Why is the gzip minimum length directive not being respected? - nginx

Question

If I understand correctly it's better not to gzip small resources as they might actually get bigger while still having a performance hit on the CPU.
So using the gzip_min_length directive is an obvious solution to that.
However, when trying this on a server that runs a REST API I'm working on this doesn't seem to work.
When I receive an empty json response, or a very small one, the Content-Encoding header is still present and reading "gzip".
HTTP Response headers
My question is why this setting is not being respected by NginX and what can I do to fix it?
The API is built on the Lumen microframework.
I have attached the Gzip setting I'm using in my nginx.conf:
# Compression
# Enable Gzip compressed.
gzip on;
# Enable compression both for HTTP/1.0 and HTTP/1.1.
gzip_http_version 1.1;
# Compression level (1-9).
# 5 is a perfect compromise between size and cpu usage, offering about
# 75% reduction for most ascii files (almost identical to level 9).
gzip_comp_level 5;
# Don't compress anything that's already small and unlikely to shrink much
# if at all (the default is 20 bytes, which is bad as that usually leads to
# larger files after gzipping).
gzip_min_length 1000;
# Compress data even for clients that are connecting to us via proxies,
# identified by the "Via" header (required for CloudFront).
gzip_proxied any;
# Tell proxies to cache both the gzipped and regular version of a resource
# whenever the client's Accept-Encoding capabilities header varies;
# Avoids the issue where a non-gzip capable client (which is extremely rare
# today) would display gibberish if their proxy gave them the gzipped version.
gzip_vary on;
# Compress all output labeled with one of the following MIME-types.
gzip_types
application/atom+xml
application/javascript
application/json
application/rss+xml
application/vnd.ms-fontobject
application/x-font-ttf
application/x-web-app-manifest+json
application/xhtml+xml
application/xml
font/opentype
image/svg+xml
image/x-icon
text/css
text/plain
text/x-component;
# text/html is always compressed by HttpGzipModule

Answer 1

Confirming my note above, this does seem to correspond to the note in the NGINX gzip module documentation stating "The length is determined only from the “Content-Length” response header field."
With gzip_min_length 1000;, my JSON responses were being gzip'ed, even if they were only 100 bytes.
I changed my application to add the Content-Length: 100 header and NGINX sends the JSON response without using the gzip encoding.
If I change the configuration to gzip_min_length 80; with the same 100-byte Content-Length, then NGINX applies the gzip encoding as expected.
Short story: you need to apply the Content-Length header for NGINX to properly handle the gzip_min_length check.