EntityFramework Core并发迁移解决方案-数字星空

场景

目前一个项目中数据持久化采用EF Core + MySQL，使用CodeFirst模式开发，并且对数据进行了分库，按照目前颗粒度分完之后，大概有一两百个库，每个库的数据都是相互隔离的。
借鉴了Github上一个开源的仓库 arch/UnitOfWork 实现UnitOfWork，核心操作就是每个api请求的时候带上库名，在执行CRUD之前先将DbContext切换到目标数据库，我们在切换数据库的时候加了一些操作，如检查数据库是否已创建、检查连接是否可用、判断是否需要 表结构迁移 等

/// <summary>
/// 切换数据库 这要求数据库在同一台机器上 注意:这只适用于MySQL。
/// </summary>
/// <param name="database">目标数据库</param>
public void ChangeDatabase(string database)
{
    // 检查连接
    ......

    // 检查数据库是否创建
    ......

    var connection = _context.Database.GetDbConnection();
    if (connection.State.HasFlag(ConnectionState.Open))
    {
        connection.ChangeDatabase(database);
    }
    else
    {
        var connectionString = Regex.Replace(connection.ConnectionString.Replace(" ", ""), @"(?<=[Dd]atabase=)\w+(?=;)", database, RegexOptions.Singleline);
        connection.ConnectionString = connectionString;
    }

    // 判断是否需要执行表结构迁移
    if(_context..Database.GetPendingMigrations().Any())
    {
        //自定义的迁移的一些逻辑
        _context.Database.Migrate(_context);
    }
}

但是当多个操作同时对一个库进行Migrate的时候，就会出现问题，比如“新增一张表”的操作已经被第一个迁移执行过了，第二个执行的迁移并不知道已经执行过了Migrate，就会报错表已存在。
于是考虑在执行Migrate的时候，加入一个锁的机制，对当前数据库执行Migrate之前先获取锁，然后再来决定接下来的操作。由于这边有的服务无法访问Redis，这里使用数据库来实现锁的机制，当然用Redis来实现更好，加入锁的机制只是一种解决问题的思路。

利用数据库实现迁移锁

1. 新增 `MigrationLocks` 表来实现迁移锁

锁的操作不依赖DbContext实例

在执行Migrate之前，尝试获取一个锁，在获取锁之前，如果表不存在则创建

CREATE TABLE IF NOT EXISTS MigrationLocks (
    LockName VARCHAR(255) PRIMARY KEY,
    LockedAt DATETIME NOT NULL
);

成功往表中插入一条记录，视为获取锁成功，主键为需要迁移的库的名称
```
INSERT INTO MigrationLocks (LockName, LockedAt) VALUES (@database, NOW());
```
迁移完成后，删除这条记录，视为释放锁成功；
```
DELETE FROM MigrationLocks WHERE LockName = @database;
```
为防止 “死锁” 发生，每次尝试获取锁之前，会对锁的状态进行检查，释放超过5分钟的锁（正常来说，上一个迁移的执行时间不会超过5分钟）。
```
SELECT COUNT(*) FROM MigrationLocks WHERE LockName = @database AND LockedAt > NOW() - INTERVAL 5 MINUTE;
```

2. 封装一下MigrateLock的实现

/// <summary>
/// 迁移锁
/// </summary>
public interface IMigrateLock
{
    /// <summary>
    /// 尝试获取锁
    /// </summary>
    /// <param name="connection"></param>
    /// <returns></returns>
    bool TryAcquireLock(IDbConnection connection);

    /// <summary>
    /// 尝试获取锁
    /// </summary>
    /// <param name="connection"></param>
    /// <returns></returns>
    Task<bool> TryAcquireLockAsync(IDbConnection connection);

    /// <summary>
    /// 释放锁
    /// </summary>
    void ReleaseLock(IDbConnection connection);

    /// <summary>
    /// 释放锁
    /// </summary>
    /// <returns></returns>
    Task ReleaseLockAsync(IDbConnection connection);
}

/// <summary>
/// 迁移锁
/// </summary>
public class MigrateLock : IMigrateLock
{
    private readonly ILogger<MigrateLock> _logger;

    public MigrateLock(ILogger<MigrateLock> logger)
    {
        _logger = logger;
    }

    private const string CreateTableSql = @"
        CREATE TABLE IF NOT EXISTS MigrationLocks (
            LockName VARCHAR(255) PRIMARY KEY,
            LockedAt DATETIME NOT NULL
        );";

    private const string CheckLockedSql = "SELECT COUNT(*) FROM MigrationLocks WHERE LockName = @database AND LockedAt > NOW() - INTERVAL 5 MINUTE;";

    private const string AcquireLockSql = "INSERT INTO MigrationLocks (LockName, LockedAt) VALUES (@database, NOW());";

    private const string ReleaseLockSql = "DELETE FROM MigrationLocks WHERE LockName = @database;";

