> For the complete documentation index, see [llms.txt](https://stephen-tsoi.gitbook.io/stephen-tsoi-docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://stephen-tsoi.gitbook.io/stephen-tsoi-docs/opentelemetry/from-missing-events-to-complete-visibility-tracing-enterprise-transactions-with-opentelemetry/day-6-34-context-propagation.md).
# Day 6/34 Context Propagation
OpenTelemetry uses a Trace ID to correlate different spans that belong to the same execution flow, enabling end‑to‑end request tracing and visualization through the APM dashboard.
📌When a transaction is initiated, the trigger point generates a Trace ID and propagates it to downstream components in a structured format known as context. The context typically includes the following information:\
\*\*Trace ID – This unique identifier represents the entire trace flow. It is initially generated at the trigger point and then propagated throughout the entire trace flow.\
\*\*Span ID – This unique identifier distinguishes each step within the trace flow, acting as a fingerprint for individual steps.\
\*\*Parent Span ID – This refers to the span ID of the previous step, establishing the sequential order of each span within the trace flow.
As a message flows end‑to‑end, the upstream application injects the trace context into the outgoing request or message, while the downstream application extracts the context and continues the trace. This mechanism ensures trace continuity across multiple components within the same application. This process is known as context propagation. The actor to serialize and deserialize the context is propagators.
📌When trace context is propagated across different applications, all participating systems must follow agreed‑upon protocol standards for injecting and extracting trace information:\
\*\*RESTful / HTTP calls:\
OpenTelemetry follows the W3C Trace Context standard (W3C Trace Context Level 2), which defines the version, trace-id, parent-id and trace flag on\
HTTP headers for propagating trace information.\
\*\*Messaging systems:\
Since there is no universal standard across messaging protocols, trace context is typically propagated via message headers. This approach is commonly used for protocols such as MQTT, where OpenTelemetry trace metadata is manually injected and extracted according to implementation conventions.
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