本文介绍了Cassandra数据库的基础操作，包括通过CQL创建键空间和列族，进行数据的增删改查，并对索引的使用进行了说明。
和cassandra数据库交互的方式有两种，一种是通过类似于cassandra-cli命令的thrift api，或者通过cassandra提供的cql(cassandra query lanugage),.
注意：cassandra-cli客户端命令从cassandra V2.2已经弃用，所以想使用cassandra-cli命令的话只能安装cassandra V2.2之前的版本。cassandra-cli命令比较难懂，和传统的sql有很大的区别，对于我们学习起来是比较头疼的事情。cql api的语法风格类似于sql，该api屏蔽了cassandra底层架构，将底层的数据结构以sql的形式展现出来。推荐学习cassandra的时候一定好好研究一下该thrift api的使用，cassandra-cli可以帮助我们深入了解cassandra的内部存储结构。
1. keyspace（键空间）
首先查看当前cluster默认的data center
[cassandra@sht-sgmhadoopcm-01 bin]$ nodetool status Datacenter: EAST ================ Status=Up/Down |/ State=Normal/Leaving/Joining/Moving -- Address Load Tokens Owns (effective) Host ID Rack UN 172.16.101.54 51.66 KB 256 100.0% d821e5e5-e99d-41a9-b11f-a0fd0d3d9b05 RAC1
可以看到当前集群默认有一个data center EAST，当前cassandra节点在RAC1机柜，该data center的定义涉及到两个配置文件
$CASSANDRA_HOME/conf/cassandra.yaml的“endpoint_snitch: PropertyFileSnitch“参数
$CASSANDRA_HOME/conf//cassandra-topology.properties
查看创建keyspace的语法并创建 mytest keyspace
[default@unknown] help create keyspace; ................... Examples: create keyspace Keyspace2 with placement_strategy = 'org.apache.cassandra.locator.SimpleStrategy' and strategy_options = {replication_factor:4}; create keyspace Keyspace3 with placement_strategy = 'org.apache.cassandra.locator.NetworkTopologyStrategy' and strategy_options={DC1:2, DC2:2}; create keyspace Keyspace4 with placement_strategy = 'org.apache.cassandra.locator.OldNetworkTopologyStrategy' and strategy_options = {replication_factor:1}; [default@unknown] create keyspace mytest with placement_strategy = 'org.apache.cassandra.locator.NetworkTopologyStrategy' and strategy_options={EAST:1}; 78c6f3e9-622c-3ae8-86d2-7733518ca7ff [default@unknown] describe mytest; WARNING: CQL3 tables are intentionally omitted from 'describe' output. See https://issues.apache.org/jira/browse/CASSANDRA-4377 for details. Keyspace: mytest: Replication Strategy: org.apache.cassandra.locator.NetworkTopologyStrategy Durable Writes: true Options: [EAST:1] Column Families:
2. column family（列族）
[default@unknown] use mytest; [default@mytest] create column family users with column_type=Standard and comparator=UTF8Type and key_validation_class=UTF8Type and default_validation_class=UTF8Type; bd4ffef6-9e93-38ec-a4fc-62b4fc00604c [default@mytest] describe users; WARNING: CQL3 tables are intentionally omitted from 'describe' output. See https://issues.apache.org/jira/browse/CASSANDRA-4377 for details. ColumnFamily: users Key Validation Class: org.apache.cassandra.db.marshal.UTF8Type Default column value validator: org.apache.cassandra.db.marshal.UTF8Type Cells sorted by: org.apache.cassandra.db.marshal.UTF8Type GC grace seconds: 864000 Compaction min/max thresholds: 4/32 Read repair chance: 0.0 DC Local Read repair chance: 0.1 Caching: KEYS_ONLY Default time to live: 0 Bloom Filter FP chance: default Index interval: default Speculative Retry: NONE Built indexes: [] Compaction Strategy: org.apache.cassandra.db.compaction.SizeTieredCompactionStrategy Compression Options: sstable_compression: org.apache.cassandra.io.compress.LZ4Compressor
通过上述创建users列族可以看到，我们仅仅是指定了列族级别的属性（相当于表的属性），但是该列族没有任何列。
我们也可以通过meta_data选项在创建column family的时候加入预定义的列以及索引信息
[default@mytest] create column family students ... with column_type = 'Standard' ... and comparator = UTF8Type ... and key_validation_class=UTF8Type ... and default_validation_class=UTF8Type ... and column_metadata = [ ... {column_name: age, validation_class: UTF8Type} ... {column_name: birthday, validation_class: UTF8Type, index_type: KEYS,index_name: IDXbirthday} ... {column_name: first, validation_class: UTF8Type} ... {column_name: last, validation_class: UTF8Type} ... ];
[default@mytest] describe students; WARNING: CQL3 tables are intentionally omitted from 'describe' output. See https://issues.apache.org/jira/browse/CASSANDRA-4377 for details. ColumnFamily: students Key Validation Class: org.apache.cassandra.db.marshal.UTF8Type Default column value validator: org.apache.cassandra.db.marshal.UTF8Type Cells sorted by: org.apache.cassandra.db.marshal.UTF8Type GC grace seconds: 864000 Compaction min/max thresholds: 4/32 Read repair chance: 0.0 DC Local Read repair chance: 0.1 Caching: KEYS_ONLY Default time to live: 0 Bloom Filter FP chance: default Index interval: default Speculative Retry: NONE Built indexes: [students.IDXbirthday] Column Metadata: Column Name: first Validation Class: org.apache.cassandra.db.marshal.UTF8Type Column Name: birthday Validation Class: org.apache.cassandra.db.marshal.UTF8Type Index Name: IDXbirthday Index Type: KEYS Column Name: age Validation Class: org.apache.cassandra.db.marshal.UTF8Type Column Name: last Validation Class: org.apache.cassandra.db.marshal.UTF8Type Compaction Strategy: org.apache.cassandra.db.compaction.SizeTieredCompactionStrategy Compression Options: sstable_compression: org.apache.cassandra.io.compress.LZ4Compressor
