Context Propagation

Context Propagation — Rules for Connected Traces

Every internal method takes ctx: Context as its first parameter (Go-style). The @trace.span() decorator detects ctx as args[0], uses it as the parent span, and replaces it with a child context before calling the method body. This enables hierarchical OTEL traces without Zone.js.

For how the tracing system works internally (TracingBackend, RemoteSpan, TRACE_PROCESSOR, all APIs), see the tracing skill.

import { Context } from '@dxos/context';

Why This Matters

The @trace.span() decorator reads a parent span from the incoming ctx attribute (TRACE_SPAN_ATTRIBUTE). If ctx has no parent span (e.g., from Context.default()), the decorator creates an orphaned root span — disconnected from the trace hierarchy. Every method between an entry point and a @trace.span() method must accept and forward ctx, even if the intermediate method itself has no @trace.span().

Which Methods Need ctx

ctx: Context is always the first parameter. Not in an options bag, not as the last parameter, not optional.

Add ctx: Context as first parameter to methods on any call chain that reaches:

• EdgeHttpClient methods — all HTTP calls to EDGE services.
• EdgeWsConnection.send / EdgeClient.send — all WebSocket messages to EDGE.
• Swarm / WebRTC connections (mesh replication, peer discovery).
• Feed replication (Hypercore feed read/write over the network).
• Credential notarization (writes that hit the control pipeline and propagate to peers).

Every method in the call chain — from entry point through signal managers, messengers, replicators, down to the terminal networking call — must accept ctx: Context as its first parameter and forward it.

Do not add ctx to:

• Public user-facing APIs — these create ctx = Context.default() internally and forward it.
• RPC service methods (proto-generated interface) — the signature is fixed by the proto definition; you cannot add ctx as a parameter. Instead, read options.ctx which is provided by RpcPeer via ContextRpcCodec with the caller's trace context already decoded. Forward options.ctx (or options?.ctx ?? Context.default()) to internal methods.
• React components and hooks — UI code should not propagate ctx; create Context.default() at the boundary.
• Infrastructure / plumbing — worker setup, service registry wiring, serialization.
• Pure local operations — in-memory data transforms, UI state, local database reads.
• Leaf utility methods — small methods that don't call other methods (getters, simple lookups, validation helpers).
• Test utilities — test builders, test helpers (unless they call production code that requires ctx, in which case pass Context.default()).

Key entry points

Rules

1. Direct calls: forward the caller's ctx

Any sync or async call inside a method should receive the method's ctx.

@trace.span()
async open(ctx: Context): Promise<void> {
  await this._initPipeline(ctx);
  await this._loadData(ctx);
}

For public user-facing methods, create a root context and forward it:

async createSpace(options: CreateSpaceOptions = {}): Promise<Space> {
  const ctx = Context.default();
  return this._createSpaceInternal(ctx, options);
}

For RPC service methods, use options.ctx which carries the caller's trace context propagated across the wire by ContextRpcCodec:

testCall: async (req: TestRequest, options?: RequestOptions) => {
  const ctx = options?.ctx ?? Context.default();
  return this._handleTestCall(ctx, req);
},

2. Event subscriptions and callbacks: use lifecycle ctx

Callbacks are new entry points without a caller-provided ctx. Use this._ctx from Resource or a manually managed Context.

this._spaceStateMachine.onFeedAdmitted.set(async (info) => {
  await this._handleFeedAdmitted(this._ctx, info);
});

this.stateUpdate.on(this._ctx, () => {
  this._refresh(this._ctx);
});

Lifecycle ctx is correct because it is scoped to the object's lifetime. When the class uses @trace.resource({ lifecycle: true }), this._ctx carries the lifecycle span's trace context, so callbacks appear as children of the lifecycle span rather than orphaned roots or stale children of the _open span.

3. Detached async work: use class lifecycle ctx

setTimeout, scheduleTask, DeferredTask, scheduleMicroTask, queueMicrotask run outside the caller's call stack. Use this._ctx.

this._updateTask = new DeferredTask(this._ctx, () => this._executeQueries(this._ctx));

scheduleMicroTask(this._ctx, async () => {
  await this._handleFeedAdmission(this._ctx, info);
});

Never use Context.default() for detached work — it creates an orphaned context nobody disposes.

