Apache Cassandra

Alpakka Cassandra offers an Akka Streams API on top of a CqlSession from the Datastax Java Driver version 4.0+. The driver configuration is provided in the same config format as Akka uses and can be placed in the same application.conf as your Akka settings.

Artifacts

The Akka dependencies are available from Akka’s library repository. To access them there, you need to configure the URL for this repository.

sbt

resolvers += "Akka library repository".at("https://repo.akka.io/maven")

Maven

<project>
  ...
  <repositories>
    <repository>
      <id>akka-repository</id>
      <name>Akka library repository</name>
      <url>https://repo.akka.io/maven</url>
    </repository>
  </repositories>
</project>

Gradle

repositories {
    mavenCentral()
    maven {
        url "https://repo.akka.io/maven"
    }
}

Additionally, add the dependencies as below.

sbt

libraryDependencies += "com.lightbend.akka" %% "akka-stream-alpakka-cassandra" % "9.0.0"

Maven

<properties>
  <scala.binary.version>2.13</scala.binary.version>
</properties>
<dependencies>
  <dependency>
    <groupId>com.lightbend.akka</groupId>
    <artifactId>akka-stream-alpakka-cassandra_${scala.binary.version}</artifactId>
    <version>9.0.0</version>
  </dependency>
</dependencies>

Gradle

def versions = [
  ScalaBinary: "2.13"
]
dependencies {
  implementation "com.lightbend.akka:akka-stream-alpakka-cassandra_${versions.ScalaBinary}:9.0.0"
}

The table below shows direct dependencies of this module and the second tab shows all libraries it depends on transitively.

Direct dependencies

Organization	Artifact	Version
com.datastax.oss	java-driver-core	4.17.0
com.fasterxml.jackson.core	jackson-core	2.17.2
com.fasterxml.jackson.core	jackson-databind	2.17.2
com.typesafe.akka	akka-stream_2.13	2.10.0
org.scala-lang	scala-library	2.13.12

