
# `nextflow.trace`

The `nextflow.trace` package defines the trace observer interface and implements several built-in trace observers.

## Class Diagram

```mermaid
classDiagram
    %%
    %% nextflow.trace
    %%
    Session --> TraceObserverFactory : init

    TraceObserverFactory "1" --> "*" TraceObserver : create
    TraceObserver <|-- AnsiLogObserver
    TraceObserver <|-- GraphObserver
    TraceObserver <|-- ReportObserver
    TraceObserver <|-- TimelineObserver
    TraceObserver <|-- TraceFileObserver
    TraceObserver <|-- WorkflowStatsObserver
```

:::note
Some classes may be excluded from the above diagram for brevity.
:::

## Notes

The `TraceObserver` interface defines a set of hooks into the workflow execution, such as when a workflow starts and completes, when a task starts and completes, and when an output file is published. The `Session` maintains a list of all observers and triggers each hook when the corresponding event occurs. Implementing classes can use these hooks to perform custom behaviors. In fact, this interface is used to implement several core features, including the various execution reports, DAG renderer, and the integration with Seqera Platform.
