Context

Cloudserver recently gained OpenTelemetry tracing (CLDSRV-884, cloudserver PR scality/cloudserver#6140). That gives us a single end-to-end trace per S3 request spanning cloudserver → vault → hdproxy → hyperiod.

But many state mutations in the S3 stack are written to MongoDB and consumed asynchronously by worker services via the oplog:

• Backbeat — replication, lifecycle expiration/transition, garbage collection
• Sorbet — cold-storage restore completion
• Other oplog consumers

Today the oplog entry carries no trace context, so async work performed hours or days after the S3 request cannot be tied back to the request that caused it. A user's POST …?restore produces a trace that ends at the metadata write; the actual cold-tier retrieval by Sorbet is invisible from the original trace.

Solution

Stamp a W3C trace context (traceparent/tracestate) on every mutating metadata write, auto-derived from the currently-active OTEL context.

The field lives on ObjectMDData next to originOp:

traceContext?: {
    traceparent?: string;
    tracestate?: string;
};

A new helper captureCurrentTraceContext() reads the active OTEL context via @opentelemetry/api and serializes it via propagation.inject(). It's invoked at the three MongoClientInterface sites that currently set originOp:

• internalPutObject
• repair
• internalDeleteObject

Consumers (backbeat, sorbet — out of scope for this PR) will later:

1. Read value.traceContext.traceparent from the oplog entry
2. propagation.extract() it to reconstruct a parent context
3. Wrap per-entry processing in context.with(extractedCtx, ...)

This closes the loop on end-to-end tracing across the async boundary.

Design decisions

1. Auto-inject at the arsenal write boundary (vs. param plumbing).
The alternative was to thread a new traceContext parameter through the ~15 cloudserver call sites that already set originOp. Rejected because:

• @opentelemetry/context-async-hooks (loaded by the OTEL SDK at cloudserver startup) already propagates the request context through arsenal's async call chain for free. Re-reading it at the mongo write is idiomatic OTEL.
• Zero cloudserver surface change — we can iterate on arsenal independently.
• Parallels existing behavior where arsenal reads originOp out of params and attaches it to the ObjectMD.
• Avoids replicating the maintenance cost that makes originOp easy to forget on new API endpoints today.

2. Only @opentelemetry/api as a runtime dep (~10 KB, no SDK).
The api package is specifically designed to be safe for libraries to depend on. When no SDK is registered (today's state for anyone not opting into cloudserver OTEL), context.active() returns a no-op context, trace.getSpan() returns undefined, and captureCurrentTraceContext() returns undefined. setTraceContext(undefined) no-ops; the field stays absent in the write. Zero runtime cost when OTEL is off.

3. Optional field, no schema migration.
traceContext is absent by default — consistent with other optional ObjectMD fields like legalHold, retentionDate. No default-value change in the ObjectMDData construction. Existing consumers that don't understand the field simply ignore it; fully backward compatible.

4. No-op when no traceparent is present.
setTraceContext(undefined) and setTraceContext({ tracestate: 'x' }) (no traceparent) both no-op — the field only appears when the caller had an active trace. This prevents junk entries from any edge cases in the OTEL API.

Interaction with async flows (cold restore as example)

Cold storage restore exercises both a sync and an async metadata write:

1. ObjectRestore:Post (sync, user-initiated via POST /bucket/key?restore) — cloudserver sets originOp; arsenal now also stamps traceContext pointing to the user's request trace. ✅
2. ObjectRestore:Completed (async, Sorbet-triggered via cloudserver's /_/backbeat/* route) — behavior depends on Sorbet:
   • If Sorbet forwards traceparent on its HTTP call (ideally extracted from step 1's stored traceContext): arsenal picks it up via context.with(remoteContext, ...) in cloudserver's server.js, and the Completed write ties back to the original user restore. ✅
   • If Sorbet calls without traceparent (today's state): safely no-ops — Completed write has no traceContext, no regression. ✅

Same pattern applies to lifecycle expiration/transition, CRR replication, and GC.

Verification

Unit tests

tests/unit/models/ObjectMD.spec.js — 4 new tests covering:

• Valid traceparent round-trips through set/getTraceContext
• setTraceContext(undefined) is a no-op
• setTraceContext({ tracestate: ... }) without traceparent is a no-op
• getValue() serializes traceContext only when set

All 128 ObjectMD tests pass (was 124 + 4 new).

End-to-end (follow-up on a test cluster)

1. Bump cloudserver's arsenal pin to a build of this branch; roll the cloudserver deployment on an Artesca cluster with ENABLE_OTEL=true
2. Make a PutObject; note the trace ID in Jaeger
3. Inspect the MongoDB oplog: o.value.traceContext.traceparent should be present, with characters 3-35 matching the Jaeger trace ID
4. Negative test with ENABLE_OTEL=false → field should be absent

Issue: ARSN-572