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Idempotency

The idempotency utility provides a simple solution to convert your Lambda functions into idempotent operations which are safe to retry.

Terminology

The property of idempotency means that an operation does not cause additional side effects if it is called more than once with the same input parameters.

Idempotent operations will return the same result when they are called multiple times with the same parameters. This makes idempotent operations safe to retry. Read more about idempotency.

Idempotency key is a hash representation of either the entire event or a specific configured subset of the event, and invocation results are JSON serialized and stored in your persistence storage layer.

Key features

  • Prevent Lambda handler function from executing more than once on the same event payload during a time window
  • Ensure Lambda handler returns the same result when called with the same payload
  • Select a subset of the event as the idempotency key using JMESPath expressions
  • Set a time window in which records with the same payload should be considered duplicates

Getting started

Installation

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<dependencies>
    ...
    <dependency>
        <groupId>software.amazon.lambda</groupId>
        <artifactId>powertools-idempotency</artifactId>
        <version>1.12.3</version>
    </dependency>
    ...
</dependencies>

<!-- configure the aspectj-maven-plugin to compile-time weave (CTW) the aws-lambda-powertools-java aspects into your project -->
<build>
    <plugins>
        ...
        <plugin>
             <groupId>org.codehaus.mojo</groupId>
             <artifactId>aspectj-maven-plugin</artifactId>
             <version>1.14.0</version>
             <configuration>
                 <source>1.8</source>
                 <target>1.8</target>
                 <complianceLevel>1.8</complianceLevel>
                 <aspectLibraries>
                     <aspectLibrary>
                         <groupId>software.amazon.lambda</groupId>
                         <artifactId>powertools-idempotency</artifactId>
                     </aspectLibrary>
                     ...
                 </aspectLibraries>
             </configuration>
             <executions>
                 <execution>
                     <goals>
                         <goal>compile</goal>
                     </goals>
                 </execution>
             </executions>
        </plugin>
        ...
    </plugins>
</build>

Required resources

Before getting started, you need to create a persistent storage layer where the idempotency utility can store its state - your Lambda functions will need read and write access to it.

As of now, Amazon DynamoDB is the only supported persistent storage layer, so you'll need to create a table first.

Default table configuration

If you're not changing the default configuration for the DynamoDB persistence layer, this is the expected default configuration:

Configuration Value Notes
Partition key id
TTL attribute name expiration This can only be configured after your table is created if you're using AWS Console

Tip: You can share a single state table for all functions

You can reuse the same DynamoDB table to store idempotency state. We add your function name in addition to the idempotency key as a hash key.

AWS Serverless Application Model (SAM) example
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Resources:
  IdempotencyTable:
    Type: AWS::DynamoDB::Table
    Properties:
      AttributeDefinitions:
        - AttributeName: id
          AttributeType: S
      KeySchema:
        - AttributeName: id
          KeyType: HASH
      TimeToLiveSpecification:
        AttributeName: expiration
        Enabled: true
      BillingMode: PAY_PER_REQUEST

  IdempotencyFunction:
    Type: AWS::Serverless::Function
    Properties:
      CodeUri: Function
      Handler: helloworld.App::handleRequest
      Policies:
        - DynamoDBCrudPolicy:
            TableName: !Ref IdempotencyTable
      Environment:
        Variables:
          IDEMPOTENCY_TABLE: !Ref IdempotencyTable

Warning: Large responses with DynamoDB persistence layer

When using this utility with DynamoDB, your function's responses must be smaller than 400KB. Larger items cannot be written to DynamoDB and will cause exceptions.

Info: DynamoDB

Each function invocation will generally make 2 requests to DynamoDB. If the result returned by your Lambda is less than 1kb, you can expect 2 WCUs per invocation. For retried invocations, you will see 1WCU and 1RCU. Review the DynamoDB pricing documentation to estimate the cost.

Idempotent annotation

You can quickly start by initializing the DynamoDBPersistenceStore and using it with the @Idempotent annotation on your Lambda handler.

Important

Initialization and configuration of the DynamoDBPersistenceStore must be performed outside the handler, preferably in the constructor.

