Rollback

Sometimes things don’t go as planned when attempting an upgrade. This section explains how to perform a rollback to an earlier HBase release. Note that this should only be needed between Major and some Minor releases. You should always be able to downgrade between HBase Patch releases within the same Minor version. These instructions may require you to take steps before you start the upgrade process, so be sure to read through this section beforehand.

12.1. Caveats

Rollback vs Downgrade
This section describes how to perform a rollback on an upgrade between HBase minor and major versions. In this document, rollback refers to the process of taking an upgraded cluster and restoring it to the old version while losing all changes that have occurred since upgrade. By contrast, a cluster downgrade would restore an upgraded cluster to the old version while maintaining any data written since the upgrade. We currently only offer instructions to rollback HBase clusters. Further, rollback only works when these instructions are followed prior to performing the upgrade.
When these instructions talk about rollback vs downgrade of prerequisite cluster services (i.e. HDFS), you should treat leaving the service version the same as a degenerate case of downgrade.

Replication
Unless you are doing an all-service rollback, the HBase cluster will lose any configured peers for HBase replication. If your cluster is configured for HBase replication, then prior to following these instructions you should document all replication peers. After performing the rollback you should then add each documented peer back to the cluster. For more information on enabling HBase replication, listing peers, and adding a peer see Managing and Configuring Cluster Replication. Note also that data written to the cluster since the upgrade may or may not have already been replicated to any peers. Determining which, if any, peers have seen replication data as well as rolling back the data in those peers is out of the scope of this guide.

Data Locality
Unless you are doing an all-service rollback, going through a rollback procedure will likely destroy all locality for Region Servers. You should expect degraded performance until after the cluster has had time to go through compactions to restore data locality. Optionally, you can force a compaction to speed this process up at the cost of generating cluster load.

Configurable Locations
The instructions below assume default locations for the HBase data directory and the HBase znode. Both of these locations are configurable and you should verify the value used in your cluster before proceeding. In the event that you have a different value, just replace the default with the one found in your configuration * HBase data directory is configured via the key ‘hbase.rootdir’ and has a default value of ‘/hbase’. * HBase znode is configured via the key ‘zookeeper.znode.parent’ and has a default value of ‘/hbase’.

12.2. All service rollback
If you will be performing a rollback of both the HDFS and ZooKeeper services, then HBase’s data will be rolled back in the process.

Requirements
Ability to rollback HDFS and ZooKeeper

Before upgrade
No additional steps are needed pre-upgrade. As an extra precautionary measure, you may wish to use distcp to back up the HBase data off of the cluster to be upgraded. To do so, follow the steps in the ‘Before upgrade’ section of ‘Rollback after HDFS downgrade’ but copy to another HDFS instance instead of within the same instance.

Performing a rollback
Stop HBase

Perform a rollback for HDFS and ZooKeeper (HBase should remain stopped)

Change the installed version of HBase to the previous version

Start HBase

Verify HBase contents—use the HBase shell to list tables and scan some known values.

12.3. Rollback after HDFS rollback and ZooKeeper downgrade
If you will be rolling back HDFS but going through a ZooKeeper downgrade, then HBase will be in an inconsistent state. You must ensure the cluster is not started until you complete this process.

Requirements
Ability to rollback HDFS

Ability to downgrade ZooKeeper

Before upgrade
No additional steps are needed pre-upgrade. As an extra precautionary measure, you may wish to use distcp to back up the HBase data off of the cluster to be upgraded. To do so, follow the steps in the ‘Before upgrade’ section of ‘Rollback after HDFS downgrade’ but copy to another HDFS instance instead of within the same instance.

Performing a rollback
Stop HBase

Perform a rollback for HDFS and a downgrade for ZooKeeper (HBase should remain stopped)

Change the installed version of HBase to the previous version

Clean out ZooKeeper information related to HBase. WARNING: This step will permanently destroy all replication peers. Please see the section on HBase Replication under Caveats for more information.
Clean HBase information out of ZooKeeper

[hpnewton@gateway_node.example.com ~]$ zookeeper-client -server zookeeper1.example.com:2181,zookeeper2.example.com:2181,zookeeper3.example.com:2181
Welcome to ZooKeeper!
JLine support is disabled
rmr /hbase
quit
Quitting...

Start HBase

Verify HBase contents—use the HBase shell to list tables and scan some known values.

12.4. Rollback after HDFS downgrade
If you will be performing an HDFS downgrade, then you’ll need to follow these instructions regardless of whether ZooKeeper goes through rollback, downgrade, or reinstallation.

