Cross-wiki capabilities are becoming more and more important for Wikimedia sites, as well as for MediaWiki in general. In particular, we need a way to test propagation of changes across multiple wikis, in the presence of cross-wiki use of information (e.g. for a media repo, wikibase, global user pages, etc).

Phester could support this as follows:
Each request in a test (or fixture) supports an optional site or target key. That key would specify a symbolic name of a target site, which would be resolved to a base URL based on configuration. Specifying an unknown target site would be an error and would cause the test to fail.

Note that a single test (that is, a single interaction sequence) can involve requests to multiple different target sites.

Note that this depends on support for delays/retries, since propagation across site boundaries is not going to be instantanious, even in a testing environment.

This could perhaps be part of T119169: Create a test system to validate DB integrity for saving edits in MW from multiple servers/DCs.