Context Propagation

It can be convenient to attach metadata to each element in the stream.

For example, when reading from an external data source it can be useful to keep track of the read offset, so it can be marked as processed when the element reaches the SinkSink.

For this use case we provide the SourceWithContextSourceWithContext and FlowWithContextFlowWithContext variations on SourceSource and FlowFlow.

Essentially, a FlowWithContextFlowWithContext is just a FlowFlow that contains tuplespairs of element and context, but the advantage is in the operators: most operators on FlowWithContextFlowWithContext will work on the element rather than on the tuplepair, allowing you to focus on your application logic rather without worrying about the context.


Not all operations that are available on FlowFlow are also available on FlowWithContextFlowWithContext. This is intentional: in the use case of keeping track of a read offset, if the FlowWithContextFlowWithContext was allowed to arbitrarily filter and reorder the stream, the SinkSink would have no way to determine whether an element was skipped or merely reordered and still in flight.

For this reason, FlowWithContextFlowWithContext allows filtering operations (such as filter, filterNot, collect, etc) and grouping operations (such as grouped, sliding, etc) but not reordering operations (such as mapAsyncUnordered and statefulMapConcat). Finally, also ‘one-to-n’ operations such as mapConcat are allowed.

Filtering operations will drop the context along with dropped elements, while grouping operations will keep all contexts from the elements in the group. Streaming one-to-many operations such as mapConcat associate the original context with each of the produced elements.

As an escape hatch, there is a via operator that allows you to insert an arbitrary FlowFlow that can process the tuplespairs of elements and context in any way desired. When using this operator, it is the responsibility of the implementor to make sure this FlowFlow does not perform any operations (such as reordering) that might break assumptions made by the SinkSink consuming the context elements.


The simplest way to create a SourceWithContextSourceWithContext is to first create a regular SourceSource with elements from which the context can be extracted, and then use Source.asSourceWithContext.


When you have a SourceWithContextSourceWithContext source that produces elements of type Foo with a context of type Ctx, and a FlowFlow flow from Foo to Bar, you cannot simply source.via(flow) to arrive at a SourceWithContextSourceWithContext that produces elements of type Bar with contexts of type Ctx. The reason for this is that flow might reorder the elements flowing through it, making via challenging to implement.

There is a Flow.asFlowWithContext which can be used when the types used in the inner FlowFlow have room to hold the context. If this is not the case, a better solution is usually to build the flow from the ground up as a FlowWithContextFlowWithContext, instead of first building a FlowFlow and trying to convert it to FlowWithContextFlowWithContext after-the-fact.

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