本文為英文版的機器翻譯版本，如內容有任何歧義或不一致之處，概以英文版為準。

# Amazon DocumentDB 中的交易
<a name="transactions"></a>

Amazon DocumentDB （具有 MongoDB 相容性） 現在支援 MongoDB 4.0 相容性，包括交易。您可以跨多個文件、陳述式、集合和資料庫執行交易。交易可讓您在 Amazon DocumentDB 叢集內的一或多個文件上執行原子、一致、隔離和耐久 (ACID) 操作，以簡化應用程式開發。交易的常見使用案例包括財務處理、履行和管理訂單，以及建立多玩家遊戲。

交易無需額外費用。您只需支付您在交易中所使用的讀取和寫入 IOs。

**Topics**
+ [要求](#transactions-requirements)
+ [最佳實務](#transactions-best-practices)
+ [限制](#transactions-limitations)
+ [監控和診斷](#transactions-monitoring)
+ [交易隔離層級](#transactions-isolation-level)
+ [使用案例](#transactions-usecases)
+ [支援的命令](#transactions-supported-commands)
+ [不支援的功能](#transactions-unsupported-commands)
+ [工作階段](#transactions-sessions)
+ [交易錯誤](#transactions-errors)

## 要求
<a name="transactions-requirements"></a>

若要使用交易功能，您需要符合下列要求：
+ 您必須使用 Amazon DocumentDB 4.0 引擎。
+ 您必須使用與 MongoDB 4.0 或更新版本相容的驅動程式。

## 最佳實務
<a name="transactions-best-practices"></a>

以下是一些最佳實務，讓您可以充分利用與 Amazon DocumentDB 的交易。
+ 交易完成後，一律遞交或終止交易。讓交易處於未完成狀態，會與資料庫資源產生關聯，並可能導致寫入衝突。
+ 建議將交易保持在所需的最小命令數量。如果您有多個陳述式的交易可以分成多個較小的交易，建議您這樣做，以減少逾時的可能性。一律以建立短交易為目標，而非長時間執行的讀取。

## 限制
<a name="transactions-limitations"></a>
+ Amazon DocumentDB 不支援交易中的游標。
+ Amazon DocumentDB 無法在交易中建立新的集合，也無法針對不存在的集合進行查詢/更新。
+ 文件層級寫入鎖定會經過 1 分鐘的逾時，使用者無法設定。
+ Amazon DocumentDB 不支援可重試寫入、可重試遞交和可重試中止命令。如果您使用的是舊版 mongo shell （非 mongosh)，請勿在任何程式碼字串中包含 `retryWrites=false`命令。根據預設，停用可重試寫入。包含 `retryWrites=false`可能會導致正常讀取命令失敗。
+ 每個 Amazon DocumentDB 執行個體對於執行個體一次開啟的並行交易數量都有上限。如需限制，請參閱 [執行個體限制](limits.md#limits.instance)。
+ 對於指定的交易，交易日誌大小必須小於 32MB。
+ Amazon DocumentDB 確實支援交易`count()`，但並非所有驅動程式都支援此功能。另一種方法是使用 `countDocuments()` API，將計數查詢轉譯為用戶端的彙總查詢。
+ 交易有一分鐘的執行限制，工作階段有 30 分鐘的逾時。如果交易逾時，將會中止，而且在現有交易的工作階段中發出的任何後續命令都會產生下列錯誤：

  ```
  WriteCommandError({
  "ok" : 0,
  "operationTime" : Timestamp(1603491424, 627726),
  "code" : 251,
  "errmsg" : "Given transaction number 0 does not match any in-progress transactions."
  }
  ```

## 監控和診斷
<a name="transactions-monitoring"></a>

透過支援 Amazon DocumentDB 4.0 中的交易，新增了額外的 CloudWatch 指標，以協助您監控交易。

新的 CloudWatch 指標
+ `DatabaseTransactions`：在一分鐘期間內進行的開啟交易數量。
+ `DatabaseTransactionsAborted`：在一分鐘期間內進行的中止交易數量。
+ `DatabaseTransactionsMax`：一分鐘內開啟的交易數量上限。
+ `TransactionsAborted`：在一分鐘期間內在執行個體上中止的交易數量。
+ `TransactionsCommitted`：在一分鐘期間內對執行個體遞交的交易數量。
+ `TransactionsOpen`：在一分鐘期間內執行個體上開啟的交易數量。
+ `TransactionsOpenMax`：在一分鐘期間內執行個體上開啟的交易數量上限。
+ `TransactionsStarted`：在一分鐘期間內在執行個體上啟動的交易數量。

**注意**  
如需 Amazon DocumentDB 的 CloudWatch 指標，請前往 [使用 CloudWatch 監控 Amazon DocumentDB](cloud_watch.md)。

此外，新欄位已新增至`currentOp`「`idle transaction`」`lsid``transactionThreadId`和`serverStatus`交易的新狀態：`currentActive`、`currentInactive`、、`totalCommitted`、 `currentOpen` `totalAborted`和 `totalStarted`。

## 交易隔離層級
<a name="transactions-isolation-level"></a>

開始交易時，您可以同時指定 `readConcern`和 `writeConcern`，如以下範例所示：

`mySession.startTransaction({readConcern: {level: 'snapshot'}, writeConcern: {w: 'majority'}});`

對於 `readConcern`，Amazon DocumentDB 預設支援快照隔離。如果指定本機、可用或大多數`readConcern`的 ，Amazon DocumentDB 會將`readConcern`關卡升級為快照。Amazon DocumentDB 不支援線性化`readConcern`，且指定此類讀取問題將導致錯誤。

對於 `writeConcern`，Amazon DocumentDB 預設支援大多數，當資料有四個複本保留在三個可用AZs時，就會達到寫入規定人數。如果`writeConcern`指定較低版本，Amazon DocumentDB 會將 `writeConcern` 升級至大多數版本。此外，所有 Amazon DocumentDB 寫入都會進行日誌記錄，且無法停用日誌記錄。

## 使用案例
<a name="transactions-usecases"></a>

在本節中，我們將逐步介紹交易的兩個使用案例：多陳述式和多集合。

### 多陳述式交易
<a name="transactions-usecases-multistatement"></a>

Amazon DocumentDB 交易是多陳述式，這表示您可以撰寫跨越多個陳述式且具有明確遞交或轉返的交易。您可以將 `insert`、`delete`、 `update`和 `findAndModify`動作分組為單一原子操作。

多陳述式交易的常見使用案例是扣款額度交易。例如：您欠朋友衣服的錢。因此，您需要從您的帳戶扣款 （提取） 500 美元，並將 500 美元 （存款） 的額度計入朋友的帳戶。若要執行該操作，您可以在單一交易中同時執行扣款和付款操作，以確保原子性。這樣做可以防止從您的帳戶中扣除 500 美元，但不會記入朋友帳戶的情況。以下是此使用案例的外觀：

```
// *** Transfer $500 from Alice to Bob inside a transaction: Success Scenario***
// Setup bank account for Alice and Bob. Each have $1000 in their account

var databaseName = "bank";
var collectionName = "account";
var amountToTransfer = 500;

var session = db.getMongo().startSession({causalConsistency: false});   
var bankDB = session.getDatabase(databaseName);
var accountColl = bankDB[collectionName];
accountColl.drop();

accountColl.insert({name: "Alice", balance: 1000});
accountColl.insert({name: "Bob", balance: 1000});

session.startTransaction();

// deduct $500 from Alice's account
var aliceBalance = accountColl.find({"name": "Alice"}).next().balance;
var newAliceBalance = aliceBalance - amountToTransfer;
accountColl.update({"name": "Alice"},{"$set": {"balance": newAliceBalance}});
var findAliceBalance = accountColl.find({"name": "Alice"}).next().balance;

// add $500 to Bob's account
var bobBalance = accountColl.find({"name": "Bob"}).next().balance;
var newBobBalance = bobBalance + amountToTransfer;
accountColl.update({"name": "Bob"},{"$set": {"balance": newBobBalance}});
var findBobBalance = accountColl.find({"name": "Bob"}).next().balance;

session.commitTransaction();

accountColl.find();

// *** Transfer $500 from Alice to Bob inside a transaction: Failure Scenario***

// Setup bank account for Alice and Bob. Each have $1000 in their account
var databaseName = "bank";
var collectionName = "account";
var amountToTransfer = 500;

var session = db.getMongo().startSession({causalConsistency: false});   
var bankDB = session.getDatabase(databaseName);
var accountColl = bankDB[collectionName];
accountColl.drop();

accountColl.insert({name: "Alice", balance: 1000});
accountColl.insert({name: "Bob", balance: 1000});

session.startTransaction();

// deduct $500 from Alice's account
var aliceBalance = accountColl.find({"name": "Alice"}).next().balance;
var newAliceBalance = aliceBalance - amountToTransfer;
accountColl.update({"name": "Alice"},{"$set": {"balance": newAliceBalance}});
var findAliceBalance = accountColl.find({"name": "Alice"}).next().balance;

session.abortTransaction();
```