4. Parallel fan-out: share the parent ctx

Pass the same ctx to each branch. The @trace.span() decorator derives independent child contexts.

@trace.span()
async initializeAll(ctx: Context): Promise<void> {
  await Promise.all([
    this._initA(ctx),
    this._initB(ctx),
    this._initC(ctx),
  ]);
}

5. Reassign ctx to the return value of trace.spanStart()

The manual span API trace.spanStart() returns a derived Context carrying the new span on TRACE_SPAN_ATTRIBUTE. Callers MUST reassign the local ctx to the returned value so that downstream @trace.span() methods see this span as their parent.

// ✅ CORRECT — ctx reassigned, downstream @trace.span methods nest under the manual span.
ctx = trace.spanStart({
  id: spanId,
  instance: this,
  methodName: 'acceptInvitation',
  parentCtx: ctx,
  op: 'invitation.guest',
}) ?? ctx;
ctx.onDispose(() => trace.spanEnd(spanId));
await this._handleGuestFlow(ctx, ...); // inherits the manual span as parent

// ❌ WRONG — return value ignored; downstream spans don't see this one.
trace.spanStart({ id: spanId, instance: this, methodName: 'acceptInvitation', parentCtx: ctx, ... });
ctx.onDispose(() => trace.spanEnd(spanId));
await this._handleGuestFlow(ctx, ...); // ctx unchanged → handleGuestFlow's @trace.span attaches to ctx's parent, becoming a sibling of the manual span (or a new root if ctx had no parent)

When parentCtx is not null and the span is actually created, the returned ctx is a parentCtx.derive({ attributes: { [TRACE_SPAN_ATTRIBUTE]: newSpanContext } }) — i.e., the original parentCtx with TRACE_SPAN_ATTRIBUTE overridden to the new span. Leaving ctx unchanged means downstream @trace.span methods read the OLD TRACE_SPAN_ATTRIBUTE from parentCtx and attach to that span, so they end up as siblings of the manual span rather than children. They only become a new root trace when the old ctx had no parent span at all. Either way, the onDispose(spanEnd) pattern remains correct — disposing the original parentCtx cascades to the derived one.

6. Complete call chain: every method between entry point and @trace.span must forward ctx

Critical rule: every intermediate method between an entry point and a @trace.span() method must accept and forward ctx, even if the intermediate method itself has no @trace.span(). Otherwise the trace chain is broken.

// ❌ WRONG — breaks trace chain with Context.default() at the last step
class SpaceList {
  private _setupSpacesStream(): void {
    stream.subscribe((data) => {
      scheduleMicroTask(this._ctx, async () => {
        await spaceProxy._processUpdate(data); // no ctx
      });
    });
  }
}

class SpaceProxy {
  async _processUpdate(data: Data): Promise<void> {
    // no ctx
    await this._initialize(); // no ctx
  }

  private async _initialize(): Promise<void> {
    // no ctx
    await this._initializeDb(Context.default()); // orphaned root!
  }

  @trace.span()
  private async _initializeDb(_ctx: Context): Promise<void> {
    // _ctx has no parent span — creates disconnected root span
  }
}

// ✅ CORRECT — ctx flows through entire chain
class SpaceList {
  private _setupSpacesStream(): void {
    stream.subscribe((data) => {
      scheduleMicroTask(this._ctx, async () => {
        await spaceProxy._processUpdate(this._ctx, data); // lifecycle ctx
      });
    });
  }
}

class SpaceProxy {
  async _processUpdate(ctx: Context, data: Data): Promise<void> {
    await this._initialize(ctx); // forwards ctx
  }

  private async _initialize(ctx: Context): Promise<void> {
    await this._initializeDb(ctx); // forwards ctx
  }

  @trace.span()
  private async _initializeDb(_ctx: Context): Promise<void> {
    // _ctx has parent span from lifecycle context — connected hierarchy
  }
}

How Context Flows

User code (no ctx)
  → public API creates ctx = Context.default()
    → intermediate method forwards ctx (no @trace.span — passes as-is)
      → @trace.span() method receives ctx
        decorator: reads parent span, creates child span, replaces ctx
        → method body receives child ctx
          → next method forwards child ctx
            → @trace.span() method — creates grandchild span
              → ...

