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Original Description

Description:

I'm not sure we need to document this, since we'll definitely revisit readConcern for sharded transactions later this release as we add support for more readConcern levels, but putting "Needed" just in case. This ticket explicitly disallowed starting a sharded transaction with any readConcern level other than "snapshot" (so local and majority are rejected, unlike on mongod in 4.0). If a user provides no readConcern level when starting a transaction though, mongos will use snapshot as the default.

Engineering Ticket Description:

If the first statement in a transaction does not specify snapshot readConcern, mongod will implicitly upconvert it to snapshot. Mongos currently does not have this behavior, and because many core/txns tests rely on this, either mongos will need to upconvert readConcerns to snapshot as well (until we support transactions with other readConcern levels) or the tests need to be updated to explicitly choose snapshot readConcern.

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Description

Scope of changes (files that need work and how much)

Impact to other docs outside of this product

MVP (work and date?)

Resources (e.g. Scope Docs, Invision)