### 多集合交易
<a name="transactions-usecases-multicollection"></a>

我們的交易也是多集合，這表示它們可以用來在單一交易內和多個集合之間執行多個操作。這可提供一致的資料檢視，並維護資料的完整性。當您將命令遞交為單一 時`<>`，交易是all-or-nothing執行，其中，它們都會成功或全部失敗。

以下是多集合交易的範例，使用與多陳述式交易範例相同的案例和資料。

```
// *** Transfer $500 from Alice to Bob inside a transaction: Success Scenario***

// Setup bank account for Alice and Bob. Each have $1000 in their account
var amountToTransfer = 500;
var collectionName = "account";

var session = db.getMongo().startSession({causalConsistency: false});   
var accountCollInBankA = session.getDatabase("bankA")[collectionName];
var accountCollInBankB = session.getDatabase("bankB")[collectionName];

accountCollInBankA.drop();
accountCollInBankB.drop();

accountCollInBankA.insert({name: "Alice", balance: 1000});
accountCollInBankB.insert({name: "Bob", balance: 1000});

session.startTransaction();

// deduct $500 from Alice's account
var aliceBalance = accountCollInBankA.find({"name": "Alice"}).next().balance;
var newAliceBalance = aliceBalance - amountToTransfer;
accountCollInBankA.update({"name": "Alice"},{"$set": {"balance": newAliceBalance}});
var findAliceBalance = accountCollInBankA.find({"name": "Alice"}).next().balance;

// add $500 to Bob's account
var bobBalance = accountCollInBankB.find({"name": "Bob"}).next().balance;
var newBobBalance = bobBalance + amountToTransfer;
accountCollInBankB.update({"name": "Bob"},{"$set": {"balance": newBobBalance}});
var findBobBalance = accountCollInBankB.find({"name": "Bob"}).next().balance;

session.commitTransaction();

accountCollInBankA.find(); // Alice holds $500 in bankA
accountCollInBankB.find(); // Bob holds $1500 in bankB

// *** Transfer $500 from Alice to Bob inside a transaction: Failure Scenario***

// Setup bank account for Alice and Bob. Each have $1000 in their account
var collectionName = "account";
var amountToTransfer = 500;

var session = db.getMongo().startSession({causalConsistency: false});   
var accountCollInBankA = session.getDatabase("bankA")[collectionName];
var accountCollInBankB = session.getDatabase("bankB")[collectionName];

accountCollInBankA.drop();
accountCollInBankB.drop();

accountCollInBankA.insert({name: "Alice", balance: 1000});
accountCollInBankB.insert({name: "Bob", balance: 1000});

session.startTransaction();

// deduct $500 from Alice's account
var aliceBalance = accountCollInBankA.find({"name": "Alice"}).next().balance;
var newAliceBalance = aliceBalance - amountToTransfer;
accountCollInBankA.update({"name": "Alice"},{"$set": {"balance": newAliceBalance}});
var findAliceBalance = accountCollInBankA.find({"name": "Alice"}).next().balance;

// add $500 to Bob's account
var bobBalance = accountCollInBankB.find({"name": "Bob"}).next().balance;
var newBobBalance = bobBalance + amountToTransfer;
accountCollInBankB.update({"name": "Bob"},{"$set": {"balance": newBobBalance}});
var findBobBalance = accountCollInBankB.find({"name": "Bob"}).next().balance;

session.abortTransaction();

accountCollInBankA.find(); // Alice holds $1000 in bankA
accountCollInBankB.find(); // Bob holds $1000 in bankB
```

### 回呼 API 的交易 API 範例
<a name="transactions-usecases-code-samples"></a>

回呼 API 僅適用於 4.2\$1 驅動程式。

------
#### [ Javascript ]

下列程式碼示範如何搭配 Javascript 使用 Amazon DocumentDB 交易 API。

```
// *** Transfer $500 from Alice to Bob inside a transaction: Success ***
// Setup bank account for Alice and Bob. Each have $1000 in their account
var databaseName = "bank";
var collectionName = "account";
var amountToTransfer = 500;
 
var session = db.getMongo().startSession({causalConsistency: false});  
var bankDB = session.getDatabase(databaseName);
var accountColl = bankDB[collectionName];
accountColl.drop();
 
accountColl.insert({name: "Alice", balance: 1000});
accountColl.insert({name: "Bob", balance: 1000});
 
session.startTransaction();
 
// deduct $500 from Alice's account
var aliceBalance = accountColl.find({"name": "Alice"}).next().balance;
assert(aliceBalance >= amountToTransfer);
var newAliceBalance = aliceBalance - amountToTransfer;
accountColl.update({"name": "Alice"},{"$set": {"balance": newAliceBalance}});
var findAliceBalance = accountColl.find({"name": "Alice"}).next().balance;
assert.eq(newAliceBalance, findAliceBalance);
 
// add $500 to Bob's account
var bobBalance = accountColl.find({"name": "Bob"}).next().balance;
var newBobBalance = bobBalance + amountToTransfer;
accountColl.update({"name": "Bob"},{"$set": {"balance": newBobBalance}});
var findBobBalance = accountColl.find({"name": "Bob"}).next().balance;
assert.eq(newBobBalance, findBobBalance);
 
session.commitTransaction();
 
accountColl.find();
```

------
#### [ Node.js ]

下列程式碼示範如何搭配 Node.js 使用 Amazon DocumentDB 交易 API。

```
// Node.js callback API:
                    
const bankDB = await mongoclient.db("bank");
var accountColl = await bankDB.createCollection("account");
var amountToTransfer = 500;

const session = mongoclient.startSession({causalConsistency: false});
await accountColl.drop();

await accountColl.insertOne({name: "Alice", balance: 1000}, { session });
await accountColl.insertOne({name: "Bob", balance: 1000}, { session });

const transactionOptions = {
    readConcern: { level: 'snapshot' },
    writeConcern: { w: 'majority' } 
    };

// deduct $500 from Alice's account
var aliceBalance = await accountColl.findOne({name: "Alice"}, {session});
assert(aliceBalance.balance >= amountToTransfer);
var newAliceBalance = aliceBalance - amountToTransfer;
session.startTransaction(transactionOptions);
await accountColl.updateOne({name: "Alice"}, {$set: {balance: newAliceBalance}}, {session });
await session.commitTransaction();
aliceBalance = await accountColl.findOne({name: "Alice"}, {session});
assert(newAliceBalance == aliceBalance.balance);

// add $500 to Bob's account
var bobBalance = await accountColl.findOne({name: "Bob"}, {session});
var newBobBalance = bobBalance.balance + amountToTransfer;
session.startTransaction(transactionOptions);
await accountColl.updateOne({name: "Bob"}, {$set: {balance: newBobBalance}}, {session });
await session.commitTransaction();
bobBalance = await accountColl.findOne({name: "Bob"}, {session});
assert(newBobBalance == bobBalance.balance);
```

------
#### [ C\$1 ]

下列程式碼示範如何使用 Amazon DocumentDB 交易 API 搭配 C\$1。

```
// C# Callback API
                    
var dbName = "bank";
var collName = "account";
var amountToTransfer = 500;

using (var session = client.StartSession(new ClientSessionOptions{CausalConsistency = false}))
{
    var bankDB = client.GetDatabase(dbName);
    var accountColl = bankDB.GetCollection<BsonDocument>(collName);
    bankDB.DropCollection(collName);
    accountColl.InsertOne(session, new BsonDocument { {"name", "Alice"}, {"balance", 1000 } });
    accountColl.InsertOne(session, new BsonDocument { {"name", "Bob"}, {"balance", 1000 } });
    
    // start transaction
    var transactionOptions = new TransactionOptions(
            readConcern: ReadConcern.Snapshot,
            writeConcern: WriteConcern.WMajority);
    var result = session.WithTransaction(
        (sess, cancellationtoken) =>
        {
            // deduct $500 from Alice's account
            var aliceBalance = accountColl.Find(sess, Builders<BsonDocument>.Filter.Eq("name", "Alice")).FirstOrDefault().GetValue("balance");
            Debug.Assert(aliceBalance >= amountToTransfer);
            var newAliceBalance = aliceBalance.AsInt32 - amountToTransfer;
            accountColl.UpdateOne(sess, Builders<BsonDocument>.Filter.Eq("name", "Alice"), 
                                    Builders<BsonDocument>.Update.Set("balance", newAliceBalance));
            aliceBalance = accountColl.Find(sess, Builders<BsonDocument>.Filter.Eq("name", "Alice")).FirstOrDefault().GetValue("balance");
            Debug.Assert(aliceBalance == newAliceBalance);

            // add $500 from Bob's account
            var bobBalance = accountColl.Find(sess, Builders<BsonDocument>.Filter.Eq("name", "Bob")).FirstOrDefault().GetValue("balance");
            var newBobBalance = bobBalance.AsInt32 + amountToTransfer;
            accountColl.UpdateOne(sess, Builders<BsonDocument>.Filter.Eq("name", "Bob"), 
                                    Builders<BsonDocument>.Update.Set("balance", newBobBalance));
            bobBalance = accountColl.Find(sess, Builders<BsonDocument>.Filter.Eq("name", "Bob")).FirstOrDefault().GetValue("balance");
            Debug.Assert(bobBalance == newBobBalance);

            return "Transaction committed";
        }, transactionOptions);
   // check values outside of transaction
    var aliceNewBalance = accountColl.Find(Builders<BsonDocument>.Filter.Eq("name", "Alice")).FirstOrDefault().GetValue("balance");
    var bobNewBalance = accountColl.Find(Builders<BsonDocument>.Filter.Eq("name", "Bob")).FirstOrDefault().GetValue("balance");
    Debug.Assert(aliceNewBalance ==  500);
    Debug.Assert(bobNewBalance ==  1500);
}
```

