Java驱动器2.1：Apache Cassandra的CQL3与二进制协议指南

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"Java Driver 2.1 for Apache Cassandra 是专为使用 Cassandra 查询语言版本3 (CQL3) 和 Cassandra 1.2 引入的新二进制协议设计的驱动程序。该驱动程序允许Java应用程序高效地与Cassandra集群交互。文档涵盖了连接集群、执行CQL语句、异步I/O、故障转移、批处理、集群配置等多个方面，并提供了对象映射API用于基本的CRUD操作。" Java Driver 2.1 for Apache Cassandra 是一个关键的开发工具，它专为Cassandra数据库提供Java支持。这个版本的驱动程序是针对CQL3设计的，这意味着开发者可以利用CQL3的强大功能来查询和操作数据。CQL3是一种更高级、更接近SQL的查询语言，使得数据操作更加直观。驱动程序的依赖性是理解其工作方式的关键。在Java Driver 2.1中，它依赖于Cassandra的二进制协议，这个协议自Cassandra 1.2版本引入，提高了与数据库的通信效率。开发者需要确保他们的Cassandra集群至少是1.2版本或更高，以便使用此驱动。连接到Cassandra集群是开发过程的第一步。通过创建Session对象，开发者可以执行CQL语句。Session提供了异步执行CQL语句的能力，这是Java Driver的一个显著特性，它可以提高应用程序的性能和响应速度。 Bound Statements是预编译的CQL语句，允许绑定变量，从而提供更好的性能和防止SQL注入。它们可以预先准备，然后多次使用，只需更改绑定的值即可。文档中还介绍了如何配置集群，包括调优策略，如连接选项、故障转移策略以及批量语句的处理。连接选项涉及到网络设置，如端口、SSL和认证。而故障转移策略确保了在节点失败时，驱动程序能够自动切换到其他可用节点。 CQL数据类型与Java类型之间的映射是另一个重要主题。驱动程序处理CQL类型（如ASCII、INT、TIMESTAMP等）与Java类型（如String、Integer、Date等）之间的转换，方便在Java代码中使用CQL。此外，Java Driver 2.1提供了对象映射API，简化了基本的创建（Create）、读取（Read）、更新（Update）和删除（Delete）操作。这包括对用户定义类型（UDT）的支持，可以通过接口和访问器注解进行映射，直接操作对象字段。对于开发环境的设置，文档指导开发者如何正确配置Java开发环境以充分利用驱动程序。例如，使用Tupletypes和User-definedtypes可以更有效地处理复杂的数据结构。在问题解答部分，文档涵盖了常见的问题，如如何检查条件语句（轻量级事务）是否成功，以及什么是参数化的语句。 Java Driver 2.1 for Apache Cassandra文档提供了一个全面的指南，帮助开发者理解和使用这个驱动程序，以便高效地构建与Cassandra集群交互的Java应用程序。

Writing your first client

for ( Host host : metadata.getAllHosts() ) {

System.out.printf("Datatacenter: %s; Host: %s; Rack: %s\n",

host.getDatacenter(), host.getAddress(), host.getRack());

}

public void close() {

cluster.close();

}

public static void main(String[] args) {

SimpleClient client = new SimpleClient();

client.connect("127.0.0.1");

client.close();

}

When run the client program prints out this metadata on the cluster's constituent nodes in the console

pane:

Connected to cluster: xerxes

Datatacenter: datacenter1; Host: /127.0.0.1; Rack: rack1

Datatacenter: datacenter1; Host: /127.0.0.2; Rack: rack1

Datatacenter: datacenter1; Host: /127.0.0.3; Rack: rack1

Using a session to execute CQL statements

Once you have connected to a Cassandra cluster using a cluster object, you retrieve a session, which

allows you to execute CQL statements to read and write data.

Before you begin

This tutorial uses a CQL3 schema which is described in a post on the DataStax developer blog. Reading

that post, could help with some of the new CQL3 concepts used here.

About this task

Getting metadata for the cluster is good, but you also want to be able to read and write data to the cluster.

The Java driver lets you execute CQL statements using a session instance that you retrieve from the

Cluster object. You will add code to your client for:

• creating tables

• inserting data into those tables

• querying the tables

• printing the results

Procedure

1. Modify your SimpleClient class.

a) Add a Session instance field.

private Session session;

b) Declare and implement a getter for the new session field.

public Session getSession() {

return this.session;

Writing your first client

}

c) Get a session from your cluster and store the reference to it.

Add the following line to the end of the connect method:

session = cluster.connect();

You can execute queries by calling the execute method on your session object. The session maintains

multiple connections to the cluster nodes, provides policies to choose which node to use for each query

(round-robin on all nodes of the cluster by default), and handles retries for failed queries when it makes

sense.

Session instances are thread-safe and usually a single instance is all you need per application.

However, a given session can only be set to one keyspace at a time, so one instance per keyspace

is necessary. Your application typically only needs a single cluster object, unless you're dealing with

multiple physical clusters.

2. Add an instance method, createSchema, to the SimpleClient class implementation.

public void createSchema() { }

3. Add the code to create a new schema.

a) Execute a statement that creates a new keyspace.

Add to the createSchema method:

session.execute("CREATE KEYSPACE IF NOT EXISTS simplex WITH replication

" +

"= {'class':'SimpleStrategy', 'replication_factor':3};");

In this example, you create a new keyspace, simplex.

b) Execute statements to create two new tables, songs and playlists.

Add to the createSchema method:

session.execute(

"CREATE TABLE IF NOT EXISTS simplex.songs (" +

"id uuid PRIMARY KEY," +

"title text," +

"album text," +

"artist text," +

"tags set<text>," +

"data blob" +

");");

session.execute(

"CREATE TABLE IF NOT EXISTS simplex.playlists (" +

"id uuid," +

"title text," +

"album text, " +

"artist text," +

"song_id uuid," +

"PRIMARY KEY (id, title, album, artist)" +

");");

4. Add an instance method, loadData, to the SimpleCient class implementation.

public void loadData() { }

5. Add the code to insert data into the new schema.

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