Token bucket with upstream stats

nginx.conf

worker_processes 1;
error_log logs/error.log;
events {
    worker_connections 1024;
}
http {
    lua_shared_dict token_bucket 10M;
    lua_shared_dict stats 5M;

    init_by_lua '
        ngx.shared.token_bucket:set("tokens_avail", 0)
        ngx.shared.token_bucket:set("last_timestamp", os.time())
        ngx.shared.stats:set("upstream_time-sum", 0)
        ngx.shared.stats:set("upstream_time-count", 0)
    ';

    upstream backend {
        server localhost:1337;
    }

    server {
        listen 8098;
        location / {
            proxy_pass http://backend;
            default_type text/html;

            access_by_lua '
                local TOKEN_ACCUM_RATE = 10.0
                local BUCKET_SIZE = 50
                local TOKENS_PER_REQ = 1
                local last_timestamp = ngx.shared.token_bucket:get("last_timestamp")
                local cur_timestamp = os.time()
                local time_elapsed = cur_timestamp - last_timestamp
                local tokens_accum = time_elapsed * TOKEN_ACCUM_RATE
                local tokens_avail = ngx.shared.token_bucket:get("tokens_avail")
                tokens_avail = math.min(tokens_avail + tokens_accum, BUCKET_SIZE)
                if tokens_avail >= TOKENS_PER_REQ then
                    ngx.shared.token_bucket:set("tokens_avail", tokens_avail - TOKENS_PER_REQ)
                    ngx.shared.token_bucket:set("last_timestamp", cur_timestamp)
                    ngx.exit(ngx.OK)
                else
                    -- we have to set the status here because we call ngx.say(),
                    -- forcing response headers to be sent immediately
                    -- https://github.com/openresty/lua-resty-redis/issues/15#issuecomment-14747652
                    ngx.status = 429
                    ngx.say(string.format("%.1f tokens left", tokens_avail))
                    ngx.say("Request failed")
                    ngx.exit(429)
                end
            ';

            log_by_lua '
                if ngx.var.upstream_response_time then
                    local upstream_time = tonumber(ngx.var.upstream_response_time)
                    local sum = ngx.shared.stats:get("upstream_time-sum")
                    sum = sum + upstream_time
                    ngx.shared.stats:set("upstream_time-sum", sum)
                    ngx.shared.stats:incr("upstream_time-count", 1)
                end
            ';
        }

        location = /status {
            content_by_lua '
                local sum = ngx.shared.stats:get("upstream_time-sum")
                local count = ngx.shared.stats:get("upstream_time-count")

                if count > 0 then
                    local avg_ms = (sum*1000) / count
                    ngx.say("average upstream response time: ", math.floor(avg_ms+0.5), "ms (", count, " reqs)")
                else
                    ngx.say("no data yet")
                end
            ';
        }
    }
}