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Lagom: is number of read side shards set per node or within the whole cluster?


By : Colombus
Date : October 16 2020, 06:10 PM
this will help Lagom allows splitting read side processor into shards to scale events processing. , It's the total number of shards for the whole cluster.
code :


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Elasticsearch: on full cluster restart - Shards are not recovering and remaining as unassigned shards


By : Shadow C
Date : March 29 2020, 07:55 AM
should help you out When you are doing full cluster restart you need to start at least quorum of your data nodes. This is configured through index.recovery.initial_shards which prevents allocation stale shard copy.

CQRS (Lagom) elasticsearch read-side


By : teeohdoor
Date : March 29 2020, 07:55 AM
I think the issue was by ths following , I don't have a huge amount of experience running ES in production, but essentially, ensuring that when you persist data, it stays persisted, especially in a distributed system, is hard. There are many, many edge cases that are very hard to get right, and it takes time for a database to mature and sort those edge cases out. A less durable database is one that probably hasn't ironed all these issues out.
Of course, ElasticSearch is popular open source database with a thriving community maintaining it, so there's likely no well defined cases where "your data will be lost in this circumstance", rather, there's likely cases that either haven't been come across yet, or when they have been come across by users in the wild, the users that came across them didn't care enough to debug it because they were only using ES as a secondary data store and were able to rebuild it from their primary data store. Whenever a case is identified that ES loses data under well understood circumstances, the maintainers of ES would be quick to fix that.

How can we use lagom's Read-side processor with Dgraph?


By : N.A.
Date : March 29 2020, 07:55 AM
With these it helps Lagom does not provide out-of-the-box support for Dgraph. If you have to use Lagom's Read-Side processor with Dgraph, then you have to use Lagom's Generic Read Side support. Like this:
code :
/**
 * Read side processor for Dgraph.
 */
public class FriendEventProcessor extends ReadSideProcessor<FriendEvent> {
    private static void createModel() {
        //TODO: Initialize schema in Dgraph
    }

    @Override
    public ReadSideProcessor.ReadSideHandler<FriendEvent> buildHandler() {
        return new ReadSideHandler<FriendEvent>() {
            private final Done doneInstance = Done.getInstance();

            @Override
            public CompletionStage<Done> globalPrepare() {
                createModel();
                return CompletableFuture.completedFuture(doneInstance);
            }

            @Override
            public CompletionStage<Offset> prepare(final AggregateEventTag<FriendEvent> tag) {
                return CompletableFuture.completedFuture(Offset.NONE);
            }

            @Override
            public Flow<Pair<FriendEvent, Offset>, Done, ?> handle() {
                return Flow.<Pair<FriendEvent, Offset>>create()
                        .mapAsync(1, eventAndOffset -> {
                                    if (eventAndOffset.first() instanceof FriendCreated) {
                                        //TODO: Add Friend in Dgraph;
                                    }

                                    return CompletableFuture.completedFuture(doneInstance);
                                }
                        );
            }
        };
    }

    @Override
    public PSequence<AggregateEventTag<FriendEvent>> aggregateTags() {
        return FriendEvent.TAG.allTags();
    }
}
int NUM_SHARDS = 20;

  AggregateEventShards<FriendEvent> TAG =
          AggregateEventTag.sharded(FriendEvent.class, NUM_SHARDS);

  @Override
  default AggregateEventShards<FriendEvent> aggregateTag() {
    return TAG;
  }

On an Elasticsearch cluster, how to have a node not allocate shards on it?


By : user1949688
Date : March 29 2020, 07:55 AM
may help you . This can be done via this command (e.g. for a cluster node with IP 10.0.0.1):
code :
PUT _cluster/settings
{
  "transient" : {
    "cluster.routing.allocation.exclude._ip" : "10.0.0.1"
  }
}

How to configure number of shards per cluster in elasticsearch


By : Jessica Ho
Date : March 29 2020, 07:55 AM
will help you So first off I'd start reading about indexes, primary shards, replica shards and nodes to understand the differences:
http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/glossary.html
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