= Cassandra Client for Vert.x

A Vert.x client allowing applications to interact with an http://cassandra.apache.org/[Apache Cassandra] service.

WARNING: This module has _Tech Preview_ status, this means the API can change between versions.

== Getting started

To use this module, add the following to the _dependencies_ section of your Maven POM file:

[source,xml,subs="+attributes"]
----
<dependency>
 <groupId>io.vertx</groupId>
 <artifactId>vertx-cassandra-client</artifactId>
 <version>3.9.12</version>
</dependency>
----

Or, if you use Gradle:

[source,groovy,subs="+attributes"]
----
compile 'io.vertx:vertx-cassandra-client:3.9.12'
----

WARNING: The Cassandra client is not compatible with the Vert.x Dropwizard Metrics library.
Both are using a different major version of the Dropwizard Metrics library and the Datastax Java driver https://github.com/datastax/java-driver/pull/943[won't upgrade] to the most recent version due to the drop of Java 7.
The next major version (4.x) of the driver will use a more recent Dropwizard Metrics library  version.

== Creating a client

=== Client options

Cassandra is a distributed system, and it can have many nodes.
To connect to Cassandra you need to specify the addresses of some cluster nodes when creating a `link:../../apidocs/io/vertx/cassandra/CassandraClientOptions.html[CassandraClientOptions]` object:

[source,kotlin]
----
var options = CassandraClientOptions(
  contactPoints = listOf("node1.address", "node2.address", "node3.address"))
var client = CassandraClient.create(vertx, options)

----

By default, the Cassandra client for Vert.x connects to the local machine's port `9042` and is not tied to any specific keyspace.
But you can set either or both of these options:

[source,kotlin]
----
var options = CassandraClientOptions(
  port = 9142,
  keyspace = "my_keyspace")
var client = CassandraClient.create(vertx, options)

----

TIP: For fine tuning purposes, `link:../../apidocs/io/vertx/cassandra/CassandraClientOptions.html[CassandraClientOptions]` exposes a `com.datastax.driver.core.Cluster.Builder` instance.

=== Shared clients

If you deploy multiple instances of your verticle or have different verticles interacting with the same database, it is recommended to create a shared client:

[source,kotlin]
----
var options = CassandraClientOptions(
  contactPoints = listOf("node1.address", "node2.address", "node3.address"),
  keyspace = "my_keyspace")
var client = CassandraClient.createShared(vertx, "sharedClientName", options)

----

Shared clients with the same name will use a single underlying `com.datastax.driver.core.Session`.

=== Client lifecycle

After the client is created, it is not connected until the first query is executed.

TIP: A shared client can be connected after creation if another client with the same name has already executed a query.

Clients created inside a verticle are automatically stopped when the verticle is undeployed.
In other words, you do not need to invoke `link:../../apidocs/io/vertx/cassandra/CassandraClient.html#close--[close]` in the verticle `stop` method.

In all other cases, you must manually close the client.

NOTE: When a shared client is closed, the driver dession is not closed if other clients with the same name are still running.

== Using the API

The client API is represented by `link:../../apidocs/io/vertx/cassandra/CassandraClient.html[CassandraClient]`.

=== Querying

You can get query results using three different ways.

==== Streaming

The streaming API is most appropriate when you need to consume results iteratively, e.g you want to process each item.
This is very efficient specially for large amount of rows.

In order to give you some inspiration and ideas on how you can use the API, we'd like to you to consider this example:

[source,kotlin]
----
cassandraClient.queryStream("SELECT my_string_col FROM my_keyspace.my_table where my_key = 'my_value'", { queryStream ->
  if (queryStream.succeeded()) {
    var stream = queryStream.result()

    // resume stream when queue is ready to accept buffers again
    response.drainHandler({ v ->
      stream.resume()
    })

    stream.handler({ row ->
      var value = row.getString("my_string_col")
      response.write(value)

      // pause row stream when we buffer queue is full
      if (response.writeQueueFull()) {
        stream.pause()
      }
    })

    // end request when we reached end of the stream
    stream.endHandler({ end ->
      response.end()
    })

  } else {
    queryStream.cause().printStackTrace()
    // response with internal server error if we are not able to execute given query
    response.setStatusCode(500).end("Unable to execute the query")
  }
})

----

In the example, we are executing a query, and stream results via HTTP.

==== Bulk fetching

This API should be used when you need to process all the rows at the same time.

[source,kotlin]
----
cassandraClient.executeWithFullFetch("SELECT * FROM my_keyspace.my_table where my_key = 'my_value'", { executeWithFullFetch ->
  if (executeWithFullFetch.succeeded()) {
    var rows = executeWithFullFetch.result()
    for (row in rows) {
      // handle each row here
    }
  } else {
    println("Unable to execute the query")
    executeWithFullFetch.cause().printStackTrace()
  }
})

----

CAUTION: Use bulk fetching only if you can afford to load the full result set in memory.

=== Collector queries

You can use Java collectors with the query API:

[source,kotlin]
----
Code not translatable
----

==== Low level fetch

This API provides greater control over loading at the expense of being a bit lower-level than the streaming and bulk fetching APIs.

