Snapshots

Features

• Implements the Pekko Persistence snapshot store plugin API.

Schema

The keyspace and tables needs to be created before using the plugin.

The default keyspace used by the plugin is pekko_snapshot, it should be created with the NetworkTopology replication strategy with a replication factor of at least 3:

CREATE KEYSPACE IF NOT EXISTS pekko_snapshot WITH replication = {'class': 'NetworkTopologyStrategy', '<your_dc_name>' : 3 }; 

For local testing, and the default if you enable pekko.persistence.cassandra.snapshot.keyspace-autocreate you can use the following:

copysourceCREATE KEYSPACE IF NOT EXISTS pekko_snapshot
 WITH REPLICATION = { 'class' : 'SimpleStrategy','replication_factor':1 };

A single table is required. This needs to be created before starting your application. For local testing you can enable pekko.persistence.cassandra.snapshot.tables-autocreate. The default table definitions look like this:

copysourceCREATE TABLE IF NOT EXISTS pekko_snapshot.snapshots (
  persistence_id text,
  sequence_nr bigint,
  timestamp bigint,
  ser_id int,
  ser_manifest text,
  snapshot_data blob,
  snapshot blob,
  meta_ser_id int,
  meta_ser_manifest text,
  meta blob,
  PRIMARY KEY (persistence_id, sequence_nr))
  WITH CLUSTERING ORDER BY (sequence_nr DESC) AND gc_grace_seconds =864000
  AND compaction = {
    'class' : 'SizeTieredCompactionStrategy',
    'enabled' : true,
    'tombstone_compaction_interval' : 86400,
    'tombstone_threshold' : 0.2,
    'unchecked_tombstone_compaction' : false,
    'bucket_high' : 1.5,
    'bucket_low' : 0.5,
    'max_threshold' : 32,
    'min_threshold' : 4,
    'min_sstable_size' : 50
    };

ScyllaDB

ScyllaDB does not support unchecked_tombstone_compaction. You should adjust the pekko.messages table definition to remove this.

Consistency

By default, the snapshot store uses ONE for all reads and writes, since snapshots should only be used as an optimization to reduce number of replayed events. If a recovery doesn’t see the latest snapshot it will just start from an older snapshot and replay events from there. Be careful to not delete events too eagerly after storing snapshots since the deletes may be visible before the snapshot is visible. Keep a few snapshots and corresponding events before deleting older events and snapshots.

The consistency level for snapshots can be changed with:

datastax-java-driver.profiles {
  pekko-persistence-cassandra-snapshot-profile {
    basic.request.consistency = QUORUM
  }
}

Configuration

To activate the snapshot-store plugin, add the following line to your Pekko application.conf:

pekko.persistence.snapshot-store.plugin = "pekko.persistence.cassandra.snapshot"

This will run the snapshot store with its default settings. The default settings can be changed with the configuration properties defined in reference.conf. Journal configuration is under pekko.persistence.cassandra.snapshot.

Limitations

The snapshot is stored in a single row so the maximum size of a serialized snapshot is the Cassandra configured max_mutation_size_in_kb which is 16MB by default.

Delete all snapshots

The CleanupCleanup tool can be used for deleting all events and/or snapshots given list of persistenceIds without using persistent actors. It’s important that the actors with corresponding persistenceId are not running at the same time as using the tool. See Database Cleanup for more details.