AWS Developer Tools Blog

Using Improved Conditional Writes in DynamoDB

Last month the Amazon DynamoDB team announced a new pair of features: Improved Query Filtering and Conditional Updates.  In this post, we’ll show how to use the new and improved conditional writes feature of DynamoDB to speed up your app.

Let’s say you’re building a racing game, where two players advance in position until they reach the finish line.  To manage the state in DynamoDB, each game could be stored in its own Item in DynamoDB, in a Game table with GameId as the primary key, and each player position stored in a different attribute.  Here’s an example of what a Game item could look like:

        "GameId": "abc",
        "Status": "IN_PROGRESS",
        "Player1-Position": 0,
        "Player2-Position": 0

To make players move, you can use the atomic counters feature of DynamoDB in the UpdateItem API to send requests like, “increase the player position by 1, regardless of its current value”.  To prevent players from advancing before the game starts, you can use conditional writes to make the same request as before, but only “as long as the game status is IN_PROGRESS.”  Conditional writes are a way of instructing DynamoDB to perform a given write request only if certain attribute values in the item match what you expect them to be at the time of the request.

But this isn’t the whole story.  How do you determine the winner of the game, and prevent players from moving once the game is over?  In other words, we need a way to atomically make it so that all players stop once one reaches the end of the race (no ties allowed!).

This is where the new improved conditional writes come in handy.  Before, the conditional writes feature supported tests for equality (attribute “x” equals “20”).  With improved conditions, DynamoDB supports tests for inequality (attribute “x” is less than “20”).  This is useful for the game application, because now the request can be, “increase the player position by 1 as long as the status of the game equals IN_PROGRESS, and the positions of player 1 and player 2 are less than 20.”  During player movement, one player will eventually reach the finish line first, and any future moves after that will be blocked by the conditional writes.  Here’s the code:

    public static void main(String[] args) {

        // To run this example, first initialize the client, and create a table
        // named 'Game' with a primary key of type hash / string called 'GameId'.
        AmazonDynamoDB dynamodb; // initialize the client
        try {
            // First set up the example by inserting a new item
            // To see different results, change either player's
            // starting positions to 20, or set player 1's location to 19.
            Integer player1Position = 15;
            Integer player2Position = 12;
            dynamodb.putItem(new PutItemRequest()
                    .addItemEntry("GameId", new AttributeValue("abc"))
                        new AttributeValue().withN(player1Position.toString()))
                        new AttributeValue().withN(player2Position.toString()))
                    .addItemEntry("Status", new AttributeValue("IN_PROGRESS")));
            // Now move Player1 for game "abc" by 1,
            // as long as neither player has reached "20".
            UpdateItemResult result = dynamodb.updateItem(new UpdateItemRequest()
                .addKeyEntry("GameId", new AttributeValue("abc"))
                     "Player1-Position", new AttributeValueUpdate()
                         .withValue(new AttributeValue().withN("1"))
                     "Player1-Position", new ExpectedAttributeValue()
                         .withValue(new AttributeValue().withN("20"))
                     "Player2-Position", new ExpectedAttributeValue()
                         .withValue(new AttributeValue().withN("20"))
                     "Status", new ExpectedAttributeValue()
                         .withValue(new AttributeValue().withS("IN_PROGRESS"))
            if ("20".equals(result.getAttributes().get("Player1-Position").getN())) {
                System.out.println("Player 1 wins!");
            } else {
                System.out.println("The game is still in progress: "
                    + result.getAttributes());
        } catch (ConditionalCheckFailedException e) {
            System.out.println("Failed to move player 1 because the game is over");

With this algorithm, player movement now takes only one write operation to DynamoDB.  What would it have taken without improved conditions?  Using only equality conditions, the app would have needed to follow the read-modify-write pattern:

  1. Read each item, making note of each player’s position, and verify that neither player already reached the end of the race.
  2. Advance the player’s position by 1, with a condition that both players were still in the position we read in step 1).

Notice that this algorithm requires two round-trips to DynamoDB, whereas with improved conditions, it can be done in only one round-trip.  This reduces both latency and cost.

You can find more information about conditional writes in Amazon DynamoDB in the Developer Guide.