持續交付項目中，項目會不斷的迭代上線，這是會出現數據庫的改動問題。在一個還未上線的項目中，我更改數據庫可以使用簡單的直接對sql進行更改，但是面對一個已投入使用的系統而言，每次部署都對數據庫進行init顯得有點不太可能。於是便有了數據庫Migration的誕生。
我所在的項目是SpringMVC配合Mybatis的開發框架。作爲一個數據庫集成框架，Mybatis在使用上還是比較方便的。言歸正傳，爲了項目需要，於是便開始了Mybatis Migration的初探，文章中出現的不妥之處還請大家不吝賜教。
Mybatis Migration 安裝
安裝過程主要參照Mybatis的官網： http://mybatis.github.io/migrations/index.html，以及Mybatis源代碼授權地址: https://github.com/mybatis/migrations，參照github上的README文件可以輕易的完成其安裝過程。README文件中所有的Mybatis包可以直接在其Git repo的releases中找到，我選擇了3.2.0進行下載安裝。安裝完成後就可以直接在terminal中運行migrate相關的命令了。
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
初始化之後的目錄下面會自動生成一些和Migration相關的項目文件及目錄 README, drivers, environments, scripts。drivers中放置連接數據庫需要的jdbc，environments主要放置連接數據庫的地址以及授權信息等，scripts目錄下則放置數據庫相關的sql，init的時候就已經生成了上述顯示的create_changelog.sql和first_migration.sql文件。
爲了以實例說明問題，本次測試採用了mysql數據庫，從mysql官網下載了mysql-connector-java-5.1.32，並對environment的數據庫信息進行了簡單的配置，配置如下：
## JDBC connection properties.
driver=com.mysql.jdbc.Driver
url=jdbc:mysql://localhost:3306/arctic
username=root
password=
這裏的arctic是已經存在本地的一個mysql數據庫。
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
------------------------------------------------------------------------
運行之後，scripts文件中將會生成另外一個bootstrap文件。主要可以進行一些數據庫初始化工作。
Migrate new
new命令能夠創建新的sql腳本，我們只需要填充數據表結構信息，便可以方便的創建我們所需的數據表
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文件夾中會創建帶有時間戳的sql文件：20140817142201_create_blog_table.sql， 其內容如下：
-- // create blog table
-- Migration SQL that makes the change goes here.
-- //@UNDO
-- SQL to undo the change goes here.
文件內容主要包含create table和undo，分別表示表的創建和刪除命令：
-- // 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;
已經將表結構寫在了sql裏面，那麼怎樣執行這個腳本，從而改變數據庫中的表結構呢。首先我們需要了解當前數據庫的狀態。
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
------------------------------------------------------------------------
由於我們沒有運行過migration, 所以所有migration腳本都處於pending狀態，包括changelog table本身。當我們運行migrate up命令之後，腳本的狀態便會發生相應的改變。
Migrate up, down
爲了顯示更多up, down以及後續的version操作，再次新建一個數據表migrate new 'create table author'此時的author和blog應該都處於pending狀態。
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命令會將所有的sql進行up,所有處於pending狀態的SQL腳本都會進行執行。對應的migrate down則是undo sql腳本的執行，也就是執行sql中用戶自定義的UNDO部分。不同的是migrate down只能回退一步，所以回退到初始狀態則需要運行多次的migrate down操作。當然也可以直接指定參數，本例中我們指定回退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
回滾到指定的version狀態，後面需要加上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腳本先創建但是後提交，同步到一臺服務器上就有可能出現中間某個文件處於pending狀態，這時使用migrate up是不能使其執行的。
這種情況如果這個sql基本和其他sql無依賴時，可以直接使用migrate up使處於pending狀態的腳本被執行。官方解釋這是不推薦的做法。另外一種做法是使用migrate version [ID]回到pending版本之前的狀態，然後再次執行migrate up，從而完成整個執行操作，這種做法是推薦的比較安全的。
Migrate script
使用方法爲migrate script <V1> <V2> > file.sql
migrate 20140817142201 20140817164341 > do.sql這個do腳本的執行將會應用上述兩個狀態的執行author和book。
migrate 20140817164341 20140817142201 > undo.sql 這個將undo腳本執行可以回滾V1和V2之間的即上述狀態。
如果要在首尾之間回滾，則可以用 0 代替原始版本ID
migrate 0 20140817164341 > do.sql
migrate 20140817164341 0 > undo.sql
但是對於上述的script腳本直接使用 migrate do.sql執行卻一直報錯！官方也沒有對單個的腳本執行有說明。後續弄清楚了再補充上來。