AWS News Blog

New – Amazon DynamoDB Transactions

March 13, 2020: Post updated to clarify how to use transactions with global tables and the increase in the maximum number of items per transaction from 10 to 25.

Over the years, customers have used Amazon DynamoDB for lots of different use cases, from building microservices and mobile backends to implementing gaming and Internet of Things (IoT) solutions. For example, Capital One uses DynamoDB to reduce the latency of their mobile applications by moving their mainframe transactions to a serverless architecture. Tinder migrated user data to DynamoDB with zero downtime, to get the scalability they need to support their global user base.

Developers sometimes need to implement business logic that requires multiple, all-or-nothing operations across one or more tables. This requirement can add unnecessary complexity to their implementation. Today, we are making these use cases easier to build on DynamoDB with native support for transactions!

Introducing Amazon DynamoDB Transactions
DynamoDB transactions provide developers atomicity, consistency, isolation, and durability (ACID) across one or more tables within a single AWS account and region. You can use transactions when building applications that require coordinated inserts, deletes, or updates to multiple items as part of a single logical business operation.

Transactions bring the scale, performance, and enterprise benefits of DynamoDB to a broader set of workloads. Many use cases are easier and faster to implement using transactions, for example:

  • Processing financial transactions
  • Fulfilling and managing orders
  • Building multiplayer game engines
  • Coordinating actions across distributed components and services

Two new DynamoDB operations have been introduced for handling transactions:

  • TransactWriteItems, a batch operation that contains a write set, with one or more PutItem, UpdateItem, and DeleteItem operations. TransactWriteItems can optionally check for prerequisite conditions that must be satisfied before making updates. These conditions may involve the same or different items than those in the write set. If any condition is not met, the transaction is rejected.
  • TransactGetItems, a batch operation that contains a read set, with one or more GetItem operations. If a TransactGetItems request is issued on an item that is part of an active write transaction, the read transaction is canceled. To get the previously committed value, you can use a standard read.

Each transaction can include up to 25 unique items or up to 4 MB of data, including conditions.

With this new feature, DynamoDB offers multiple read and write options to meet different application requirements, providing huge flexibility to developers implementing complex, data-driven business logic:

  • Three options for reads—eventual consistency, strong consistency, and transactional.
  • Two for writes—standard and transactional.

For example, imagine you are building a game where players can buy items with virtual coins:

  • In the players table, each player has a number of coins and an inventory of purchased items.
  • In the items table, each item has a price and is marked as available (or not) with a Boolean value.

To purchase an item, you can now implement a single atomic transaction:

  1. First, check that the item is available and the player has the necessary coins.
  2. If those conditions are satisfied, the item is marked as not available and owned by the player.
  3. The purchased item is then added to the player inventory list.

In JavaScript, using the AWS SDK for JavaScript in Node.js, you have code similar to this:

data = await dynamoDb.transactWriteItems({
    TransactItems: [
        {
            Update: {
                TableName: 'items',
                Key: { id: { S: itemId } },
                ConditionExpression: 'available = :true',
                UpdateExpression: 'set available = :false, ' +
                    'ownedBy = :player',
                ExpressionAttributeValues: {
                    ':true': { BOOL: true },
                    ':false': { BOOL: false },
                    ':player': { S: playerId }
                }
            }
        },
        {
            Update: {
                TableName: 'players',
                Key: { id: { S: playerId } },
                ConditionExpression: 'coins >= :price',
                UpdateExpression: 'set coins = coins - :price, ' +
                    'inventory = list_append(inventory, :items)',
                ExpressionAttributeValues: {
                    ':items': { L: [{ S: itemId }] },
                    ':price': { N: itemPrice.toString() }
                }
            }
        }
    ]
}).promise();

You can also use transactions with the simplified, higher-level programming interfaces for DynamoDB provided by AWS SDKs. For example:

  • DocumentClient in the AWS JavaScript SDK with transactGet and transactWrite;
  • DynamoDBMapper in the AWS Java SDK with transactionWrite and transactionLoad.

Using Transactions
Items are not locked during a transaction. DynamoDB transactions provide serializable isolation. If an item is modified outside of a transaction while the transaction is in progress, the transaction is canceled and an exception is thrown with details about which item or items caused the exception.

Transactional operations work within one or more tables within a region, and are not supported across regions in global tables. To use transactional APIs with global tables, please review the limitations in our developer guide.

When creating an AWS Identity and Access Management (IAM) policy, you should give ConditionCheck permissions to check for prerequisite conditions that must be satisfied before making transactional updates. Existing DynamoDB UpdateItem, PutItem, DeleteItem, and GetItem actions authorize the use of those operations also within transactions. For example, if an IAM user has only PutItem permission, they can send a transaction with one or more put, but if they add a delete to the write set, it will get rejected because they do not have DeleteItem permission.

Pricing, Monitoring, and Availability
There is no additional cost to enable transactions for DynamoDB tables. You only pay for the reads or writes that are part of your transaction. DynamoDB performs two underlying reads or writes of every item in the transaction, one to prepare the transaction and one to commit the transaction. The two underlying read/write operations are visible in your Amazon CloudWatch metrics. You should plan your costs, capacity, and performance needs assuming each transactional read performs two reads and each transactional write performs two writes.

DynamoDB transactions are available globally in all commercial regions.

More information on transactions is available in the DynamoDB developer guide.

I am really intrigued by these new capabilities. Please let me know what you are going to use them for!

Danilo Poccia

Danilo Poccia

Danilo works with startups and companies of any size to support their innovation. In his role as Chief Evangelist (EMEA) at Amazon Web Services, he leverages his experience to help people bring their ideas to life, focusing on serverless architectures and event-driven programming, and on the technical and business impact of machine learning and edge computing. He is the author of AWS Lambda in Action from Manning.