    /// <summary>
    /// 尝试获取锁
    /// </summary>
    /// <param name="connection"></param>
    /// <returns></returns>
    public bool TryAcquireLock(IDbConnection connection)
    {
        try  
        {
            CheckLocked(connection);

            var result = connection.Execute(AcquireLockSql, new { database = connection.Database });
            if (result == 1)
            {
                _logger.LogInformation("Lock acquired: {LockName}", connection.Database);

                return true;
            }

            _logger.LogWarning("Failed to acquire lock: {LockName}", connection.Database);

            return false;
        }
        catch (Exception ex)
        {
            if (ex.Message.StartsWith("Duplicate"))
            {
                _logger.LogWarning("Failed acquiring lock due to duplicate entry: {LockName}", connection.Database);
            }
            else
            {
                _logger.LogError(ex, "Error acquiring lock: {LockName}", connection.Database);
            }

            return false;
        }
    }

    /// <summary>
    /// 尝试获取锁
    /// </summary>
    /// <param name="connection"></param>
    /// <returns></returns>
    public async Task<bool> TryAcquireLockAsync(IDbConnection connection)
    {
        try
        {
            await CheckLockedAsync(connection);

            var result = await connection.ExecuteAsync(AcquireLockSql, new { database = connection.Database });
            if (result == 1)
            {
                _logger.LogInformation("Lock acquired: {LockName}", connection.Database);

                return true;
            }

            _logger.LogWarning("Failed to acquire lock: {LockName}", connection.Database);

            return false;
        }
        catch (Exception ex)
        {
            if (ex.Message.StartsWith("Duplicate"))
            {
                _logger.LogWarning("Failed acquiring lock due to duplicate entry: {LockName}", connection.Database);
            }
            else
            {
                _logger.LogError(ex, "Error acquiring lock: {LockName}", connection.Database);
            }

            return false;
        }
    }

    /// <summary>
    /// 释放锁
    /// </summary>
    public void ReleaseLock(IDbConnection connection)
    {
        try
        {
            connection.ExecuteAsync(ReleaseLockSql, new { database = connection.Database });
            _logger.LogInformation("Lock released: {LockName}", connection.Database);
        }
        catch (Exception ex)
        {
            _logger.LogError(ex, "Error releasing lock: {LockName}", connection.Database);
        }
    }

    /// <summary>
    /// 释放锁
    /// </summary>
    public async Task ReleaseLockAsync(IDbConnection connection)
    {
        try
        {
            await connection.ExecuteAsync(ReleaseLockSql, new { database = connection.Database });
            _logger.LogInformation("Lock released: {LockName}", connection.Database);
        }
        catch (Exception ex)
        {
            _logger.LogError(ex, "Error releasing lock: {LockName}", connection.Database);
        }
    }

    /// <summary>
    /// 检查锁
    /// </summary>
    private void CheckLocked(IDbConnection connection)
    {
        connection.Execute(CreateTableSql); 

        var databaseParam = new
        {
            database = connection.Database
        };

        var lockExists = connection.QueryFirstOrDefault<int>(CheckLockedSql, databaseParam);
        if (lockExists <= 0)
        {
            return;
        }

        _logger.LogWarning("Lock exists and is older than 5 minutes. Releasing old lock.");
        connection.Execute(ReleaseLockSql, databaseParam);
    }

    /// <summary>
    /// 检查锁
    /// </summary>
    private async Task CheckLockedAsync(IDbConnection connection)
    {
        await connection.ExecuteAsync(CreateTableSql);
         
        var databaseParam = new
        {
            database = connection.Database
        };

        var lockExists = await connection.QueryFirstOrDefaultAsync<int>(CheckLockedSql, databaseParam);
        if (lockExists <= 0)
        {
            return;
        }

        _logger.LogWarning("Lock exists and is older than 5 minutes. Releasing old lock.");
        await connection.ExecuteAsync(ReleaseLockSql, databaseParam);
    }
}

