Assertion limitations

Assertions cover attributes and delivery (where items go, and how many). Everything below is outside what Tracelet v0.1.0 can assert.

Fields that can't be assertedCopy Anchor Link

These fields still reach the Collector and drive your processors: a filter on severity_number, a sampling policy on status.code, or a transform on the body. You can test their effect on attributes and delivery, but not the fields themselves.

Ways to work around this:

• Status or severity filters: assert dropped: true or a count on the alerting exporter.
• Body parsing: assert on the attributes the parser produced, as the log redaction example does.
• Aggregation: assert the resulting data-point count, and that dropped labels are absent.
• Metric renames: not testable today.

Value checksCopy Anchor Link

• equals must hold for every item. There is no "one of" check. An attribute that differs between items (status 200 on one data point and 500 on another) can only be checked with present or matches.
• There is no way to assert an attribute's type without its exact value.
• matches works on strings only.
• Only scalar values can be compared. Arrays, maps and bytes can't be matched with equals.

Timing-dependent outputCopy Anchor Link

• Connectors that emit on a timer (span_metrics, service_graph) need metrics_flush_interval below the test timeout, and emit repeatedly. Avoid count on their output.
• groupbytrace discards pending traces at shutdown, so dropped can pass for the wrong reason when its wait_duration isn't shorter than the timeout. Tracelet warns.

Stateful processorsCopy Anchor Link

One test injects one fixture into a fresh Collector. Processors whose behavior depends on several requests over time, such as cumulative_to_delta, rate limiting or probabilistic sampling rates, can't be exercised meaningfully. A fixture can still hold many items, such as a whole trace or several data points, so per-request behavior is testable.

Not observable by designCopy Anchor Link

• Anything the receiver would have done, because receivers are replaced.
• Anything the exporter or backend would have done, because exporters are replaced and no backend is contacted.

See Known limitations for limitations outside the assertion engine.