Dependency tree

com.datastax.oss    java-driver-core    4.17.0
    com.datastax.oss    java-driver-shaded-guava    25.1-jre-graal-sub-1    Apache 2
    com.datastax.oss    native-protocol    1.5.1    Apache 2
    com.fasterxml.jackson.core    jackson-core    2.17.2    The Apache Software License, Version 2.0
    com.fasterxml.jackson.core    jackson-databind    2.17.2    The Apache Software License, Version 2.0
        com.fasterxml.jackson.core    jackson-annotations    2.17.2    The Apache Software License, Version 2.0
        com.fasterxml.jackson.core    jackson-core    2.17.2    The Apache Software License, Version 2.0
    com.github.jnr    jnr-posix    3.1.15    Eclipse Public License - v 2.0
        com.github.jnr    jnr-constants    0.10.3    The Apache Software License, Version 2.0
        com.github.jnr    jnr-ffi    2.2.11    The Apache Software License, Version 2.0
            com.github.jnr    jffi    1.3.9    The Apache Software License, Version 2.0
            com.github.jnr    jnr-a64asm    1.0.0    The Apache Software License, Version 2.0
            com.github.jnr    jnr-x86asm    1.0.2    MIT License
            org.ow2.asm    asm-analysis    9.2    BSD-3-Clause
                org.ow2.asm    asm-tree    9.2    BSD-3-Clause
                    org.ow2.asm    asm    9.2    BSD-3-Clause
            org.ow2.asm    asm-commons    9.2    BSD-3-Clause
                org.ow2.asm    asm-analysis    9.2    BSD-3-Clause
                    org.ow2.asm    asm-tree    9.2    BSD-3-Clause
                        org.ow2.asm    asm    9.2    BSD-3-Clause
                org.ow2.asm    asm-tree    9.2    BSD-3-Clause
                    org.ow2.asm    asm    9.2    BSD-3-Clause
                org.ow2.asm    asm    9.2    BSD-3-Clause
            org.ow2.asm    asm-tree    9.2    BSD-3-Clause
                org.ow2.asm    asm    9.2    BSD-3-Clause
            org.ow2.asm    asm-util    9.2    BSD-3-Clause
                org.ow2.asm    asm-analysis    9.2    BSD-3-Clause
                    org.ow2.asm    asm-tree    9.2    BSD-3-Clause
                        org.ow2.asm    asm    9.2    BSD-3-Clause
                org.ow2.asm    asm-tree    9.2    BSD-3-Clause
                    org.ow2.asm    asm    9.2    BSD-3-Clause
                org.ow2.asm    asm    9.2    BSD-3-Clause
            org.ow2.asm    asm    9.2    BSD-3-Clause
    com.github.stephenc.jcip    jcip-annotations    1.0-1    Apache License, Version 2.0
    com.typesafe    config    1.4.3    Apache-2.0
    io.dropwizard.metrics    metrics-core    4.1.18
        org.slf4j    slf4j-api    1.7.30
    io.netty    netty-handler    4.1.94.Final
        io.netty    netty-buffer    4.1.94.Final
            io.netty    netty-common    4.1.94.Final
        io.netty    netty-codec    4.1.94.Final
            io.netty    netty-buffer    4.1.94.Final
                io.netty    netty-common    4.1.94.Final
            io.netty    netty-common    4.1.94.Final
            io.netty    netty-transport    4.1.94.Final
                io.netty    netty-buffer    4.1.94.Final
                    io.netty    netty-common    4.1.94.Final
                io.netty    netty-common    4.1.94.Final
                io.netty    netty-resolver    4.1.94.Final
                    io.netty    netty-common    4.1.94.Final
        io.netty    netty-common    4.1.94.Final
        io.netty    netty-resolver    4.1.94.Final
            io.netty    netty-common    4.1.94.Final
        io.netty    netty-transport-native-unix-common    4.1.94.Final
            io.netty    netty-buffer    4.1.94.Final
                io.netty    netty-common    4.1.94.Final
            io.netty    netty-common    4.1.94.Final
            io.netty    netty-transport    4.1.94.Final
                io.netty    netty-buffer    4.1.94.Final
                    io.netty    netty-common    4.1.94.Final
                io.netty    netty-common    4.1.94.Final
                io.netty    netty-resolver    4.1.94.Final
                    io.netty    netty-common    4.1.94.Final
        io.netty    netty-transport    4.1.94.Final
            io.netty    netty-buffer    4.1.94.Final
                io.netty    netty-common    4.1.94.Final
            io.netty    netty-common    4.1.94.Final
            io.netty    netty-resolver    4.1.94.Final
                io.netty    netty-common    4.1.94.Final
    org.hdrhistogram    HdrHistogram    2.1.12    Public Domain, per Creative Commons CC0
    org.reactivestreams    reactive-streams    1.0.4    MIT-0
    org.slf4j    slf4j-api    1.7.30
com.fasterxml.jackson.core    jackson-core    2.17.2    The Apache Software License, Version 2.0
com.fasterxml.jackson.core    jackson-databind    2.17.2    The Apache Software License, Version 2.0
    com.fasterxml.jackson.core    jackson-annotations    2.17.2    The Apache Software License, Version 2.0
    com.fasterxml.jackson.core    jackson-core    2.17.2    The Apache Software License, Version 2.0
com.typesafe.akka    akka-stream_2.13    2.10.0    BUSL-1.1
    com.typesafe.akka    akka-actor_2.13    2.10.0    BUSL-1.1
        com.typesafe    config    1.4.3    Apache-2.0
        org.scala-lang    scala-library    2.13.12    Apache-2.0
    com.typesafe.akka    akka-protobuf-v3_2.13    2.10.0    BUSL-1.1
    org.reactivestreams    reactive-streams    1.0.4    MIT-0
    org.scala-lang    scala-library    2.13.12    Apache-2.0
org.scala-lang    scala-library    2.13.12    Apache-2.0

Sessions

Cassandra is accessed through CassandraSessionCassandraSessions which are managed by the CassandraSessionRegistryCassandraSessionRegistry Akka extension. This way a session is shared across all usages within the actor system and properly shut down after the actor system is shut down.

