作者：杨文
DBA，负责客户我的项目的需要与保护，会点数据库，不限于MySQL、Redis、Cassandra、GreenPlum、ClickHouse、Elastic、TDSQL等等。
本文起源：原创投稿
*爱可生开源社区出品，原创内容未经受权不得随便应用，转载请分割小编并注明起源。
一、前情提要：
咱们晓得 cassandra 具备分区容错性和强一致性，然而当数据所在主机产生故障时，该主机对应的数据副本该何去何从呢？是否跟宿主机一样变得不可用呢？想晓得答案的话，就跟我一起往下看吧。
二、试验环境：
集群模式下跨数据中心：
| 数据中心 | 节点IP | 种子节点 |
|---|---|---|
| DC1 | 10.186.60.61、10.186.60.7、10.186.60.118、10.186.60.67 | 10.186.60.61、10.186.60.7 |
| DC2 | 10.186.60.53、10.186.60.65、10.186.60.94、10.186.60.68 | 10.186.60.53、10.186.60.65 |
首先一起来瞅一瞅节点退出集群过程中的 owns 变动：
[cassandra@data01 ~]$ nodetool statusDatacenter: dc1=================Status=Up/Down|/ State=Normal/Leaving/Joining/Moving-- Address Load Tokens Owns (effective) Host ID RackUN 10.186.60.7 88.29 KiB 16 46.0% 4702178e-9878-48dc-97e7-9211b7c9f2e7 rack1UN 10.186.60.118 69.07 KiB 16 37.7% c920c611-2e8b-472d-93a4-34f1abd5b207 rack1UN 10.186.60.61 88.25 KiB 16 34.2% af2e0c42-3a94-4647-9716-c484b690899i rack1Datacenter: dc2=================Status=Up/Down|/ State=Normal/Leaving/Joining/Moving-- Address Load Tokens Owns (effective) Host ID RackUN 10.186.60.65 69.04 KiB 16 41.4% 89683bf8-aff8-4fdc-9525-c14764cf2d4f rack2UN 10.186.60.53 83.18 KiB 16 41.7% 7c91c707-abac-44f2-811O-b18f03f03d13 rack2[cassandra@data01 ~]$ nodetool statusDatacenter: dc1=================Status=Up/Down|/ State=Normal/Leaving/Joining/Moving-- Address Load Tokens Owns (effective) Host ID RackUN 10.186.60.67 74.01 KiB 16 24.7% 9d6d759b-c00c-488b-938d-3e1ef9b92b02 rack1UN 10.186.60.7 88.29 KiB 16 27.5% 4702178e-9878-48dc-97e7-9211b7c9f2e7 rack1UN 10.186.60.118 83.16 KiB 16 28.9% c920c611-2e8b-472d-93a4-34f1abd5b207 rack1UN 10.186.60.61 88.25 KiB 16 30.3% af2e0c42-3a94-4647-9716-c484b690899i rack1Datacenter: dc2=================Status=Up/Down|/ State=Normal/Leaving/Joining/Moving-- Address Load Tokens Owns (effective) Host ID RackUN 10.186.60.65 83.17 KiB 16 27.7% 89683bf8-aff8-4fdc-9525-c14764cf2d4f rack2UN 10.186.60.53 83.18 KiB 16 29.8% 7c91c707-abac-44f2-811O-b18f03f03d13 rack2UN 10.186.60.94 69.05 KiB 16 31.1% c8fa86e4-ee9a-4c62-b00b-d15edc967b9f rack2[cassandra@data01 ~]$ nodetool statusDatacenter: dc1=================Status=Up/Down|/ State=Normal/Leaving/Joining/Moving-- Address Load Tokens Owns (effective) Host ID RackUN 10.186.60.67 74.01 KiB 16 21.4% 9d6d759b-c00c-488b-938d-3e1ef9b92b02 rack1UN 10.186.60.7 88.29 KiB 16 25.2% 4702178e-9878-48dc-97e7-9211b7c9f2e7 rack1UN 10.186.60.118 83.16 KiB 16 27.1% c920c611-2e8b-472d-93a4-34f1abd5b207 rack1UN 10.186.60.61 83.19 KiB 16 28.9% af2e0c42-3a94-4647-9716-c484b690899i rack1Datacenter: dc2=================Status=Up/Down|/ State=Normal/Leaving/Joining/Moving-- Address Load