Hey MA, I've checked, and while not explicitly disallowed, the fact that this could work is more of an implementation detail that shouldn't really be relied on.

The sections and where the instances are on them are organized to maintain the service, and are not supposed to be depended on since they could change.

Even if the mappings are public and fairly stable, there could be a point where a change in the implementation/organization is made -like with this new architecture- and those in-section cross-db joins would stop working.

On Wed, Nov 11, 2020 at 10:31 PM MusikAnimal <musikanimal@gmail.com> wrote:
Technically, cross-wiki joins aren't completely disallowed, you just have to make sure each of the db names are on the same slice/section, right?

~ MA

On Wed, Nov 11, 2020 at 4:11 PM Maarten Dammers <maarten@mdammers.nl> wrote:

Hi Joaquin,

On 10-11-2020 21:26, Joaquin Oltra Hernandez wrote:
TLDR: Wiki Replicas' architecture is being redesigned for stability and performance. Cross database JOINs will not be available and a host connection will only allow querying its associated DB. See [1] for more details.

If you only think of Wikipedia, not a lot will break probably, but if you take into account Commons and Wikidata a lot will break. A quick grep in my folder with Commons queries returns 123 lines with cross database joins. So yes, stuff will break and tools will be abandoned. This follows the practice that seems to have become standard for the WMF these days: Decisions are made with a small group within the WMF without any community involved. Only after the decision has been made, it's announced.

Unhappy and disappointed,

Maarten

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Joaquin Oltra Hernandez
Developer Advocate - Wikimedia Foundation