head

The highest sequence of a stream in partition, in one aggregation over the collections this log knows; a placeholder is no stream, and a reader that reads the tail up to the head waits for a stream there. The counter is the blocks reserved, not the streams written. An append in flight may still land below it: one concurrent with this read, which a subscription starting from the head may count as committed before it.

The aggregation also reads the context's registry: a collection another node registered since this log last read it (a new aggregate type's first commit, written by the partition's previous owner) makes it read the registry again and look once more, so the head never misses the streams of a collection it has not seen yet.