------
#### [ Ruby ]

下列程式碼示範如何搭配 Ruby 使用 Amazon DocumentDB 交易 API。

```
// Ruby Callback API
                    
dbName = "bank"
collName = "account"
amountToTransfer = 500

session = client.start_session(:causal_consistency=> false)
bankDB = Mongo::Database.new(client, dbName)
accountColl = bankDB[collName]
accountColl.drop()

accountColl.insert_one({"name"=>"Alice", "balance"=>1000})
accountColl.insert_one({"name"=>"Bob", "balance"=>1000})
    
    # start transaction 
    session.with_transaction(read_concern: {level: :snapshot}, write_concern: {w: :majority}) do
        # deduct $500 from Alice's account
        aliceBalance = accountColl.find({"name"=>"Alice"}, :session=> session).first['balance']
        assert aliceBalance >= amountToTransfer
        newAliceBalance = aliceBalance - amountToTransfer
        accountColl.update_one({"name"=>"Alice"}, { "$set" => {"balance"=>newAliceBalance} }, :session=> session)
        aliceBalance = accountColl.find({"name"=>>"Alice"}, :session=> session).first['balance']
        assert_equal(newAliceBalance, aliceBalance)

        # add $500 from Bob's account
        bobBalance = accountColl.find({"name"=>"Bob"}, :session=> session).first['balance']
        newBobBalance = bobBalance + amountToTransfer
        accountColl.update_one({"name"=>"Bob"}, { "$set" => {"balance"=>newBobBalance} }, :session=> session)
        bobBalance = accountColl.find({"name"=>"Bob"}, :session=> session).first['balance']
        assert_equal(newBobBalance, bobBalance)
    end
   
   # check results outside of transaction
    aliceBalance = accountColl.find({"name"=>"Alice"}).first['balance']
    bobBalance = accountColl.find({"name"=>"Bob"}).first['balance']
    assert_equal(aliceBalance, 500)
    assert_equal(bobBalance, 1500)

session.end_session
```

------
#### [ Go ]

下列程式碼示範如何使用 Amazon DocumentDB 交易 API 搭配 Go。

```
// Go - Callback API
type Account struct {
    Name string
    Balance  int
}

ctx := context.TODO()

dbName := "bank"
collName := "account"
amountToTransfer := 500

session, err := client.StartSession(options.Session().SetCausalConsistency(false))
assert.NilError(t, err)
defer session.EndSession(ctx)

bankDB := client.Database(dbName)
accountColl := bankDB.Collection(collName)
accountColl.Drop(ctx)

_, err = accountColl.InsertOne(ctx, bson.M{"name" : "Alice", "balance":1000})
_, err = accountColl.InsertOne(ctx, bson.M{"name" : "Bob", "balance":1000})

transactionOptions := options.Transaction().SetReadConcern(readconcern.Snapshot()).
                            SetWriteConcern(writeconcern.New(writeconcern.WMajority()))
_, err = session.WithTransaction(ctx, func(sessionCtx mongo.SessionContext) (interface{}, error) {
    var result Account
    // deduct $500 from Alice's account
    err = accountColl.FindOne(sessionCtx, bson.M{"name": "Alice"}).Decode(&result)
    aliceBalance := result.Balance
    newAliceBalance := aliceBalance - amountToTransfer
    _, err = accountColl.UpdateOne(sessionCtx, bson.M{"name": "Alice"}, bson.M{"$set": bson.M{"balance": newAliceBalance}})
    err = accountColl.FindOne(sessionCtx, bson.M{"name": "Alice"}).Decode(&result)
    aliceBalance = result.Balance
    assert.Equal(t, aliceBalance, newAliceBalance)

    // add $500 to Bob's account
    err = accountColl.FindOne(sessionCtx, bson.M{"name": "Bob"}).Decode(&result)
    bobBalance := result.Balance
    newBobBalance := bobBalance + amountToTransfer
    _, err = accountColl.UpdateOne(sessionCtx, bson.M{"name": "Bob"}, bson.M{"$set": bson.M{"balance": newBobBalance}})
    err = accountColl.FindOne(sessionCtx, bson.M{"name": "Bob"}).Decode(&result)
    bobBalance = result.Balance
    assert.Equal(t, bobBalance, newBobBalance)

    if err != nil {
        return nil, err
    }
    return "transaction committed", err
}, transactionOptions)

// check results outside of transaction
var result Account
err = accountColl.FindOne(ctx, bson.M{"name": "Alice"}).Decode(&result)
aliceNewBalance := result.Balance
err = accountColl.FindOne(ctx, bson.M{"name": "Bob"}).Decode(&result)
bobNewBalance := result.Balance
assert.Equal(t, aliceNewBalance, 500)
assert.Equal(t, bobNewBalance, 1500)
```

------
#### [ Java ]

下列程式碼示範如何搭配 Java 使用 Amazon DocumentDB 交易 API。

```
// Java (sync) - Callback API
MongoDatabase bankDB = mongoClient.getDatabase("bank");
MongoCollection accountColl = bankDB.getCollection("account");
accountColl.drop();
int amountToTransfer = 500;

// add sample data
accountColl.insertOne(new Document("name", "Alice").append("balance", 1000));
accountColl.insertOne(new Document("name", "Bob").append("balance", 1000));

TransactionOptions txnOptions = TransactionOptions.builder()
        .readConcern(ReadConcern.SNAPSHOT)
        .writeConcern(WriteConcern.MAJORITY)
        .build();
ClientSessionOptions sessionOptions = ClientSessionOptions.builder().causallyConsistent(false).build();
try ( ClientSession clientSession = mongoClient.startSession(sessionOptions) ) {
    clientSession.withTransaction(new TransactionBody<Void>() {
        @Override
        public Void execute() {
            // deduct $500 from Alice's account
            List<Document> documentList = new ArrayList<>();
            accountColl.find(clientSession, new Document("name", "Alice")).into(documentList);
            int aliceBalance = (int) documentList.get(0).get("balance");
            int newAliceBalance = aliceBalance - amountToTransfer;

            accountColl.updateOne(clientSession, new Document("name", "Alice"), new Document("$set", new Document("balance", newAliceBalance)));

            // check Alice's new balance
            documentList = new ArrayList<>();
            accountColl.find(clientSession, new Document("name", "Alice")).into(documentList);
            int updatedBalance = (int) documentList.get(0).get("balance");
            Assert.assertEquals(updatedBalance, newAliceBalance);

            // add $500 to Bob's account
            documentList = new ArrayList<>();
            accountColl.find(clientSession, new Document("name", "Bob")).into(documentList);
            int bobBalance = (int) documentList.get(0).get("balance");
            int newBobBalance = bobBalance + amountToTransfer;

            accountColl.updateOne(clientSession, new Document("name", "Bob"), new Document("$set", new Document("balance", newBobBalance)));

            // check Bob's new balance
            documentList = new ArrayList<>();
            accountColl.find(clientSession, new Document("name", "Bob")).into(documentList);
            updatedBalance = (int) documentList.get(0).get("balance");
            Assert.assertEquals(updatedBalance, newBobBalance);

            return null;
        }
    }, txnOptions);
}
```

------
#### [ C ]

