Cloud formation custom resource creation fails - amazon-web-services

Question

I am trying to create a custom resource which points to a lambda function and then invoke it to generate random Priority or my ELB Listener.
Code for Lambda function is as follows.
[assembly: LambdaSerializer(typeof(Amazon.Lambda.Serialization.SystemTextJson.DefaultLambdaJsonSerializer))]
namespace aws_listenser_rule_priority_generator {
public class Function {
public async Task<int> FunctionHandler(FunctionParams input, ILambdaContext context) {
AmazonElasticLoadBalancingV2Client elbV2Client = new AmazonElasticLoadBalancingV2Client(RegionEndpoint.EUWest1);
var describeRulesResponse = await elbV2Client.DescribeRulesAsync(new DescribeRulesRequest {
ListenerArn = input.ListenerArn
});
var priority = 0;
var random = new Random();
do {
priority = random.Next(1, 50000);
}
while(describeRulesResponse.Rules.Exists(r => r.Priority == priority.ToString()));
return priority;
}
}
public class FunctionParams {
public string ListenerArn { get; set; }
}
}
I have tested this lambda on AWS console with the following parameters and it returns successfully.
{
"ListenerArn": "arn:aws:elasticloadbalancing:eu-west-1:706137030892:listener/app/Cumulus/dfcf28e0393cbf77/cdfe928b0285d5f0"
}
But as soon as I try to use this with Cloud Formation. The Custom Resource is stuck at creation in progress.
Resources:
ListenerPriority:
Type: Custom::Number
Properties:
ServiceToken: "arn:aws:lambda:eu-west-1:706137030892:function:GenerateListenerPriority"
ListenerArn: "arn:aws:elasticloadbalancing:eu-west-1:706137030892:listener/app/Cumulus/dfcf28e0393cbf77/cdfe928b0285d5f0"

Answer 1

The issue with the previous approach was the data format. When we are working with the Custom Resources, Cloud Formation sends a request in a specified format and response is expected asynchronously at a specified response URL.
I had to make following updates to the code:
[assembly: LambdaSerializer(typeof(Amazon.Lambda.Serialization.SystemTextJson.DefaultLambdaJsonSerializer))]
namespace aws_listenser_rule_priority_generator
{
public class Function
{
public async Task FunctionHandler(CustomResourceRequest<CustomResourceRequestProperties> crfRequest, ILambdaContext context)
{
var jsonResponse = string.Empty;
if(crfRequest.RequestType.ToUpperInvariant() != "CREATE") {
jsonResponse = JsonSerializer.Serialize(new CustomResourceResponse<object> {
Status = "SUCCESS",
Reason = string.Empty,
PhysicalResourceId = Guid.NewGuid().ToString(),
StackId = crfRequest.StackId,
RequestId = crfRequest.RequestId,
LogicalResourceId = crfRequest.LogicalResourceId,
Data = new {
Dummy = "Dummy"
}
});
}
else {
AmazonElasticLoadBalancingV2Client elbV2Client = new AmazonElasticLoadBalancingV2Client(RegionEndpoint.EUWest1);
var describeRulesResponse = await elbV2Client.DescribeRulesAsync(new DescribeRulesRequest {
ListenerArn = crfRequest.ResourceProperties.ListenerArn
});
var priority = 0;
var random = new Random();
do {
priority = random.Next(1, 50000);
}
while(describeRulesResponse.Rules.Exists(r => r.Priority == priority.ToString()));
jsonResponse = JsonSerializer.Serialize(new CustomResourceResponse<object> {
Status = "SUCCESS",
Reason = string.Empty,
PhysicalResourceId = Guid.NewGuid().ToString(),
StackId = crfRequest.StackId,
RequestId = crfRequest.RequestId,
LogicalResourceId = crfRequest.LogicalResourceId,
Data = new {
Priority = priority
}
});
}
var byteArray = Encoding.UTF8.GetBytes(jsonResponse);
var webRequest = WebRequest.Create(crfRequest.ResponseURL) as HttpWebRequest;
webRequest.Method = "PUT";
webRequest.ContentType = string.Empty;
webRequest.ContentLength = byteArray.Length;
using(Stream datastream = webRequest.GetRequestStream()) {
await datastream.WriteAsync(byteArray, 0, byteArray.Length);
}
await webRequest.GetResponseAsync();
}
}
public class CustomResourceRequest<T> where T : ICustomResourceRequestProperties {
public string RequestType { get; set; }
public string ResponseURL { get; set; }
public string StackId { get; set; }
public string RequestId { get; set; }
public string ResourceType { get; set; }
public string LogicalResourceId{ get; set; }
public string PhysicalResourceId { get; set; }
public T ResourceProperties { get; set; }
public T OldResourceProperties { get; set; }
}
public class CustomResourceResponse<T> {
public string Status { get; set; }
public string Reason { get; set; }
public string PhysicalResourceId { get; set; }
public string StackId { get; set; }
public string RequestId { get; set; }
public string LogicalResourceId { get; set; }
public bool NoEcho { get; set; }
public T Data { get; set; }
}
public interface ICustomResourceRequestProperties {
public string ServiceToken { get; set; }
}
public class CustomResourceRequestProperties : ICustomResourceRequestProperties
{
string ICustomResourceRequestProperties.ServiceToken { get; set; }
public string ListenerArn { get; set; }
}
}
The above code expects everything to work without any issues and there are no try catch blocks. Best practice would be to have try catch blocks and send a failure response.
Following URLs can be referred for more details:
Custom resources
Custom resource request objects
Custom resource response objects
Using AWS Lambda with AWS CloudFormation