Context Propagation

Context Propagation in opentelemetry-go-auto-instrumentation is inspired by Apache-Skywalking. Context in OpenTelemetry is a design for propagating trace-related information in distributed systems. Based on the propagation of Context, distributed services (AKA Spans) can be linked together to form a complete call chain (AKA Trace). OpenTelemetry saves trace-related information in Golang's context.Context and requires users to correctly pass context.Context. If context.Context is not passed correctly in the call chain, the call chain will be broken. To solve this problem, when opentelemetry-go-auto-instrumentation create a span, opentelemetry-go-auto-instrumentation save it to Golang's coroutine structure (i.e. GLS), and when opentelemetry-go-auto-instrumentation create a new coroutine, opentelemetry-go-auto-instrumentation also copy the corresponding data structure from the current coroutine. When opentelemetry-go-auto-instrumentation need to create a new span later, opentelemetry-go-auto-instrumentation will query the most recently created span from GLS as the parent, so that opentelemetry-go-auto-instrumentation have the opportunity to protect the integrity of the call chain.

Baggage is a data structure in OpenTelemetry used to share key-value pairs in Trace. Baggage is stored in context.Context and is propagated along with the context.Context. If context.Context is not correctly propagated in the call chain, subsequent services will not be able to read Baggage. To solve this problem, when opentelemetry-go-auto-instrumentation save the baggage to context.Context, opentelemetry-go-auto-instrumentation also save it to GLS. When context.Context is not passed correctly, opentelemetry-go-auto-instrumentation try to read the baggage from GLS, which allows the baggage to be read in this case.