Requirements
Ability to downgrade HDFS

Pre-upgrade cluster must be able to run MapReduce jobs

HDFS super user access

Sufficient space in HDFS for at least two copies of the HBase data directory

Before upgrade
Before beginning the upgrade process, you must take a complete backup of HBase’s backing data. The following instructions cover backing up the data within the current HDFS instance. Alternatively, you can use the distcp command to copy the data to another HDFS cluster.
Stop the HBase cluster

Copy the HBase data directory to a backup location using the distcp command as the HDFS super user (shown below on a security enabled cluster)
Using distcp to backup the HBase data directory

[hpnewton@gateway_node.example.com ~]$ kinit -k -t hdfs.keytab hdfs@EXAMPLE.COM
[hpnewton@gateway_node.example.com ~]$ hadoop distcp /hbase /hbase-pre-upgrade-backup

Distcp will launch a mapreduce job to handle copying the files in a distributed fashion. Check the output of the distcp command to ensure this job completed successfully.

Performing a rollback
Stop HBase

Perform a downgrade for HDFS and a downgrade/rollback for ZooKeeper (HBase should remain stopped)

Change the installed version of HBase to the previous version

Restore the HBase data directory from prior to the upgrade as the HDFS super user (shown below on a security enabled cluster). If you backed up your data on another HDFS cluster instead of locally, you will need to use the distcp command to copy it back to the current HDFS cluster.
Restore the HBase data directory

[hpnewton@gateway_node.example.com ~]$ kinit -k -t hdfs.keytab hdfs@EXAMPLE.COM
[hpnewton@gateway_node.example.com ~]$ hdfs dfs -mv /hbase /hbase-upgrade-rollback
[hpnewton@gateway_node.example.com ~]$ hdfs dfs -mv /hbase-pre-upgrade-backup /hbase

Clean out ZooKeeper information related to HBase. WARNING: This step will permanently destroy all replication peers. Please see the section on HBase Replication under Caveats for more information.
Clean HBase information out of ZooKeeper

[hpnewton@gateway_node.example.com ~]$ zookeeper-client -server zookeeper1.example.com:2181,zookeeper2.example.com:2181,zookeeper3.example.com:2181
Welcome to ZooKeeper!
JLine support is disabled
rmr /hbase
quit
Quitting...

Start HBase

Verify HBase contents–use the HBase shell to list tables and scan some known values.

savepoint test;
rollback to savepoint test;
转载于:https://www.cnblogs.com/seasonsstory/p/3504099.html

不用rollback()表面和用了rollback()效果一样，但是不用rollback()可能导致被锁住的数据不能及时的释放（需要等事物超时释放）,会影响下一次的事物操作

文章目录
1 概述
2 语法
2.1 rollback
2.2 rollback to savepoint xx
2.3 rollback work force 'string'
3 扩展
3.1 rollback 官方解释

1 概述
1. rollback 是什么？
   (1) 回滚
   (2) 事务管理语句之一
   
2. rollback 的作用？
   (1) 撤销（'undo'）当前的事务
   (2) 与它相反的是 commit（提交当前的事务）

2 语法


可选子句解释：




关键字
解释
备注




work
为实现SQL标准兼容性而提供的
写与不写，结果都一样


to savepoint clause
将当前事务回滚至 clause
若省略该子句，则当前事务都会被回滚


force string
仅回滚指定的分布式事务，不会对当前的事务产生影响
string：事务 id，可通过 dba_2pc_pending 进行查询


基础数据准备：
-- 学生信息表
create table student_info (
   sno   number(10) constraint pk_student_info_sno primary key,
   name varchar2(30),
   sex  varchar2(2)
);

2.1 rollback
-- *****************************************************************
-- 直接 回滚 事务
-- *****************************************************************
declare
begin
  insert into student_info (sno, name, sex) values (1, '张三', '女');
  insert into student_info (sno, name, sex) values (2, '李四', '男');

  rollback;
end;

运行结果：


2.2 rollback to savepoint xx
-- *****************************************************************
-- 1. 回滚到指定的回滚点 xx
-- 2. 若回滚点同名，则回滚至最近的一个回滚点（不建议同名，不方便排查问题）
-- *****************************************************************
declare
begin
   insert into student_info (sno, name, sex) values (1, '张三', '女');
   savepoint sp1;
   
   insert into student_info (sno, name, sex) values (2, '李四', '女');
   savepoint sp2; -- 若也是 ps1，则回滚至此处（最近）
   
   -- 模拟报错：违反唯一性约束
   insert into student_info (sno, name, sex) values (2, '王五', '男');
   savepoint sp3;
   
   commit;
exception 
  when others then
    -- 演示!
    rollback to sp1;
    -- 报错的行数
    dbms_output.put_line(dbms_utility.format_error_backtrace);
    dbms_output.put_line(sqlcode ||' : ' || sqlerrm);
    commit;
end;

运行结果：


2.3 rollback work force ‘string’
1. 使用场景
   (1) 分布式事务
   
2. 具体操作
   (1) select * from dba_2pc_pending;  -- 查询
   (2) rollback work force '25.32.87';  -- 回滚

3 扩展
3.1 rollback 官方解释

Oralce 官方解释：rollback