cassandra学习笔记4--Cassandra Java客户端
Cassandra在设计的时候，就是支持Thrift的，这意味着我们可以使用多种语言开发。
对于Cassandra的开发本身而言，这是使用Thrift的好处：支持多语言。坏处也是显而易见的：Thrift API功能过于简单，不具备在生产环境使用的条件。
在Cassandra Wiki页面上，也有基于Thrift API开发的更加高级的API，各个语言都有，具体信息可以参考：http://wiki.apache.org/cassandra/ClientExamples。
这次只谈谈下面两类Java的客户端：
1 Thrift Java API
2 hector
Thrift Java API这个是Cassandra自带的最简单的一类API，这个文件在apache-cassandra-0.5.1.jar(现在已经升级到0.6.1版本)中包含了。可以直接使用。我们也可以自己安装一个Thrift，然后通过cassandra.thrift文件自动生成。
如果你要使用Cassandra，那么我们必须要了解Thrift API，毕竟所有的其他更加高级的API都是基于这个来包装的。
提供的功能插入数据插入数据需要指定keyspace，ColumnFamily， Column，Key，Value，timestamp和数据同步级别。
/**
* Insert a Column consisting of (column_path.column, value, timestamp) at the given column_path.column_family and optional
* column_path.super_column. Note that column_path.column is here required, since a SuperColumn cannot directly contain binary
* values -- it can only contain sub-Columns.
*
* @param keyspace
* @param key
* @param column_path
* @param value
* @param timestamp
* @param consistency_level
*/
public void insert(String keyspace, String key, ColumnPath column_path, byte[] value, long timestamp, int consistency_level) throws InvalidRequestException, UnavailableException, TimedOutException, TException;
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/**
* Insert Columns or SuperColumns across different Column Families for the same row key. batch_mutation is a
* map<string, list<ColumnOrSuperColumn>> -- a map which pairs column family names with the relevant ColumnOrSuperColumn
* objects to insert.
*
* @param keyspace
* @param key
* @param cfmap
* @param consistency_level
*/
public void batch_insert(String keyspace, String key, Map<String,List<ColumnOrSuperColumn>> cfmap, int consistency_level) throws InvalidRequestException, UnavailableException, TimedOutException, TException;读取数据
获取一个查询条件精确的值。
/**
* Get the Column or SuperColumn at the given column_path. If no value is present, NotFoundException is thrown. (This is
* the only method that can throw an exception under non-failure conditions.)
*
* @param keyspace
* @param key
* @param column_path
* @param consistency_level
*/
public ColumnOrSuperColumn get(String keyspace, String key, ColumnPath column_path, int consistency_level) throws InvalidRequestException, NotFoundException, UnavailableException, TimedOutException, TException;
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/**
* Perform a get for column_path in parallel on the given list<string> keys. The return value maps keys to the
* ColumnOrSuperColumn found. If no value corresponding to a key is present, the key will still be in the map, but both
* the column and super_column references of the ColumnOrSuperColumn object it maps to will be null.
*
* @param keyspace
* @param keys
* @param column_path
* @param consistency_level
*/
public Map<String,ColumnOrSuperColumn> multiget(String keyspace, List<String> keys, ColumnPath column_path, int consistency_level) throws InvalidRequestException, UnavailableException, TimedOutException, TException;
获取某一个keyspace，Key，ColumnFamily，SuperColumn（如果有的话需要指定）下面的相关数据：只查询Column的name符合条件的相关数据（SlicePredicate）。
/**
* Get the group of columns contained by column_parent (either a ColumnFamily name or a ColumnFamily/SuperColumn name
* pair) specified by the given SlicePredicate. If no matching values are found, an empty list is returned.
*
* @param keyspace
* @param key
* @param column_parent
* @param predicate
* @param consistency_level
*/
public List<ColumnOrSuperColumn> get_slice(String keyspace, String key, ColumnParent column_parent, SlicePredicate predicate, int consistency_level) throws InvalidRequestException, UnavailableException, TimedOutException, TException;
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/**
* Performs a get_slice for column_parent and predicate for the given keys in parallel.
*
* @param keyspace
* @param keys
* @param column_parent
* @param predicate
* @param consistency_level
*/
public Map<String,List<ColumnOrSuperColumn>> multiget_slice(String keyspace, List<String> keys, ColumnParent column_parent, SlicePredicate predicate, int consistency_level) throws InvalidRequestException, UnavailableException, TimedOutException, TException;
查询Key的取值范围（使用这个功能需要使用order-preserving partitioner）。
/**
* @deprecated; use get_range_slice instead
*
* @param keyspace
* @param column_family
* @param start
* @param finish
* @param count
* @param consistency_level
*/
public List<String> get_key_range(String keyspace, String column_family, String start, String finish, int count, int consistency_level) throws InvalidRequestException, UnavailableException, TimedOutException, TException;
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/**
* returns a subset of columns for a range of keys.
