Use NGINX to replace body from proxy_pass

Question

I am new to openresty/nginx and am trying to replace the body the comes from an earlier call to proxy_pass with code like this:

body_filter_by_lua_block {
  ngx.arg[1] = '{"count"' .. ":2}'
  ngx.arg[2] = true
}

I had originally, not had the line for ngx.arg[2] = true, but I would get back the body twice (does that mean the body I'm replacing has 2 chunks?). I added in the ngx.arg[2] = true line and now the output from the request looks good. When I deploy the nginx file, the service looks good for a few requests then the nginx requests start to hang. Is the correct/best practice for replacing the response body? What should I do instead?

Answer 1

Within body_filter_by_lua_block:

ngx.arg[1] is current chunk of response body.

ngx.arg[2] is end-of-file flag. I often see this flag set in separate body_filter_by_lua_block invocation, when ngx.arg[1] is nil.

It is why you saw your string twice.

EOF flag above has read/write access, so if you set it to true it means just throw away any remaining chunk data.

Changing body content works fine for me, even did some performance testing, never see hangs while using this.

PS: Don't forget to include header_filter_by_lua_block { ngx.header.content_length = nil } if you change the body.

Also there may be some tricks with compressed content.