數據庫遷移框架Flyway介紹

官方文檔

https://flywaydb.org/getstarted/firststeps/api[https://flywaydb.org/getstarted/firststeps/api]

入門示例

Java代碼

package foobar;
import org.flywaydb.core.Flyway;
public class App {
 public static void main(String[] args) {
 Flyway flyway = new Flyway();
 // 指定數據源
 flyway.setDataSource("jdbc:mysql://localhost/test", "root", "root");
 // 開始數據遷移
 flyway.migrate();
 }
 }
 

在classpath下添加SQL文件 db/migration/V1__Create_person_table.sql

create table PERSON (
    ID int not null,
    NAME varchar(100) not null
);

運行程序，在test數據庫會自動創建PERSON表。

后續新增表和字段，只需在db/migration目錄下新增SQL文件，格式為V${version}__${name}.sql，version值依次增加，比如V2__name.sql，V3__name.sql。

原理介紹

Flyway的數據庫遷移的實現原理是，從classpath或文件系統中找到符合規則的數據庫遷移腳本，比如db/migration目錄下命名規則為V${version}__${name}.sql的文件，將腳本按照version進行排序，依次執行。執行過的腳本會作為一條記錄，存儲在schema_version表中。當下次執行遷移時，判斷腳本已經執行，則跳過。

MigrationResolver接口負責查找數據庫遷移腳本，方法為resolveMigrations()，數據庫遷移腳本用ResolvedMigration對象表示。MigrationResolver包含多種實現類，比如SqlMigrationResolver會從classpath下查找sql文件。查詢通過Scanner類實現，Location類指定查詢路徑，sql文件的命名規則需要符合V${version}__${name}.sql，規則中的前綴V、后綴.sql、分隔符__均在FlywayConfiguration接口中定義。另一種實現類JdbcMigrationResolver會從classpath下查找實現JdbcMigration接口的類，類的命名規則需要符合V${version}__${name}。需要擴展自己的實現類，可以繼承BaseMigrationResolver。

MetaDataTable接口負責查找已執行的數據庫遷移腳本，方法為findAppliedMigrations()，已執行的數據庫遷移腳本用AppliedMigration對象表示。MetaDataTable只有一種實現類MetaDataTableImpl，從數據庫schema_version表查詢所有記錄。

ResolvedMigration集合包含了已經執行的AppliedMigration集合，在執行ResolvedMigration前，需要對比AppliedMigration，找到已執行和未執行的ResolvedMigration，對比通過MigrationInfoServiceImpl.refresh()實現。已執行的ResolvedMigration需要校驗文件有沒有發生變化，有變更則提示錯誤。未執行的ResolvedMigration依次執行，執行結果記錄在schema_version表中。

Flyway的主要方法：

public class Flyway {
   /**數據庫遷移*/
   public int migrate();
   /**校驗已執行的遷移操作的變更情況*/
   public void validate();
   /**清理數據庫*/
   public void clean();
   /**設置數據庫的基准版本*/
   public void baseline();
   /**刪除執行錯誤的遷移記錄*/
   public void repair();
   /**准備執行環境，並執行Command操作，以上方法都調用了execute()來執行操作*/
   <T> T execute(Command<T> command);
}

接下來我們分析Flyway.migrate()代碼執行的主邏輯。

public void migrate() {
  //由Flyway.execute()准備Command.execute()執行所需要的參數
  return execute(new Command<Integer>() {
    public Integer execute(Connection connectionMetaDataTable, MigrationResolver migrationResolver, MetaDataTable metaDataTable, DbSupport dbSupport, Schema[] schemas, FlywayCallback[] flywayCallbacks) {
      //為了簡化代碼，忽略參數傳遞
      doMigrate();
}

});
 }
private void doMigrate() {
 //校驗已執行的遷移操作的變更情況
 if (validateOnMigrate) {
 doValidate(connectionMetaDataTable, dbSupport, migrationResolver,
 metaDataTable, schemas, flywayCallbacks, true);
 }
//如果尚未進行數據遷移，即schema_version表中不存在數據，
 //並且數據庫不為空，則插入一條baseline信息
 if (!metaDataTable.exists()) {
 //數據庫不為空
 if (!nonEmptySchemas.isEmpty()) {
 //插入一條baseline信息
 new DbBaseline(connectionMetaDataTable, dbSupport, metaDataTable,
 schemas[0], baselineVersion, baselineDescription, flywayCallbacks).baseline();
 }
 }
//進行數據遷移
 DbMigrate dbMigrate = new DbMigrate(connectionUserObjects, dbSupport,
 metaDataTable,schemas[0], migrationResolver, ignoreFailedFutureMigration,
 Flyway.this);
 return dbMigrate.migrate();
 }
 

接下來看DbMigrate.migrate()的代碼片段。

MigrationInfoServiceImpl對比ResolvedMigration和AppliedMigration對象，找出需要執行數據庫遷移腳本，通過pending()方法返回。最后執行數據庫遷移腳本。

public int migrate() {
  int migrationSuccessCount = 0;
  while (true) {
    int count = metaDataTable.lock(new Callable<Integer>() {
  <span class="hljs-meta">@Override</span>
  <span class="hljs-function"><span class="hljs-keyword">public</span> Integer <span class="hljs-title">call</span><span class="hljs-params">()</span> </span>{
    <span class="hljs-comment">//為了簡化代碼，忽略參數傳遞</span>
    <span class="hljs-keyword">return</span> doMigrate();
  }
}
<span class="hljs-keyword">if</span> (count == <span class="hljs-number">0</span>) {
  <span class="hljs-comment">// No further migrations available</span>
  <span class="hljs-keyword">break</span>;
}
migrationSuccessCount += count;

