Called a lambda function once, it's executed twice

Question

This is more of a concern than a question, but still, has anyone experienced this before? Does anyone know how to prevent it?

I have a lambda function (L1) which calls a second lambda function (L2) all written in NodeJs (runtime: Node.Js 8.10, and aws-sdk should be v2.488.0 - but I'm just pasting that from the documentation). The short story is that L1 is supposed to call L2, and when it does L2 is executed twice! I discovered this by writing logs to CloudWatch and I could see one L1 log and two L2 logs.

Here's a simplified version of L1 and L2.

L1:

const AWS = require('aws-sdk');
const lambda = new AWS.Lambda();

module.exports = {
    handler: async (event, context, callback) => {
        const payload: { rnd: Math.random() };
        const lambdaParams = {
            FunctionName: 'L2',
            Qualifier: `dev`,
            Payload: JSON.stringify(payload),
        };

        console.log(`L1 calling: ${JSON.stringify(payload)}`);
        return await lambda.invoke(lambdaParams).promise();
    },
};

L2:

module.exports = {
    handler: async (event, context, callback) => {
        console.log(`L2 called: ${JSON.stringify(event)}`);
    },
};

In CloudWatch I can see one L1 calling {"rnd": 0.012072353149807702} and two L2 called: {"rnd": 0.012072353149807702}!

BTW, this does not happen all the time. This is part of a step function process which was going to call L1 10k times. My code is written in a way that if L2 is executed twice (per one call), it will fail the whole process (because L2 inserts a record to DB only if it does not exist and fails if it does). So far, I managed to log this behaviour three times. All of them processing the same 10k items, facing the issue at a different iteration each time.

Does anyone have the same experience? Or even better, knows how to make sure one call leads to exactly one execution?

Answer 1

Your lambda function must be idempotent, because it can be called twice in different situations.

https://aws.amazon.com/premiumsupport/knowledge-center/lambda-function-idempotent/

https://cloudonaut.io/your-lambda-function-might-execute-twice-deal-with-it/