[AWS CDK] Granting Permissions

Allow Lambda to read/write our DynamoDB table

Let’s give our Lambda’s execution role permissions to read/write from our table.

Go back to hitcounter.ts and add the following highlighted lines:

import * as cdk from 'aws-cdk-lib';
import * as lambda from 'aws-cdk-lib/aws-lambda';
import * as dynamodb from 'aws-cdk-lib/aws-dynamodb';
import { Construct } from 'constructs';

export interface HitCounterProps {
  /** the function for which we want to count url hits **/
  downstream: lambda.IFunction;
}

export class HitCounter extends Construct {

  /** allows accessing the counter function */
  public readonly handler: lambda.Function;

  constructor(scope: Construct, id: string, props: HitCounterProps) {
    super(scope, id);

    const table = new dynamodb.Table(this, 'Hits', {
        partitionKey: { name: 'path', type: dynamodb.AttributeType.STRING }
    });

    this.handler = new lambda.Function(this, 'HitCounterHandler', {
      runtime: lambda.Runtime.NODEJS_14_X,
      handler: 'hitcounter.handler',
      code: lambda.Code.fromAsset('lambda'),
      environment: {
        DOWNSTREAM_FUNCTION_NAME: props.downstream.functionName,
        HITS_TABLE_NAME: table.tableName
      }
    });

    // grant the lambda role read/write permissions to our table
    table.grantReadWriteData(this.handler);
  }
}

Deploy

Save & deploy:

cdk deploy

Test again

Okay, deployment is complete. Let’s run our test again (either use curl or your web browser):

curl -i https://xxxxxxxxxx.execute-api.us-east-1.amazonaws.com/prod/

Again?

HTTP/2 502 Bad Gateway
...

{"message": "Internal server error"}

😢

Still getting this pesky 5xx error! Let’s look at our CloudWatch logs again (click “Refresh”):

{
    "errorMessage": "User: arn:aws:sts::585695036304:assumed-role/CdkWorkshopStack-HelloHitCounterHitCounterHandlerS-TU5M09L1UBID/CdkWorkshopStack-HelloHitCounterHitCounterHandlerD-144HVUNEWRWEO is not authorized to perform: lambda:InvokeFunction on resource: arn:aws:lambda:us-east-1:585695036304:function:CdkWorkshopStack-HelloHandler2E4FBA4D-149MVAO4969O7",
    "errorType": "AccessDeniedException",
    "stackTrace": [
        "Object.extractError (/var/runtime/node_modules/aws-sdk/lib/protocol/json.js:48:27)",
        "Request.extractError (/var/runtime/node_modules/aws-sdk/lib/protocol/rest_json.js:52:8)",
        "Request.callListeners (/var/runtime/node_modules/aws-sdk/lib/sequential_executor.js:105:20)",
        "Request.emit (/var/runtime/node_modules/aws-sdk/lib/sequential_executor.js:77:10)",
        "Request.emit (/var/runtime/node_modules/aws-sdk/lib/request.js:683:14)",
        "Request.transition (/var/runtime/node_modules/aws-sdk/lib/request.js:22:10)",
        "AcceptorStateMachine.runTo (/var/runtime/node_modules/aws-sdk/lib/state_machine.js:14:12)",
        "/var/runtime/node_modules/aws-sdk/lib/state_machine.js:26:10",
        "Request.<anonymous> (/var/runtime/node_modules/aws-sdk/lib/request.js:38:9)",
        "Request.<anonymous> (/var/runtime/node_modules/aws-sdk/lib/request.js:685:12)"
    ]
}

Another access denied, but this time, if you take a close look:

User: <VERY-LONG-STRING> is not authorized to perform: lambda:InvokeFunction on resource: <VERY-LONG-STRING>"

So it seems like our hit counter actually managed to write to the database. We can confirm by going to the DynamoDB Console:

But, we must also give our hit counter permissions to invoke the downstream lambda function.

Grant invoke permissions

Add the highlighted lines to lib/hitcounter.ts:

import * as cdk from 'aws-cdk-lib';
import * as lambda from 'aws-cdk-lib/aws-lambda';
import * as dynamodb from 'aws-cdk-lib/aws-dynamodb';
import { Construct } from 'constructs';

export interface HitCounterProps {
  /** the function for which we want to count url hits **/
  downstream: lambda.IFunction;
}

export class HitCounter extends Construct {

  /** allows accessing the counter function */
  public readonly handler: lambda.Function;

  constructor(scope: Construct, id: string, props: HitCounterProps) {
    super(scope, id);

    const table = new dynamodb.Table(this, 'Hits', {
        partitionKey: { name: 'path', type: dynamodb.AttributeType.STRING }
    });

    this.handler = new lambda.Function(this, 'HitCounterHandler', {
      runtime: lambda.Runtime.NODEJS_14_X,
      handler: 'hitcounter.handler',
      code: lambda.Code.fromAsset('lambda'),
      environment: {
        DOWNSTREAM_FUNCTION_NAME: props.downstream.functionName,
        HITS_TABLE_NAME: table.tableName
      }
    });

    // grant the lambda role read/write permissions to our table
    table.grantReadWriteData(this.handler);

    // grant the lambda role invoke permissions to the downstream function
    props.downstream.grantInvoke(this.handler);
  }
}

Diff

You can check what this did using cdk diff:

cdk diff

The Resource section should look something like this, which shows the IAM statement was added to the role:

Resources
[~] AWS::IAM::Policy HelloHitCounter/HitCounterHandler/ServiceRole/DefaultPolicy HelloHitCounterHitCounterHandlerServiceRoleDefaultPolicy1487A60A
 └─ [~] PolicyDocument
     └─ [~] .Statement:
         └─ @@ -19,5 +19,15 @@
            [ ]         "Arn"
            [ ]       ]
            [ ]     }
            [+]   },
            [+]   {
            [+]     "Action": "lambda:InvokeFunction",
            [+]     "Effect": "Allow",
            [+]     "Resource": {
            [+]       "Fn::GetAtt": [
            [+]         "HelloHandler2E4FBA4D",
            [+]         "Arn"
            [+]       ]
            [+]     }
            [ ]   }
            [ ] ]

Which is exactly what we wanted.

Deploy

Okay… let’s give this another shot:

cdk deploy

Then hit your endpoint with curl or with your web browser:

curl -i https://xxxxxxxxxx.execute-api.us-east-1.amazonaws.com/prod/

Output should look like this:

HTTP/2 200 OK
...

Hello, CDK! You've hit /

If you still get 5xx, give it a few seconds and try again. Sometimes API Gateway takes a little bit to “flip” the endpoint to use the new deployment.

😲