}
 return migrationSuccessCount;
 }
private int doMigrate() {
 //收集已經入庫的數據庫遷移記錄，和以文件形式存在的數據庫遷移腳本
 MigrationInfoServiceImpl infoService = new MigrationInfoServiceImpl(
 migrationResolver, metaDataTable, configuration.getTarget(),
 configuration.isOutOfOrder(), true, true, true);
 infoService.refresh();
//infoService.pending()記錄將要執行的數據庫遷移腳本
 LinkedHashMap<MigrationInfoImpl, Boolean> group =
 groupMigrationInfo(infoService.pending());
 if (!group.isEmpty()) {
 //執行數據庫遷移操作
 applyMigrations(group);
 }
 }
 

DbMigrate.doMigrateGroup() 執行數據庫遷移腳本

private void doMigrateGroup() {
  //執行遷移腳本
  migration.getResolvedMigration().getExecutor().execute(connectionUserObjects);
//存入數據庫
 AppliedMigration appliedMigration = new AppliedMigration(migration.getVersion(),
 migration.getDescription(), migration.getType(), migration.getScript(),
 migration.getResolvedMigration().getChecksum(), executionTime, true);
 metaDataTable.addAppliedMigration(appliedMigration);
 }
 

擴展練習

在一個已經上線的游戲項目中引入Flyway框架，對於已經存在的游戲服，只執行新增的sql語句，對於新搭建的游戲服，需要創建數據庫，並執行新增的sql語句。

在這個需求中，需要做的是對於新搭建的游戲服，需要創建數據庫。我們可以通過FlywayCallback實現這一點，如果指定數據庫為空，則執行初始化數據庫的語句。

代碼如下：

public static void main(String[] args) {
    final Flyway flyway = new Flyway();
    flyway.setDataSource("jdbc:mysql://localhost/test", "root", "root");
FlywayCallback flywayCallback = <span class="hljs-keyword">new</span> BaseFlywayCallback() {
    
    <span class="hljs-meta">@Override</span>
    <span class="hljs-function"><span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">beforeMigrate</span><span class="hljs-params">(Connection connection)</span> </span>{
        DbSupport dbSupport = DbSupportFactory.createDbSupport(connection, <span class="hljs-keyword">false</span>);
        <span class="hljs-keyword">if</span>(!hasTable(dbSupport)) {
            initDb(dbSupport);
        }
    }

    <span class="hljs-function"><span class="hljs-keyword">private</span> <span class="hljs-keyword">boolean</span> <span class="hljs-title">hasTable</span><span class="hljs-params">(DbSupport dbSupport)</span> </span>{
        Schema&lt;?&gt; schema = dbSupport.getOriginalSchema();
        Table[] tables = schema.allTables();
        <span class="hljs-keyword">if</span>(tables == <span class="hljs-keyword">null</span> || tables.length == <span class="hljs-number">0</span>) {
            <span class="hljs-keyword">return</span> <span class="hljs-keyword">false</span>;
        }
        <span class="hljs-comment">//忽略表 schema_version</span>
        <span class="hljs-keyword">if</span>(tables.length == <span class="hljs-number">1</span> &amp;&amp; 
            tables[<span class="hljs-number">0</span>].getName().equalsIgnoreCase(<span class="hljs-string">"schema_version"</span>)) {
            <span class="hljs-keyword">return</span> <span class="hljs-keyword">false</span>;
        }
        <span class="hljs-keyword">return</span> <span class="hljs-keyword">true</span>;
    }
    

    <span class="hljs-function"><span class="hljs-keyword">private</span>  <span class="hljs-keyword">void</span> <span class="hljs-title">initDb</span><span class="hljs-params">(DbSupport dbSupport)</span> </span>{
        Scanner scanner = <span class="hljs-keyword">new</span> Scanner(
          Thread.currentThread().getContextClassLoader());
        Resource[] resources = scanner.scanForResources(
          <span class="hljs-keyword">new</span> Location(<span class="hljs-string">"db/init"</span>), <span class="hljs-string">""</span>, <span class="hljs-string">".sql"</span>);
        <span class="hljs-keyword">if</span>(resources == <span class="hljs-keyword">null</span> || resources.length == <span class="hljs-number">0</span>) {
            <span class="hljs-keyword">throw</span> <span class="hljs-keyword">new</span> RuntimeException(
              <span class="hljs-string">"db/init/*.sql not found in the classpath. "</span>);
        }
        <span class="hljs-keyword">for</span>(Resource resource : resources) {
          SqlMigrationExecutor executor = <span class="hljs-keyword">new</span> SqlMigrationExecutor(
              dbSupport, resource, PlaceholderReplacer.NO_PLACEHOLDERS, flyway);
          executor.execute(dbSupport.getJdbcTemplate().getConnection());
        }
    }
};

List&lt;FlywayCallback&gt; callbacks = <span class="hljs-keyword">new</span> ArrayList&lt;FlywayCallback&gt;(
  Arrays.asList(flyway.getCallbacks()));
callbacks.add(flywayCallback);
flyway.setCallbacks(callbacks.toArray(<span class="hljs-keyword">new</span> FlywayCallback[callbacks.size()]));

flyway.setBaselineOnMigrate(<span class="hljs-keyword">true</span>);
flyway.repair();
flyway.migrate();

}

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