下列程式碼示範如何搭配 C 使用 Amazon DocumentDB 交易 API。

```
// Sample Code for C with Callback
                    
#include <bson.h>
#include <mongoc.h>
#include <stdio.h>
#include <string.h>
#include <assert.h>

typedef struct {
    int64_t balance;
    bson_t *account;
    bson_t *opts;
    mongoc_collection_t *collection;
} ctx_t;

bool callback_session (mongoc_client_session_t *session, void *ctx, bson_t **reply, bson_error_t *error)
{
    bool r = true;
    ctx_t *data = (ctx_t *) ctx;
    bson_t local_reply;
    bson_t *selector = data->account;
    bson_t *update = BCON_NEW ("$set", "{", "balance", BCON_INT64 (data->balance), "}");

    mongoc_collection_update_one (data->collection, selector, update, data->opts, &local_reply, error);
    
    *reply = bson_copy (&local_reply);
    bson_destroy (&local_reply);
    bson_destroy (update);
    return r;
}

void test_callback_money_transfer(mongoc_client_t* client, mongoc_collection_t* collection, int amount_to_transfer){
    
    bson_t reply;
    bool r = true;
    const bson_t *doc;
    bson_iter_t iter;
    ctx_t alice_ctx;
    ctx_t bob_ctx;
    bson_error_t error;

    // find query
    bson_t *alice_query = bson_new ();
    BSON_APPEND_UTF8(alice_query, "name", "Alice");

    bson_t *bob_query = bson_new ();
    BSON_APPEND_UTF8(bob_query, "name", "Bob");

    // create session
    // set causal consistency to false
    mongoc_session_opt_t *session_opts = mongoc_session_opts_new ();
    mongoc_session_opts_set_causal_consistency (session_opts, false);
    // start the session
    mongoc_client_session_t *client_session = mongoc_client_start_session (client, session_opts, &error);

    // add session to options
    bson_t *opts = bson_new();
    mongoc_client_session_append (client_session, opts, &error);

    // deduct 500 from Alice
    // find account balance of Alice
    mongoc_cursor_t *cursor = mongoc_collection_find_with_opts (collection, alice_query, NULL, NULL);
    mongoc_cursor_next (cursor, &doc);
    bson_iter_init (&iter, doc);
    bson_iter_find (&iter, "balance");
    int64_t alice_balance = (bson_iter_value (&iter))->value.v_int64; 
    assert(alice_balance >= amount_to_transfer);
    int64_t new_alice_balance = alice_balance - amount_to_transfer;

    // set variables which will be used by callback function
    alice_ctx.collection = collection;
    alice_ctx.opts = opts;
    alice_ctx.balance = new_alice_balance;
    alice_ctx.account = alice_query;

    // callback
    r = mongoc_client_session_with_transaction (client_session, &callback_session, NULL, &alice_ctx, &reply, &error);
    assert(r);

    // find account balance of Alice after transaction
    cursor = mongoc_collection_find_with_opts (collection, alice_query, NULL, NULL);
    mongoc_cursor_next (cursor, &doc);
    bson_iter_init (&iter, doc);
    bson_iter_find (&iter, "balance");
    alice_balance = (bson_iter_value (&iter))->value.v_int64;
    assert(alice_balance == new_alice_balance);
    assert(alice_balance == 500);

        // add 500 to bob's balance
    // find account balance of Bob
    cursor = mongoc_collection_find_with_opts (collection, bob_query, NULL, NULL);
    mongoc_cursor_next (cursor, &doc);
    bson_iter_init (&iter, doc);
    bson_iter_find (&iter, "balance");
    int64_t bob_balance = (bson_iter_value (&iter))->value.v_int64;
    int64_t new_bob_balance = bob_balance + amount_to_transfer;

    bob_ctx.collection = collection;
    bob_ctx.opts = opts;
    bob_ctx.balance = new_bob_balance;
    bob_ctx.account = bob_query;
    
    // set read & write concern
    mongoc_read_concern_t *read_concern = mongoc_read_concern_new ();
    mongoc_write_concern_t *write_concern = mongoc_write_concern_new ();
    mongoc_transaction_opt_t *txn_opts = mongoc_transaction_opts_new ();
    
    mongoc_write_concern_set_w(write_concern, MONGOC_WRITE_CONCERN_W_MAJORITY);
    mongoc_read_concern_set_level(read_concern, MONGOC_READ_CONCERN_LEVEL_SNAPSHOT);
    mongoc_transaction_opts_set_write_concern (txn_opts, write_concern);
    mongoc_transaction_opts_set_read_concern (txn_opts, read_concern);

    // callback
    r = mongoc_client_session_with_transaction (client_session, &callback_session, txn_opts, &bob_ctx, &reply, &error);
    assert(r);
	
	// find account balance of Bob after transaction
    cursor = mongoc_collection_find_with_opts (collection, bob_query, NULL, NULL);
    mongoc_cursor_next (cursor, &doc);
    bson_iter_init (&iter, doc);
    bson_iter_find (&iter, "balance");
    bob_balance = (bson_iter_value (&iter))->value.v_int64;
    assert(bob_balance == new_bob_balance);
    assert(bob_balance == 1500);

    // cleanup
    bson_destroy(alice_query);
    bson_destroy(bob_query);
    mongoc_client_session_destroy(client_session);
    bson_destroy(opts);
    mongoc_transaction_opts_destroy(txn_opts);
    mongoc_read_concern_destroy(read_concern);
    mongoc_write_concern_destroy(write_concern);
    mongoc_cursor_destroy(cursor);
    bson_destroy(doc);
}
int main(int argc, char* argv[]) {
    mongoc_init ();
    mongoc_client_t* client = mongoc_client_new (<connection uri>);
    bson_error_t error;

    // connect to bank db
    mongoc_database_t *database = mongoc_client_get_database (client, "bank");
    // access account collection
    mongoc_collection_t* collection = mongoc_client_get_collection(client, "bank", "account");
    // set amount to transfer
    int64_t amount_to_transfer = 500;
    // delete the collection if already existing
    mongoc_collection_drop(collection, &error);

    // open Alice account
    bson_t *alice_account = bson_new ();
    BSON_APPEND_UTF8(alice_account, "name", "Alice");
    BSON_APPEND_INT64(alice_account, "balance", 1000);

    // open Bob account
    bson_t *bob_account = bson_new ();
    BSON_APPEND_UTF8(bob_account, "name", "Bob");
    BSON_APPEND_INT64(bob_account, "balance", 1000);

    bool r = true;
    
    r = mongoc_collection_insert_one(collection, alice_account, NULL, NULL, &error);
    if (!r) {printf("Error encountered:%s", error.message);}
    r = mongoc_collection_insert_one(collection, bob_account, NULL, NULL, &error);
    if (!r) {printf("Error encountered:%s", error.message);}

    test_callback_money_transfer(client, collection, amount_to_transfer);

}
```

------
#### [ Python ]

下列程式碼示範如何使用 Amazon DocumentDB 交易 API 搭配 Python。

```
// Sample Python code with callback api
                    
import pymongo

def callback(session, balance, query):
    collection.update_one(query, {'$set': {"balance": balance}}, session=session)

client = pymongo.MongoClient(<connection uri>)
rc_snapshot = pymongo.read_concern.ReadConcern('snapshot')
wc_majority = pymongo.write_concern.WriteConcern('majority')

# To start, drop and create an account collection and insert balances for both Alice and Bob
collection = client.get_database("bank").get_collection("account")
collection.drop()
collection.insert_one({"_id": 1, "name": "Alice", "balance": 1000})
collection.insert_one({"_id": 2, "name": "Bob", "balance": 1000})

amount_to_transfer = 500

# deduct 500 from Alice's account
alice_balance = collection.find_one({"name": "Alice"}).get("balance")
assert alice_balance >= amount_to_transfer
new_alice_balance = alice_balance - amount_to_transfer

with client.start_session({'causalConsistency':False}) as session:
    session.with_transaction(lambda s: callback(s, new_alice_balance, {"name": "Alice"}), read_concern=rc_snapshot, write_concern=wc_majority)

updated_alice_balance = collection.find_one({"name": "Alice"}).get("balance")
assert updated_alice_balance == new_alice_balance

# add 500 to Bob's account
bob_balance = collection.find_one({"name": "Bob"}).get("balance")
assert bob_balance >= amount_to_transfer
new_bob_balance = bob_balance + amount_to_transfer

with client.start_session({'causalConsistency':False}) as session:
    session.with_transaction(lambda s: callback(s, new_bob_balance, {"name": "Bob"}), read_concern=rc_snapshot, write_concern=wc_majority)

updated_bob_balance = collection.find_one({"name": "Bob"}).get("balance")
assert updated_bob_balance == new_bob_balance
```

------

### 核心 API 的交易 API 範例
<a name="transactions-usecases-code-samples"></a>

------
#### [ Javascript ]