3. 封装一下MigrateExecutor的实现

/// <summary>
/// 数据库迁移执行器
/// </summary>
public interface IMigrateExcutor
{
    /// <summary>
    /// 执行迁移
    /// </summary>
    /// <param name="dbContext"></param>
    void Migrate(DbContext dbContext);

    /// <summary>
    /// 执行迁移
    /// </summary>
    /// <param name="dbContext"></param>
    /// <returns></returns>
    Task MigrateAsync(DbContext dbContext);

    /// <summary>
    /// 并发场景执行迁移
    /// </summary>
    /// <param name="dbContext"></param>
    /// <param name="wait">是否等待至正在进行中的迁移完成</param>
    void ConcurrentMigrate(DbContext dbContext, bool wait = true);
     
    /// <summary>
    /// 并发场景执行迁移
    /// </summary>
    /// <param name="dbContext"></param>
    /// <param name="wait">是否等待至正在进行中的迁移完成</param>
    /// <returns></returns>
    Task ConcurrentMigrateAsync(DbContext dbContext, bool wait = true);

    /// <summary>
    /// 并发场景执行迁移
    /// </summary>
    /// <param name="dbContext"></param>
    /// <param name="connection"></param>
    /// <param name="wait">是否等待至正在进行中的迁移完成</param>
    void ConcurrentMigrate(DbContext dbContext, IDbConnection connection, bool wait = true);

    /// <summary>
    /// 并发场景执行迁移
    /// </summary>
    /// <param name="dbContext"></param>
    /// <param name="connection"></param>
    /// <param name="wait">是否等待至正在进行中的迁移完成</param>
    Task ConcurrentMigrateAsync(DbContext dbContext, IDbConnection connection, bool wait = true);
}

/// <summary>
/// 数据库迁移执行器
/// </summary>
public class MigrateExcutor : IMigrateExcutor
{
    private readonly IMigrateLock _migrateLock;
    private readonly ILogger<MigrateExcutor> _logger;

    public MigrateExcutor(
        IMigrateLock migrateLock,
        ILogger<MigrateExcutor> logger)
    {
        _migrateLock = migrateLock;
        _logger = logger;
    }

    /// <summary>
    /// 执行迁移
    /// </summary>
    /// <param name="dbContext"></param>
    /// <returns></returns>
    public void Migrate(DbContext dbContext)
    {
        try
        {
            if (dbContext.Database.GetPendingMigrations().Any())
            {
                dbContext.Database.Migrate();
            }
        }
        catch (Exception e)
        {
            _logger.LogError(e, "Migration failed");

            HandleError(dbContext, e);
        }
    }

    /// <summary>
    /// 执行迁移
    /// </summary>
    /// <param name="dbContext"></param>
    /// <returns></returns>
    public async Task MigrateAsync(DbContext dbContext)
    {
        try
        {
            if ((await dbContext.Database.GetPendingMigrationsAsync()).Any())
            {
                await dbContext.Database.MigrateAsync();
            }
        }
        catch (Exception e)
        {
            _logger.LogError(e, "Migration failed");

            await HandleErrorAsync(dbContext, e);
        }
    }

    /// <summary>
    /// 并发场景执行迁移
    /// </summary>
    /// <param name="dbContext"></param>
    /// <param name="wait">是否等待至正在进行中的迁移完成</param>
    /// <returns></returns>
    public void ConcurrentMigrate(DbContext dbContext, bool wait = true)
    {
        if (!dbContext.Database.GetPendingMigrations().Any())
        {
            return;
        }

        using var connection = MySqlConnectionHelper.CreateConnection(dbContext.Database.GetDbConnection().Database);

        ConcurrentMigrate(dbContext, connection, wait);
    }

    /// <summary>
    /// 并发场景执行迁移
    /// </summary>
    /// <param name="dbContext"></param>
    /// <param name="wait">是否等待至正在进行中的迁移完成</param>
    /// <returns></returns>
    public async Task ConcurrentMigrateAsync(DbContext dbContext, bool wait = true)
    {
        if ((await dbContext.Database.GetPendingMigrationsAsync()).Any())
        {
            return;
        }

        await using var connection = await MySqlConnectionHelper.CreateConnectionAsync(dbContext.Database.GetDbConnection().Database);

        await ConcurrentMigrateAsync(dbContext, connection, wait);
    }

    /// <summary>
    /// 并发场景执行迁移（供数据同步相关服务使用，”迁移锁“ 使用传入的 <see cref="IDbConnection"/> 对象来完成）
    /// </summary>
    /// <param name="dbContext"></param>
    /// <param name="connection"></param>
    /// <param name="wait">是否等待至正在进行中的迁移完成</param>
    public void ConcurrentMigrate(DbContext dbContext, IDbConnection connection, bool wait = true)
    {
        if (!dbContext.Database.GetPendingMigrations().Any())
        {
            return;
        }

