Logical Component Relationships

• Vertices are components.
• Edges are ownership relationships between components. Each edge is labeled by the kind of "smart pointer" that could implement that kind of relationship.
• Components containing a padlock are protected by a mutex.

Objects

• Span has a beginning, end, and tags. It is associated with a TraceSegment.
• TraceSegment is part of a trace. It makes sampling decisions, detects when it is finished, and sends itself to the Collector.
• Collector receives trace segments. It provides a callback to deliver sampler modifications, if applicable.
• Tracer is responsible for creating trace segments. It contains the instances of, and configuration for, the Collector, TraceSampler, and SpanSampler. A tracer is created from a TracerConfig.
• TraceSampler is used by trace segments to decide when to keep or drop themselves.
• SpanSampler is used by trace segments to decide which spans to keep when the segment is dropped.
• TracerConfig contains all of the information needed to configure the collector, trace sampler, and span sampler, as well as defaults for span properties.

Intended usage is:

1. Create a TracerConfig.
2. Use the TracerConfig to create a Tracer.
3. Use the Tracer to create and/or extract local root Spans.
4. Use Span to create children and/or inject context.
5. Use a Span's TraceSegment to perform trace-wide operations.
6. When all Spans in TraceSegment are finished, the segment is sent to the Collector.

Different instances of Tracer are independent of each other. If an application wishes to reconfigure tracing at runtime, it can create another Tracer using the new configuration.