下列程式碼示範如何搭配 Javascript 使用 Amazon DocumentDB 交易 API。

```
// *** Transfer $500 from Alice to Bob inside a transaction: Success ***
// Setup bank account for Alice and Bob. Each have $1000 in their account
var databaseName = "bank";
var collectionName = "account";
var amountToTransfer = 500;
 
var session = db.getMongo().startSession({causalConsistency: false});  
var bankDB = session.getDatabase(databaseName);
var accountColl = bankDB[collectionName];
accountColl.drop();
 
accountColl.insert({name: "Alice", balance: 1000});
accountColl.insert({name: "Bob", balance: 1000});
 
session.startTransaction();
 
// deduct $500 from Alice's account
var aliceBalance = accountColl.find({"name": "Alice"}).next().balance;
assert(aliceBalance >= amountToTransfer);
var newAliceBalance = aliceBalance - amountToTransfer;
accountColl.update({"name": "Alice"},{"$set": {"balance": newAliceBalance}});
var findAliceBalance = accountColl.find({"name": "Alice"}).next().balance;
assert.eq(newAliceBalance, findAliceBalance);
 
// add $500 to Bob's account
var bobBalance = accountColl.find({"name": "Bob"}).next().balance;
var newBobBalance = bobBalance + amountToTransfer;
accountColl.update({"name": "Bob"},{"$set": {"balance": newBobBalance}});
var findBobBalance = accountColl.find({"name": "Bob"}).next().balance;
assert.eq(newBobBalance, findBobBalance);
 
session.commitTransaction();
 
accountColl.find();
```

------
#### [ C\$1 ]

下列程式碼示範如何使用 Amazon DocumentDB 交易 API 搭配 C\$1。

```
// C# Core API
                    
public void TransferMoneyWithRetry(IMongoCollection<bSondocument> accountColl, IClientSessionHandle session) 
{
    var amountToTransfer = 500;
    
    // start transaction
   var transactionOptions = new TransactionOptions(
                readConcern: ReadConcern.Snapshot,
                writeConcern: WriteConcern.WMajority);
    session.StartTransaction(transactionOptions);
   try
    {
        // deduct $500 from Alice's account
        var aliceBalance = accountColl.Find(session, Builders<bSondocument>.Filter.Eq("name", "Alice")).FirstOrDefault().GetValue("balance");
        Debug.Assert(aliceBalance >= amountToTransfer);
        var newAliceBalance = aliceBalance.AsInt32 - amountToTransfer;
        accountColl.UpdateOne(session, Builders<bSondocument>.Filter.Eq("name", "Alice"), 
                                Builders<bSondocument>.Update.Set("balance", newAliceBalance));
        aliceBalance = accountColl.Find(session, Builders<bSondocument>.Filter.Eq("name", "Alice")).FirstOrDefault().GetValue("balance");
        Debug.Assert(aliceBalance == newAliceBalance);

        // add $500 from Bob's account
        var bobBalance = accountColl.Find(session, Builders<bSondocument>.Filter.Eq("name", "Bob")).FirstOrDefault().GetValue("balance");
        var newBobBalance = bobBalance.AsInt32 + amountToTransfer;
        accountColl.UpdateOne(session, Builders<bSondocument>.Filter.Eq("name", "Bob"), 
                                Builders<bSondocument>.Update.Set("balance", newBobBalance));
        bobBalance = accountColl.Find(session, Builders<bSondocument>.Filter.Eq("name", "Bob")).FirstOrDefault().GetValue("balance");
        Debug.Assert(bobBalance == newBobBalance);

    }
    catch (Exception e)
    {
        session.AbortTransaction();
        throw;
    }

    session.CommitTransaction();
 }

}
public void DoTransactionWithRetry(MongoClient client)
{
    var dbName = "bank";
    var collName = "account";
    using (var session = client.StartSession(new ClientSessionOptions{CausalConsistency = false}))
    {
        try 
        {
            var bankDB = client.GetDatabase(dbName);
            var accountColl = bankDB.GetCollection<bSondocument>(collName);
            bankDB.DropCollection(collName);
            accountColl.InsertOne(session, new BsonDocument { {"name", "Alice"}, {"balance", 1000 } });
            accountColl.InsertOne(session, new BsonDocument { {"name", "Bob"}, {"balance", 1000 } });

            while(true) {
                try 
                {
                        TransferMoneyWithRetry(accountColl, session);
                        break;
                }
                catch (MongoException e) 
                {
                    if(e.HasErrorLabel("TransientTransactionError"))
                    {
                        continue;
                    }
                    else
                    {
                        throw;
                    }
                }
            }
            
            // check values outside of transaction
            var aliceNewBalance = accountColl.Find(Builders<bSondocument>.Filter.Eq("name", "Alice")).FirstOrDefault().GetValue("balance");
            var bobNewBalance = accountColl.Find(Builders<bSondocument>.Filter.Eq("name", "Bob")).FirstOrDefault().GetValue("balance");
            Debug.Assert(aliceNewBalance ==  500);
            Debug.Assert(bobNewBalance ==  1500);
        }
        catch (Exception e)
        {
            Console.WriteLine("Error running transaction: " + e.Message);
        }
    }        
}
```

------
#### [ Ruby ]

下列程式碼示範如何搭配 Ruby 使用 Amazon DocumentDB 交易 API。

```
# Ruby Core API
                    
def transfer_money_w_retry(session, accountColl)
    amountToTransfer = 500

    session.start_transaction(read_concern: {level: :snapshot}, write_concern: {w: :majority})
    # deduct $500 from Alice's account
    aliceBalance = accountColl.find({"name"=>"Alice"}, :session=> session).first['balance']
    assert aliceBalance >= amountToTransfer
    newAliceBalance = aliceBalance - amountToTransfer
    accountColl.update_one({"name"=>"Alice"}, { "$set" => {"balance"=>newAliceBalance} }, :session=> session)
    aliceBalance = accountColl.find({"name"=>"Alice"}, :session=> session).first['balance']
    assert_equal(newAliceBalance, aliceBalance)

    # add $500 to Bob's account
    bobBalance = accountColl.find({"name"=>"Bob"}, :session=> session).first['balance']
    newBobBalance = bobBalance + amountToTransfer
    accountColl.update_one({"name"=>"Bob"}, { "$set" => {"balance"=>newBobBalance} }, :session=> session)
    bobBalance = accountColl.find({"name"=>"Bob"}, :session=> session).first['balance']
    assert_equal(newBobBalance, bobBalance)

    session.commit_transaction

end

def do_txn_w_retry(client)
     dbName = "bank"
    collName = "account"
   
    session = client.start_session(:causal_consistency=> false)
    bankDB = Mongo::Database.new(client, dbName)
    accountColl = bankDB[collName]
    accountColl.drop()

    accountColl.insert_one({"name"=>"Alice", "balance"=>1000})
    accountColl.insert_one({"name"=>"Bob", "balance"=>1000})

    begin
        transferMoneyWithRetry(session, accountColl)
        puts "transaction committed"
    rescue Mongo::Error => e
        if e.label?('TransientTransactionError')
            retry
        else
            puts "transaction failed"
            raise
        end
    end

    # check results outside of transaction
    aliceBalance = accountColl.find({"name"=>"Alice"}).first['balance']
    bobBalance = accountColl.find({"name"=>"Bob"}).first['balance']
    assert_equal(aliceBalance, 500)
    assert_equal(bobBalance, 1500)
   
end
```

------
#### [ Go ]

下列程式碼示範如何使用 Amazon DocumentDB 交易 API 搭配 Go。

```
// Go - Core API
type Account struct {
    Name string
    Balance  int
}

func transferMoneyWithRetry(sessionContext mongo.SessionContext, accountColl *mongo.Collection, t *testing.T) error {
    amountToTransfer := 500

    transactionOptions := options.Transaction().SetReadConcern(readconcern.Snapshot()).
                            SetWriteConcern(writeconcern.New(writeconcern.WMajority()))
    if err := sessionContext.StartTransaction(transactionOptions); err != nil {
        panic(err)
    }

    var result Account
    // deduct $500 from Alice's account
    err := accountColl.FindOne(sessionContext, bson.M{"name": "Alice"}).Decode(&result)
    aliceBalance := result.Balance
    newAliceBalance := aliceBalance - amountToTransfer
    _, err = accountColl.UpdateOne(sessionContext, bson.M{"name": "Alice"}, bson.M{"$set": bson.M{"balance": newAliceBalance}})
    if err != nil {
        sessionContext.AbortTransaction(sessionContext)
    }
    err = accountColl.FindOne(sessionContext, bson.M{"name": "Alice"}).Decode(&result)
    aliceBalance = result.Balance
    assert.Equal(t, aliceBalance, newAliceBalance)

    // add $500 to Bob's account
    err = accountColl.FindOne(sessionContext, bson.M{"name": "Bob"}).Decode(&result)
    bobBalance := result.Balance
    newBobBalance := bobBalance + amountToTransfer
    _, err = accountColl.UpdateOne(sessionContext, bson.M{"name": "Bob"}, bson.M{"$set": bson.M{"balance": newBobBalance}})
    if err != nil {
        sessionContext.AbortTransaction(sessionContext)
    }
    err = accountColl.FindOne(sessionContext, bson.M{"name": "Bob"}).Decode(&result)
    bobBalance = result.Balance
    assert.Equal(t, bobBalance, newBobBalance)
  
    err = sessionContext.CommitTransaction(sessionContext)
    return err
}

func doTransactionWithRetry(t *testing.T) {
    ctx := context.TODO()

    dbName := "bank"
    collName := "account"
    bankDB := client.Database(dbName)
    accountColl := bankDB.Collection(collName)

    client.UseSessionWithOptions(ctx, options.Session().SetCausalConsistency(false), func(sessionContext mongo.SessionContext) error {
        accountColl.Drop(ctx)
        accountColl.InsertOne(sessionContext, bson.M{"name" : "Alice", "balance":1000})
        accountColl.InsertOne(sessionContext, bson.M{"name" : "Bob", "balance":1000})
        for {
            err := transferMoneyWithRetry(sessionContext, accountColl, t)
            if err == nil {
                println("transaction committed")
                return nil
            } 
            if mongoErr := err.(mongo.CommandError); mongoErr.HasErrorLabel("TransientTransactionError") {
                continue
            }
            println("transaction failed")
            return err
        }
    })
    
    // check results outside of transaction
    var result Account
    accountColl.FindOne(ctx, bson.M{"name": "Alice"}).Decode(&result)
    aliceBalance := result.Balance
    assert.Equal(t, aliceBalance, 500)
    accountColl.FindOne(ctx, bson.M{"name": "Bob"}).Decode(&result)
    bobBalance := result.Balance
    assert.Equal(t, bobBalance, 1500)
}
```