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public class App implements RequestHandler<Subscription, SubscriptionResult> {

  public App() {
    // we need to initialize idempotency store before the handleRequest method is called
    Idempotency.config().withPersistenceStore(
      DynamoDBPersistenceStore.builder()
        .withTableName(System.getenv("TABLE_NAME"))
        .build()
      ).configure();
  }

  @Idempotent
  public SubscriptionResult handleRequest(final Subscription event, final Context context) {
    SubscriptionPayment payment = createSubscriptionPayment(
      event.getUsername(),
      event.getProductId()
    );

    return new SubscriptionResult(payment.getId(), "success", 200);
  }
}
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{
  "username": "xyz",
  "product_id": "123456789"
}

Idempotent annotation on another method

You can use the @Idempotent annotation for any synchronous Java function, not only the handleRequest one.

When using @Idempotent annotation on another method, you must tell which parameter in the method signature has the data we should use:

  • If the method only has one parameter, it will be used by default.
  • If there are 2 or more parameters, you must set the @IdempotencyKey on the parameter to use.

The parameter must be serializable in JSON. We use Jackson internally to (de)serialize objects

This example also demonstrates how you can integrate with Batch utility, so you can process each record in an idempotent manner.

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public class AppSqsEvent implements RequestHandler<SQSEvent, String> {

  public AppSqsEvent() {
    Idempotency.config()
      .withPersistenceStore(
          DynamoDBPersistenceStore.builder()
            .withTableName(System.getenv("TABLE_NAME"))
            .build()
      ).withConfig(
           IdempotencyConfig.builder()
             .withEventKeyJMESPath("messageId") // see Choosing a payload subset section
             .build()
      ).configure();
    }

  @Override
  @SqsBatch(SampleMessageHandler.class)
  public String handleRequest(SQSEvent input, Context context) {
    dummy("hello", "world");
    return "{\"statusCode\": 200}";
  }

  @Idempotent
  private String dummy(String argOne, @IdempotencyKey String argTwo) {
    return "something";
  }

  public static class SampleMessageHandler implements SqsMessageHandler<Object> {
    @Override
    @Idempotent
    // no need to use @IdempotencyKey as there is only one parameter
    public String process(SQSMessage message) {
      String returnVal = doSomething(message.getBody());
      return returnVal;
    }
  }
}
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{
    "Records": [
        {
            "messageId": "059f36b4-87a3-44ab-83d2-661975830a7d",
            "receiptHandle": "AQEBwJnKyrHigUMZj6rYigCgxlaS3SLy0a...",
            "body": "Test message.",
            "attributes": {
                "ApproximateReceiveCount": "1",
                "SentTimestamp": "1545082649183",
                "SenderId": "AIDAIENQZJOLO23YVJ4VO",
                "ApproximateFirstReceiveTimestamp": "1545082649185"
            },
            "messageAttributes": {
                "testAttr": {
                "stringValue": "100",
                "binaryValue": "base64Str",
                "dataType": "Number"
                }
            },
            "md5OfBody": "e4e68fb7bd0e697a0ae8f1bb342846b3",
            "eventSource": "aws:sqs",
            "eventSourceARN": "arn:aws:sqs:us-east-2:123456789012:my-queue",
            "awsRegion": "us-east-2"
        }
    ]
}

Choosing a payload subset for idempotency

Tip: Dealing with always changing payloads

When dealing with an elaborate payload (API Gateway request for example), where parts of the payload always change, you should configure the EventKeyJMESPath.

Use IdempotencyConfig to instruct the Idempotent annotation to only use a portion of your payload to verify whether a request is idempotent, and therefore it should not be retried.

Payment scenario

In this example, we have a Lambda handler that creates a payment for a user subscribing to a product. We want to ensure that we don't accidentally charge our customer by subscribing them more than once.

Imagine the function executes successfully, but the client never receives the response due to a connection issue. It is safe to retry in this instance, as the idempotent decorator will return a previously saved response.

Warning: Idempotency for JSON payloads

The payload extracted by the EventKeyJMESPath is treated as a string by default, so will be sensitive to differences in whitespace even when the JSON payload itself is identical.

To alter this behaviour, you can use the JMESPath built-in function powertools_json() to treat the payload as a JSON object rather than a string.