其中
- column_type定义列的类型，默认为标准列
- comparator 在查询数据时返回的列的排序方式，注意这里是列名，不是列的值，可以根据long、byte、UTF8等其他排序方式进行，同时也意味着当列名显示在命令行界面时，它们将显示为UTF8Type（可读）文本。在Cassandra中是无法按照关系型数据中的值来排序的，必须按照列名来排序，以便能从一个很宽的行里高效取出一列，而无需把每列都读进内存。
- key_validation_class 定义row key（主键）的数据类型
- default_validation_class 定义value的数据类型
3. set&get/list&del/count（插入更新&查询&删除&统计）
语法 set column family[‘row key‘]['key']='value'
[default@mytest] set users['zhangpeng']['first']='zhang'; Value inserted. Elapsed time: 3.83 msec(s). [default@mytest] set users['zhangpeng']['last']='peng'; Value inserted. Elapsed time: 2.69 msec(s). [default@mytest] set users['zhangpeng']['age']='18'; Value inserted. Elapsed time: 1.97 msec(s). [default@mytest] set users['wangxing']['first']='wang'; Value inserted. Elapsed time: 1.64 msec(s). [default@mytest] set users['wangxing']['last']='xing'; Value inserted. Elapsed time: 1.4 msec(s). [default@mytest] set users['wangxing']['age']='19'; Value inserted. Elapsed time: 1.38 msec(s). [default@mytest] set users['wangxing']['sex']='male'; Value inserted. Elapsed time: 1.38 msec(s). [default@mytest] get users['zhangpeng']; => (name=age, value=18, timestamp=1527690907938000) => (name=first, value=zhang, timestamp=1527690887611000) => (name=last, value=peng, timestamp=1527690898435000) Returned 3 results. Elapsed time: 3.68 msec(s). [default@mytest] get users['wangxing']; => (name=age, value=19, timestamp=1527690946003000) => (name=first, value=wang, timestamp=1527690926424000) => (name=last, value=xing, timestamp=1527690934083000) => (name=sex, value=male, timestamp=1527690962938000) Returned 4 results. Elapsed time: 3.46 msec(s). [default@mytest] set users[wangxing]['age']='17'; Value inserted. Elapsed time: 10 msec(s). [default@mytest] get users['wangxing']; => (name=age, value=17, timestamp=1527691267345000) => (name=first, value=wang, timestamp=1527690926424000) => (name=last, value=xing, timestamp=1527690934083000) => (name=sex, value=male, timestamp=1527690962938000) Returned 4 results. Elapsed time: 3.67 msec(s). [default@mytest] del users['wangxing']['sex']; cell removed. Elapsed time: 14 msec(s). [default@mytest] get users['wangxing']; => (name=age, value=17, timestamp=1527691267345000) => (name=first, value=wang, timestamp=1527690926424000) => (name=last, value=xing, timestamp=1527690934083000) Returned 3 results. Elapsed time: 4.84 msec(s). [default@mytest] count users['wangxing']; 3 cells [default@mytest] count users['zhangpeng']; 3 cells [default@mytest] count users['wangxing']; 3 cells [default@mytest] list users; Using default limit of 100 Using default cell limit of 100 ------------------- RowKey: wangxing => (name=age, value=19, timestamp=1527692441512000) => (name=first, value=wang, timestamp=1527692429554000) => (name=last, value=xing, timestamp=1527692435344000) ------------------- RowKey: zhangpeng => (name=age, value=18, timestamp=1527692421898000) => (name=first, value=zhang, timestamp=1527692409752000) => (name=last, value=peng, timestamp=1527692416150000) 2 Rows Returned. Elapsed time: 6.55 msec(s).
4. index
默认情况下我们不可以通过where限制条件查询数据，否则会报如下错误，我们可以为需要where条件的列增加索引，但是只局限于where的等值查询
[default@mytest] get users where age='17'; No indexed columns present in index clause with operator EQ [default@mytest] update column family users with column_metadata=[{column_name : age, validation_class : UTF8Type, index_type : KEYS,index_name : IDXage}]; b2a8c0af-4162-3590-acda-70ccdb7d363c [default@mytest] get users where age='17'; ------------------- RowKey: wangxing => (name=age, value=17, timestamp=1527692751954000) => (name=first, value=wang, timestamp=1527692429554000) => (name=last, value=xing, timestamp=1527692435344000) 1 Row Returned. Elapsed time: 70 msec(s).
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