Streams
A stream is a sequence of messages, with each message having the same key.
Different streams might processed by the same set of blocks inside a flow, message processing might be interleaved but their relative order (inside the same stream) is kept.
A practical example of the concept of streams is the sequence of messages coming from several similar sensors, which can share the same processing infrastructure but must be stored as different time series.
Streams will play an important role in load balancing and automatic consistent sharding, therefore messages that belong to a stream are always processed in-order by the same set of computing resources.