log the size of the response headers

backend/server/routes.go

@@ -43,3 +43,13 @@ func (s *Server) baseHandler(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 }
+
+func (s *Server) largeHandler(w http.ResponseWriter, r *http.Request) {
+	b := make([]byte, 8192)
+	for i := range b {
+		b[i] = 'A'
+	}
+
+	w.Header().Add("X-Large-Header", string(b))
+	fmt.Fprintf(w, "very large\n")
+}

backend/server/server.go

@@ -30,6 +30,8 @@ func (s *Server) Run() error {
 
 	mux := http.NewServeMux()
 	mux.HandleFunc("/", s.baseHandler)
+	mux.HandleFunc("/large", s.largeHandler)
+	mux.HandleFunc("/large_error", s.largeHandler)
 
 	hs := &http.Server{
 		Addr:           fmt.Sprintf("%s:%d", s.Address, s.Port),

outer-proxy/conf/conf.d/default.conf

@@ -21,12 +21,16 @@ server {
         proxy_pass http://inner-proxy;
     }
 
-    #error_page  404              /404.html;
+    location /large {
+        proxy_set_header  X-Forwarded-For  $proxy_add_x_forwarded_for;
+        proxy_set_header  X-Real-IP        $remote_addr;
+        proxy_pass http://backend;
+        proxy_buffer_size 9k;
+    }
 
-    # redirect server error pages to the static page /50x.html
-    #
-    error_page   500 502 503 504  /50x.html;
-    location = /50x.html {
-        root   /usr/share/nginx/html;
+    location /large_error {
+        proxy_set_header  X-Forwarded-For  $proxy_add_x_forwarded_for;
+        proxy_set_header  X-Real-IP        $remote_addr;
+        proxy_pass http://backend;
     }
 }

outer-proxy/conf/lua/example.lua

@@ -3,7 +3,7 @@
 local _M = {}
 
 function _M.run()
-    ngx.var.example_var = "foo"
+    -- ngx.var.example_var = "foo"
 end
 
 return _M

outer-proxy/conf/lua/log_headers_size.lua

@@ -0,0 +1,20 @@
+-- log the total size of the response's headers.
+-- https://github.com/openresty/lua-nginx-module#ngxrespget_headers
+
+local _M = {}
+
+function _M.run()
+    local h, err = ngx.resp.get_headers()
+    if err == "truncated" then
+        ngx.log(ngx.ERR, "truncated headers from lua")
+        return
+    end
+
+    local size=0
+    for k, v in pairs(h) do
+        size = size + string.len(v)
+    end
+    ngx.var.headers_size = size
+end
+
+return _M

outer-proxy/conf/nginx.conf

@@ -32,7 +32,8 @@ http {
                 '"headers":{'
                     '"x_forwarded_for": "$http_x_forwarded_for",'
                     '"x_forwarded_host": "$http_x_forwarded_host",'
-                    '"x_forwarded_proto": "$http_x_forwarded_proto"'
+                    '"x_forwarded_proto": "$http_x_forwarded_proto",'
+                    '"response_size": $headers_size'
                 '},'
                 '"host": "$host",'
                 '"method": "$request_method", '
@@ -88,11 +89,17 @@ http {
     # Set default values for variables manipulated by Lua scripts.
     map $request $example_var { default ""; }
 
+    map $request $headers_size { default 0; }
+
     # Run the following Lua scripts in the log phase.
     log_by_lua '
         require("example").run()
     ';
 
+    header_filter_by_lua_block {
+        require("log_headers_size").run()
+    }
+
     # Misc settings
     sendfile on;
     keepalive_timeout 65;