The CassandraSession is provided to the stream factory methods as an implicit parameter.

Scala

copysourceimport akka.stream.alpakka.cassandra.CassandraSessionSettings
import akka.stream.alpakka.cassandra.scaladsl.CassandraSession
import akka.stream.alpakka.cassandra.scaladsl.CassandraSessionRegistry

val system: ActorSystem = // ???
val sessionSettings = CassandraSessionSettings()
implicit val cassandraSession: CassandraSession =
  CassandraSessionRegistry.get(system).sessionFor(sessionSettings)

val version: Future[String] =
  cassandraSession
    .select("SELECT release_version FROM system.local;")
    .map(_.getString("release_version"))
    .runWith(Sink.head)

Java

copysource
import akka.actor.ActorSystem;
import akka.stream.Materializer;
import akka.stream.alpakka.cassandra.CassandraSessionSettings;
import akka.stream.alpakka.cassandra.javadsl.CassandraSession;
import akka.stream.alpakka.cassandra.javadsl.CassandraSessionRegistry;

ActorSystem system = // ???
CassandraSessionSettings sessionSettings = CassandraSessionSettings.create();
CassandraSession cassandraSession =
    CassandraSessionRegistry.get(system).sessionFor(sessionSettings);

CompletionStage<String> version =
    cassandraSession
        .select("SELECT release_version FROM system.local;")
        .map(row -> row.getString("release_version"))
        .runWith(Sink.head(), system);

See custom session creation below for tweaking this.

Reading from Cassandra

CassandraSourceCassandraSource provides factory methods to get Akka Streams Sources from CQL queries and from com.datastax.oss.driver.api.core.cql.Statements.

Dynamic parameters can be provided to the CQL as variable arguments.

Scala

copysourceimport akka.stream.alpakka.cassandra.scaladsl.CassandraSource

val ids: Future[immutable.Seq[Int]] =
  CassandraSource(s"SELECT id FROM $intTable").map(row => row.getInt("id")).runWith(Sink.seq)

val idsWhere: Future[Int] =
  CassandraSource(s"SELECT * FROM $intTable WHERE id = ?", value).map(_.getInt("id")).runWith(Sink.head)

Java

copysourceimport akka.stream.alpakka.cassandra.javadsl.CassandraSource;

CompletionStage<List<Integer>> select =
    CassandraSource.create(cassandraSession, "SELECT id FROM " + idtable + ";")
        .map(r -> r.getInt("id"))
        .runWith(Sink.seq(), system);

CompletionStage<Integer> select =
    CassandraSource.create(
            cassandraSession, "SELECT * FROM " + idtable + " WHERE id = ?;", value)
        .map(r -> r.getInt("id"))
        .runWith(Sink.head(), system);

If the statement requires specific settings, you may pass any com.datastax.oss.driver.api.core.cql.Statement.

Scala

copysourceimport com.datastax.oss.driver.api.core.cql.{Row, SimpleStatement}

val stmt = SimpleStatement.newInstance(s"SELECT * FROM $intTable").setPageSize(20)

val rows: Future[immutable.Seq[Row]] = CassandraSource(stmt).runWith(Sink.seq)

Java

copysourceimport com.datastax.oss.driver.api.core.cql.SimpleStatement;
import com.datastax.oss.driver.api.core.cql.Statement;

Statement<?> stmt =
    SimpleStatement.newInstance("SELECT * FROM " + idtable + ";").setPageSize(20);

CompletionStage<List<Integer>> select =
    CassandraSource.create(cassandraSession, stmt)
        .map(r -> r.getInt("id"))
        .runWith(Sink.seq(), system);

Here we used a basic sink to complete the stream by collecting all of the stream elements into a collection. The power of streams comes from building larger data pipelines which leverage backpressure to ensure efficient flow control. Feel free to edit the example code and build more advanced stream topologies.

