Eric Allam
d3906241a5
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## Summary
The realtime runs feed hydrates run rows from read replicas, which means
it needs a replica-lag gate to avoid serving a run's previous state
right after a write. Setting
`REALTIME_BACKEND_NATIVE_RUN_READS_FROM_PRIMARY=1` reads those rows from
each run store's primary instead, so there is no lag to gate against: no
probe, no wake delay, no stale-read retries. Off by default, so nothing
changes unless you set it.
## Design
The run stores already decide replica-vs-primary from the *brand* on the
read client they are handed: a branded replica keeps the read on the
owning store's replica, an unbranded writer escalates it to that store's
own primary. So this is a one-line choice at the hydrator, and it stays
correct across topologies. With the run-ops split on, each leg lands on
its own writer and the caller's client is never forwarded across
databases; with the split off, it is the single database's primary.
```ts
const runReader = new RunHydrator({
readClient: runReadsFromPrimary ? prisma : $replica,
runStore,
});
```
The same flag skips constructing the lag estimator, since probing a
replica the feed no longer reads would be measuring the wrong thing.
Independently, `AuroraReplicaLagSource` detected Aurora by letting
`aurora_replica_status()` fail, on the assumption that the app-level
catch made that free. It isn't: an unresolvable function is a query
error the driver reports to the error log on every sample, so a
non-Aurora replica produced a continuous stream of error events while
the estimator quietly fell through to its next candidate. It now
resolves the function with `to_regproc` and memoizes the answer, so the
unparseable call never reaches the wire.