Also, there's a setting called
"cassandra-journal.pubsub-minimum-interval", which if set will cause the
journal to notify the persistence query side of new writes, so it can
only poll when needed instead of doing so periodically.
Cheers,
Michal
On 12/03/17 19:15, Patrik Nordwall wrote:
Tal describes it very well. In the future we'll hopefully be able to
get notifications from Cassandra when something has changed instead of
using the primitive poll approach. There is already Change Data
Capture (CDC) in Cassandra but it's an extremely low level api so far.
Improvement suggestions based on CDC or something else very much welcome.
/Patrik
sön 12 mars 2017 kl. 15:24 skrev Tal Pressman <[email protected]
<mailto:[email protected]>>:
Hi,
I think the name for Persistence Query is a little misleading -
when I first heard it I imagined something like an SQL query that
can run over the data source, but that's not what it is. What it
actually does is set up an Akka stream source that handles the
events from the data store. The stream can be finite (like
currentEventsByPersistenceId will stop once it has caught up with
all the events) or infinite (like eventsByPersistenceId). In the
second case, whenever there a new message is written to the data
store (and the stream isn't back-pressured / busy) the source will
emit the new message for the stream to handle.
Now, as you've mentioned, there is no "push" mechanism for
Cassandra, but that doesn't mean that everything breaks down. It
simply means that when you use the CassandraReadJournal there is a
thread that polls the DB and when there are new events there
passes them down the stream.
HTH,
Tal
On Sunday, March 12, 2017 at 4:09:02 PM UTC+2, kant kodali wrote:
Hi All,
I am trying to understand event sourcing in detail and I am
new to Akka. One of book I was reading says a Persistence
Actor will write events to a Journal so in my case the Journal
is Cassandra and one can have Persistence Query Actor listen
to the events from the Journal and this is where I don't
understand how it works. More specifically, How does
Persistence Query Actor know they are new events that it needs
to Query for? I assume Persistence Actor and Persistence Query
Actor are two separate things (so How does Persistence Query
Actor gets notified that there are new events written by
Persistence Actor)
I would like to get live stream from Cassandra but I as far as
I know I don't think there is any push based mechanism in
Cassandra where Cassandra itself will push new events to some
message bus or application. please correct me if I am wrong.
Thanks,
kant
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