Context Propagation

What Is Context Propagation?

Context propagation is the mechanism that carries trace context across service boundaries. Without it, each service creates an independent trace. With it, all spans from a single request are connected into one trace.

# Without propagation
Service A -> trace-1 (span-1)
Service B -> trace-2 (span-2)  # disconnected!
Service C -> trace-3 (span-3)  # disconnected!

# With propagation
Service A -> trace-1 (span-1)
Service B -> trace-1 (span-2)  # connected via parent
Service C -> trace-1 (span-3)  # connected via parent

W3C Traceparent

The W3C Trace Context standard defines the traceparent header format. It carries the trace ID, parent span ID, and trace flags. This is the default propagator in OpenTelemetry and the recommended format for all new implementations.

traceparent: 00-abc123def456789a1b2c3d4e5f6a7b8c-d4e5f6a7b8c9-01

# Version:     00
# Trace ID:    abc123def456789a1b2c3d4e5f6a7b8c  (32 hex chars)
# Parent ID:   d4e5f6a7b8c9  (16 hex chars)
# Trace Flags: 01  (sampled)

Baggage

Baggage carries key-value pairs across service boundaries. Unlike trace context, baggage is application-defined. Use it to propagate user IDs, tenant IDs, or feature flags. Baggage is sent in the baggage header.

baggage: tenant_id=acme, user_role=admin, feature_flag=new-checkout

HTTP Propagation

HTTP propagation injects trace context into outgoing HTTP headers and extracts it from incoming headers. The OTel SDK handles this automatically for HTTP clients and servers when configured with the correct propagator.

gRPC Propagation

gRPC propagation uses metadata instead of HTTP headers. The OTel SDK provides gRPC interceptors that automatically inject and extract trace context from gRPC metadata.

Messaging Propagation

Message queues (Kafka, RabbitMQ, SQS) require special propagation. Trace context is carried in message headers or attributes. The producer injects context, and the consumer extracts it when processing the message.

# Producer injects traceparent in message headers
# Consumer extracts traceparent from message headers

# OTel Kafka instrumentation handles this automatically
producers:
  - topic: orders
    header propagation:
      enabled: true

consumers:
  - topic: orders
    header propagation:
      enabled: true