MongoDB 事务，回调 API 方式

在阅读了许多有关 mongoDB 事务的文档/文章后，我仍然需要进一步澄清。

这里说：

MonGoDB provides two apis to use transactions. The first is the core API which has similar syntax to relational databases. The second, the callback API, is the recommended approach to using transactions in MongoDB.

但它继续介绍了核心 API 方法，而根本没有触及推荐的方法。

这里的官方文档说，

This example highlights the key components of the transactions API. In particular, it uses the callback API. The callback API:

starts a transaction
executes the specified operations
commits the result (or aborts on error)

但是，当谈到最重要的“执行指定操作”步骤时，该示例没有显示任何相关代码。即，正如 MongoDB Transactions In nodejs 中所问的，我“仍然需要一个真实的示例”。

// WithTransactionExample is an example of using the Session.WithTransaction function.
func WithTransactionExample(ctx context.Context) error {
    // For a replica set, include the replica set name and a seedlist of the members in the URI string; e.g.
    // uri := "mongodb://mongodb0.example.com:27017,mongodb1.example.com:27017/?replicaSet=myRepl"
    // For a sharded cluster, connect to the mongos instances; e.g.
    // uri := "mongodb://mongos0.example.com:27017,mongos1.example.com:27017/"
    uri := mtest.ClusterURI()
    clientOpts := options.Client().ApplyURI(uri)
    client, err := mongo.Connect(ctx, clientOpts)
    if err != nil {
        return err
    }
    defer func() { _ = client.Disconnect(ctx) }()
    // Prereq: Create collections.
    wcMajority := writeconcern.Majority()
    wcMajority.WTimeout = 1 * time.Second
    wcMajorityCollectionOpts := options.Collection().SetWriteConcern(wcMajority)
    fooColl := client.Database("mydb1").Collection("foo", wcMajorityCollectionOpts)
    barColl := client.Database("mydb1").Collection("bar", wcMajorityCollectionOpts)
    // Step 1: Define the callback that specifies the sequence of operations to perfORM inside the transaction.
    callback := func(sessCtx mongo.SessionContext) (interface{}, error) {
        // Important: You must pass sessCtx as the Context parameter to the operations for them to be executed in the
        // transaction.
        if _, err := fooColl.InsertOne(sessCtx, bson.D{{"abc", 1}}); err != nil {
            return nil, err
        }
        if _, err := barColl.InsertOne(sessCtx, bson.D{{"xyz", 999}}); err != nil {
            return nil, err
        }
        return nil, nil
    }
    // Step 2: Start a session and run the callback using WithTransaction.
    session, err := client.StartSession()
    if err != nil {
        return err
    }
    defer session.EndSession(ctx)
    result, err := session.WithTransaction(ctx, callback)
    if err != nil {
        return err
    }
    log.Printf("result: %v\n", result)
    return nil
}

从示例中我看来，最重要的“执行指定操作”步骤是在回调中完成的。是这样吗？如果是这样，官方文档确实需要强调这一点。

正确答案

示例已完成。它包含以下关键评论：

// Step 2: Start a session and run the callback using WithTransaction.

所以callback函数是由Session执行的.WithTransaction() 方法。你将 callback 函数传递给它，它将被 Session.WithTransaction() 方法调用。

该实现确保回调函数中完成的操作将作为事务执行（即，要么应用所有操作，要么不应用任何操作）。如果回调函数返回非nil错误，则事务将被中止，否则事务将被提交。

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