Parent-Child Streams

The Tap SDK supports parent-child streams, by which one stream type can be declared to be a parent to another stream, and the child stream will automatically receive context from a parent record each time the child stream is invoked.

If you do want to utilize parent-child streams

  1. Set parent_stream_type in the child-stream’s class to the class of the parent.

  2. Implement one of the below methods to pass context from the parent to the child:

    1. If using get_records(context) you can simply return a tuple instead of a record dictionary. A tuple return value will be interpreted by the SDK as (record: dict, child_context: dict).

    2. Override get_child_context(record, context: Dict) -> dict to return a new child context object based on records and any existing context from the parent stream.

  3. If the parent stream’s replication key won’t get updated when child items are changed, indicate this by adding ignore_parent_replication_key = True in the child stream class declaration.

  4. If the number of parent items is very large (thousands or tens of thousands), you can optionally set state_partitioning_keys on the child stream to specify a subset of context keys to use in state bookmarks. (When not set, the number of bookmarks will be equal to the number of parent items.)

Example parent-child implementation

Here is an abbreviated example from the Gitlab sample (also in this repo) which uses the above techniques. In this example, EpicIssuesStream is a child of EpicsStream.

class GitlabStream(RESTStream):
    # Base stream definition with auth and pagination logic 
    # This logic works for other base classes as well, including Stream, GraphQLStream, etc.

class EpicsStream(GitlabStream):

    name = "epics"

    # ...

    def get_child_context(self, record: dict, context: Optional[dict]) -> dict:
        """Return a context dictionary for child streams."""
        return {
            "group_id": record["group_id"],
            "epic_id": record["id"],
            "epic_iid": record["iid"],

class EpicIssuesStream(GitlabStream):
    # Note that this class inherits from the GitlabStream base class, and not from 
    # the EpicsStream class.

    name = "epic_issues"

    # EpicIssues streams should be invoked once per parent epic:
    parent_stream_type = EpicsStream  

    # Assume epics don't have `updated_at` incremented when issues are changed:
    ignore_parent_replication_keys = True

    # Path is auto-populated using parent context keys:
    path = "/groups/{group_id}/epics/{epic_iid}/issues"

    # ...