[source,kotlin]
----
cassandraClient.execute("SELECT * FROM my_keyspace.my_table where my_key = 'my_value'", { execute ->
  if (execute.succeeded()) {
    var resultSet = execute.result()

    resultSet.one({ one ->
      if (one.succeeded()) {
        var row = one.result()
        println("One row successfully fetched")
      } else {
        println("Unable to fetch a row")
        one.cause().printStackTrace()
      }
    })

    resultSet.fetchMoreResults({ fetchMoreResults ->
      if (fetchMoreResults.succeeded()) {
        var availableWithoutFetching = resultSet.getAvailableWithoutFetching()
        println("Now we have ${availableWithoutFetching} rows fetched, but not consumed!")
        if (resultSet.isFullyFetched()) {
          println("The result is fully fetched, we don't need to call this method for one more time!")
        } else {
          println("The result still does not fully fetched")
        }
      } else {
        println("Unable to fetch more results")
        fetchMoreResults.cause().printStackTrace()
      }
    })

  } else {
    println("Unable to execute the query")
    execute.cause().printStackTrace()
  }
})

----

=== Prepared queries

For security and efficiency reasons, it is a good idea to use prepared statements for all the queries you are using more than once.

You can prepare a query:

[source,kotlin]
----
cassandraClient.prepare("SELECT * FROM my_keyspace.my_table where my_key = ? ", { preparedStatementResult ->
  if (preparedStatementResult.succeeded()) {
    println("The query has successfully been prepared")
    var preparedStatement = preparedStatementResult.result()
    // now you can use this PreparedStatement object for the next queries
  } else {
    println("Unable to prepare the query")
    preparedStatementResult.cause().printStackTrace()
  }
})

----

And then use the https://docs.datastax.com/en/drivers/java/3.5/com/datastax/driver/core/PreparedStatement.html[`PreparedStatement`] for all the next queries:

[source,kotlin]
----
// You can execute you prepared statement using any way to execute queries.

// Low level fetch API
cassandraClient.execute(preparedStatement.bind("my_value"), { done ->
  var results = done.result()
  // handle results here
})

// Bulk fetching API
cassandraClient.executeWithFullFetch(preparedStatement.bind("my_value"), { done ->
  var results = done.result()
  // handle results here
})

// Streaming API
cassandraClient.queryStream(preparedStatement.bind("my_value"), { done ->
  var results = done.result()
  // handle results here
})

----

=== Batching

In case you'd like to execute several queries at once, you can use https://docs.datastax.com/en/drivers/java/3.5/com/datastax/driver/core/BatchStatement.html[`BatchStatement`] for that:

[source,kotlin]
----
var batchStatement = com.datastax.driver.core.BatchStatement().add(com.datastax.driver.core.SimpleStatement("INSERT INTO NAMES (name) VALUES ('Pavel')")).add(com.datastax.driver.core.SimpleStatement("INSERT INTO NAMES (name) VALUES ('Thomas')")).add(com.datastax.driver.core.SimpleStatement("INSERT INTO NAMES (name) VALUES ('Julien')"))

cassandraClient.execute(batchStatement, { result ->
  if (result.succeeded()) {
    println("The given batch executed successfully")
  } else {
    println("Unable to execute the batch")
    result.cause().printStackTrace()
  }
})

----

=== Object Mapper

You can use the object `link:../../apidocs/io/vertx/cassandra/Mapper.html[Mapper]` to map between domain classes and queries.

First, add the following to the _dependencies_ section of your Maven POM file:

[source,xml,subs="+attributes"]
----
<dependency>
 <groupId>com.datastax.cassandra</groupId>
 <artifactId>cassandra-driver-mapping</artifactId>
 <version>3.5.0</version>
</dependency>
----

Or, if you use Gradle:

[source,groovy,subs="+attributes"]
----
compile 'com.datastax.cassandra:cassandra-driver-mapping:3.5.0'
----

Consider the following entity:

[source, java]
----
@Table(keyspace = "test", name = "names")
public class MappedClass {
 @PartitionKey
 private String name;

 public MappedClass(String name) {
   this.name = name;
 }

 MappedClass() {
   // Required for mapping
 }

 // getters / setters
}
----

Create a mapper for it and you may save, retrieve or delete data from the corresponding table:

[source, kotlin]
----
var mappingManager = MappingManager.create(cassandraClient)
var mapper = mappingManager.mapper(examples.CassandraClientExamples.MappedClass.`class`)

var value = examples.CassandraClientExamples.MappedClass("foo")

mapper.save(value, { handler ->
  // Entity saved
})

mapper.get(java.util.Collections.singletonList("foo"), { handler ->
  // Entity loaded
})

mapper.delete(java.util.Collections.singletonList("foo"), { handler ->
  // Entity deleted
})

----

TIP: It is safe to reuse mapping manager and mapper instances for a given Cassandra client.

ifeval::["kotlin" == "java"]
include::override/rxjava2.adoc[]
endif::[]