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The internal documentation that we do have lives in cockroachdb/cockroach/docs. At the time of writing, most of this documentation covers the high-level architecture of the system. Only a few documents hone in on specifics, and even those only cover the features that were found to cause significant developer frustration. For most first-party packages, you'll need to read the source for usage instructions.
Protip: If you prefer Go-generated HTML documentation to reading the source directly, take a look at godoc.org/github.com/cockroachdb/cockroach.
For our internal docs, I recommend the following reading order.
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CR, "code review"
PR, "pull request"—you probably knew that one.
PTAL, "please take another look"
RFAL, "ready for another look"—as in, I made some changes, PTAL.
LGTM, "looks good to me"—i.e., ship it!
, "science dog"—i.e., I have no idea what this code does.
TF[YT]R, "thanks for your/the review"
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