/*
  For a replica set, include the replica set name and a seedlist of the members in the URI string; e.g.
  String uri = "mongodb://mongodb0.example.com:27017,mongodb1.example.com:27017/admin?replicaSet=myRepl";
  For a sharded cluster, connect to the mongos instances.
  For example:
  String uri = "mongodb://mongos0.example.com:27017,mongos1.example.com:27017:27017/admin";
 */

final MongoClient client = MongoClients.create(uri);

/*
    Create collections.
 */

client.getDatabase("mydb1").getCollection("foo")
        .withWriteConcern(WriteConcern.MAJORITY).insertOne(new Document("abc", 0));
client.getDatabase("mydb2").getCollection("bar")
        .withWriteConcern(WriteConcern.MAJORITY).insertOne(new Document("xyz", 0));

/* Step 1: Start a client session. */

final ClientSession clientSession = client.startSession();

/* Step 2: Optional. Define options to use for the transaction. */

TransactionOptions txnOptions = TransactionOptions.builder()
        .readPreference(ReadPreference.primary())
        .readConcern(ReadConcern.LOCAL)
        .writeConcern(WriteConcern.MAJORITY)
        .build();

/* Step 3: Define the sequence of operations to perform inside the transactions. */

TransactionBody txnBody = new TransactionBody<String>() {
    public String execute() {
        MongoCollection<Document> coll1 = client.getDatabase("mydb1").getCollection("foo");
        MongoCollection<Document> coll2 = client.getDatabase("mydb2").getCollection("bar");

        /*
           Important:: You must pass the session to the operations.
         */
        coll1.insertOne(clientSession, new Document("abc", 1));
        coll2.insertOne(clientSession, new Document("xyz", 999));
        return "Inserted into collections in different databases";
    }
};
try {
    /*
       Step 4: Use .withTransaction() to start a transaction,
       execute the callback, and commit (or abort on error).
    */

    clientSession.withTransaction(txnBody, txnOptions);
} catch (RuntimeException e) {
    // some error handling
} finally {
    clientSession.close();
}

Transactions and atomicity

• MongoDB supports distributed transactions, including transactions on replica sets and sharded clusters.
• 기본적으로 원자성을 보장함. 트랜잭션이 커밋되기 전까지는 변경된 데이터는 외부에 표시되지 않음.
• 그러나 샤딩의 경우 모든 쓰기작업이 끝날때까지 기다리지는 않음.
• 분산트랜잭션이 단일 문서 쓰기보다 대부분의 경우 비용이 많이 발생함
• 데이터를 적절히 모델링하면 분산 트랜잭션의 필요성을 최소화할 수 있음.

Transaction and Operations

create collections and indexes in a transaction

You can perform the following operations in a distributed transaction

if the transaction is not a cross-shard write transaction:

• create collection
  • insert/update를 통한 암시적인 생성
  • explicitly create by create operation
• create indexes on new empty collections created earlier in the same transaction

제한

• 교차 샤드 쓰기 트랜잭션에서는 새 컬렉션 생성 불가

• 한 샤드의 기존 컬렉션에 쓰고 다른 샤드에서 컬렉션을 암시적으로 생성하는 경우

• 샤드된 컬렉션을 대상으로 하는 경우 $graphLookup 을 사용할 수 없습니다.

• 트랜잭션 내에서 컬렉션 또는 인덱스를 명시적으로 생성하려면 concern 수준이 local이어야함

• 명시적 생성 commands