Tokens Owns (effective) Host ID RackUN 10.186.60.68 88.55 KiB 16 21.6% a7307228-62bb-4354-9853-990cac9614ab rack2UN 10.186.60.65 83.17 KiB 16 24.0% 89683bf8-aff8-4fdc-9525-c14764cf2d4f rack2UN 10.186.60.53 83.18 KiB 16 25.4% 7c91c707-abac-44f2-811O-b18f03f03d13 rack2UN 10.186.60.94 69.05 KiB 16 26.4% c8fa86e4-ee9a-4c62-b00b-d15edc967b9f rack2能够看到，刚建设的集群，owns 的总和时刻放弃在 200% ，但单个数据中心的 owns 不是 100% 。
三、具体试验：
3.1、试验1：
[cassandra@data01 ~]$ cqlsh 10.186.60.61 -u cassandra -p cassandraCREATE KEYSPACE "dcdatabase" WITH REPLICATION = {'class':'NetworkTopologyStrategy', 'dc1' : 4, 'dc2' : 4};use dcdatabase;create table test (id int, user_name varchar, primary key (id) );insert into test (id,name) VALUES (1,'test1');insert into test (id,name) VALUES (2,'test2');insert into test (id,name) VALUES (3,'test3');insert into test (id,name) VALUES (4,'test4');insert into test (id,name) VALUES (5,'test5');查看集群状态：
[cassandra@data01 ~]$ nodetool statusDatacenter: dc1=================Status=Up/Down|/ State=Normal/Leaving/Joining/Moving-- Address Load Tokens owns (effective) Host ID RackUN 10.186.60.67 96.55 KiB 16 100.0% 9d6d759b-c00c-488b-938d-3e1ef9b92b02 rack1UN 10.186.60.7 88.29 KiB 16 100.0% 4702178e-9878-48dc-97e7-9211b7c9f2e7 rack1UN 10.186.60.118 88.33 KiB 16 100.0% c920c611-2e8b-472d-93a4-34f1abd5b207 rack1UN 10.186.60.61 88.37 KiB 16 100.0% af2e0c42-3a94-4647-9716-c484b690899i rack1Datacenter: dc2=================Status=Up/Down|/ State=Normal/Leaving/Joining/Moving-- Address Load Tokens owns (effective) Host ID RackUN 10.186.60.68 74.23 KiB 16 100.0% a7307228-62bb-4354-9853-990cac9614ab rack2UN 10.186.60.65 83.17 KiB 16 100.0% 89683bf8-aff8-4fdc-9525-c14764cf2d4f rack2UN 10.186.60.53 88.36 KiB 16 100.0% 7c91c707-abac-44f2-811O-b18f03f03d13 rack2UN 10.186.60.94 74.23 KiB 16 100.0% c8fa86e4-ee9a-4c62-b00b-d15edc967b9f rack2能够看到集群中，每个数据中心的 owns 都是 400% ，合乎四正本的设置；
查看数据在节点上的散布状况：
[cassandra@data01 ~]$ nodetool getendpoints dcdatabase test 110.186.60.710.186.60.9410.186.60.6510.186.60.11810.186.60.6710.186.60.6110.186.60.5310.186.60.68[cassandra@data03 ~]$ nodetool getendpoints dcdatabase test 510.186.60.6710.186.60.9410.186.60.710.186.60.5310.186.60.6510.186.60.11810.186.60.6110.186.60.68能够看到集群数据分布在所有数据中心的所有节点上，合乎数据的散布原理。
测试并查看集群中呈现故障节点后的数据分布状况：
