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- 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        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']='';

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']='';

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']='';

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='';

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='';

-------------------

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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