Hello,
following the improvement suggestions in the latest cluster sharding
activator example I have added a persistent view and a replicated journal
using
https://github.com/ddevore/akka-persistence-mongo/.
As usual everything works as announced and I think persistence/cluster
sharding is really great and we'll be using it a lot in the future.
Now, I am not too sure how to best use persistent views:
I have added an AuthorListingView (pls. see code at the end of this post).
The view is also sharded and I have deactivated auto updates as I think in
most cases I will not find the correct update interval - either the update
is tried too often without any changes happened or the delay between a
change happening and updating the view would be too long.
So, I create the AuthorListing with the view's shardRegion:
ClusterSharding(system).start(
typeName = AuthorListing.shardName,
entryProps =
Some(AuthorListing.props(ClusterSharding(system).shardRegion(AuthorListingView.shardName)))
and in AuthorListing when it gets a PostSummary msg I update the
corresponding view "manually" via the shardRegion:
def receive = {
case Persistent(s: PostSummary, _) =>
posts :+= s
log.info("Post added to {}'s list: {}", s.author, s.title)
view ! ListingChanged(s.author)
Forgetting about the duplicate sending of DoUpdate msgs when replaying ( I
think Views will be mostly used with EventSourced processors anyway) this
works
but it still feels not correct somehow - I just could send the view the
persisted msg (command or event) directly without the Update, saving a
journal query.
Also, it is probably not a good solution when the same event has to be sent
to more than one view (using auto update would of course fix this
problem...).
Would it be a good idea to update a view's state by just sending "normal"
events(and never use auto-update or Update msgs), but of course use the
replay and snapshot functionality of a view for starting/restarting?
When updating Views by accessing the journal and not using "messaging" will
the journal not become a bottleneck at a certain point?
In the moment (we are using eventsourced) we do it the classical way -
command side persisting to the event store and updating the view side by
emitting events and the view persists the view model again. But I really
like the idea to also keep the view in memory when needed, update it, shut
it down when not used and bring it back easily by replaying the
corresponding processor or snapshotting the view.
I think I forgot all other things I additionally wanted to ask by now...
Thanks in advance for any suggestions,
michael
object AuthorListingView {
case class GetPosts(author: String)
case class Posts(list: immutable.IndexedSeq[PostSummary])
case class ListingChanged(author: String)
val idExtractor: ShardRegion.IdExtractor = {
case p @ Persistent(s: PostSummary, _) => (s.author, p)
case ListingChanged(author) => (author, Update(await =
true))
case m: GetPosts => (m.author, m)
}
val shardResolver: ShardRegion.ShardResolver = msg => msg match {
case Persistent(s: PostSummary, _) => (math.abs(s.author.hashCode) %
100).toString
case ListingChanged(author) => (math.abs(author.hashCode) %
100).toString
case GetPosts(author) => (math.abs(author.hashCode)
% 100).toString }
val shardName: String = "AuthorListingView"
}
class AuthorListingView extends View with ActorLogging {
import AuthorListingView._
log.info(s"viewId: $viewId and processorId: $processorId")
override def processorId: String = this.viewId.replace(shardName,
AuthorListing.shardName)
override def autoUpdate = false
var posts = Vector.empty[PostSummary]
override def receive: Receive = {
case Persistent(s: PostSummary, _) =>
log.info(s"View for ${self.path.name} received summary: $s")
posts :+= s
case GetPosts(_) => sender ! Posts(posts)
}
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