持续交付项目中，项目会不断的迭代上线，这是会出现数据库的改动问题。在一个还未上线的项目中，我更改数据库可以使用简单的直接对sql进行更改，但是面对一个已投入使用的系统而言，每次部署都对数据库进行init显得有点不太可能。于是便有了数据库Migration的诞生。
我所在的项目是SpringMVC配合Mybatis的开发框架。作为一个数据库集成框架，Mybatis在使用上还是比较方便的。言归正传，为了项目需要，于是便开始了Mybatis Migration的初探，文章中出现的不妥之处还请大家不吝赐教。
Mybatis Migration 安装
安装过程主要参照Mybatis的官网： http://mybatis.github.io/migrations/index.html，以及Mybatis源代码授权地址: https://github.com/mybatis/migrations，参照github上的README文件可以轻易的完成其安装过程。README文件中所有的Mybatis包可以直接在其Git repo的releases中找到，我选择了3.2.0进行下载安装。安装完成后就可以直接在terminal中运行migrate相关的命令了。
Mybatus Migration 初始化
minggong:migration-test minggong$ migrate init
------------------------------------------------------------------------
-- MyBatis Migrations - init
------------------------------------------------------------------------
Initializing: .
Creating: environments
Creating: scripts
Creating: drivers
Creating: README
Creating: development.properties
Creating: bootstrap.sql
Creating: 20140817132704_create_changelog.sql
Creating: 20140817132705_first_migration.sql
Done!
------------------------------------------------------------------------
-- MyBatis Migrations SUCCESS
-- Total time: 2s
-- Finished at: Sun Aug 17 18:57:05 GMT+05:30 2014
-- Final Memory: 3M/493M
------------------------------------------------------------------------
minggong:migration-test minggong$ ls
README drivers environments scripts
初始化之后的目录下面会自动生成一些和Migration相关的项目文件及目录 README, drivers, environments, scripts。drivers中放置连接数据库需要的jdbc，environments主要放置连接数据库的地址以及授权信息等，scripts目录下则放置数据库相关的sql，init的时候就已经生成了上述显示的create_changelog.sql和first_migration.sql文件。
为了以实例说明问题，本次测试采用了mysql数据库，从mysql官网下载了mysql-connector-java-5.1.32，并对environment的数据库信息进行了简单的配置，配置如下：
## JDBC connection properties.
driver=com.mysql.jdbc.Driver
url=jdbc:mysql://localhost:3306/arctic
username=root
password=
这里的arctic是已经存在本地的一个mysql数据库。
Migrate bootstrap
minggong:migration-test minggong$ migrate bootstrap
------------------------------------------------------------------------
-- MyBatis Migrations - bootstrap
------------------------------------------------------------------------
========== Applying: bootstrap.sql =============================================
------------------------------------------------------------------------
-- MyBatis Migrations SUCCESS
-- Total time: 0s
-- Finished at: Sun Aug 17 19:43:29 GMT+05:30 2014
-- Final Memory: 11M/493M
------------------------------------------------------------------------
运行之后，scripts文件中将会生成另外一个bootstrap文件。主要可以进行一些数据库初始化工作。
Migrate new
new命令能够创建新的sql脚本，我们只需要填充数据表结构信息，便可以方便的创建我们所需的数据表
minggong:migration-test minggong$ migrate new "create blog table"
------------------------------------------------------------------------
-- MyBatis Migrations - new
------------------------------------------------------------------------
Creating: 20140817142201_create_blog_table.sql
Done!
------------------------------------------------------------------------
-- MyBatis Migrations SUCCESS
-- Total time: 1s
-- Finished at: Sun Aug 17 19:52:01 GMT+05:30 2014
-- Final Memory: 3M/493M
------------------------------------------------------------------------
scripts文件夹中会创建带有时间戳的sql文件：20140817142201_create_blog_table.sql， 其内容如下：
-- // create blog table
-- Migration SQL that makes the change goes here.
-- //@UNDO
-- SQL to undo the change goes here.
文件内容主要包含create table和undo，分别表示表的创建和删除命令：
-- // create blog table
-- Migration SQL that makes the change goes here.
CREATE TABLE BLOG (
ID INT,
NAME VARCHAR(255),
PRIMARY KEY(ID)
);
-- //@UNDO
-- SQL to undo the change goes here.
