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Scylla Specific Features

Shard Awareness

scylla-driver is shard aware and contains extensions that work with the TokenAwarePolicy supported by Scylla 2.3 and onwards. Using this policy, the driver can select a connection to a particular shard based on the shard's token. As a result, latency is significantly reduced because there is no need to pass data between the shards.

Details on the scylla cql protocol extensions https://github.com/scylladb/scylla/blob/master/docs/dev/protocol-extensions.md#intranode-sharding

For using it you only need to enable TokenAwarePolicy on the Cluster

See the configuration of native_shard_aware_transport_port and native_shard_aware_transport_port_ssl on scylla.yaml: https://github.com/scylladb/scylla/blob/master/docs/dev/protocols.md#cql-client-protocol

from cassandra.cluster import Cluster
from cassandra.policies import TokenAwarePolicy, RoundRobinPolicy

cluster = Cluster(load_balancing_policy=TokenAwarePolicy(RoundRobinPolicy()))

New Cluster Helpers

  • shard_aware_options

    Setting it to dict(disable=True) would disable the shard aware functionally, for cases favoring once connection per host (example, lots of processes connecting from one client host, generating a big load of connections

    Other option is to configure scylla by setting enable_shard_aware_drivers: false on scylla.yaml.

from cassandra.cluster import Cluster

cluster = Cluster(shard_aware_options=dict(disable=True))
session = cluster.connect()

assert not cluster.is_shard_aware(), "Shard aware should be disabled"

# or just disable the shard aware port logic
cluster = Cluster(shard_aware_options=dict(disable_shardaware_port=True))
session = cluster.connect()

  • cluster.is_shard_aware()

    New method available on Cluster allowing to check whether the remote cluster supports shard awareness (bool)

from cassandra.cluster import Cluster

cluster = Cluster()
session = cluster.connect()

if cluster.is_shard_aware():
    print("connected to a scylla cluster")

  • cluster.shard_aware_stats()

    New method available on Cluster allowing to check the status of shard aware connections to all available hosts (dict)

from cassandra.cluster import Cluster

cluster = Cluster()
session = cluster.connect()

stats = cluster.shard_aware_stats()
if all([v["shards_count"] == v["connected"] for v in stats.values()]):
    print("successfully connected to all shards of all scylla nodes")

New Error Types

  • SCYLLA_RATE_LIMIT_ERROR Error

    The ScyllaDB 5.1 introduced a feature called per-partition rate limiting. In case the (user defined) per-partition rate limit is exceeded, the database will start returning a Scylla-specific type of error: RateLimitReached.

from cassandra import RateLimitReached
from cassandra.cluster import Cluster

cluster = Cluster()
session = cluster.connect()
session.execute("""
    CREATE KEYSPACE IF NOT EXISTS keyspace1
    WITH replication = {'class': 'SimpleStrategy', 'replication_factor': '1'}
""")

session.execute("USE keyspace1")
session.execute("""
    CREATE TABLE tbl (pk int PRIMARY KEY, v int)
    WITH per_partition_rate_limit = {'max_writes_per_second': 1}
""")

prepared = session.prepare("""
    INSERT INTO tbl (pk, v) VALUES (?, ?)
""")

try:
    for _ in range(1000):
        self.session.execute(prepared.bind((123, 456)))
except RateLimitReached:
    raise