Define resources

Add resources to the hit counter construct

Now, let’s define the AWS Lambda function and the DynamoDB table in our HitCounter construct.

As usual, we first need to install the DynamoDB construct library (we already have the Lambda library installed):

dotnet add package Amazon.CDK.AWS.DynamoDB

Now, go back to src/CdkWorkshop/HitCounter.cs and add the following highlighted code:

using Amazon.CDK;
using Amazon.CDK.AWS.DynamoDB;
using Amazon.CDK.AWS.Lambda;
using System.Collections.Generic;

namespace CdkWorkshop
{
    public class HitCounterProps
    {
        // The function for which we want to count url hits
        public IFunction Downstream { get; set; }
    }

    public class HitCounter : Construct
    {
        public Function Handler { get; }

        public HitCounter(Construct scope, string id, HitCounterProps props) : base(scope, id)
        {
            var table = new Table(this, "Hits", new TableProps
            {
                PartitionKey = new Attribute
                {
                    Name = "path",
                    Type = AttributeType.STRING
                }
            });

            Handler = new Function(this, "HitCounterHandler", new FunctionProps
            {
                Runtime = Runtime.NODEJS_10_X,
                Handler = "hitcounter.handler",
                Code = Code.FromAsset("lambda"),
                Environment = new Dictionary<string, string>
                {
                    ["DOWNSTREAM_FUNCTION_NAME"] = props.Downstream.FunctionName,
                    ["HITS_TABLE_NAME"] = table.TableName
                }
            });
        }
    }
}

What did we do here?

This code is hopefully quite easy to understand:

  • We defined a DynamoDB table with path as the partition key (every DynamoDB table must have a single partition key).
  • We defined a Lambda function which is bound to the lambda/hitcounter.handler code.
  • We wired the Lambda’s environment variables to the FunctionName and TableName of our resources.

Late-bound values

The FunctionName and TableName properties are values that only resolve when we deploy our stack (notice that we haven’t configured these physical names when we defined the table/function, only logical IDs). This means that if you print their values during synthesis, you will get a “TOKEN”, which is how the CDK represents these late-bound values. You should treat tokens as opaque strings. This means you can concatenate them together for example, but don’t be tempted to parse them in your code.