An integration test of a web application sends requests and checks responses. When it passes, it leaves nothing behind but a green mark. When it fails, it leaves an assertion message on one side and the server's logs somewhere else, and someone has to put the two together.

The thread that ties them together already exists. OpenTelemetry carries a trace from client to server in a request header, so the server's work becomes a child of the client's request. Production systems use it across dozens of services; tests rarely do, because of the setup: a collector, a backend, agents on both sides, and a way to find one test's trace among everything else.

In one JVM it is free

A test that starts the server it tests, in the same JVM, needs none of that. This book describes the harness module of the Nasdanika SDK Runtime, with one method at its heart: Harness.inServer starts a server whose every handler is instrumented, runs your code in a span, gives it a client whose requests carry the trace, and stops the server when your code is done. Every request of the test and everything the server did for it are in the test's trace, and nothing else is.

• A server for the length of a test, on the JDK's own HTTP server, serving handlers or files.
• One trace, with the request, the server span it caused, the server's log records and any exception in one tree.
• Recording, with the runtime's telemetry model: the trace saved as a model in YAML, JSON or XMI that you can query and compare.

The last part shows what is built on it: browser walk-throughs with screenshots in the same trace, which are available in the Zephyr Book Walk-throughs with Playwright, and documents, slide decks, sequence diagrams and narrated videos generated from recorded traces, which are planned.

Who it is for

• Java developers who test web applications and services and want a failed test to say what the server did, not only what the client got.
• Developers who build demos and walk-throughs and want them reproducible.
• Architects who want end-to-end traceability from a test to the code that served it. The first part needs no code.

How it keeps you honest

Every listing is included from the module's sources or tests and runs when the book is built. Status labels mark what is PLANNED.

This is a short Nasdanika Zephyr Book, an in-depth manual for one small module of the Nasdanika SDK Runtime, and it stands on its own. The code needs Java 25; recording traces as models adds the runtime's telemetry model exporters. The text is licensed under CC BY 4.0 and the code under EPL-2.0.