## Summary
During an ElastiCache role swap (failover) or node-type change (vertical
scale), the ioredis TCP/TLS connection stays open but the server starts
answering with `READONLY` (the client is talking to a node that became a
replica) or `LOADING` (node still loading data from disk). Without an
explicit hook, those errors surface to caller code as `ReplyError`
instances — every write op on the affected connection fails until the
cluster fully cuts over.
This PR adds `reconnectOnError` to every prod ioredis client so the
disconnect + reconnect + retry cycle absorbs these errors and caller
code never sees them.
## Fix
```ts
export function defaultReconnectOnError(err: Error): boolean | 1 | 2 {
const msg = err.message ?? "";
if (msg.startsWith("READONLY") || msg.startsWith("LOADING")) return 2;
return false;
}
```
Returning `2` tells ioredis to disconnect, reconnect, and re-issue the
failed command. After reconnect, DNS / SG state routes the new socket to
a writable node.
The helper lives in `@internal/redis` and is wired into both the shared
`createRedisClient` (which covers RunQueue, schedule-engine,
redis-worker, and every other internal-package consumer) and the direct
`new Redis(...)` call sites in the webapp.
V1-only marqs files are intentionally not migrated.
## Test plan
- [x] `pnpm run typecheck --filter webapp`
- [x] `pnpm run typecheck --filter @internal/run-engine`
- [x] Verified end-to-end against a live ElastiCache vertical-scale
event — caller-surfaced errors went from tens of thousands during the
cutover window down to a handful per ioredis client
- [ ] Confirm steady-state behavior unchanged after deploy
Eric Allam
567e2a2c32