------
#### [ Java ]

下列程式碼示範如何搭配 Java 使用 Amazon DocumentDB 交易 API。

```
// Java (sync) - Core API
                    
public void transferMoneyWithRetry() {
   // connect to server
    MongoClientURI mongoURI = new MongoClientURI(uri);
    MongoClient mongoClient = new MongoClient(mongoURI);

    MongoDatabase bankDB = mongoClient.getDatabase("bank");
    MongoCollection accountColl = bankDB.getCollection("account");
    accountColl.drop();

   // insert some sample data
    accountColl.insertOne(new Document("name", "Alice").append("balance", 1000));
    accountColl.insertOne(new Document("name", "Bob").append("balance", 1000));

    while (true) {
        try {
            doTransferMoneyWithRetry(accountColl, mongoClient);
            break;
        } catch (MongoException e) {
            if (e.hasErrorLabel(MongoException.TRANSIENT_TRANSACTION_ERROR_LABEL)) {
                continue;
            } else {
                throw e;
            }
        }
    }
}

public void doTransferMoneyWithRetry(MongoCollection accountColl, MongoClient mongoClient) {
    int amountToTransfer = 500;

   TransactionOptions txnOptions = TransactionOptions.builder()
      .readConcern(ReadConcern.SNAPSHOT)
      .writeConcern(WriteConcern.MAJORITY)
      .build();
    ClientSessionOptions sessionOptions = ClientSessionOptions.builder().causallyConsistent(false).build();
    try ( ClientSession clientSession = mongoClient.startSession(sessionOptions) ) {
        clientSession.startTransaction(txnOptions);

        // deduct $500 from Alice's account
        List<Document> documentList = new ArrayList<>();
        accountColl.find(clientSession, new Document("name", "Alice")).into(documentList);
        int aliceBalance = (int) documentList.get(0).get("balance");
        Assert.assertTrue(aliceBalance >= amountToTransfer);
        int newAliceBalance = aliceBalance - amountToTransfer;
        accountColl.updateOne(clientSession, new Document("name", "Alice"), new Document("$set", new Document("balance", newAliceBalance)));

        // check Alice's new balance
        documentList = new ArrayList<>();
        accountColl.find(clientSession, new Document("name", "Alice")).into(documentList);
        int updatedBalance = (int) documentList.get(0).get("balance");
        Assert.assertEquals(updatedBalance, newAliceBalance);

        // add $500 to Bob's account
        documentList = new ArrayList<>();
        accountColl.find(clientSession, new Document("name", "Bob")).into(documentList);
        int bobBalance = (int) documentList.get(0).get("balance");
        int newBobBalance = bobBalance + amountToTransfer;
        accountColl.updateOne(clientSession, new Document("name", "Bob"), new Document("$set", new Document("balance", newBobBalance)));

        // check Bob's new balance
        documentList = new ArrayList<>();
        accountColl.find(clientSession, new Document("name", "Bob")).into(documentList);
        updatedBalance = (int) documentList.get(0).get("balance");
        Assert.assertEquals(updatedBalance, newBobBalance);

        // commit transaction
        clientSession.commitTransaction();
    }
}
// Java (async) -- Core API
public void transferMoneyWithRetry() {
    // connect to the server
    MongoClient mongoClient = MongoClients.create(uri);

    MongoDatabase bankDB = mongoClient.getDatabase("bank");
    MongoCollection accountColl = bankDB.getCollection("account");
    SubscriberLatchWrapper<Void> dropCallback = new SubscriberLatchWrapper<>();
    mongoClient.getDatabase("bank").drop().subscribe(dropCallback);
    dropCallback.await();

    // insert some sample data
    SubscriberLatchWrapper<InsertOneResult> insertionCallback = new SubscriberLatchWrapper<>();
    accountColl.insertOne(new Document("name", "Alice").append("balance", 1000)).subscribe(insertionCallback);
    insertionCallback.await();

    insertionCallback = new SubscriberLatchWrapper<>();
    accountColl.insertOne(new Document("name", "Bob").append("balance", 1000)).subscribe(insertionCallback);;
    insertionCallback.await();

    while (true) {
        try {
            doTransferMoneyWithRetry(accountColl, mongoClient);
            break;
        } catch (MongoException e) {
            if (e.hasErrorLabel(MongoException.TRANSIENT_TRANSACTION_ERROR_LABEL)) {
                continue;
            } else {
                throw e;
            }
        }
    }
}

public void doTransferMoneyWithRetry(MongoCollection accountColl, MongoClient mongoClient) {
    int amountToTransfer = 500;

    // start the transaction
    TransactionOptions txnOptions = TransactionOptions.builder()
            .readConcern(ReadConcern.SNAPSHOT)
            .writeConcern(WriteConcern.MAJORITY)
            .build();
    ClientSessionOptions sessionOptions = ClientSessionOptions.builder().causallyConsistent(false).build();

    SubscriberLatchWrapper<ClientSession> sessionCallback = new SubscriberLatchWrapper<>();
    mongoClient.startSession(sessionOptions).subscribe(sessionCallback);
    ClientSession session = sessionCallback.get().get(0);
    session.startTransaction(txnOptions);

    // deduct $500 from Alice's account
    SubscriberLatchWrapper<Document> findCallback = new SubscriberLatchWrapper<>();
    accountColl.find(session, new Document("name", "Alice")).first().subscribe(findCallback);
    Document documentFound = findCallback.get().get(0);
    int aliceBalance = (int) documentFound.get("balance");
    int newAliceBalance = aliceBalance - amountToTransfer;

    SubscriberLatchWrapper<UpdateResult> updateCallback = new SubscriberLatchWrapper<>();
    accountColl.updateOne(session, new Document("name", "Alice"), new Document("$set", new Document("balance", newAliceBalance))).subscribe(updateCallback);
    updateCallback.await();

    // check Alice's new balance
    findCallback = new SubscriberLatchWrapper<>();
    accountColl.find(session, new Document("name", "Alice")).first().subscribe(findCallback);
    documentFound = findCallback.get().get(0);
    int updatedBalance = (int) documentFound.get("balance");
    Assert.assertEquals(updatedBalance, newAliceBalance);

    // add $500 to Bob's account
    findCallback = new SubscriberLatchWrapper<>();
    accountColl.find(session, new Document("name", "Bob")).first().subscribe(findCallback);
    documentFound = findCallback.get().get(0);
    int bobBalance = (int) documentFound.get("balance");
    int newBobBalance = bobBalance + amountToTransfer;

    updateCallback = new SubscriberLatchWrapper<>();
    accountColl.updateOne(session, new Document("name", "Bob"), new Document("$set", new Document("balance", newBobBalance))).subscribe(updateCallback);
    updateCallback.await();

    // check Bob's new balance
    findCallback = new SubscriberLatchWrapper<>();
    accountColl.find(session, new Document("name", "Bob")).first().subscribe(findCallback);
    documentFound = findCallback.get().get(0);
    updatedBalance = (int) documentFound.get("balance");
    Assert.assertEquals(updatedBalance, newBobBalance);

    // commit the transaction
    SubscriberLatchWrapper<Void> transactionCallback = new SubscriberLatchWrapper<>();
    session.commitTransaction().subscribe(transactionCallback);
    transactionCallback.await();
}

public class SubscriberLatchWrapper<T> implements Subscriber<T> {

    /**
     * A Subscriber that stores the publishers results and provides a latch so can block on completion.
     *
     * @param <T> The publishers result type
     */
    private final List<T> received;
    private final List<RuntimeException> errors;
    private final CountDownLatch latch;
    private volatile Subscription subscription;
    private volatile boolean completed;

    /**
     * Construct an instance
     */
    public SubscriberLatchWrapper() {
        this.received = new ArrayList<>();
        this.errors = new ArrayList<>();
        this.latch = new CountDownLatch(1);
    }

    @Override
    public void onSubscribe(final Subscription s) {
        subscription = s;
        subscription.request(Integer.MAX_VALUE);
    }

    @Override
    public void onNext(final T t) {
        received.add(t);
    }

    @Override
    public void onError(final Throwable t) {
        if (t instanceof RuntimeException) {
            errors.add((RuntimeException) t);
        } else {
            errors.add(new RuntimeException("Unexpected exception", t));
        }
        onComplete();
    }

    @Override
    public void onComplete() {
        completed = true;
        subscription.cancel();
        latch.countDown();
    }

    /**
     * Get received elements
     *
     * @return the list of received elements
     */
    public List<T> getReceived() {
        return received;
    }

    /**
     * Get received elements.
     *
     * @return the list of receive elements
     */
    public List<T> get() {
        return await().getReceived();
    }

    /**
     * Await completion or error
     *
     * @return this
     */
    public SubscriberLatchWrapper<T> await() {
        subscription.request(Integer.MAX_VALUE);
        try {
            if (!latch.await(300, TimeUnit.SECONDS)) {
                throw new MongoTimeoutException("Publisher onComplete timed out for 300 seconds");
            }
        } catch (InterruptedException e) {
            throw new MongoInterruptedException("Interrupted waiting for observeration", e);
        }
        if (!errors.isEmpty()) {
            throw errors.get(0);
        }
        return this;
    }

    public boolean getCompleted() {
        return this.completed;
    }

    public void close() {
        subscription.cancel();
        received.clear();
    }
}
```