Writing to Cassandra

CassandraFlowCassandraFlow provides factory methods to get Akka Streams flows to run CQL statements that change data (UPDATE, INSERT). Alpakka Cassandra creates a PreparedStatement and for every stream element the statementBinder function binds the CQL placeholders to data.

The incoming elements are emitted unchanged for further processing.

Scala

copysourceimport akka.stream.alpakka.cassandra.CassandraWriteSettings
import akka.stream.alpakka.cassandra.scaladsl.CassandraFlow
import com.datastax.oss.driver.api.core.cql.{BoundStatement, PreparedStatement}

case class Person(id: Int, name: String, city: String)

val persons =
  immutable.Seq(Person(12, "John", "London"), Person(43, "Umberto", "Roma"), Person(56, "James", "Chicago"))

val statementBinder: (Person, PreparedStatement) => BoundStatement =
  (person, preparedStatement) => preparedStatement.bind(Int.box(person.id), person.name, person.city)

val written: Future[immutable.Seq[Person]] = Source(persons)
  .via(
    CassandraFlow.create(CassandraWriteSettings.defaults,
                         s"INSERT INTO $table(id, name, city) VALUES (?, ?, ?)",
                         statementBinder)
  )
  .runWith(Sink.seq)

Java

copysourceimport akka.NotUsed;
import akka.actor.ActorSystem;
import akka.japi.Function2;
import akka.japi.Pair;
import akka.stream.alpakka.cassandra.CassandraWriteSettings;
import akka.stream.alpakka.cassandra.javadsl.CassandraFlow;
import akka.stream.alpakka.testkit.javadsl.LogCapturingJunit4;
import akka.stream.javadsl.SourceWithContext;
import com.datastax.oss.driver.api.core.cql.BoundStatement;
import com.datastax.oss.driver.api.core.cql.PreparedStatement;

List<Person> persons =
    Arrays.asList(
        new Person(12, "John", "London"),
        new Person(43, "Umberto", "Roma"),
        new Person(56, "James", "Chicago"));

Function2<Person, PreparedStatement, BoundStatement> statementBinder =
    (person, preparedStatement) -> preparedStatement.bind(person.id, person.name, person.city);

CompletionStage<List<Person>> written =
    Source.from(persons)
        .via(
            CassandraFlow.create(
                cassandraSession,
                CassandraWriteSettings.defaults(),
                "INSERT INTO " + table + "(id, name, city) VALUES (?, ?, ?)",
                statementBinder))
        .runWith(Sink.seq(), system);

Update flows with context

Alpakka Cassandra flows offer “With Context”-support which integrates nicely with some other Alpakka connectors.

Scala

copysourceval personsAndHandles: SourceWithContext[Person, AckHandle, NotUsed] = // ???

val written: Future[Done] = personsAndHandles
  .via(
    CassandraFlow.withContext(
      CassandraWriteSettings.defaults,
      s"INSERT INTO $table(id, name, city) VALUES (?, ?, ?)",
      (person, preparedStatement) => preparedStatement.bind(Int.box(person.id), person.name, person.city)
    )
  )
  .asSource
  .mapAsync(1) {
    case (_, handle) => handle.ack()
  }
  .runWith(Sink.ignore)

Java

copysourceSourceWithContext<Person, AckHandle, NotUsed> from = // ???;
CompletionStage<Done> written =
    from.via(
            CassandraFlow.withContext(
                cassandraSession,
                CassandraWriteSettings.defaults(),
                "INSERT INTO " + table + "(id, name, city) VALUES (?, ?, ?)",
                (person, preparedStatement) ->
                    preparedStatement.bind(person.id, person.name, person.city)))
        .asSource()
        .mapAsync(1, pair -> pair.second().ack())
        .runWith(Sink.ignore(), system);

Custom Session creation

Session creation and configuration is controlled via settings in application.conf. The CassandraSessionSettingsCassandraSessionSettings accept a full path to a configuration section which needs to specify a session-provider setting. The CassandraSessionRegistryCassandraSessionRegistry expects a fully qualified class name to a class implementing CqlSessionProviderCqlSessionProvider.

