== Problem
Our stack comprises many components/servers that all interact with each other in other to fulfil clients' requests (or prepare data to be served to clients at a later point). For any given request, requests may be spawned to other components and their responses assembled before being returned to the client. This creates the need for having a sort of a //distributed stack trace// that allows us to pin-point problematic links in the request chain.
A certain degree of request identification does currently exist in our infrastructure, alas only on sub-system levels:
- MediaWiki's `WebRequest` relies on [the `UNIQUE_ID` env variable](https://github.com/wikimedia/mediawiki/blob/f6d582a91ee990de3ba04dad67eba055040a0e3f/includes/WebRequest.php#L266-L282) provided by [Apache's `mod_unique_id`](http://httpd.apache.org/docs/current/mod/mod_unique_id.html)
- RESTBase and the services behind it use and propagate the [`X-Request-Id` header](https://github.com/wikimedia/hyperswitch/blob/aa78fb649213bb0a4445135338341b71505cbc7d/lib/server.js#L260)
- EventBus relies on the [same `x-request-id` header](https://github.com/wikimedia/mediawiki-extensions-EventBus/blob/c36754afe5f04e72b799077c965180b96e187747/includes/EventBus.php#L393-L403) when creating events for both asynchronous updates as well as JobQueue messages
- Thumbor uses a [custom `Thumbor-Request-Id` header](https://phabricator.wikimedia.org/diffusion/THMBREXT/browse/master/wikimedia_thumbor/logging/__init__.py;f4bfba091899c22ebdad950abded7c869749d1d1$26)
There are probably more such examples.
In order to be able to trace the requests provoked by an (initial/external) request, all of the systems in our infrastructure should identify requests in the same way, use this identifier for logging and propagate it to other links in the request chain.
== Proposed Solution
Use a UUID v1/v4 `x-request-id` header/entity. Varnish f-e (soon ATS) is the main point of entry of external requests. Therefore, it can generate the request IDs and attach them to requests in the form of the `x-request-id` header, which can then be used and propagated by all entities behind it.
== See Also
- {T89562}
- {T97226}
- {T97207}
- {T117021}
- {T113817}
- {T200594}