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Mar 5, 2024
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Improve instrumentation doc #6777

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improve instrumentation doc
amarziali Mar 5, 2024
9e14099
Update docs/how_instrumentations_work.md
bm1549 Mar 5, 2024
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Update docs/how_instrumentations_work.md
bm1549 Mar 5, 2024
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9 changes: 8 additions & 1 deletion docs/how_instrumentations_work.md
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Expand Up @@ -94,7 +94,10 @@ To run muzzle on your instrumentation, run:
It checks that the types and methods used by the instrumentation are present in particular versions of libraries.
It can be subverted with `MethodHandle` and reflection, so muzzle passing is not the end of the story.

**TODO: discuss why 'name' is not always included.**
By default, all the muzzle directives are checked against all the instrumentations included in a module.
However, there can be situations in which it’s only needed to check one specific directive on an instrumentation.
At this point the instrumentation should override the method `muzzleDirective()` by returning the name of the directive to execute.


## Instrumentation classes

Expand Down Expand Up @@ -527,6 +530,8 @@ It is important to understand that even though they look like maps, since the va
retrieve a value if you use the exact same key object as when it was set. Using a different object that is “`.equals()`”
to the first will yield nothing.

Since `ContextStore` does not support null keys, null checks must be enforced _before_ using an object as a key.

## CallDepthThreadLocalMap

In order to avoid activating new spans on recursive calls to the same method
Expand All @@ -536,6 +541,8 @@ and [decremented](https://github.com/DataDog/dd-trace-java/blob/9d5c7ea524cfec98
or [reset](https://github.com/DataDog/dd-trace-java/blob/9d5c7ea524cfec982176e687a489fc8c2865e445/dd-java-agent/instrumentation/java-http-client/src/main/java11/datadog/trace/instrumentation/httpclient/SendAdvice.java#L44))
when exiting.

This only works if the methods are called on the same thread since the counter is a ThreadLocal variable.

## Span Lifecycle

In Advice classes, the `@Advice.OnMethodEnter` methods typically start spans and `@Advice.OnMethodExit` methods
Expand Down