Alpakka Cassandra includes a default implementation DefaultSessionProviderDefaultSessionProvider, which is referenced in the default configuration alpakka.cassandra.

The DefaultSessionProviderDefaultSessionProvider config section must contain:

• a settings section service-discovery which may be used to discover Cassandra contact points via Akka Discovery,
• and a reference to a Cassandra config section in datastax-java-driver-config which is used to configure the Cassandra client. For details see the Datastax Java Driver configuration and the driver’s reference.conf.

reference.conf

copysourcealpakka.cassandra {
  # The implementation of `akka.stream.alpakka.cassandra.CqlSessionProvider`
  # used for creating the `CqlSession`.
  # It may optionally have a constructor with an `ClassicActorSystemProvider` and `Config` parameters.
  session-provider = "akka.stream.alpakka.cassandra.DefaultSessionProvider"

  # Configure Akka Discovery by setting a service name
  service-discovery {
    name = ""
    lookup-timeout = 1 s
  }

  # The ExecutionContext to use for the session tasks and future composition.
  session-dispatcher = "akka.actor.default-dispatcher"

  # Full config path to the Datastax Java driver's configuration section.
  # When connecting to more than one Cassandra cluster different session configuration can be
  # defined with this property.
  # See https://docs.datastax.com/en/developer/java-driver/latest/manual/core/configuration/#quick-overview
  # and https://docs.datastax.com/en/developer/java-driver/latest/manual/core/configuration/reference/
  datastax-java-driver-config = "datastax-java-driver"
}

In simple cases your datastax-java-driver section will need to define contact-points and load-balancing-policy.local-datacenter. To make the Cassandra driver retry its initial connection attempts, add advanced.reconnect-on-init = true.

application.conf

copysourcedatastax-java-driver {
  basic {
    contact-points = [ "127.0.0.1:9042" ]
    load-balancing-policy.local-datacenter = datacenter1
  }
  advanced.reconnect-on-init = true
}

Using Akka Discovery

To use Akka Discovery make sure the akka-discovery dependency is on you classpath.

sbt

val AkkaVersion = "2.10.0"
libraryDependencies += "com.typesafe.akka" %% "akka-discovery" % AkkaVersion

Maven

<properties>
  <akka.version>2.10.0</akka.version>
  <scala.binary.version>2.13</scala.binary.version>
</properties>
<dependencies>
  <dependency>
    <groupId>com.typesafe.akka</groupId>
    <artifactId>akka-discovery_${scala.binary.version}</artifactId>
    <version>${akka.version}</version>
  </dependency>
</dependencies>

Gradle

def versions = [
  AkkaVersion: "2.10.0",
  ScalaBinary: "2.13"
]
dependencies {
  implementation "com.typesafe.akka:akka-discovery_${versions.ScalaBinary}:${versions.AkkaVersion}"
}

To enable Akka Discovery with the DefaultSessionProviderDefaultSessionProvider, set up the desired service name in the discovery mechanism of your choice and pass that name in service-discovery.name. The example below extends the alpakka.cassandra config section and only overwrites the service name.

application.conf

copysourceakka {
  discovery.method = config
}
akka.discovery.config.services = {
  cassandra-service = {
    endpoints = [
      {
        host = "127.0.0.1"
        port = 9042
      }
    ]
  }
}

// inherit defaults from `alpakka.cassandra` settings
example-with-akka-discovery: ${alpakka.cassandra} {
  service-discovery.name = "cassandra-service"
}

Use the full config section path to create the CassandraSessionSettingsCassandraSessionSettings.

Scala

copysourceval sessionSettings = CassandraSessionSettings("example-with-akka-discovery")
implicit val session: CassandraSession = CassandraSessionRegistry.get(system).sessionFor(sessionSettings)

Java

copysourceCassandraSessionSettings sessionSettings =
    CassandraSessionSettings.create("example-with-akka-discovery");
CassandraSession session = CassandraSessionRegistry.get(system).sessionFor(sessionSettings);