RPC boundary:
  caller's @trace.span() sets TRACE_SPAN_ATTRIBUTE on ctx
    → RpcPeer.call: ContextRpcCodec.encode(ctx) → W3C traceparent on wire
      ──── network ────
    → RpcPeer handler: ContextRpcCodec.decode(data) → options.ctx with parent span
      → service method reads options.ctx → forwards to internal methods
        → @trace.span() method — child of the remote caller's span

Callback / detached async work:
  event fires → callback uses this._ctx (lifecycle context)
    → intermediate method forwards this._ctx
      → @trace.span() method — creates root span tied to object lifecycle

Who Provides the Root Context

Quick Reference

No Orphaned Internal Spans

Every @trace.span() method that runs inside a service or resource MUST have a parent span. An orphaned root span (one with no parentSpanID) means the trace context chain is broken — the span is disconnected from the trace tree and provides no useful correlation.

Allowed root spans (no parent):

• Public API entry points — Client.initialize(), HaloProxy.createIdentity(), SpaceList.create() — these are user-initiated actions that start a new trace.
• Stream/subscription callbacks using this._ctx from a @trace.resource({ lifecycle: true }) class — the lifecycle span IS the root, and the callback span is its child.

Disallowed orphaned spans:

• Any @trace.span() method inside a Resource subclass that has no parent. Fix: add lifecycle: true to the class's @trace.resource() so this._ctx carries the lifecycle span's trace context.
• Any @trace.span() method called from an RPC handler that ignores options.ctx. Fix: read options?.ctx ?? Context.default() and forward it.
• Any intermediate method that creates Context.default() instead of forwarding the caller's ctx.

When adding a new @trace.span() to an internal method, verify that at least one of these is true:

1. The method accepts ctx: Context as its first parameter and the caller forwards a traced context.
2. The method runs inside a @trace.resource({ lifecycle: true }) class and uses this._ctx (which carries the lifecycle span).
3. The method is an RPC handler that reads options.ctx.

If none of these hold, the span will be orphaned. Either fix the call chain or don't add @trace.span().

Common Mistakes

• Breaking the chain with Context.default() in an intermediate method — if a method calls child(Context.default()) where child has @trace.span(), the span is an orphaned root. The intermediate method must accept ctx from its caller and forward it.
• Passing this._ctx in a direct call when the method has ctx in scope — breaks trace hierarchy, creates a new root instead of a child span.
• Passing ctx in a callback or scheduleTask — captures a stale context whose span has already ended. However, this._ctx from lifecycle contexts still works as a parent because TRACE_SPAN_ATTRIBUTE stores W3C strings that remain valid after the span ends.
• Ignoring the return value of trace.spanStart() — spanStart returns a derived ctx carrying the new span. Downstream @trace.span methods called with the unchanged ctx will see the span's parent, not the span itself, making them siblings of the manual span (or, if the parent ctx had no span, a new root). Reassign ctx = trace.spanStart(...) ?? ctx; before forwarding.
• Adding ctx to Node.js protocol methods (e.g., [Symbol.for('nodejs.util.inspect.custom')]) — fixed-signature methods that don't support extra parameters.
• Adding ctx to public APIs — user-facing methods should not expose Context; create Context.default() internally.
• Ignoring options.ctx in RPC service methods — RpcPeer provides options.ctx with the caller's trace context decoded from W3C headers. Use options?.ctx ?? Context.default(), not this._ctx or a bare Context.default(). Using this._ctx breaks the cross-process trace hierarchy; using Context.default() discards the propagated trace.

ESLint Enforcement

The dxos-plugin/require-context-param rule warns on class methods missing ctx: Context as the first parameter. Configured in eslint.config.mjs for core SDK packages with exemptions for public API classes.

Audit

See AUDIT.md for a detailed review of context propagation compliance across the codebase, including known gaps and prioritized recommendations.