94机器敞开服务：systemctl stop cassandra[cassandra@data01 ~]$ nodetool statusDatacenter: dc1=================Status=Up/Down|/ State=Normal/Leaving/Joining/Moving-- Address Load Tokens owns (effective) Host ID RackUN 10.186.60.67 96.55 KiB 16 100.0% 9d6d759b-c00c-488b-938d-3e1ef9b92b02 rack1UN 10.186.60.7 88.29 KiB 16 100.0% 4702178e-9878-48dc-97e7-9211b7c9f2e7 rack1UN 10.186.60.118 88.33 KiB 16 100.0% c920c611-2e8b-472d-93a4-34f1abd5b207 rack1UN 10.186.60.61 88.37 KiB 16 100.0% af2e0c42-3a94-4647-9716-c484b690899i rack1Datacenter: dc2=================Status=Up/Down|/ State=Normal/Leaving/Joining/Moving-- Address Load Tokens owns (effective) Host ID RackUN 10.186.60.68 74.23 KiB 16 100.0% a7307228-62bb-4354-9853-990cac9614ab rack2UN 10.186.60.65 83.17 KiB 16 100.0% 89683bf8-aff8-4fdc-9525-c14764cf2d4f rack2UN 10.186.60.53 88.36 KiB 16 100.0% 7c91c707-abac-44f2-811O-b18f03f03d13 rack2DN 10.186.60.94 74.23 KiB 16 100.0% c8fa86e4-ee9a-4c62-b00b-d15edc967b9f rack2能够看到，94节点曾经宕掉，然而 dc2 数据中心的 owns 散布并未扭转。
查看数据分布在哪个节点：
[cassandra@data01 ~]$ nodetool getendpoints dcdatabase test 510.186.60.6710.186.60.9410.186.60.710.186.60.5310.186.60.6510.186.60.11810.186.60.6110.186.60.68能够看到，数据仍散布在94节点上；
把故障节点94移除集群：
[cassandra@data02 ~]$ nodetool removenode c8fa86e4-ee9a-4c62-b00b-d15edc967b9f查看集群状态：
[cassandra@data01 ~]$ nodetool statusDatacenter: dc1=================Status=Up/Down|/ State=Normal/Leaving/Joining/Moving-- Address Load Tokens owns (effective) Host ID RackUN 10.186.60.67 96.55 KiB 16 100.0% 9d6d759b-c00c-488b-938d-3e1ef9b92b02 rack1UN 10.186.60.7 88.29 KiB 16 100.0% 4702178e-9878-48dc-97e7-9211b7c9f2e7 rack1UN 10.186.60.118 88.33 KiB 16 100.0% c920c611-2e8b-472d-93a4-34f1abd5b207 rack1UN 10.186.60.61 88.37 KiB 16 100.0% af2e0c42-3a94-4647-9716-c484b690899i rack1Datacenter: dc2=================Status=Up/Down|/ State=Normal/Leaving/Joining/Moving-- Address Load Tokens owns (effective) Host ID RackUN 10.186.60.68 74.23 KiB 16 100.0% a7307228-62bb-4354-9853-990cac9614ab rack2UN 10.186.60.65 83.17 KiB 16 100.0% 89683bf8-aff8-4fdc-9525-c14764cf2d4f rack2UN 10.186.60.53 88.36 KiB 16 100.0% 7c91c707-abac-44f2-811O-b18f03f03d13 rack2[cassandra@data02 ~]$ nodetool getendpoints dcdatabase test 510.186.60.6710.186.60.710.186.60.5310.186.60.6510.186.60.11810.186.60.6110.186.60.68能够看到，数据不在94节点上了；
阐明：对于 cassandra 进行服务或移出集群，仍是能够应用的，只是不能登入本人的 cassandra 数据库，但仍能够登录其余 cassandra 数据库。
3.2、试验2：
CREATE KEYSPACE "dcdatabase" WITH REPLICATION = {'class':'NetworkTopologyStrategy', 'dc1' : 3, 'dc2' : 3};查看集群状态：
[cassandra@data01 ~]$ nodetool statusDatacenter: dc1=================Status=Up/Down|/ State=Normal/Leaving/Joining/Moving-- Address Load Tokens owns (effective) Host ID RackUN 10.186.60.67 96.55 KiB 16 73.2% 9d6d759b-c00c-488b-938d-3e1ef9b92b02 rack1UN 10.186.60.7 89.39 KiB 16 74.7% 4702178e-9878-48dc-97e7-9211b7c9f2e7 rack1UN 10.186.60.118 88.33 KiB 16 77.4% c920c611-2e8b-472d-93a4-34f1abd5b207 rack1UN 10.186.60.61 88.42 KiB 16 74.7% af2e0c42-3a94-4647-9716-c484b690899i