DROP TABLE BLOG;
已经将表结构写在了sql里面，那么怎样执行这个脚本，从而改变数据库中的表结构呢。首先我们需要了解当前数据库的状态。
Migrate status
minggong:migration-test minggong$ migrate status
------------------------------------------------------------------------
-- MyBatis Migrations - status
------------------------------------------------------------------------
ID Applied At Description
================================================================================
20140817132704 ...pending... create changelog
20140817132705 ...pending... first migration
20140817142201 ...pending... create blog table
------------------------------------------------------------------------
-- MyBatis Migrations SUCCESS
-- Total time: 0s
-- Finished at: Sun Aug 17 20:21:06 GMT+05:30 2014
-- Final Memory: 11M/493M
------------------------------------------------------------------------
由于我们没有运行过migration, 所以所有migration脚本都处于pending状态，包括changelog table本身。当我们运行migrate up命令之后，脚本的状态便会发生相应的改变。
Migrate up, down
为了显示更多up, down以及后续的version操作，再次新建一个数据表migrate new 'create table author'此时的author和blog应该都处于pending状态。
minggong:migration-test minggong$ migrate status
------------------------------------------------------------------------
-- MyBatis Migrations - status
------------------------------------------------------------------------
ID Applied At Description
================================================================================
20140817132704 ...pending... create changelog
20140817132705 ...pending... first migration
20140817142201 ...pending... create blog table
20140817154537 ...pending... create table author
------------------------------------------------------------------------
-- MyBatis Migrations SUCCESS
-- Total time: 0s
-- Finished at: Sun Aug 17 21:21:49 GMT+05:30 2014
-- Final Memory: 11M/493M
------------------------------------------------------------------------
minggong:migration-test minggong$ migrate up
------------------------------------------------------------------------
-- MyBatis Migrations - up
------------------------------------------------------------------------
========== Applying: 20140817132704_create_changelog.sql =======================
-- Create Changelog
-- Default DDL for changelog table that will keep
-- a record of the migrations that have been run.
-- You can modify this to suit your database before
-- running your first migration.
-- Be sure that ID and DESCRIPTION fields exist in
-- BigInteger and String compatible fields respectively.
CREATE TABLE CHANGELOG (
ID NUMERIC(20,0) NOT NULL,
APPLIED_AT VARCHAR(25) NOT NULL,
DESCRIPTION VARCHAR(255) NOT NULL
)
ALTER TABLE CHANGELOG
ADD CONSTRAINT PK_CHANGELOG
PRIMARY KEY (id)
========== Applying: 20140817132705_first_migration.sql ========================
-- First migration.
-- Migration SQL that makes the change goes here.
========== Applying: 20140817142201_create_blog_table.sql ======================
-- create blog table
-- Migration SQL that makes the change goes here.
========== Applying: 20140817154537_create_table_author.sql ====================
-- create table author
-- Migration SQL that makes the change goes here.
------------------------------------------------------------------------
-- MyBatis Migrations SUCCESS
-- Total time: 0s
-- Finished at: Sun Aug 17 21:22:17 GMT+05:30 2014
-- Final Memory: 13M/493M
------------------------------------------------------------------------
migrate up命令会将所有的sql进行up,所有处于pending状态的SQL脚本都会进行执行。对应的migrate down则是undo sql脚本的执行，也就是执行sql中用户自定义的UNDO部分。不同的是migrate down只能回退一步，所以回退到初始状态则需要运行多次的migrate down操作。当然也可以直接指定参数，本例中我们指定回退4步migrate down 4:
minggong:migration-test minggong$ migrate status
————————————————————————
– MyBatis Migrations - status
————————————————————————
ID Applied At Description
================================================================================
20140817132704 2014-08-17 21:22:17 create changelog
20140817132705 2014-08-17 21:22:17 first migration
20140817142201 2014-08-17 21:22:17 create blog table
20140817154537 2014-08-17 21:26:01 create table author
------------------------------------------------------------------------
-- MyBatis Migrations SUCCESS
-- Total time: 0s
-- Finished at: Sun Aug 17 21:26:07 GMT+05:30 2014
-- Final Memory: 11M/493M
------------------------------------------------------------------------
minggong:migration-test minggong$ migrate down 4
------------------------------------------------------------------------
-- MyBatis Migrations - down
------------------------------------------------------------------------
========== Undoing: 20140817154537_create_table_author.sql =====================
-- @UNDO
-- SQL to undo the change goes here.
========== Undoing: 20140817142201_create_blog_table.sql =======================
-- @UNDO
-- SQL to undo the change goes here.
========== Undoing: 20140817132705_first_migration.sql =========================
-- @UNDO
-- SQL to undo the change goes here.
