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Session.commitTransaction()
New in version 4.0.在版本4.0中新增。
Saves the changes made by the operations in the multi-document transaction and ends the transaction.保存多文档事务中操作所做的更改并结束事务。
When commiting the transaction, the session uses the write concern specified at the transaction start. 提交事务时，会话使用在事务开始时指定的写关注点。See 请参阅Session.startTransaction().Session.startTransaction()。
If you commit using 如果使用"w: 1" write concern, your transaction can be rolled back during the failover process."w: 1"写关注点进行提交，则可以在故障转移过程中回滚事务。
When a transaction commits, all data changes made in the transaction are saved and visible outside the transaction. 当事务提交时，事务中所做的所有数据更改都将保存并在事务外部可见。That is, a transaction will not commit some of its changes while rolling back others.也就是说，事务不会提交其某些更改，而回滚其他更改。
Until a transaction commits, the data changes made in the transaction are not visible outside the transaction.在事务提交之前，在事务中所做的数据更改在事务外部不可见。
However, when a transaction writes to multiple shards, not all outside read operations need to wait for the result of the committed transaction to be visible across the shards. 然而，当事务写入多个分片时，并非所有外部读取操作都需要等待提交的事务的结果在分片中可见。For example, if a transaction is committed and write 1 is visible on shard A but write 2 is not yet visible on shard B, an outside read at read concern 例如，如果事务已提交，并且写1在分片a上可见，但写2在分片B上尚不可见，则外部读取关注点"local" can read the results of write 1 without seeing write 2."local"可以读取写1的结果，而不会看到写2。
If the commit operation encounters an error, MongoDB drivers retry the commit operation a single time regardless of whether 如果提交操作遇到错误，无论retryWrites is set to false. retryWrites是否设置为false，MongoDB驱动程序都会重试一次提交操作。For more information, see Transaction Error Handling.有关更多信息，请参阅事务错误处理。
Consider a scenario where as changes are made to an employee's record in the 考虑这样一个场景：当对hr database, you want to ensure that the events collection in the reporting database are in sync with the hr changes. 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，您希望确保这些写操作作为单个事务完成，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 以下示例打开一个事务，在Inactive in the employees status and inserts a corresponding document to the events collection, and commits the two operations as a single transaction.employees状态中将员工的状态更新为Inactive，并将相应的文档插入events集合，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.hasOwnProperty("errorLabels") && 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.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 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(); }