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nextflow/docs/developer/nextflow.script.md

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+
+# `nextflow.script`
+
+The `nextflow.script` package implements the parsing and execution of Nextflow scripts.
+
+## Class Diagram
+
+```{mermaid} diagrams/nextflow.script.mmd
+```
+
+```{note}
+Some classes may be excluded from the above diagram for brevity.
+```
+
+## Notes
+
+The execution of a Nextflow pipeline occurs in two phases. In the first phase, Nextflow parses and runs the script (using the language extensions in [nextflow.ast](nextflow.ast.md) and [nextflow.extension](nextflow.extension.md)), which produces the workflow DAG. In the second phase, Nextflow executes the workflow.
+
+```{note}
+In DSL1, there was no separation between workflow construction and execution -- dataflow operators were executed as soon as they were constructed. DSL2 introduced lazy execution in order to separate process definition from execution, and thereby facilitate subworkflows and modules.
+```