Summary

In the specification for driver support for running MongoDB behind an L4 load-balancer, we specify connection-pinning rules for distributed transactions to ensure all transaction commands target the same service behind the load balancer. Specifically, we say:

"When executing a transaction in load balancing mode, drivers MUST follow the rules outlined in Sharded Transactions with one exception: drivers MUST use the same connection for all commands in the transaction " 

I would like to clarify in the spec whether or not this implies that:

"drivers MUST NOT use the same connection for two concurrent transactions run under different sessions from the same client"

In other words, I wonder if the following situation possible:
(1) Client A begins txn with LSID x and TxnNumber 1; the driver pins connection C to serve this txn
(2) Client A sends some pieces of txn (x, 1) but doesn’t commit or abort it yet
(3) Client A begins txn with LSID y and txnNumber 1 on connection C, and sends some pieces of it
(4) Client A commits txn (x, 1)
(5) Client A commits txn (y, 1)

I believe the answer is "this is not possible", which means we could add to the spec a clarification that a pinned connection for a transaction will only run commands for that single transaction until it either commits or aborts. 

Motivation

Who is the affected end user?

Adding this to the spec would make the server work for LB support much simpler.

Is this ticket required by a downstream team?

SERVER