        while (true)
        {
            if (_migrateLock.TryAcquireLock(connection))
            {
                try
                {
                    Migrate(dbContext);

                    break;
                }
                finally
                {
                    _migrateLock.ReleaseLock(connection);
                }
            }

            if (wait)
            {
                _logger.LogWarning("Migration is locked, wait for 2 seconds");
                Thread.Sleep(20000);

                continue;
            }

            _logger.LogInformation("Migration is locked, skip");
        }
    }

    /// <summary>
    /// 并发场景执行迁移（供数据同步相关服务使用，”迁移锁“ 使用传入的 <see cref="IDbConnection"/> 对象来完成）
    /// </summary>
    /// <param name="dbContext"></param>
    /// <param name="connection"></param>
    /// <param name="wait">是否等待至正在进行中的迁移完成</param>
    public async Task ConcurrentMigrateAsync(DbContext dbContext, IDbConnection connection, bool wait = true)
    {
        if ((await dbContext.Database.GetPendingMigrationsAsync()).Any())
        {
            return;
        }

        while (true)
        {
            if (await _migrateLock.TryAcquireLockAsync(connection))
            {
                try
                {
                    await MigrateAsync(dbContext);
                    break;
                }
                finally
                {
                    await _migrateLock.ReleaseLockAsync(connection);
                }
            }

            if (wait)
            {
                _logger.LogWarning("Migration is locked, wait for 2 seconds");
                Thread.Sleep(20000);

                continue;
            }

            _logger.LogInformation("Migration is locked, skip");

            break;
        }
    }

    private void HandleError(DbContext dbContext, Exception e)
    {
        var needChangeList = dbContext.Database.GetPendingMigrations().ToList();
        var allChangeList = dbContext.Database.GetMigrations().ToList();
        var hasChangeList = dbContext.Database.GetAppliedMigrations().ToList();

        if (needChangeList.Count + hasChangeList.Count > allChangeList.Count)
        {
            int errIndex = allChangeList.Count - needChangeList.Count;

            if (hasChangeList.Count - 1 == errIndex && hasChangeList[errIndex] != needChangeList[0])
            {
                int index = needChangeList[0].IndexOf("_", StringComparison.Ordinal);
                string errSuffix = needChangeList[0].Substring(index, needChangeList[0].Length - index);
                if (hasChangeList[errIndex].EndsWith(errSuffix))
                {
                    dbContext.Database.ExecuteSqlRaw($"Update __EFMigrationsHistory set MigrationId = '{needChangeList[0]}' where MigrationId = '{hasChangeList[errIndex]}'");
                    dbContext.Database.Migrate();
                }
                else
                {
                    throw e;
                }
            }
            else
            {
                throw e;
            }
        }
        else
        {
            throw e;
        }

        _logger.LogInformation("Migration failed, but success on second try.");
    }

    private async Task HandleErrorAsync(DbContext dbContext, Exception e)
    {
        var needChangeList = (await dbContext.Database.GetPendingMigrationsAsync()).ToList();
        var allChangeList = dbContext.Database.GetMigrations().ToList();
        var hasChangeList = (await dbContext.Database.GetAppliedMigrationsAsync()).ToList();

        if (needChangeList.Count + hasChangeList.Count > allChangeList.Count)
        {
            int errIndex = allChangeList.Count - needChangeList.Count;

            if (hasChangeList.Count - 1 == errIndex && hasChangeList[errIndex] != needChangeList[0])
            {
                int index = needChangeList[0].IndexOf("_", StringComparison.Ordinal);
                string errSuffix = needChangeList[0].Substring(index, needChangeList[0].Length - index);
                if (hasChangeList[errIndex].EndsWith(errSuffix))
                {
                    await dbContext.Database.ExecuteSqlRawAsync($"Update __EFMigrationsHistory set MigrationId = '{needChangeList[0]}' where MigrationId = '{hasChangeList[errIndex]}'");
                    await dbContext.Database.MigrateAsync();
                }
                else
                {
                    throw e;
                }
            }
            else
            {
                throw e;
            }
        }
        else
        {
            throw e;
        }

        _logger.LogInformation("Migration failed, but success on second try.");
    }
}