The passthrough will probably need to:

• Have some kind of override that simulates the proxy's retry logic
• Set up at least two replica sets and run migrations between them

Open questions:

• How does the fixture know which databases exist to run migrations for? Should the override add a prefix to all databases and the fixture runs migrations for that prefix?
• Should we test concurrent migrations on different databases? I don't think this is required for the first attempt at this, but it may be worth thinking about how that could work

The passthrough should abort all tenant migrations until migrations actually copy the data over safely.