Source<EventEnvelope,NotUsed> eventsByTag(java.lang.String tag, Offset offset)Query events that have a specific tag. A tag can for example correspond to an aggregate root type (in DDD terminology).
The consumer can keep track of its current position in the event stream by storing the
offsetand restart the query from a given
offsetafter a crash/restart.
The exact meaning of the
offsetdepends on the journal and must be documented by the read journal plugin. It may be a sequential id number that uniquely identifies the position of each event within the event stream. Distributed data stores cannot easily support those semantics and they may use a weaker meaning. For example it may be a timestamp (taken when the event was created or stored). Timestamps are not unique and not strictly ordered, since clocks on different machines may not be synchronized.
In strongly consistent stores, where the
offsetis unique and strictly ordered, the stream should start from the next event after the
offset. Otherwise, the read journal should ensure that between an invocation that returned an event with the given
offset, and this invocation, no events are missed. Depending on the journal implementation, this may mean that this invocation will return events that were already returned by the previous invocation, including the event with the passed in
The returned event stream should be ordered by
offsetif possible, but this can also be difficult to fulfill for a distributed data store. The order must be documented by the read journal plugin.
The stream is not completed when it reaches the end of the currently stored events, but it continues to push new events when new events are persisted. Corresponding query that is completed when it reaches the end of the currently stored events is provided by