Lambda function handler executing DynamoDB update twice

Question

My Lambda function is supposed to increment a value in a dynamodb table, I'm using the .update API as an atomic counter. The value is incremented twice! I tried to change the increment variable value and it always increments twice. For example: the value in my DynamoDB table is 2, and the increment value is 1. I test the Lambda function, and the dynamoDB value is 4 (instead of 3).

The same code (without the handler function) works successfully from outside Lambda (executed from VSCode using the JS SDK) What could be the issue?

'use strict';

const AWS = require("aws-sdk");
AWS.config.update({ region: "me-central-1" });

const docClient = new AWS.DynamoDB.DocumentClient();

exports.handler = async (event) => {
await docClient.update(
  {
    TableName: "visitor-counter-table",
    Key: {
      "visitor-counter": "counter",
    },
    UpdateExpression: "set #num = #num + :incr",
    ExpressionAttributeNames: {
      "#num": "number",
    },
    ExpressionAttributeValues: {
      ":incr": 1,
    },
  },
  (err, data) => {
    err ? console.log(err) : console.log(data);
  }
).promise();

  return {"statusCode": 200, "body": "number added to DDB Success"}
};

Here is the code i ran on VSCode:

const AWS = require("aws-sdk");
AWS.config.update({ region: "me-central-1" });

const docClient = new AWS.DynamoDB.DocumentClient();

docClient.update(
  {
    TableName: "visitor-counter-tbl",
    Key: {
      "visitor-counter": "counter",
    },
    UpdateExpression: "set #num = #num + :incr",
    ExpressionAttributeNames: {
      "#num": "number",
    },
    ExpressionAttributeValues: {
      ":incr": 1,
    },
  },
  (err, data) => {
    err ? console.log(err) : console.log(data);
  }
);

Can you please exactly show the code that you ran locally (i.e. without the handler function)? — Ermiya Eskandary
– Ermiya Eskandary, Commented Nov 10, 2022 at 16:13
@BorislavStoilov for this purpose i use the TEST button in lambda, but i also trigger it using an API Gateway URL, which yields the same result. — b_a9f
– b_a9f, Commented Nov 10, 2022 at 16:23
@b_a9f It's because you're awaiting the promise, and using the callback at the same time - that's 2x. You should either await the promise OR use the callback not await the promise AND use the callback. I'll type up an answer soon :) glad it worked. — Ermiya Eskandary
– Ermiya Eskandary, Commented Nov 10, 2022 at 16:28

Ermiya Eskandary · Accepted Answer · 2022-11-10 20:09:17Z

1

You're passing success and failure callbacks to docClient.update while also obtaining and awaiting a generated promise.

This means that ultimately you're triggering the update twice.

The reason why your local invocation works is that you're only passing the callbacks, and not asking for a promise (.promise()) and then awaiting it. This results in triggering the update once.

Either use await, or use callbacks - not both.

This should work:

await docClient.update(
  {
    TableName: "visitor-counter-table",
    Key: {
      "visitor-counter": "counter",
    },
    UpdateExpression: "set #num = #num + :incr",
    ExpressionAttributeNames: {
      "#num": "number",
    },
    ExpressionAttributeValues: {
      ":incr": 1,
    },
  }
).promise();

answered Nov 10, 2022 at 20:09

Ermiya Eskandary

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2 Comments

jarmod Over a year ago

It's interesting. I wonder why this is happening. Why would providing both a callback function and requesting a promise result in 2x API requests? That seems like a bug in the AWS SDK, or at least a very questionable outcome that would be worth preventing, if possible. Two completions from 1x API request (one a fulfilled promise and the other a callback) I could almost understand but not 2x API requests. Might have to dig into the code here to understand more.

jarmod Over a year ago

OK, a call to service.action(params) returns an AWSRequest object that you have to call .send() on in order for the API request to actually be sent. But a call to service.action(params, callback) actually executes if (callback) request.send(callback), so the presence of the callback function sends the API request. Now, the .promise() method's code "sends the request and returns a promise", so it also sends the request. And that's why the API call happens twice if callback and promise() are both present.

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