Boost Mew's Stats

What you'll learn

Race conditions occur when parallel updates overwrite each other.
The ADD action atomically increments numeric values in DynamoDB.
Atomic increments are not idempotent: retrying an uncertain write can count twice. Use deduplication or a transaction when exact counts matter.

Section 01

The Race Condition Problem

Imagine thousands of players battling at once. A traditional read-modify-write pattern is dangerous:

Read current stats (Wins: 10)
Application adds 1 (Wins: 11)
Write back (Wins: 11)

Ready · 16 steps · Orbi

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All steps as text

Trainer Ash sends GetItem to the Pokemon table with Name = "Mew". Read the counter first.
DynamoDB hashes "Mew" to 416a585c, which routes the request to partition P0.
P0 returns Mew (BattlesWon 0).
Trainer Gary sends GetItem to the Pokemon table with Name = "Mew". Reads before A has written.
DynamoDB hashes "Mew" to 416a585c, which routes the request to partition P0.
P0 returns Mew (BattlesWon 0).
Trainer Ash sends UpdateItem to the Pokemon table with Name = "Mew". Writes back 0 + 1: SET BattlesWon = :wins
DynamoDB hashes "Mew" to 416a585c, which routes the request to partition P0.
P0 updates Mew (BattlesWon = 1).
This call consumed 1 WCU.
DynamoDB responds with success: Ash's write succeeded.
Trainer Gary sends UpdateItem to the Pokemon table with Name = "Mew". Also writes back 0 + 1: SET BattlesWon = :wins
DynamoDB hashes "Mew" to 416a585c, which routes the request to partition P0.
Trainer Gary's write to Mew lands on P0 and silently replaces the other change.
This call consumed 1 WCU.
DynamoDB responds with success: two wins recorded, but BattlesWon shows 1.

Section 02

Atomic Counters to the Rescue

DynamoDB supports atomic updates. You tell it "Add 1 to this attribute" rather than "Set this attribute to 11". Each update is atomic, so concurrent increments do not overwrite one another. A timeout can still leave the outcome uncertain; blindly retrying can increment again.

Atomic Increment ExampleJavaScript

import { DynamoDBClient } from "@aws-sdk/client-dynamodb";
import { DynamoDBDocumentClient, UpdateCommand } from "@aws-sdk/lib-dynamodb";

const client = new DynamoDBClient({ region: "ap-south-1" });
const docClient = DynamoDBDocumentClient.from(client);

await docClient.send(new UpdateCommand({
  TableName: "Pokemon",
  Key: { Name: "Mew" },
  UpdateExpression: "ADD BattlesWon :inc",
  ExpressionAttributeValues: {
    ":inc": 1
  }
}));

Practice

Your turn

Mew won another battle. Use UpdateCommand with the ADD action to increase BattlesWon by 1. It's atomic, so it beats reading the value and writing it back.

Use ADD in UpdateExpression to increment BattlesWon by 1.

// @ts-nocheck
import { DynamoDBClient } from "@aws-sdk/client-dynamodb";
import { DynamoDBDocumentClient, UpdateCommand } from "@aws-sdk/lib-dynamodb";
const client = new DynamoDBClient({ region: "ap-south-1" });
const docClient = DynamoDBDocumentClient.from(client);
async function recordBattleWin() {
  const command = new UpdateCommand({
    TableName: "Pokemon",
    Key: {
      Name: "Mew"
    },
    // ⬇️ Use the ADD action to increment BattlesWon by 1
    UpdateExpression: "", 
    ExpressionAttributeValues: {
      
    }
  });
  try {
    const response = await docClient.send(command);
    console.log("Battle recorded!");
  } catch (err) {
    console.error("Failed to update stats:", err instanceof Error ? err.message : String(err));
  }
}

// @ts-nocheck
import { DynamoDBClient } from "@aws-sdk/client-dynamodb";
import { DynamoDBDocumentClient, UpdateCommand } from "@aws-sdk/lib-dynamodb";

const client = new DynamoDBClient({ region: "ap-south-1" });
const docClient = DynamoDBDocumentClient.from(client);

async function recordBattleWin() {
  const command = new UpdateCommand({
    TableName: "Pokemon",
    Key: {
      Name: "Mew"
    },
    // ⬇️ Use the ADD action to increment BattlesWon by 1
    UpdateExpression: "", 
    ExpressionAttributeValues: {
      
    }
  });

  try {
    const response = await docClient.send(command);
    console.log("Battle recorded!");
  } catch (err) {
    console.error("Failed to update stats:", err instanceof Error ? err.message : String(err));
  }
}

Ctrl/⌘ Enter to run · simulated, no AWS callsSimulated — no AWS calls

Check

Check your understanding

Quick Quiz · one question, as many tries as you need.

Why should you use atomic counters instead of read-modify-write for game stats?

Choose one answer.