Tag Archives: AWS SAM

Access to fetch from origin has been blocked by CORS policy – AWS SAM Local

Hi everyone,

I’ve been using AWS SAM local lately and ran into a bit of an issue with CORS. It took a looong time to find a solution that worked for all of my local scenarios so hopefully this will be able to help someone else out.

Access to fetch at '' from origin 'http://localhost:3001' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.

The error above is fairly typical when working with APIs. The request will work from tools such as postman and fiddler but browsers will block it. These are a few good links that explain why CORS is necessary:

As for the solution, add the following to your template.yml:

    Type: AWS::Serverless::Function
      Handler: functions/generic/cors.getPreflightHandler
      Runtime: nodejs8.10
          Type: Api
            Path: /pages
            Method: options
            RestApiId: !Ref XXXApi
              Authorizer: NONE

If you haven’t already defined your api in your template.yml file there is a default/omitted one created for you. There are a few examples on the AWS github: https://github.com/awslabs/serverless-application-model/blob/release/v1.8.0/examples/2016-10-31/api_cognito_auth/template.yaml

The next thing to do is to create a handler for the options request:

/* Handles retrieving a specific page */
exports.getPreflightHandler = async (event, context, callback) => {
    callback(null, { body: {} }, headers: { 'content-type': 'application/json', 'Access-Control-Allow-Origin': '*', 'Access-Control-Allow-Headers': 'Content-Type,X-Amz-Date,Authorization,X-Api-Key,X-Amz-Security-Token', 'Access-Control-Allow-Methods': 'OPTIONS,GET,POST,PUT,PATCH,DELETE', 'Access-Control-Allow-Credentials': true }, statusCode: 200 })

I’ve moved all mine out to some helper methods but the above should be enough to get it working for you. Hopefully the AWS team will have a simpler solution soon but if you run into any issues in the meantime please let me know!


AWS SAM Request Extremely Slow – Fix

Hi everyone,

I’m currently using AWS SAM CLI with NodeJS and was surprised to find that the requests were significantly slower when run locally. Luckily, I came across a post that suggested adding –skip-pull-image to your start-api command:

sam local start-api --skip-pull-image

This brought my requests down to under a second. Thanks to the following link for the info: https://github.com/awslabs/aws-sam-cli/issues/134#issuecomment-406786704

Overriding Global Variables – AWS SAM Local

Hi everyone,

Today I’ve added local overrides to the global variables in my template.yml file. This was pretty finicky – a lot of conflicting suggestions out there unfortunately. These are the settings that ended up working for me.


    "Parameters": {
        "RDS_HOSTNAME_TEST": ""


          Type: String
          Default: 'DEFAULT'

aws sam local command:

sam local start-api --env-vars "local-env-var-overrides.json"


exports.createTrapHandler = async (event, context, callback) => {

	try {

        console.log("RDS_HOSTNAME_TEST: " + process.env.RDS_HOSTNAME_TEST)
        // Outputs:

This link ended up being pretty useful: https://www.npmjs.com/package/aws-sam-local

AWS SAM not re-creating DynamoDb Table that was manually deleted – CodeStar

Hi everyone,

I ran into a bit of an issue after deleting a DynamoDb table via the AWS Console that had been created via CloudFormation (using AWS SAM). After deleting it I had expected it to be re-created automatically on the next deploy. Unfortunately this didn’t happen.

I came across the following AWS article that does a pretty good job of summarising the issue: https://aws.amazon.com/premiumsupport/knowledge-center/failing-stack-updates-deleted/

To fix it, I removed all references to the table from my template.yml file (this includes the table definition and any !Ref tags). After pushing this changeset I returned all of the references and re-pushed.

Let me know if you have any issues!


Error: Invalid value for “–parameter-overrides”: ParameterKey=TABLE_NAME,ParameterValue=CatBreeds is not in valid format. It must look something like ‘ParameterKey=KeyPairName,ParameterValue=MyKey ParameterKey=InstanceType,ParameterValue=t1.micro’ – AWS SAM CLI

Hi everyone,

I ran into the following error while using the AWS SAM Cli:

Error: Invalid value for "--parameter-overrides": ParameterKey=PURPLE_FROG,ParameterValue=CatBreeds is not in valid format. It must look something like 'ParameterKey=KeyPairName,ParameterValue=MyKey ParameterKey=InstanceType,ParameterValue=t1.micro'

This is the command I was running:

sam local start-api --parameter-overrides "ParameterKey=PURPLE_FROG,ParameterValue=CatBreeds"

The error message is unfortunately very misleading. After a fair bit of testing I found that the cause is actually the underscore in the parameter key and has nothing to do with the command format. Renaming all of the parameters to something like “PURPLEFROG” works.


AWS SAM Cheat Sheet/Quick Reference

Hi everyone,

This will just be a living cheat sheet or quick reference for using AWS SAM with nodejs. I’ll add to it as I find things that are useful or I might need again.

General AWS SAM Info:

Description Example More
Validate template sam validate
Start AWS Sam Locally https://docs.aws.amazon.com/serverless-application-model/latest/developerguide/serverless-quick-start.html
Access config variable in Lambda const CAT_BREED_TABLE = process.env.CAT_BREED_TABLE; https://stackoverflow.com/a/48491845/522859
Dynamically reference another resource in template.yml CAT_BREED_TABLE: !Ref DynamoDbCatBreedTable https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/intrinsic-function-reference-ref.html

Dynamo DB Local Info:

Description Example More
Create table aws dynamodb create-table –table-name CatBreeds –attribute-definitions AttributeName=CatBreedId,AttributeType=S –key-schema AttributeName=CatBreedId,KeyType=HASH –provisioned-throughput ReadCapacityUnits=5,WriteCapacityUnits=5
List Tables aws dynamodb list-tables –endpoint-url https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/Tools.CLI.html
List All Rows in Table aws dynamodb scan –table-name PestinatorTable –endpoint-url https://stackoverflow.com/a/52236600/522859

Node.js Info:

Description Example More
Run tests npm test

Create DynamoDB Table – AWS CLI

Hi everyone,

A quick example of how to create a dynamodb table using the AWS CLI:

aws dynamodb create-table --table-name CatBreeds --attribute-definitions AttributeName=CatBreedId,AttributeType=S --key-schema AttributeName=CatBreedId,KeyType=HASH --provisioned-throughput ReadCapacityUnits=5,WriteCapacityUnits=5

For more info the following AWS page helped me: https://docs.aws.amazon.com/cli/latest/reference/dynamodb/create-table.html