Session.startTransaction()

Definition定义

Session.startTransaction(<options>)

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

Changed in version 4.2.4.2版更改。Starting in MongoDB 4.2, multi-document transactions are available for both sharded clusters and replica sets.从MongoDB 4.2开始，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. 在事务中，只能对现有集合指定读写（CRUD）操作。For example, a multi-document transaction cannot include an insert operation that would result in the creation of a new collection.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()方法可以使用以下选项获取文档：

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

Option选项	Description描述
readConcern	Optional. 可选的。A document that specifies the read concern for all operations in the transaction, overriding operation-specific read concern.为事务中的所有操作指定读取关注的文档，覆盖特定于操作的读取关注。 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有时可能会取代更强的读取关注。
writeConcern	Optional. 可选的。A document that specifies the write concern for the transaction. 指定事务的写入关注的文档。This write concern applies to the transaction commit and abort operations.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.如果您使用"w: 1"写入关注进行提交，那么您的事务可以在故障转移过程中回滚。 For MongoDB Drivers, transactions use the client-level write concern as the default. 对于MongoDB驱动程序，事务默认使用客户端级别的写入关注。

Behavior行为

Example实例

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. 考虑这样一种情况：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是作为单个事务完成的，这样两个操作要么成功，要么失败。

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，在employees状态中将员工的状态更新为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如果遇到TransientTransactionError，则运行txnFunc并重试

function runTransactionWithRetry(txnFunc, session) {
    while (true) {
        try {
            txnFunc(session);  // performs transaction执行事务
            break;
        } catch (error) {
            // If transient error, retry the whole transaction如果出现暂时错误，请重试整个事务
            if ( error.hasOwnProperty("errorLabels") && error.errorLabels.includes("TransientTransactionError")  ) {
                print("TransientTransactionError, retrying transaction ...");
                continue;
            } else {
                throw error;
            }
        }
    }
}

// Retries commit if UnknownTransactionCommitResult encountered如果遇到UnknownTransactionCommitResult，则重试提交

function commitWithRetry(session) {
    while (true) {
        try {
            session.commitTransaction(); // Uses write concern set at transaction start.在事务开始时使用写入关注集。
            print("Transaction committed.");
            break;
        } catch (error) {
            // Can retry commit
            if (error.hasOwnProperty("errorLabels") && error.errorLabels.includes("UnknownTransactionCommitResult") ) {
                print("UnknownTransactionCommitResult, retrying commit operation ...");
                continue;
            } else {
                print("Error during commit ...");
                throw error;
            }
       }
    }
}

// Updates two collections in a transactions更新事务记录中的两个集合

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.启动会话。
session = db.getMongo().startSession( { readPreference: { mode: "primary" } } );

try{
   runTransactionWithRetry(updateEmployeeInfo, session);
} catch (error) {
   // Do something with error做一些错误的事情
} finally {
   session.endSession();
}