*
* @param keyspace
* @param column_parent
* @param predicate
* @param start_key
* @param finish_key
* @param row_count
* @param consistency_level
*/
public List<KeySlice> get_range_slice(String keyspace, ColumnParent column_parent, SlicePredicate predicate, String start_key, String finish_key, int row_count, int consistency_level) throws InvalidRequestException, UnavailableException, TimedOutException, TException;
查询系统的信息。
/**
* get property whose value is of type string.
*
* @param property
*/
public String get_string_property(String property) throws TException;
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/**
* get property whose value is list of strings.
*
* @param property
*/
public List<String> get_string_list_property(String property) throws TException;
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/**
* describe specified keyspace
*
* @param keyspace
*/
public Map<String,Map<String,String>> describe_keyspace(String keyspace) throws NotFoundException, TException;
通过这些操作，我们可以了解到系统的信息。
其中一个比较有意思的查询信息是：token map，通过这个我们可以知道哪些Cassandra Service是可以提供服务的。
删除数据/**
* Remove data from the row specified by key at the granularity specified by column_path, and the given timestamp. Note
* that all the values in column_path besides column_path.column_family are truly optional: you can remove the entire
* row by just specifying the ColumnFamily, or you can remove a SuperColumn or a single Column by specifying those levels too.
*
* @param keyspace
* @param key
* @param column_path
* @param timestamp
* @param consistency_level
*/
public void remove(String keyspace, String key, ColumnPath column_path, long timestamp, int consistency_level) throws InvalidRequestException, UnavailableException, TimedOutException, TException;
这里需要注意的是，由于一致性的问题。这里的删除操作不会立即删除所有机器上的该数据，但是最终会一致。
程序范例(0.5.1)import java.util.List;
import java.io.UnsupportedEncodingException;
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import org.apache.thrift.transport.TTransport;
import org.apache.thrift.transport.TSocket;
import org.apache.thrift.protocol.TProtocol;
import org.apache.thrift.protocol.TBinaryProtocol;
import org.apache.thrift.TException;
import org.apache.cassandra.service.*;
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public class CClient
{????public static void main(String[] args)
????throws TException, InvalidRequestException, UnavailableException, UnsupportedEncodingException, NotFoundException
????{????????TTransport tr = new TSocket("localhost", 9160);????????TProtocol proto = new TBinaryProtocol(tr);
????????Cassandra.Client client = new Cassandra.Client(proto);
????????tr.open();
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????????String key_user_id = "逖靖寒的世界";
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????????// insert data
????????long timestamp = System.currentTimeMillis();
????????client.insert("Keyspace1",??????????????????????key_user_id,
??????????????????????new ColumnPath("Standard1", null, "网址".getBytes("UTF-8")),??????????????????????"http://gpcuster.cnblogs.com".getBytes("UTF-8"),??????????????????????timestamp,
??????????????????????ConsistencyLevel.ONE);
????????client.insert("Keyspace1",??????????????????????key_user_id,
??????????????????????new ColumnPath("Standard1", null, "作者".getBytes("UTF-8")),??????????????????????"逖靖寒".getBytes("UTF-8"),??????????????????????timestamp,
??????????????????????ConsistencyLevel.ONE);
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????????// read single column
????????ColumnPath path = new ColumnPath("Standard1", null, "name".getBytes("UTF-8"));????????System.out.println(client.get("Keyspace1", key_user_id, path, ConsistencyLevel.ONE));?
????????// read entire row
????????SlicePredicate predicate = new SlicePredicate(null, new SliceRange(new byte[0], new byte[0], false, 10));
????????ColumnParent parent = new ColumnParent("Standard1", null);????????List<ColumnOrSuperColumn> results = client.get_slice("Keyspace1", key_user_id, parent, predicate, ConsistencyLevel.ONE);????????for (ColumnOrSuperColumn result : results)
????????{????????????Column column = result.column;
????????????System.out.println(new String(column.name, "UTF-8") + " -> " + new String(column.value, "UTF-8"));
????????}
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????????tr.close();
????}
}优点与缺点
优点：简单高效
缺点：功能简单，无法提供连接池，错误处理等功能，不适合直接在生产环境使用。