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Go backends on Now

Posted on 20 August 2019

ZEIT's Now [1] service is great for deploying apps and APIs that are able to make use of serverless execution models, and I use it for many of my projects (including this website, at the time of writing).

I recently needed to deploy a backend written in Go and kept running into problems when trying to read data from the HTTP request body. The client-side app I was developing to communicate with the backend is also written in Go and everything seemed to work fine when running the backend locally (using `now dev`), but the exact same requests failed when running it in production. The client's request body was available when in development, but returned empty strings when running in production.

It eventually boiled down to two things:

Now's dev environment does not seem to care about discrepancies between the `Content-Length` header and the actual request body. In production, however, if the `Content-Length` does not exist (or is `0`) then the request body will not be read even if it exists (and I admit that this should probably be the expected behaviour and is probably a symptom of the underlying cloud architecture rather than Now itself).
When creating new client requests, Go's `http` package [2] automatically sets the `Content-Length` header based on the length of the request body (and will overwrite/remove the header even if it is set manually), but it will only do this under certain circumstances [3]. In my scenario, a request was being generated with a valid body but without the `Content-Length` header. To fix the issue, rather than passing in an instance of `os.File` directly (which I was using to form my request), I needed to read the contents into a buffer before passing it into the request.

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