Transactions

Overview

In this guide, you can learn how to use the Kotlin Sync driver driver to perform transactions. Transactions allow you to run a series of operations that do not change any data until the transaction is committed. If any operation in the transaction returns an error, the driver cancels the transaction and discards all data changes before they ever become visible.

In MongoDB, transactions run within logical sessions. A session is a grouping of related read or write operations that you intend to run sequentially. Sessions enable causal consistency for a group of operations and allow you to run operations in an ACID-compliant transaction, which is a transaction that meets an expectation of atomicity, consistency, isolation, and durability. MongoDB guarantees that the data involved in your transaction operations remains consistent, even if the operations encounter unexpected errors.

When using the Kotlin Sync driver, you can create a new session from a MongoClient instance as a ClientSession type. We recommend that you reuse your MongoClient for multiple sessions and transactions instead of creating a new client each time.

Example

The following example demonstrates how to create a session, create a transaction, and insert documents into multiple collections in one transaction through the following steps:

1. Create a session from the client by using the startSession() method.

2. Use the withTransaction() method to start a transaction.

3. Insert multiple documents. The withTransaction() method runs the insert operation and commits the transaction. If any operation results in errors, withTransaction() cancels the transaction.

4. Close the connection to the server by using the client.close() method.

// Creates a new MongoClient with a connection string
val client = MongoClients.create("<connection string>")
// Gets the database and collection
val restaurantsDb = client.getDatabase("sample_restaurants")
val restaurantsCollection: MongoCollection<Document> = restaurantsDb.getCollection("restaurants")
fun insertDocuments(session: ClientSession) {
    // Sets options for the collection operation within the transaction
    val restaurantsCollectionWithOptions = restaurantsCollection
        .withWriteConcern(WriteConcern("majority"))
        .withReadConcern(ReadConcern.LOCAL)
    // Inserts documents within the transaction
    restaurantsCollectionWithOptions.insertOne(
        session,
        Document("name", "Kotlin Sync Pizza").append("cuisine", "Pizza")
    )
    restaurantsCollectionWithOptions.insertOne(
        session,
        Document("name", "Kotlin Sync Burger").append("cuisine", "Burger")
    )
}
// Starts a client session
client.startSession().use { session ->
    try {
        val txnOptions = TransactionOptions.builder()
            .readConcern(ReadConcern.LOCAL)
            .writeConcern(WriteConcern.MAJORITY)
            .build()
        // Use the withTransaction method to start a transaction and execute the lambda function
        session.withTransaction({
            insertDocuments(session)
            println("Transaction succeeded")
        }, txnOptions)
    } catch (e: Exception) {
        println("Transaction failed: ${e.message}")
    }
}
// Closes the MongoClient
client.close()

Additional Information

To learn more about the concepts mentioned in this guide, see the following pages in the Server manual:

• Transactions

• Server Sessions

• Read Isolation, Consistency, and Recency

To learn more about ACID compliance, see the What are ACID Properties in Database Management Systems? article on the MongoDB website.