------
#### [ C ]

下列程式碼示範如何搭配 C 使用 Amazon DocumentDB 交易 API。

```
// Sample C code with core session
                    
bool core_session(mongoc_client_session_t *client_session, mongoc_collection_t* collection, bson_t *selector, int64_t balance){
    bool r = true;
    bson_error_t error;
    bson_t *opts = bson_new();
    bson_t *update = BCON_NEW ("$set", "{", "balance", BCON_INT64 (balance), "}");
    
    // set read & write concern
    mongoc_read_concern_t *read_concern = mongoc_read_concern_new ();
    mongoc_write_concern_t *write_concern = mongoc_write_concern_new ();
    mongoc_transaction_opt_t *txn_opts = mongoc_transaction_opts_new ();

    mongoc_write_concern_set_w(write_concern, MONGOC_WRITE_CONCERN_W_MAJORITY);
    mongoc_read_concern_set_level(read_concern, MONGOC_READ_CONCERN_LEVEL_SNAPSHOT);
    mongoc_transaction_opts_set_write_concern (txn_opts, write_concern);
    mongoc_transaction_opts_set_read_concern (txn_opts, read_concern);

    mongoc_client_session_start_transaction (client_session, txn_opts, &error);
    mongoc_client_session_append (client_session, opts, &error);

    r = mongoc_collection_update_one (collection, selector, update, opts, NULL, &error);

    mongoc_client_session_commit_transaction (client_session, NULL, &error);
    bson_destroy (opts);
    mongoc_transaction_opts_destroy(txn_opts);
    mongoc_read_concern_destroy(read_concern);
    mongoc_write_concern_destroy(write_concern);
    bson_destroy (update);
    return r;
}

void test_core_money_transfer(mongoc_client_t* client, mongoc_collection_t* collection, int amount_to_transfer){
    
    bson_t reply;
    bool r = true;
    const bson_t *doc;
    bson_iter_t iter;
    bson_error_t error;

    // find query
    bson_t *alice_query = bson_new ();
    BSON_APPEND_UTF8(alice_query, "name", "Alice");

    bson_t *bob_query = bson_new ();
    BSON_APPEND_UTF8(bob_query, "name", "Bob");

    // create session
    // set causal consistency to false
    mongoc_session_opt_t *session_opts = mongoc_session_opts_new ();
    mongoc_session_opts_set_causal_consistency (session_opts, false);
    // start the session
    mongoc_client_session_t *client_session = mongoc_client_start_session (client, session_opts, &error);

    // add session to options
    bson_t *opts = bson_new();
    mongoc_client_session_append (client_session, opts, &error);

    // deduct 500 from Alice
    // find account balance of Alice
    mongoc_cursor_t *cursor = mongoc_collection_find_with_opts (collection, alice_query, NULL, NULL);
    mongoc_cursor_next (cursor, &doc);
    bson_iter_init (&iter, doc);
    bson_iter_find (&iter, "balance");
    int64_t alice_balance = (bson_iter_value (&iter))->value.v_int64; 
    assert(alice_balance >= amount_to_transfer);
    int64_t new_alice_balance = alice_balance - amount_to_transfer;

    // core
    r = core_session (client_session, collection, alice_query, new_alice_balance);
    assert(r);

    // find account balance of Alice after transaction
    cursor = mongoc_collection_find_with_opts (collection, alice_query, NULL, NULL);
    mongoc_cursor_next (cursor, &doc);
    bson_iter_init (&iter, doc);
    bson_iter_find (&iter, "balance");
    alice_balance = (bson_iter_value (&iter))->value.v_int64;
    assert(alice_balance == new_alice_balance);
    assert(alice_balance == 500);

    // add 500 to Bob's balance
    // find account balance of Bob
    cursor = mongoc_collection_find_with_opts (collection, bob_query, NULL, NULL);
    mongoc_cursor_next (cursor, &doc);
    bson_iter_init (&iter, doc);
    bson_iter_find (&iter, "balance");
    int64_t bob_balance = (bson_iter_value (&iter))->value.v_int64;
    int64_t new_bob_balance = bob_balance + amount_to_transfer;

    //core
    r = core_session (client_session, collection, bob_query, new_bob_balance);
    assert(r);

    // find account balance of Bob after transaction
    cursor = mongoc_collection_find_with_opts (collection, bob_query, NULL, NULL);
    mongoc_cursor_next (cursor, &doc);
    bson_iter_init (&iter, doc);
    bson_iter_find (&iter, "balance");
    bob_balance = (bson_iter_value (&iter))->value.v_int64;
    assert(bob_balance == new_bob_balance);
    assert(bob_balance == 1500);

    // cleanup
    bson_destroy(alice_query);
    bson_destroy(bob_query);
    mongoc_client_session_destroy(client_session);
    bson_destroy(opts);
    mongoc_cursor_destroy(cursor);
    bson_destroy(doc);
}

int main(int argc, char* argv[]) {    
    mongoc_init ();
    mongoc_client_t* client = mongoc_client_new (<connection uri>);
    bson_error_t error;

    // connect to bank db
    mongoc_database_t *database = mongoc_client_get_database (client, "bank");
    // access account collection
    mongoc_collection_t* collection = mongoc_client_get_collection(client, "bank", "account");
    // set amount to transfer
    int64_t amount_to_transfer = 500;
    // delete the collection if already existing
    mongoc_collection_drop(collection, &error);

    // open Alice account
    bson_t *alice_account = bson_new ();
    BSON_APPEND_UTF8(alice_account, "name", "Alice");
    BSON_APPEND_INT64(alice_account, "balance", 1000);

    // open Bob account
    bson_t *bob_account = bson_new ();
    BSON_APPEND_UTF8(bob_account, "name", "Bob");
    BSON_APPEND_INT64(bob_account, "balance", 1000);

    bool r = true;
    
    r = mongoc_collection_insert_one(collection, alice_account, NULL, NULL, &error);
    if (!r) {printf("Error encountered:%s", error.message);}
    r = mongoc_collection_insert_one(collection, bob_account, NULL, NULL, &error);
    if (!r) {printf("Error encountered:%s", error.message);}

    test_core_money_transfer(client, collection, amount_to_transfer);

}
```

------
#### [ Scala ]

