AWS Lambda duplicate invocation specifications

Question

I would like to clarify the specification that AWS Lambda is invoked multiple times in a single call or event.  
**However, except in the case of duplicate invocation due to errors**.

Are the following specifications correct?  
※https://docs.aws.amazon.com/ja_jp/lambda/latest/dg/lambda-services.html

1. Synchronous invocation by AWS Lambda  
    ex.) 
    ```
    aws lambda invoke --invocation-type RequestResponse
    ```
    → Exactly once??.
2. Asynchronous invocation by AWS Lambda  
    ex.) 
    ```
    aws lambda invoke --invocation-type Event
    ```
    → *At least once*.
3. Event-driven; synchronous invocation  
    ex.) 
    ```
    Cognito
    API Gateway(synchronous invocation)
    ```
    → Exactly once??.
4. Event-driven; asynchronous invocation  
    ex.) 
    ```
    Cloudwatch Event
    Cloudwatch Logs
    S3
    ```
    → *At least once*.
5. Lambda polling1  
     ex.) 
    ```
    SQS(basic)
    ```
    → *At least once*.
6. Lambda polling2  
     ex.) 
    ```
    SQS(FIFO)
    DynamoDB(Streasm)
    ```
    → Exactly once??.

Accepted Answer

For all the synchronous invocations (1 and 3), the number of invocations depends on the client. If the client invokes the function once, the function will run exactly once.

For asynchronous invocations (2 and 4), events are added to an internal queue and the functions will run at least once.

For polling use cases (5 and 6, but also Kinesis Data streams, DynamoDB streams, MSK and MQ), the invocation is actually synchronous, however, SQS may deliver a message more than once, so the function may run more than once. With FIFO, messages are delivered exactly once, but due to function errors, we may invoke the function more than once. Same for the other cases.

AWS Lambda duplicate invocation specifications

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