Python: port API graph doc comment

Author: asgerf

python/ql/lib/semmle/python/ApiGraphs.qll

@@ -12,12 +12,83 @@ import semmle.python.dataflow.new.DataFlow
 private import semmle.python.internal.CachedStages
 
 /**
- * Provides classes and predicates for working with APIs used in a database.
+ * Provides classes and predicates for working with the API boundary between the current
+ * codebase and external libraries.
+ *
+ * See `API::Node` for more in-depth documentation.
  */
 module API {
   /**
-   * An abstract representation of a definition or use of an API component such as a function
-   * exported by a Python package, or its result.
+   * A node in the API graph, representing a value that has crossed the boundary between this
+   * codebase and an external library (or in general, any external codebase).
+   *
+   * ### Basic usage
+   *
+   * API graphs are typically used to identify "API calls", that is, calls to an external function
+   * whose implementation is not necessarily part of the current codebase.
+   *
+   * The most basic use of API graphs is typically as follows:
+   * 1. Start with `API::moduleImport` for the relevant library.
+   * 2. Follow up with a chain of accessors such as `getMember` describing how to get to the relevant API function.
+   * 3. Map the resulting API graph nodes to data-flow nodes, using `asSource` or `asSink`.
+   *
+   * For example, a simplified way to get arguments to `json.dumps` would be
+   * ```ql
+   * API::moduleImport("json").getMember("dumps").getParameter(0).asSink()
+   * ```
+   *
+   * The most commonly used accessors are `getMember`, `getParameter`, and `getReturn`.
+   *
+   * ### API graph nodes
+   *
+   * There are two kinds of nodes in the API graphs, distinguished by who is "holding" the value:
+   * - **Use-nodes** represent values held by the current codebase, which came from an external library.
+   *   (The current codebase is "using" a value that came from the library).
+   * - **Def-nodes** represent values held by the external library, which came from this codebase.
+   *   (The current codebase "defines" the value seen by the library).
+   *
+   * API graph nodes are associated with data-flow nodes in the current codebase.
+   * (Since external libraries are not part of the database, there is no way to associate with concrete
+   * data-flow nodes from the external library).
+   * - **Use-nodes** are associated with data-flow nodes where a value enters the current codebase,
+   *   such as the return value of a call to an external function.
+   * - **Def-nodes** are associated with data-flow nodes where a value leaves the current codebase,
+   *   such as an argument passed in a call to an external function.
+   *
+   *
+   * ### Access paths and edge labels
+   *
+   * Nodes in the API graph are associated with a set of access paths, describing a series of operations
+   * that may be performed to obtain that value.
+   *
+   * For example, the access path `API::moduleImport("json").getMember("dumps")` represents the action of
+   * importing `json` and then accessing the member `dumps` on the resulting object.
+   *
+   * Each edge in the graph is labelled by such an "operation". For an edge `A->B`, the type of the `A` node
+   * determines who is performing the operation, and the type of the `B` node determines who ends up holding
+   * the result:
+   * - An edge starting from a use-node describes what the current codebase is doing to a value that
+   *   came from a library.
+   * - An edge starting from a def-node describes what the external library might do to a value that
+   *   came from the current codebase.
+   * - An edge ending in a use-node means the result ends up in the current codebase (at its associated data-flow node).
+   * - An edge ending in a def-node means the result ends up in external code (its associated data-flow node is
+   *   the place where it was "last seen" in the current codebase before flowing out)
+   *
+   * Because the implementation of the external library is not visible, it is not known exactly what operations
+   * it will perform on values that flow there. Instead, the edges starting from a def-node are operations that would
+   * lead to an observable effect within the current codebase; without knowing for certain if the library will actually perform
+   * those operations. (When constructing these edges, we assume the library is somewhat well-behaved).
+   *
+   * For example, given this snippet:
+   * ```python
+   * import foo
+   * foo.bar(lambda x: doSomething(x))
+   * ```
+   * A callback is passed to the external function `foo.bar`. We can't know if `foo.bar` will actually invoke this callback.
+   * But _if_ the library should decide to invoke the callback, then a value will flow into the current codebase via the `x` parameter.
+   * For that reason, an edge is generated representing the argument-passing operation that might be performed by `foo.bar`.
+   * This edge is going from the def-node associated with the callback to the use-node associated with the parameter `x`.
    */
   class Node extends Impl::TApiNode {
     /**