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public class PaymentFunction implements RequestHandler<APIGatewayProxyRequestEvent, APIGatewayProxyResponseEvent> {

  public PaymentFunction() {
    Idempotency.config()
    .withConfig(
        IdempotencyConfig.builder()
          .withEventKeyJMESPath("powertools_json(body)")
          .build())
    .withPersistenceStore(
        DynamoDBPersistenceStore.builder()
          .withTableName(System.getenv("TABLE_NAME"))
          .build())
    .configure();
}

@Idempotent
public APIGatewayProxyResponseEvent handleRequest(final APIGatewayProxyRequestEvent event, final Context context) {
  APIGatewayProxyResponseEvent response = new APIGatewayProxyResponseEvent();

  try {
    Subscription subscription = JsonConfig.get().getObjectMapper().readValue(event.getBody(), Subscription.class);

    SubscriptionPayment payment = createSubscriptionPayment(
         subscription.getUsername(),
         subscription.getProductId()
    );

    return response
             .withStatusCode(200)
             .withBody(String.format("{\"paymentId\":\"%s\"}", payment.getId()));

  } catch (JsonProcessingException e) {
    return response.withStatusCode(500);
  }
}
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{
  "version":"2.0",
  "body":"{\"username\":\"xyz\",\"productId\":\"123456789\"}",
  "routeKey":"ANY /createpayment",
  "rawPath":"/createpayment",
  "rawQueryString":"",
  "headers": {
    "Header1": "value1",
    "Header2": "value2"
  },
  "requestContext":{
    "accountId":"123456789012",
    "apiId":"api-id",
    "domainName":"id.execute-api.us-east-1.amazonaws.com",
    "domainPrefix":"id",
    "http":{
      "method":"POST",
      "path":"/createpayment",
      "protocol":"HTTP/1.1",
      "sourceIp":"ip",
      "userAgent":"agent"
    },
    "requestId":"id",
    "routeKey":"ANY /createpayment",
    "stage":"$default",
    "time":"10/Feb/2021:13:40:43 +0000",
    "timeEpoch":1612964443723
  },
  "isBase64Encoded":false
}

Idempotency request flow

This sequence diagram shows an example flow of what happens in the payment scenario:

Idempotent sequence

The client was successful in receiving the result after the retry. Since the Lambda handler was only executed once, our customer hasn't been charged twice.

Note

Bear in mind that the entire Lambda handler is treated as a single idempotent operation. If your Lambda handler can cause multiple side effects, consider splitting it into separate functions.

Handling exceptions

If you are using the @Idempotent annotation on your Lambda handler or any other method, any unhandled exceptions that are thrown during the code execution will cause the record in the persistence layer to be deleted. This means that new invocations will execute your code again despite having the same payload. If you don't want the record to be deleted, you need to catch exceptions within the idempotent function and return a successful response.

Idempotent sequence exception

If an Exception is raised outside the scope of a decorated method and after your method has been called, the persistent record will not be affected. In this case, idempotency will be maintained for your decorated function. Example:

Exception not affecting idempotency record sample
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  public SubscriptionResult handleRequest(final Subscription event, final Context context) {
    // If an exception is thrown here, no idempotent record will ever get created as the
    // idempotent function does not get called 
    doSomeStuff();

    result = idempotentMethod(event);

    // This exception will not cause the idempotent record to be deleted, since it
    // happens after the decorated function has been successfully called    
    throw new Exception();
  }

  @Idempotent
  private String idempotentMethod(final Subscription subscription) {
    // perform some operation with no exception thrown
  }

Warning

We will throw an IdempotencyPersistenceLayerException if any of the calls to the persistence layer fail unexpectedly.

As this happens outside the scope of your decorated function, you are not able to catch it.

Persistence stores

DynamoDBPersistenceStore

This persistence store is built-in, and you can either use an existing DynamoDB table or create a new one dedicated for idempotency state (recommended).

Use the builder to customize the table structure:

Customizing DynamoDBPersistenceStore to suit your table structure
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DynamoDBPersistenceStore.builder()
                        .withTableName(System.getenv("TABLE_NAME"))
                        .withKeyAttr("idempotency_key")
                        .withExpiryAttr("expires_at")
                        .withStatusAttr("current_status")
                        .withDataAttr("result_data")
                        .withValidationAttr("validation_key")
                        .build()

When using DynamoDB as a persistence layer, you can alter the attribute names by passing these parameters when initializing the persistence layer:

Parameter Required Default Description
TableName Y Table name to store state
KeyAttr id Partition key of the table. Hashed representation of the payload (unless SortKeyAttr is specified)
ExpiryAttr expiration Unix timestamp of when record expires
StatusAttr status Stores status of the Lambda execution during and after invocation
DataAttr data Stores results of successfully idempotent methods
ValidationAttr validation Hashed representation of the parts of the event used for validation
SortKeyAttr Sort key of the table (if table is configured with a sort key).
StaticPkValue idempotency#{LAMBDA_FUNCTION_NAME} Static value to use as the partition key. Only used when SortKeyAttr is set.

