doesn't give me deeper detail, but Grape middle-ware only calls are taking up to 500ms to execute. This is with DB hits, application code, and other externalities removed in reporting.

A couple extra points:

- This behavior does not occur on my local development machine (a recent macbook pro).

- The Gemfile.lock is identical on my local dev machine and on Heroku (it is attached).

- I'm using Unicorn.

- I've locked my version of Rails (principally used for ActiveRecord) and Ruby.

- I am running two Heroku 2x dynos (1g of ram)

- Heroku and New Relic report that memory is holding stable about about 40% of the available and that the CPU is not going wild.

I'm at a bit of a loss to explain what's happening. Does anyone have any insight into this?

Best regards,

Adam Hiatt

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I found another person on Stackoverflow who has the same issue, but there is no resolution their either:

http://stackoverflow.com/questions/26517180/new-relic-custom-instrumentation-for-grapemiddlewareformattercall-method-in/26538116#26538116

[Daniel Doubrovkine]

First, identify which formatter is being used. I am going to guess you're returning JSON, so this is https://github.com/intridea/grape/blob/master/lib/grape/formatter/json.rb.

This is what it does:

return object.to_json if object.respond_to?(:to_json)

MultiJson.dump(object)

Move this part into your API to debug, explicitly call 'to_json' on whatever your API is returning and shortcut the formatter by doing the following:

formatter :json, lambda { |object, env| object }

(this will prevent a call to to_json twice).

This should help you debug.

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