rack1Datacenter: dc2=================Status=Up/Down|/ State=Normal/Leaving/Joining/Moving-- Address Load Tokens owns (effective) Host ID RackUN 10.186.60.68 74.22 KiB 16 100.0% a7307228-62bb-4354-9853-990cac9614ab rack2UN 10.186.60.65 84.14 KiB 16 100.0% 89683bf8-aff8-4fdc-9525-c14764cf2d4f rack2UN 10.186.60.53 88.30 KiB 16 100.0% 7c91c707-abac-44f2-811O-b18f03f03d13 rack2能够看到集群中，每个数据中心的 owns 都是 300% ，合乎三正本的设置；
测试并查看集群中呈现故障节点后的数据分布状况：
94机器敞开服务，并移除集群：
[cassandra@data02 ~]$ nodetool removenode c8fa86e4-ee9a-4c62-b00b-d15edc967b9f查看集群状态：
[cassandra@data01 ~]$ nodetool statusDatacenter: dc1=================Status=Up/Down|/ State=Normal/Leaving/Joining/Moving-- Address Load Tokens owns (effective) Host ID RackUN 10.186.60.67 96.55 KiB 16 73.2% 9d6d759b-c00c-488b-938d-3e1ef9b92b02 rack1UN 10.186.60.7 89.39 KiB 16 74.7% 4702178e-9878-48dc-97e7-9211b7c9f2e7 rack1UN 10.186.60.118 88.33 KiB 16 77.4% c920c611-2e8b-472d-93a4-34f1abd5b207 rack1UN 10.186.60.61 88.42 KiB 16 74.7% af2e0c42-3a94-4647-9716-c484b690899i rack1Datacenter: dc2=================Status=Up/Down|/ State=Normal/Leaving/Joining/Moving-- Address Load Tokens owns (effective) Host ID RackUN 10.186.60.68 74.22 KiB 16 100.0% a7307228-62bb-4354-9853-990cac9614ab rack2UN 10.186.60.65 84.14 KiB 16 100.0% 89683bf8-aff8-4fdc-9525-c14764cf2d4f rack2UN 10.186.60.53 88.30 KiB 16 100.0% 7c91c707-abac-44f2-811O-b18f03f03d13 rack2此时，数据不在94节点上了，故障节点上的数据已挪动到其余节点上，因而能够看到，在 dc1 数据中心中，数据随机仍只散布在其中三个节点上，而 dc2 数据中心的数据将散布在了仅有的三个节点上，产生了数据转移；
如果此时 dc2 数据中心还有节点持续故障，那么故障节点上的数据不可能再挪动到其余节点上了，dc1 是不变的，owns 还是300% ，然而 dc2 的 owns都是100% ，没方法故障转移了，只能存在本身的数据了；
此时重启所有主机，所有主机 Cassandra 服务都会开启，包含之前故障模拟的节点也会自启，那么此时就会达到了另一种成果：故障模拟节点后的状态，再增加到了集群中，那么此时数据又会进行了主动的散发。
查看集群状态：
[cassandra@data01 ~]$ nodetool statusDatacenter: dc1=================Status=Up/Down|/ State=Normal/Leaving/Joining/Moving-- Address Load Tokens owns (effective) Host ID RackUN 10.186.60.67 96.55 KiB 16 73.2% 9d6d759b-c00c-488b-938d-3e1ef9b92b02 rack1UN 10.186.60.7 89.39 KiB 16 74.7% 4702178e-9878-48dc-97e7-9211b7c9f2e7 rack1UN 10.186.60.118 88.33 KiB 16 77.4% c920c611-2e8b-472d-93a4-34f1abd5b207 rack1UN 10.186.60.61 88.42 KiB 16 74.7% af2e0c42-3a94-4647-9716-c484b690899i rack1Datacenter: dc2=================Status=Up/Down|/ State=Normal/Leaving/Joining/Moving-- Address Load Tokens owns (effective) Host ID RackUN 10.186.60.68 74.22 KiB 16 73.2% a7307228-62bb-4354-9853-990cac9614ab rack2UN 10.186.60.65 84.14 KiB 16 74.7% 89683bf8-aff8-4fdc-9525-c14764cf2d4f rack2UN 10.186.60.53 88.30 KiB 16 74.7% 7c91c707-abac-44f2-811O-b18f03f03d13 rack2UN 10.186.60.94 90.12 KiB 16 77.4% c8fa86e4-ee9a-4c62-b00b-d15edc967b9f rack2