Advanced

Customizing the default behavior

Idempotency behavior can be further configured with IdempotencyConfig using a builder:

Customizing IdempotencyConfig
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IdempotencyConfig.builder()
                .withEventKeyJMESPath("id")
                .withPayloadValidationJMESPath("paymentId")
                .withThrowOnNoIdempotencyKey(true)
                .withExpiration(Duration.of(5, ChronoUnit.MINUTES))
                .withUseLocalCache(true)
                .withLocalCacheMaxItems(432)
                .withHashFunction("SHA-256")
                .build()

These are the available options for further configuration:

Parameter Default Description
EventKeyJMESPath "" JMESPath expression to extract the idempotency key from the event record. See available built-in functions
PayloadValidationJMESPath "" JMESPath expression to validate whether certain parameters have changed in the event
ThrowOnNoIdempotencyKey False Throw exception if no idempotency key was found in the request
ExpirationInSeconds 3600 The number of seconds to wait before a record is expired
UseLocalCache false Whether to locally cache idempotency results (LRU cache)
LocalCacheMaxItems 256 Max number of items to store in local cache
HashFunction MD5 Algorithm to use for calculating hashes, as supported by java.security.MessageDigest (eg. SHA-1, SHA-256, ...)

These features are detailed below.

Handling concurrent executions with the same payload

This utility will throw an IdempotencyAlreadyInProgressException if we receive multiple invocations with the same payload while the first invocation hasn't completed yet.

Info

If you receive IdempotencyAlreadyInProgressException, you can safely retry the operation.

This is a locking mechanism for correctness. Since we don't know the result from the first invocation yet, we can't safely allow another concurrent execution.

Using in-memory cache

By default, in-memory local caching is disabled, to avoid using memory in an unpredictable way.

Warning

Be sure to configure the Lambda memory according to the number of records and the potential size of each record.

You can enable it as seen before with:

Enable local cache
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    IdempotencyConfig.builder()
        .withUseLocalCache(true)
        .build()
When enabled, we cache a maximum of 256 records in each Lambda execution environment - You can change it with the LocalCacheMaxItems parameter.

Note: This in-memory cache is local to each Lambda execution environment

This means it will be effective in cases where your function's concurrency is low in comparison to the number of "retry" invocations with the same payload, because cache might be empty.

Expiring idempotency records

Note

By default, we expire idempotency records after an hour (3600 seconds).

In most cases, it is not desirable to store the idempotency records forever. Rather, you want to guarantee that the same payload won't be executed within a period of time.

You can change this window with the ExpirationInSeconds parameter:

Customizing expiration time
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IdempotencyConfig.builder()
    .withExpiration(Duration.of(5, ChronoUnit.MINUTES))
    .build()

Records older than 5 minutes will be marked as expired, and the Lambda handler will be executed normally even if it is invoked with a matching payload.

Note: DynamoDB time-to-live field

This utility uses expiration as the TTL field in DynamoDB, as demonstrated in the SAM example earlier.

Payload validation

Question: What if your function is invoked with the same payload except some outer parameters have changed?

Example: A payment transaction for a given productID was requested twice for the same customer, however the amount to be paid has changed in the second transaction.

By default, we will return the same result as it returned before, however in this instance it may be misleading; we provide a fail fast payload validation to address this edge case.

With PayloadValidationJMESPath, you can provide an additional JMESPath expression to specify which part of the event body should be validated against previous idempotent invocations

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public App() {
  Idempotency.config()
    .withPersistenceStore(DynamoDBPersistenceStore.builder()
        .withTableName(System.getenv("TABLE_NAME"))
        .build())
    .withConfig(IdempotencyConfig.builder()
        .withEventKeyJMESPath("[userDetail, productId]")
        .withPayloadValidationJMESPath("amount")
        .build())
    .configure();
}

