We recently produced a set of recommendations for our services architecture, and during the process, received feedback that some of them should be captured outside the scope of this effort, to be referred to more generally. There seemed to be some consensus that Requests_for_comment/Standards_for_external_services might be the right place for this. Some initial efforts at incorporating these has been made at User:EEvans_(WMF)/Scratch/Standards_for_external_services (diff). It would be great to get a proper review of these changes though, including whether the structure best represents what the original authors intended, before submitting to TechCom for approval.
It seems that the relevant section of Requests_for_comment/Standards_for_external_services has been moved to Wikimedia services guideline. I'd recommend adding the two subsections (Abstractions: A priori versus a posteriori and Pre-generating page-derived data) under Development policies.
The content itself looks good! I made a few small edits, but I think it's ready for a proposal. One thing to consider: Is there something we could add to the bulleted list under Development policies to represent the new sections? Something along the lines of: "Avoid preemptive caching unless required by service latency targets" and "Build narrow, simple interfaces in order to decouple from non-abstract systems"?