## Summary
Stamp every Prisma span with `db.datasource: "writer" | "replica"` so
traces can distinguish which client the query went through.
Both `PrismaClient` instances share the same global
`@prisma/instrumentation`, so their spans come out with identical names
and attributes today. This makes them trivially filterable.
## How
Two pieces in `apps/webapp/app/`:
1. **`v3/tracer.server.ts`** — a `DatasourceAttributeSpanProcessor`
reads an OTel context key in `onStart` and calls
`span.setAttribute("db.datasource", value)`. Registered as the first
span processor.
2. **`db.server.ts`** — `tagDatasource(datasource, client)` wraps each
`PrismaClient` with `$extends({ query: { $allOperations } })`. The
middleware sets the context key around the query and directly tags the
active span (to catch `prisma:client:operation`, which Prisma creates
before the middleware fires).
### Context-propagation gotcha
`PrismaPromise` is lazy — `query(args)` returns a thenable that only
starts when someone `.then()`s it. The naive `context.with(ctx, () =>
query(args))` restores ALS synchronously, so when Prisma's internal code
awaits the thenable later, the engine spans fire with the original ALS.
Wrapping as `async () => await query(args)` forces the `.then()` inside
the `context.with` callback, so ALS stays on our context for the engine
spans.
### Coverage
- **Tagged**: all `prisma:engine:*` (`connection`, `db_query`,
`serialize`, `query`, etc.), `prisma:client:operation`,
`prisma:client:serialize`, `prisma:client:connect`
- **Not tagged**: `prisma:client:load_engine` — one-time startup, fires
before any query
Concurrent `Promise.all([writer.x, replica.y])` correctly tags each pool
separately (ALS isolates per-Promise chain).
### Performance
One `context.with` (~200ns) and one `setAttribute` per span (effectively
free per OTel JS benchmarks) per Prisma op. Negligible against a query
path measured in milliseconds.
## Test plan
- [ ] Verify `db.datasource` appears on `prisma:engine:connection` spans
after the webapp is restarted
- [ ] Spot-check a handful of real traces carry the attribute
Eric Allam
7c95ee498e