@Idempotent
public SubscriptionResult handleRequest(final Subscription input, final Context context) {
    // Creating a subscription payment is a side
    // effect of calling this function!
    SubscriptionPayment payment = createSubscriptionPayment(
      input.getUserDetail().getUsername(),
      input.getProductId(),
      input.getAmount()
    )
    // ...
    return new SubscriptionResult(
        "success", 200,
        payment.getId(),
        payment.getAmount()
    );
}
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{
    "userDetail": {
        "username": "User1",
        "user_email": "user@example.com"
    },
    "productId": 1500,
    "charge_type": "subscription",
    "amount": 500
}
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{
    "userDetail": {
        "username": "User1",
        "user_email": "user@example.com"
    },
    "productId": 1500,
    "charge_type": "subscription",
    "amount": 1
}

In this example, the userDetail and productId keys are used as the payload to generate the idempotency key, as per EventKeyJMESPath parameter.

Note

If we try to send the same request but with a different amount, we will raise IdempotencyValidationException.

Without payload validation, we would have returned the same result as we did for the initial request. Since we're also returning an amount in the response, this could be quite confusing for the client.

By using withPayloadValidationJMESPath("amount"), we prevent this potentially confusing behavior and instead throw an Exception.

Making idempotency key required

If you want to enforce that an idempotency key is required, you can set ThrowOnNoIdempotencyKey to True.

This means that we will throw IdempotencyKeyException if the evaluation of EventKeyJMESPath is null.

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public App() {
  Idempotency.config()
    .withPersistenceStore(DynamoDBPersistenceStore.builder()
        .withTableName(System.getenv("TABLE_NAME"))
        .build())
    .withConfig(IdempotencyConfig.builder()
        // Requires "user"."uid" and "orderId" to be present
        .withEventKeyJMESPath("[user.uid, orderId]")
        .withThrowOnNoIdempotencyKey(true)
        .build())
    .configure();
}

@Idempotent
public OrderResult handleRequest(final Order input, final Context context) {
  // ...
}
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{
    "user": {
        "uid": "BB0D045C-8878-40C8-889E-38B3CB0A61B1",
        "name": "Foo"
    },
    "orderId": 10000
}

Notice that orderId is now accidentally within user key

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{
    "user": {
        "uid": "DE0D000E-1234-10D1-991E-EAC1DD1D52C8",
        "name": "Joe Bloggs",
        "orderId": 10000
    },
}

Customizing DynamoDB configuration

When creating the DynamoDBPersistenceStore, you can set a custom DynamoDbClient if you need to customize the configuration:

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public App() {
    DynamoDbClient customClient = DynamoDbClient.builder()
        .region(Region.US_WEST_2)
        .overrideConfiguration(ClientOverrideConfiguration.builder()
            .addExecutionInterceptor(new TracingInterceptor())
            .build()
        )
        .build();

    Idempotency.config().withPersistenceStore(
      DynamoDBPersistenceStore.builder()
            .withTableName(System.getenv("TABLE_NAME"))
            .withDynamoDbClient(customClient)
            .build()
  ).configure();
}

Default configuration is the following:

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DynamoDbClient.builder()
    .credentialsProvider(EnvironmentVariableCredentialsProvider.create())
    .httpClient(UrlConnectionHttpClient.builder().build())
    .region(Region.of(System.getenv(AWS_REGION_ENV)))
    .build();

Using a DynamoDB table with a composite primary key

When using a composite primary key table (hash+range key), use SortKeyAttr parameter when initializing your persistence store.

With this setting, we will save the idempotency key in the sort key instead of the primary key. By default, the primary key will now be set to idempotency#{LAMBDA_FUNCTION_NAME}.

You can optionally set a static value for the partition key using the StaticPkValue parameter.

Reusing a DynamoDB table that uses a composite primary key
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Idempotency.config().withPersistenceStore(
     DynamoDBPersistenceStore.builder()
       .withTableName(System.getenv("TABLE_NAME"))
       .withSortKeyAttr("sort_key")
       .build())
   .configure();

Data would then be stored in DynamoDB like this:

id sort_key expiration status data
idempotency#MyLambdaFunction 1e956ef7da78d0cb890be999aecc0c9e 1636549553 COMPLETED {"id": 12391, "message": "success"}
idempotency#MyLambdaFunction 2b2cdb5f86361e97b4383087c1ffdf27 1636549571 COMPLETED {"id": 527212, "message": "success"}
idempotency#MyLambdaFunction f091d2527ad1c78f05d54cc3f363be80 1636549585 IN_PROGRESS

Bring your own persistent store

This utility provides an abstract base class, so that you can implement your choice of persistent storage layer.

