Hi kant,
You can browse the sources on github. The implementation doesn't seem to
leverage anything from Cassandra underneath, the journal plugin does the
work itself. When asked to persist events, it emits (throttled)
notifications via akka pubsub which the CassandraReadJournal subscribes to.
Cheers,
Michal
On 21/03/17 02:20, kant kodali wrote:
Hi Michael,
Any idea how that that is implemented? I understand this setting
"cassandra-journal.pubsub-minimum-interval" is for
Akka-Cassandra-Persistence but I wonder what exactly it leverages
underneath from Cassandra or Cassandra driver?
Thanks,
kant
On Mon, Mar 13, 2017 at 1:57 AM, 'Michal Borowiecki' via Akka User
List <[email protected] <mailto:[email protected]>>
wrote:
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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