# Collapsed Forwarding Plugin¶

This is a plugin for Apache Traffic Server that allows to achieve effective connection collapse by blocking all but one of the multiple concurrent requests for the same object from going to the Origin.

## Installation¶

To make this plugin available, you must either enable experimental plugins when building Traffic Server:

./configure --enable-experimental-plugins


Or use tsxs to compile the plugin against your current Traffic Server build. To do this, you must ensure that:

1. Development packages for Traffic Server are installed.
2. The tsxs binary is in your path.
3. The version of this plugin you are building, and the version of Traffic Server against which you are building it are compatible.

Once those conditions are satisfied, enter the source directory for the plugin and perform the following:

make -f Makefile.tsxs
make -f Makefile.tsxs install


## Using the plugin¶

This plugin can function as a per remap plugin or a global plugin, and it takes two optional arguments for specifying the delay between successive retries and a max number of retries.

To activate the plugin in per remap mode, in remap.config, simply append the below to the specific remap line:

@plugin=collapsed_forwarding.so @pparam=--delay=<delay> @pparam=--retries=<retries>


To activate the plugin globally, in plugin.config, add the following line:

collapsed_forwarding.so --delay=<delay> --retries=<retries>


If the plugin is enabled both globally and per remap, Traffic Server will issue an error on startup.

## Functionality¶

Traffic Server 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:

:ts:cv:proxy.config.http.cache.open_write_fail_action        1
:ts:cv:proxy.config.cache.enable_read_while_writer           1
:ts:cv:proxy.config.http.number_of_redirections             10
:ts:cv:proxy.config.http.redirect_use_orig_cache_key         1
:ts:cv:proxy.config.http.background_fill_active_timeout      0
:ts:cv: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.

## Description¶

Traffic Server has been affected severely by the Thundering Herd problem caused by its inability to do effective connection collapse of multiple concurrent requests for the same segment. This is especially critical when Traffic Server is used as a solution to use cases such as delivering a large scale video live streaming. This problem results in a specific behavior where multiple number of requests for the same file are leaked upstream to the Origin layer choking the upstream bandwidth due to the duplicated large file downloads or process intensive file at the Origin layer. This ultimately can cause stability problems on the origin layer disrupting the overall network performance.

Traffic Server supports several kind of connection collapse mechanisms including Read-While-Writer (RWW), Stale-While-Revalidate (SWR) etc each very effective dealing with a majority of the use cases that can result in the Thundering herd problem.

For a large scale Video Streaming scenario, there’s a combination of a large number of revalidations (e.g. media playlists) and cache misses (e.g. media segments) that occur for the same file. Traffic Server’s RWW works great in collapsing the concurrent requests in such a scenario, however, as described in _admin-configuration-reducing-origin-requests, Traffic Server’s implementation of RWW has a significant limitation, which restricts its ability to invoke RWW only when the response headers are already received. This means that any number of concurrent requests for the same file that are received before the response headers arrive are leaked upstream, which can result in a severe Thundering herd problem, depending on the network latencies (which impact the TTFB for the response headers) at a given instant of time.

To address this limitation, Traffic Server supports a few Cache tuning solutions, such as Open Read Retry, and a new feature called Open Write Fail action from 6.0. To understand how these approaches work, it is important to understand the high level flow of how Traffic Server handles a GET request.

On receiving a HTTP GET request, Traffic Server generates the cache key (basically, a hash of the request URL) and looks up for the directory entry (dirent) using the generated index. On a cache miss, the lookup fails and Traffic Server then tries to just get a write lock for the cache object and proceeds to the origin to download the object. On the Other hand, if the lookup is successful, meaning, the dirent exists for the generated cache key, Traffic Server tries to obtain a read lock on the cache object to be able to serve it from the cache. If the read lock is not successful (possibly, due to the fact that the object’s being written to at that same instant and the response headers are not in the cache yet), Traffic Server then moves to the next step of trying to obtain an exclusive write lock. If the write lock is already held exclusively by another request (transaction), the attempt fails and at this point Traffic Server simply disables the cache on that transaction and downloads the object in a proxy-only mode:

1). Cache Lookup (lookup for the dirent using the request URL as cache key).
1.1). If lookup fails (cache miss), goto (3).
1.2). If lookup succeeds, try to obtain a read lock, goto (2).
2.1). If read lock succeeds, serve from cache, goto (4).
2.2). If read lock fails, goto (3).
3). Open Cache Write (try to obtain write lock).
3.1). If write lock succeeds, download the object into cache and to the client in parallel
3.2). If write lock fails, disable cache, and download to the client in a proxy-only mode.
4). Done


As can be seen above, if a majority of concurrent requests arrive before response headers are received, they hit (2.2) and (3.2) above. Open Read Retry can help to repeat (2) after a configured delay on 2.2, thereby increasing the chances for obtaining a read lock and being able to serve from the cache.

However, the Open Read Retry can not help with the concurrent requests that hit (1.1) above, jumping to (3) directly. Only one such request will be able to obtain the exclusive write lock and all other requests are leaked upstream. This is where, the recently developed Traffic Server feature Open Write Fail Action will help. The feature detects the write lock failure and can return a stale copy for a Cache Revalidation or a 5xx status code for a Cache Miss with a special internal header <@Ats-Internal> that allows a TS plugin to take other special actions depending on the use-case.

collapsed_forwarding plugin catches that error in SEND_RESPONSE_HDR_HOOK and performs an internal 3xx Redirect back to the same host, the configured number of times with the configured amount of delay between consecutive retries, allowing to be able to initiate RWW, whenever the response headers are received for the request that was allowed to go to the Origin.

More details are available at