下列程式碼示範如何搭配 Scala 使用 Amazon DocumentDB 交易 API。

```
// Scala Core API
def transferMoneyWithRetry(sessionObservable: SingleObservable[ClientSession] , database: MongoDatabase ): Unit = {
    val accountColl = database.getCollection("account")
    var amountToTransfer = 500

    var transactionObservable: Observable[ClientSession] = sessionObservable.map(clientSession => {
    clientSession.startTransaction()

    // deduct $500 from Alice's account
    var aliceBalance = accountColl.find(clientSession, Document("name" -> "Alice")).await().head.getInteger("balance")
    assert(aliceBalance >= amountToTransfer)
    var newAliceBalance = aliceBalance - amountToTransfer
    accountColl.updateOne(clientSession, Document("name" -> "Alice"), Document("$set" -> Document("balance" -> newAliceBalance))).await()
    aliceBalance = accountColl.find(clientSession, Document("name" -> "Alice")).await().head.getInteger("balance")
    assert(aliceBalance == newAliceBalance)

    // add $500 to Bob's account
    var bobBalance = accountColl.find(clientSession, Document("name" -> "Bob")).await().head.getInteger("balance")
    var newBobBalance = bobBalance + amountToTransfer     
    accountColl.updateOne(clientSession, Document("name" -> "Bob"), Document("$set" -> Document("balance" -> newBobBalance))).await()
    bobBalance = accountColl.find(clientSession, Document("name" -> "Bob")).await().head.getInteger("balance")
    assert(bobBalance == newBobBalance)

    clientSession   
    })
    
    transactionObservable.flatMap(clientSession => clientSession.commitTransaction()).await()
}

def doTransactionWithRetry(): Unit = {
    val client: MongoClient = MongoClientWrapper.getMongoClient()
    val database: MongoDatabase = client.getDatabase("bank")
    val accountColl = database.getCollection("account")
    accountColl.drop().await()
    
    val sessionOptions = ClientSessionOptions.builder().causallyConsistent(false).build()
    var  sessionObservable: SingleObservable[ClientSession] = client.startSession(sessionOptions)
    accountColl.insertOne(Document("name" -> "Alice", "balance" -> 1000)).await()
    accountColl.insertOne(Document("name" -> "Bob", "balance" -> 1000)).await()
    
    var retry = true
    while (retry) {
        try {
        transferMoneyWithRetry(sessionObservable, database)
        println("transaction committed")
        retry = false
        }
        catch {
        case e: MongoException if e.hasErrorLabel(MongoException.TRANSIENT_TRANSACTION_ERROR_LABEL) => {
            println("retrying transaction") 
        }
        case other: Throwable => {
            println("transaction failed")
            retry = false
            throw other
            
        }
        }
    }
    
    // check results outside of transaction
    assert(accountColl.find(Document("name" -> "Alice")).results().head.getInteger("balance") == 500)
    assert(accountColl.find(Document("name" -> "Bob")).results().head.getInteger("balance") == 1500)
    
    accountColl.drop().await()

}
```

------
#### [ Python ]

下列程式碼示範如何使用 Amazon DocumentDB 交易 API 搭配 Python。

```
// Sample Python code with Core api

import pymongo
                        
client = pymongo.MongoClient(<connection_string>)
rc_snapshot = pymongo.read_concern.ReadConcern('snapshot')
wc_majority = pymongo.write_concern.WriteConcern('majority')
                        
# To start, drop and create an account collection and insert balances for both Alice and Bob
collection = client.get_database("bank").get_collection("account")
collection.drop()
collection.insert_one({"_id": 1, "name": "Alice", "balance": 1000})
collection.insert_one({"_id": 2, "name": "Bob", "balance": 1000})

amount_to_transfer = 500
                        
# deduct 500 from Alice's account
alice_balance = collection.find_one({"name": "Alice"}).get("balance")
assert alice_balance >= amount_to_transfer
new_alice_balance = alice_balance - amount_to_transfer
                        
with client.start_session({'causalConsistency':False}) as session:
    session.start_transaction(read_concern=rc_snapshot, write_concern=wc_majority)
    collection.update_one({"name": "Alice"}, {'$set': {"balance": new_alice_balance}}, session=session)
    session.commit_transaction()
                        
updated_alice_balance = collection.find_one({"name": "Alice"}).get("balance")
assert updated_alice_balance == new_alice_balance
                        
# add 500 to Bob's account
bob_balance = collection.find_one({"name": "Bob"}).get("balance")
assert bob_balance >= amount_to_transfer
new_bob_balance = bob_balance + amount_to_transfer
                        
with client.start_session({'causalConsistency':False}) as session:
    session.start_transaction(read_concern=rc_snapshot, write_concern=wc_majority)
    collection.update_one({"name": "Bob"}, {'$set': {"balance": new_bob_balance}}, session=session)
    session.commit_transaction()
                        
updated_bob_balance = collection.find_one({"name": "Bob"}).get("balance")
assert updated_bob_balance == new_bob_balance
```

------

## 支援的命令
<a name="transactions-supported-commands"></a>


| Command | 支援 | 
| --- | --- | 
|  `abortTransaction`  |  是  | 
|  `commitTransaction`  |  是  | 
|  `endSessions`  |  是  | 
|  `killSession`  |  是  | 
|  `killAllSession`  |  是  | 
|  `killAllSessionsByPattern`  |  否  | 
|  `refreshSessions`  |  否  | 
|  `startSession`  |  是  | 

## 不支援的功能
<a name="transactions-unsupported-commands"></a>


| 方法 | 階段或命令 | 
| --- | --- | 
|  `db.collection.aggregate()`  |  `$collStats` `$currentOp` `$indexStats` `$listSessions` `$out`  | 
|  `db.collection.count()` `db.collection.countDocuments()`  |  `$where` `$near` `$nearSphere`  | 
|  `db.collection.insert()`  |  `insert` 如果未針對現有集合執行，則不支援 。如果此方法以預先存在的集合為目標，則支援此方法。  | 

## 工作階段
<a name="transactions-sessions"></a>

MongoDB 工作階段是一種架構，用於支援可重試的寫入、因果一致性、交易和管理跨資料庫的操作。建立工作階段時，用戶端會產生邏輯工作階段識別符 (lsid)，並在傳送命令至伺服器時用來標記該工作階段中的所有操作。

Amazon DocumentDB 支援使用工作階段來啟用交易，但不支援因果一致性或可重試的寫入。

在 Amazon DocumentDB 內使用交易時，將使用 `session.startTransaction()` API 從工作階段內啟動交易，並且工作階段一次支援單一交易。同樣地，交易會使用遞交 (`session.commitTransaction()`) 或中止 (`session.abortTransaction()`) APIs 來完成。

### 因果一致性
<a name="transactions-causal-consistency"></a>

因果一致性保證在單一用戶端工作階段中，用戶端將遵守read-after-write一致性、單原子讀取/寫入和寫入將遵循讀取，而這些保證適用於叢集中的所有執行個體，而不只是主要執行個體。Amazon DocumentDB 不支援因果一致性，下列陳述式將導致錯誤。

```
var mySession = db.getMongo().startSession();
var mySessionObject = mySession.getDatabase('test').getCollection('account');
 
mySessionObject.updateOne({"_id": 2}, {"$inc": {"balance": 400}});
//Result:{ "acknowledged" : true, "matchedCount" : 1, "modifiedCount" : 1 }
 
mySessionObject.find()
//Error: error: {
//         "ok" : 0,
//         "code" : 303,
//         "errmsg" : "Feature not supported: 'causal consistency'",
//         "operationTime" : Timestamp(1603461817, 493214)
//}
 
mySession.endSession()
```

您可以在工作階段中停用因果一致性。請注意，這樣做可讓您利用工作階段架構，但不會為讀取提供因果一致性保證。使用 Amazon DocumentDB 時，主要 的讀取會是read-after-write一致，而複本執行個體的讀取最終會一致。交易是使用工作階段的主要使用案例。

```
var mySession = db.getMongo().startSession({causalConsistency: false});
var mySessionObject = mySession.getDatabase('test').getCollection('account');
 
mySessionObject.updateOne({"_id": 2}, {"$inc": {"balance": 400}});
//Result:{ "acknowledged" : true, "matchedCount" : 1, "modifiedCount" : 1 }
 
mySessionObject.find()
//{ "_id" : 1, "name" : "Bob", "balance" : 100 }
//{ "_id" : 2, "name" : "Alice", "balance" : 1700 }
```

### 可重試寫入
<a name="transactions-retryable-writes"></a>

可重試寫入是用戶端嘗試重試寫入操作一次、發生網路錯誤或用戶端找不到主要操作的功能。在 Amazon DocumentDB 中，不支援可重試的寫入，且必須停用。您可以使用連線字串中的 命令 (`retryWrites=false`) 來停用它。

**注意**  
如果您使用的是舊版 mongo shell （非 mongosh)，請勿在任何程式碼字串中包含 `retryWrites=false`命令。根據預設，停用可重試寫入。包含 `retryWrites=false`可能會導致正常讀取命令失敗。

## 交易錯誤
<a name="transactions-errors"></a>

使用交易時，有些案例可能會產生錯誤，指出交易號碼不符合任何進行中的交易。

錯誤可以在至少兩種不同的案例中產生：
+ 一分鐘交易逾時之後。
+ 執行個體重新啟動後 （由於修補、當機復原等），即使交易成功遞交，也可能收到此錯誤。在執行個體重新啟動期間，資料庫無法分辨成功完成的交易與中止的交易之間的差異。換言之，交易完成狀態不明確。

處理此錯誤的最佳方式是使交易更新具有等冪性，例如，使用 mutator `$set` 而非增量/減量操作。請參閱以下內容：

```
{ "ok" : 0,
"operationTime" : Timestamp(1603938167, 1), 
"code" : 251,
"errmsg" : "Given transaction number 1 does not match any in-progress transactions." 
}
```