BatchAccumulator
BatchAccumulator is used internally by the CDK to implement io.airbyte.cdk.load.write.LoadStrategys. Connector devs should never need to implement this interface.
It is the glue that connects a specific step in a specific pipeline to the generic pipeline on the back end. (For example, in a three-stage pipeline like bulk load, step 1 is to create a part, step 2 is to upload it, and step 3 is to load it from object storage into a table.)
S is a state type that will be threaded through accumulator calls.
K is a key type associated the input data. (NOTE: Currently, there is no support for key-mapping, so the key is always io.airbyte.cdk.load.message.StreamKey). Specifically, state will always be managed per-key.
T is the input data type
U is the output data type
The first time data is seen for a given key, start is called (with the partition number). The state returned by start will be passed per input to accept.
If accept returns IntermediateOutput or FinalOutput, the output will be forwarded to the next stage (if applicable) and/or trigger bookkeeping (iff the output type implements io.airbyte.cdk.load.message.WithBatchState).
If accept returns a NoOutput or IntermediateOutput, the nextState will be passed to the next call to accept. If accept returns a FinalOutput state, any state will be discarded and a new one will be created on the next input by a new call to start.
When the input stream is exhausted, finish will be called with any remaining state iff at least one input was seen for that key. This means that finish will not be called on empty keys or on keys where the last call to accept yielded a null (finished) state.