========== Undoing: 20140817132704_create_changelog.sql ========================
-- @UNDO
DROP TABLE CHANGELOG
Changelog doesn't exist. No further migrations will be undone (normal for the last migration).
------------------------------------------------------------------------
-- MyBatis Migrations SUCCESS
-- Total time: 0s
-- Finished at: Sun Aug 17 21:29:53 GMT+05:30 2014
-- Final Memory: 14M/493M
------------------------------------------------------------------------
minggong:migration-test minggong$ migrate status
------------------------------------------------------------------------
-- MyBatis Migrations - status
------------------------------------------------------------------------
ID Applied At Description
================================================================================
20140817132704 ...pending... create changelog
20140817132705 ...pending... first migration
20140817142201 ...pending... create blog table
20140817154537 ...pending... create table author
------------------------------------------------------------------------
-- MyBatis Migrations SUCCESS
-- Total time: 0s
-- Finished at: Sun Aug 17 21:30:09 GMT+05:30 2014
-- Final Memory: 11M/493M
------------------------------------------------------------------------
Migrate verison
回滚到指定的version状态，后面需要加上version的ID： migrate verison 20140817132705
minggong:migration-test minggong$ migrate status
------------------------------------------------------------------------
-- MyBatis Migrations - status
------------------------------------------------------------------------
ID Applied At Description
================================================================================
20140817132704 2014-08-17 22:23:05 create changelog
20140817132705 2014-08-17 22:23:25 first migration
20140817142201 2014-08-17 22:23:25 create blog table
20140817154537 2014-08-17 22:23:25 create table author
20140817164341 2014-08-17 22:23:25 create table book
------------------------------------------------------------------------
-- MyBatis Migrations SUCCESS
-- Total time: 0s
-- Finished at: Sun Aug 17 22:23:36 GMT+05:30 2014
-- Final Memory: 11M/493M
------------------------------------------------------------------------
minggong:migration-test minggong$ migrate version 20140817142201
------------------------------------------------------------------------
-- MyBatis Migrations - version
------------------------------------------------------------------------
Downgrading to: 20140817142201
========== Undoing: 20140817164341_create_table_book.sql =======================
-- @UNDO
-- SQL to undo the change goes here.
========== Undoing: 20140817154537_create_table_author.sql =====================
-- @UNDO
-- SQL to undo the change goes here.
------------------------------------------------------------------------
-- MyBatis Migrations SUCCESS
-- Total time: 0s
-- Finished at: Sun Aug 17 22:23:43 GMT+05:30 2014
-- Final Memory: 13M/493M
------------------------------------------------------------------------
minggong:migration-test minggong$ migrate status
------------------------------------------------------------------------
-- MyBatis Migrations - status
------------------------------------------------------------------------
ID Applied At Description
================================================================================
20140817132704 2014-08-17 22:23:05 create changelog
20140817132705 2014-08-17 22:23:25 first migration
20140817142201 2014-08-17 22:23:25 create blog table
20140817154537 ...pending... create table author
20140817164341 ...pending... create table book
------------------------------------------------------------------------
-- MyBatis Migrations SUCCESS
-- Total time: 0s
-- Finished at: Sun Aug 17 22:23:47 GMT+05:30 2014
-- Final Memory: 11M/493M
------------------------------------------------------------------------
Migrate pending
migrate up 命令只能向上upgrade，而在多人合作中如果migration sql脚本先创建但是后提交，同步到一台服务器上就有可能出现中间某个文件处于pending状态，这时使用migrate up是不能使其执行的。
这种情况如果这个sql基本和其他sql无依赖时，可以直接使用migrate up使处于pending状态的脚本被执行。官方解释这是不推荐的做法。另外一种做法是使用migrate version [ID]回到pending版本之前的状态，然后再次执行migrate up，从而完成整个执行操作，这种做法是推荐的比较安全的。
Migrate script
使用方法为migrate script <V1> <V2> > file.sql
migrate 20140817142201 20140817164341 > do.sql这个do脚本的执行将会应用上述两个状态的执行author和book。
migrate 20140817164341 20140817142201 > undo.sql 这个将undo脚本执行可以回滚V1和V2之间的即上述状态。
如果要在首尾之间回滚，则可以用 0 代替原始版本ID
migrate 0 20140817164341 > do.sql
migrate 20140817164341 0 > undo.sql
但是对于上述的script脚本直接使用 migrate do.sql执行却一直报错！官方也没有对单个的脚本执行有说明。后续弄清楚了再补充上来。