Session.startTransaction() (mongosh method方法)

Definition定义

Session.startTransaction(<options>)

Starts a multi-document transaction associated with the session. At any given time, you can have at most one open transaction for a session.启动与会话关联的多文档事务。在任何给定时间，会话最多只能有一个打开的事务。

Note

This operation is a no-op. The transaction won't start on the server until the first command is sent on the session. Therefore, the snapshot time of the transaction won't be set until the first command is sent on the session.此操作是无操作。在会话上发送第一个命令之前，事务不会在服务器上启动。因此，在会话上发送第一个命令之前，不会设置事务的快照时间。

Multi-document transactions are available for both sharded clusters and replica sets.多文档事务可用于分片集群和副本集。

Important

Within a transaction, you can only specify read and write (CRUD) operations on existing collections. For example, a multi-document transaction cannot include an insert operation that would result in the creation of a new collection.在事务中，您只能对现有集合指定读写（CRUD）操作。例如，多文档事务不能包含会导致创建新集合的插入操作。

The Session.startTransaction() method can take a document with the following options:Session.startTransaction()方法可以接受具有以下选项的文档：

{ readConcern: { level: <level>}, writeConcern: { w: <value>, j: <boolean>, wtimeout: <number> } }

Option选项	Description描述
readConcern	Optional. A document that specifies the read concern for all operations in the transaction, overriding operation-specific read concern.可选。一个文档，指定事务中所有操作的读取关注，覆盖特定于操作的读取关心点。 You can specify one of the following read concern levels:您可以指定以下读取关注级别之一： "snapshot" "local" "majority" For "local" and "majority" read concern, MongoDB may sometimes substitute a stronger read concern.对于"local"和"majority"读取关注，MongoDB有时可能会用更强的读取关注代替。
writeConcern	Optional. A document that specifies the write concern for the transaction. This write concern applies to the transaction commit and abort operations.可选。指定事务写入关注的文档。此写入关注适用于事务提交和中止操作。 The operations within the transaction use "w: 1", overriding operation-specific write concern.事务中的操作使用"w: 1"，覆盖特定于操作的写入关注。 If you commit using "w: 1" write concern, your transaction can be rolled back during the failover process.如果使用"w: 1"写关注提交，则可以在故障转移过程中回滚事务。 For MongoDB Drivers, transactions use the client-level write concern as the default.对于MongoDB驱动程序，事务使用客户端级写入关注作为默认值。

Compatibility兼容性

This method is available in deployments hosted in the following environments:此方法在以下环境中托管的部署中可用：

• MongoDB Atlas: The fully managed service for MongoDB deployments in the cloud：云中MongoDB部署的完全托管服务

• MongoDB Enterprise: The subscription-based, self-managed version of MongoDB：MongoDB的基于订阅的自我管理版本
• MongoDB Community: The source-available, free-to-use, and self-managed version of MongoDB：MongoDB的源代码可用、免费使用和自我管理版本

Behavior行为

Example示例

Consider a scenario where as changes are made to an employee's record in the hr database, you want to ensure that the events collection in the reporting database are in sync with the hr changes. 考虑一种情况，当对hr数据库中的员工记录进行更改时，您希望确保reporting数据库中的events集合与hr更改同步。That is, you want to ensure that these writes are done as a single transaction, such that either both operations succeed or fail.也就是说，您希望确保这些写入作为单个事务完成，以便两个操作都成功或失败。

The employees collection in the hr database has the following documents:hr数据库中的employees集合包含以下文档：

{ "_id" : ObjectId("5af0776263426f87dd69319a"), "employee" : 3, "name" : { "title" : "Mr.", "name" : "Iba Ochs" }, "status" : "Active", "department" : "ABC" }
{ "_id" : ObjectId("5af0776263426f87dd693198"), "employee" : 1, "name" : { "title" : "Miss", "name" : "Ann Thrope" }, "status" : "Active", "department" : "ABC" }
{ "_id" : ObjectId("5af0776263426f87dd693199"), "employee" : 2, "name" : { "title" : "Mrs.", "name" : "Eppie Delta" }, "status" : "Active", "department" : "XYZ" }

The events collection in the reporting database has the following documents:reporting数据库中的events集合包含以下文档：

{ "_id" : ObjectId("5af07daa051d92f02462644a"), "employee" : 1, "status" : { "new" : "Active", "old" : null }, "department" : { "new" : "ABC", "old" : null } }
{ "_id" : ObjectId("5af07daa051d92f02462644b"), "employee" : 2, "status" : { "new" : "Active", "old" : null }, "department" : { "new" : "XYZ", "old" : null } }
{ "_id" : ObjectId("5af07daa051d92f02462644c"), "employee" : 3, "status" : { "new" : "Active", "old" : null }, "department" : { "new" : "ABC", "old" : null } }

The following example opens a transaction, updates an employee's status to Inactive in the employees status and inserts a corresponding document to the events collection, and commits the two operations as a single transaction.以下示例打开一个事务，将employees状态更新为Inactive，并将相应的文档插入到events集合中，然后将这两个操作作为单个事务提交。

// Runs the txnFunc and retries if TransientTransactionError encountered

function runTransactionWithRetry(txnFunc, session) {
    while (true) {
        try {
            txnFunc(session);  // performs transaction
            break;
        } catch (error) {
            // If transient error, retry the whole transaction
            if (error?.errorLabels?.includes("TransientTransactionError")  ) {
                print("TransientTransactionError, retrying transaction ...");
                continue;
            } else {
                throw error;
            }
        }
    }
}

// Retries commit if UnknownTransactionCommitResult encountered

function commitWithRetry(session) {
    while (true) {
        try {
            session.commitTransaction(); // Uses write concern set at transaction start.
            print("Transaction committed.");
            break;
        } catch (error) {
            // Can retry commit
            if (error?.errorLabels?.includes("UnknownTransactionCommitResult") ) {
                print("UnknownTransactionCommitResult, retrying commit operation ...");
                continue;
            } else {
                print("Error during commit ...");
                throw error;
            }
       }
    }
}

// Updates two collections in a transactions

function updateEmployeeInfo(session) {
    employeesCollection = session.getDatabase("hr").employees;
    eventsCollection = session.getDatabase("reporting").events;

    session.startTransaction( { readConcern: { level: "snapshot" }, writeConcern: { w: "majority" } } );

    try{
        employeesCollection.updateOne( { employee: 3 }, { $set: { status: "Inactive" } } );
        eventsCollection.insertOne( { employee: 3, status: { new: "Inactive", old: "Active" } } );
    } catch (error) {
        print("Caught exception during transaction, aborting.");
        session.abortTransaction();
        throw error;
    }

    commitWithRetry(session);
}

// Start a session.
session = db.getMongo().startSession( { readPreference: { mode: "primary" } } );

try{
   runTransactionWithRetry(updateEmployeeInfo, session);
} catch (error) {
   // Do something with error
} finally {
   session.endSession();
}