| //////////////////////////////////////////////////////////////////////////////// |
| // collapsed_forwarding:: |
| // |
| // ATS plugin to allow collapsed forwarding of concurrent requests for the same |
| // object. This plugin is based on open_write_fail_action feature, which detects |
| // cache open write failure on a cache miss and returns a 502 error along with a |
| // special @-header indicating the reason for 502 error. The plugin acts on the |
| // error by using an internal redirect follow back to itself, essentially blocking |
| // the request until a response arrives, at which point, relies on read-while-writer |
| // feature to start downloading the object to all waiting clients. The following |
| // config parameters are assumed to be set for this plugin to work: |
| //////////////////////////////////////////////////////////////////////////////////// |
| // proxy.config.http.cache.open_write_fail_action 1 ///////////////////////// |
| // proxy.config.cache.enable_read_while_writer 1 ///////////////////////// |
| // proxy.config.http.number_of_redirections 10 ///////////////////////// |
| // proxy.config.http.redirect_use_orig_cache_key 1 ///////////////////////// |
| // proxy.config.http.background_fill_active_timeout 0 ///////////////////////// |
| // proxy.config.http.background_fill_completed_threshold 0 ///////////////////////// |
| //////////////////////////////////////////////////////////////////////////////////// |
| // Additionally, given that collapsed forwarding works based on cache write |
| // lock failure detection, the plugin requires cache to be enabled and ready. |
| // On a restart, Traffic Server typically takes a few seconds to initialize |
| // the cache depending on the cache size and number of dirents. While the |
| // cache is not ready yet, collapsed forwarding can not detect the write lock |
| // contention and so can not work. The setting proxy.config.http.wait_for_cache |
| // may be enabled which allows blocking incoming connections from being |
| // accepted until cache is ready. |
| |
| More details are available at |
| |
| https://docs.trafficserver.apache.org/en/latest/admin-guide/configuration/cache-basics.en.html#reducing-origin-server-requests-avoiding-the-thundering-herd |
| |
| Configuration: |
| |
| Add @plugin=collapsed_forwarding.so to your remap.config rules. The plugin optionally |
| accepts a --delay=<delay> parameter and a --retries=<retries> parameter that can help tune |
| the total number of retries and the delay between consecutive retries. |