You can extend the BasePersistenceStore class and implement the abstract methods getRecord, putRecord, updateRecord and deleteRecord. You can have a look at DynamoDBPersistenceStore as an implementation reference.

Danger

Pay attention to the documentation for each method - you may need to perform additional checks inside these methods to ensure the idempotency guarantees remain intact.

For example, the putRecord method needs to throw an exception if a non-expired record already exists in the data store with a matching key.

Compatibility with other utilities

Validation utility

The idempotency utility can be used with the @Validation annotation from the validation module. Ensure that idempotency is the innermost annotation.

Using Idempotency with JSONSchema Validation utility
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@Validation(inboundSchema = "classpath:/schema_in.json")
@Idempotent
public APIGatewayProxyResponseEvent handleRequest(APIGatewayProxyRequestEvent input, Context context) {
  // ...
}

Tip: JMESPath Powertools functions are also available

Built-in functions like powertools_json, powertools_base64, powertools_base64_gzip are also available to use in this utility. See JMESPath Powertools functions

Testing your code

The idempotency utility provides several routes to test your code.

Disabling the idempotency utility

When testing your code, you may wish to disable the idempotency logic altogether and focus on testing your business logic. To do this, you can set the environment variable POWERTOOLS_IDEMPOTENCY_DISABLED to true. If you prefer setting this for specific tests, and are using JUnit 5, you can use junit-pioneer library:

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@Test
@SetEnvironmentVariable(key = Constants.IDEMPOTENCY_DISABLED_ENV, value = "true")
public void testIdempotencyDisabled_shouldJustRunTheFunction() {
    MyFunction func = new MyFunction();
    func.handleRequest(someInput, mockedContext);
}

You can also disable the idempotency for all tests using maven-surefire-plugin and adding the environment variable:

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<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-surefire-plugin</artifactId>
    <configuration>
        <environmentVariables>
            <POWERTOOLS_IDEMPOTENCY_DISABLED>true</POWERTOOLS_IDEMPOTENCY_DISABLED>
        </environmentVariables>
    </configuration>
</plugin>

Testing with DynamoDB Local

Unit tests

To unit test your function with DynamoDB Local, you can refer to this guide to setup with Maven.

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<dependencies>
    <!-- maven dependency for DynamoDB local -->
    <dependency>
       <groupId>com.amazonaws</groupId>
       <artifactId>DynamoDBLocal</artifactId>
       <version>[1.12,2.0)</version>
        <scope>test</scope>
    </dependency>
    <!-- Needed when building locally on M1 Mac -->
    <dependency>
        <groupId>io.github.ganadist.sqlite4java</groupId>
        <artifactId>libsqlite4java-osx-aarch64</artifactId>
        <version>1.0.392</version>
        <scope>test</scope>
        <type>dylib</type>
    </dependency>
</dependencies>
<repositories>
    <!-- custom repository to get the dependency -->
    <!-- see https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/DynamoDBLocal.DownloadingAndRunning.html#apache-maven -->
    <repository>
       <id>dynamodb-local-oregon</id>
       <name>DynamoDB Local Release Repository</name>
       <url>https://s3-us-west-2.amazonaws.com/dynamodb-local/release</url>
    </repository>
</repositories>
<plugins>
    <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-surefire-plugin</artifactId>
        <version>3.0.0-M5</version>
        <configuration>
            <!-- need sqlite native libs -->
            <systemPropertyVariables>
                <sqlite4java.library.path>${project.build.directory}/native-libs</sqlite4java.library.path>
            </systemPropertyVariables>
            <!-- environment variables for the tests -->
            <environmentVariables>
                <IDEMPOTENCY_TABLE_NAME>idempotency</IDEMPOTENCY_TABLE_NAME>
                <AWS_REGION>eu-central-1</AWS_REGION>
            </environmentVariables>
        </configuration>
    </plugin>
    <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-dependency-plugin</artifactId>
        <executions>
            <execution>
                <id>copy</id>
                <phase>test-compile</phase>
                <goals>
                    <goal>copy-dependencies</goal>
                </goals>
                <configuration>
                    <includeScope>test</includeScope>
                    <includeTypes>so,dll,dylib</includeTypes>
                    <outputDirectory>${project.build.directory}/native-libs</outputDirectory>
                </configuration>
            </execution>
        </executions>
    </plugin>
</plugins>
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public class AppTest {
    @Mock
    private Context context;
    private App app;
    private static DynamoDbClient client;

