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.

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 Constructs;
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_14_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.