Make nginx Return a JSON Response

This is more of a “write here so I do not forget” kind of post.

I set out trying to make a simple “maintenance” mode for a REST API (that speaks only JSON) without having to handle it in the code base.

nginx has a really cool support for maintenance mode that is dead easy to set up:

if (-f $document_root/maintenance.html) {
    return 503;

Obviously, life is not that easy.
In my case, the same URL is used to show static content (the API docs) and only sub-dirs in the URL are used for the API resources:

I already handle the API resource definitions in nginx, and I only need to override the root maintenance definition. In addition, the JSON response should also have the correct Content-Type header set.

In this case, is probably “easier done than said”, and is all done in here:

if (-f $document_root/maintenance.html) {
  more_set_headers "Content-Type: application/json; charset=UTF-8";
  return 503 '{"code": 503, "reason": "Service maintenance."}';

The trick is in having the nginx_headers_more module enabled and installed in your system.

On a Debian based distribution, the module is in the nginx-extras package.