    @BeforeAll
    public static void setupDynamoLocal() {
        int port = getFreePort();

        // Initialize DynamoDBLocal
        try {
            DynamoDBProxyServer dynamoProxy = ServerRunner.createServerFromCommandLineArgs(new String[]{
                    "-inMemory",
                    "-port",
                    Integer.toString(port)
            });
            dynamoProxy.start();
        } catch (Exception e) {
            throw new RuntimeException();
        }

        // Initialize DynamoDBClient
        client = DynamoDbClient.builder()
                .httpClient(UrlConnectionHttpClient.builder().build())
                .region(Region.EU_WEST_1)
                .endpointOverride(URI.create("http://localhost:" + port))
                .credentialsProvider(StaticCredentialsProvider.create(
                        AwsBasicCredentials.create("FAKE", "FAKE")))
                .build();

        // create the table (use same table name as in pom.xml)
        client.createTable(CreateTableRequest.builder()
                .tableName("idempotency")
                .keySchema(KeySchemaElement.builder().keyType(KeyType.HASH).attributeName("id").build())
                .attributeDefinitions(
                        AttributeDefinition.builder().attributeName("id").attributeType(ScalarAttributeType.S).build()
                )
                .billingMode(BillingMode.PAY_PER_REQUEST)
                .build());
    }

    private static int getFreePort() {
        try {
            ServerSocket socket = new ServerSocket(0);
            int port = socket.getLocalPort();
            socket.close();
            return port;
        } catch (IOException ioe) {
            throw new RuntimeException(ioe);
        }
    }

    @BeforeEach
    void setUp() {
        MockitoAnnotations.openMocks(this);
        app = new App(client);
    }

    @Test
    public void testApp() {
        app.handleRequest(..., context);
        // ... assert
    }
}
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public class App implements RequestHandler<Subscription, SubscriptionResult> {

public App(DynamoDbClient ddbClient) {
    Idempotency.config().withPersistenceStore(
            DynamoDBPersistenceStore.builder()
                    .withTableName(System.getenv("IDEMPOTENCY_TABLE_NAME"))
                    .withDynamoDbClient(ddbClient)
                    .build()
    ).configure();
}

public App() {
    this(null);
}

@Idempotent
public SubscriptionResult handleRequest(final Subscription event, final Context context) {
    // ...
}

SAM Local

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public class App implements RequestHandler<Subscription, SubscriptionResult> {

  public App() {
    DynamoDbClientBuilder ddbBuilder = DynamoDbClient.builder()
       .credentialsProvider(EnvironmentVariableCredentialsProvider.create())
       .httpClient(UrlConnectionHttpClient.builder().build());

    if (System.getenv("AWS_SAM_LOCAL") != null) {
      ddbBuilder.endpointOverride(URI.create("http://dynamo:8000"));
    } else {
      ddbBuilder.region(Region.of(System.getenv("AWS_REGION")));
    }

    Idempotency.config().withPersistenceStore(
       DynamoDBPersistenceStore.builder()
          .withTableName(System.getenv("IDEMPOTENCY_TABLE_NAME"))
          .withDynamoDbClient(ddbBuilder.build())
          .build()
    ).configure();
  }

  @Idempotent
  public SubscriptionResult handleRequest(final Subscription event, final Context context) {
    // ...
  }
}
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# use or create a docker network 
docker network inspect sam-local || docker network create sam-local

# start dynamodb-local with docker
docker run -d --rm -p 8000:8000 \
    --network sam-local \
    --name dynamo \
    amazon/dynamodb-local

# create the idempotency table
aws dynamodb create-table 
    --table-name idempotency \
    --attribute-definitions AttributeName=id,AttributeType=S \
    --key-schema AttributeName=id,KeyType=HASH \
    --billing-mode PAY_PER_REQUEST \
    --endpoint-url http://localhost:8000

# invoke the function locally
sam local invoke IdempotentFunction \
    --event event.json \
    --env-vars env.json \
    --docker-network sam-local
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{
    "IdempotentFunction": {
        "IDEMPOTENCY_TABLE_NAME": "idempotency"
    }
}

Extra resources

If you're interested in a deep dive on how Amazon uses idempotency when building our APIs, check out this article.