Commenting System Using AWS SAM + ASP.NET Core

This is a walkthrough of how I built my commenting system for my blog with ASP.NET Core Web API, Lambda and AWS DynamoDB using AWS SAM

I wanted to create a commenting system for my blog. My friend Paul encouraged me to build my own instead of using an off-the-shelve one. The reason why I agreed is because it sounded like fun challenge as I had never created one before. I thought it will be a good exercise to know how the commenting systems are actually built and work inside out. Below I share the steps involved in getting this system up and running!

What’s Covered

  • Serverless Template
  • Create the ASP.NET Core Web API project
  • Integrating Swagger UI
  • Connecting to AWS DynamoDB
  • Getting all Comments from AWS DynamoDB By Post
  • Inserting Comment into AWS DynamoDB
  • Deleting Comment From DynamoDB

Technologies Used

  • .NET Core - Cross platform compatible
  • AWS SAM - Cost effective and quick to build infrastructure (as code)
  • Swagger - Test API interactively through browser


  • AWS account
  • Knowledge of C#, Javascript, HTML and CSS
  • .NET Core SDK and Runtime installed (if you haven’t got them, install them from the given links)
  • .NET CLI


Serverless Template

The below is the template code used to create all the resources needed.

        "AWSTemplateFormatVersion": "2010-09-09",
        "Transform": "AWS::Serverless-2016-10-31",
        "Description": "An AWS Serverless Application that uses the ASP.NET Core framework running in Amazon Lambda.",
        "Resources": {
            "AspNetCoreFunction": {
                "Type": "AWS::Serverless::Function",
                "Properties": {
                    "Handler": "CommentsAPI::CommentsAPI.LambdaEntryPoint::FunctionHandlerAsync",
                    "Runtime": "dotnetcore3.1",
                    "CodeUri": "",
                    "MemorySize": 256,
                    "Timeout": 30,
                    "Role": null,
                    "Policies": [
                    "Events": {
                        "ProxyResource": {
                            "Type": "Api",
                            "Properties": {
                            "Path": "/{proxy+}",
                            "Method": "ANY"
                        "RootResource": {
                            "Type": "Api",
                            "Properties": {
                            "Path": "/",
                            "Method": "ANY"
            "CommentsTable": {
                "Type": "AWS::DynamoDB::Table",
                "Properties": {
                    "AttributeDefinitions": [
                        "AttributeName": "id",
                        "AttributeType": "S"
                    "KeySchema" : [ 
                        "AttributeName": "id",
                        "KeyType": "HASH"
                    "ProvisionedThroughput": {
                    "ReadCapacityUnits": "1",
                    "WriteCapacityUnits": "1"
        "Outputs": {
            "ApiURL": {
            "Description": "API endpoint URL for Prod environment",
                "Value": {
                    "Fn::Sub": "https://${ServerlessRestApi}.execute-api.${AWS::Region}"

The above template creates from the command dotnet new -i Amazon.Lambda.Templates and what it creates is a serverless function, API Gateway and DynamoDB table. Lambda (serverless) function is triggered by API Gateway which in turn performs CRUD operations in DynamoDB.

Create the ASP.NET Core Web API Project

  1. Open up terminal
  2. Check the dotnet verison installed (I had 3.1)
    dotnet --version
  3. Install Lambda and serverless template
    dotnet new -i Amazon.Lambda.Templates
  4. Create a ASP.NET Core Web API using the template installed above:
    dotnet new serverless.AspNetCoreWebAPI --name CommentsAPI
  5. cd into the src and then project’s folder
    cd src
    cd CommentsAPI
  6. Restore all the dependencies
    dotnet restore
  7. Run the project to make sure it’s working without errors
    dotnet run
  8. Use VS Code or your favourite text editor and open the project folder
  9. Notice four new files in addition to the normal files new .NET project area created.

  • aws-lambda-tools-defaults.json - This file is read by lambda tooling by default. Your function handler is specified in this file. So, if you update the function’s name, you need to update it in this file too.
  • LambdaEntryPoint - Pretty much self explanatory (file from which lambda executes your application code) when you deploy to AWS
  • LocalEntryProint- Local testing entry point when you test the application locally
  • serverless.template - SAM template file in JSON which defines insfrastructure as code.

Integrating Swagger UI

We will be using Swagger to document our Web API and test it from the browser. I like Swagger because it generates a UI with which you can interect and test your API in the browser.

  1. Add the NuGet(like npm but for .NET development) package to use Swagger in your application
    dotnet add package Swashbuckle.AspNetCore
  2. To use it, you need to register it by adding the below code in the ConfigureServices method in Startup.cs file
    services.AddSwaggerGen(c => {
         c.SwaggerDoc("v1", new OpenApiInfo { Title = "Comments API", Version = "v1" });
  3. Add import of Microsoft.OpenApi.Models on the top of the page which allows you to describes your API’s endpoints
    using Microsoft.OpenApi.Models;
  4. Then, enable it by inserting the below code in the Configure method of Startup.cs file. Add the below code above the app.UseHttpsRedirection(); Otherwise, Swagger UI won’t load.
    app.UseSwaggerUI(c =>
         c.SwaggerEndpoint("v1/swagger.json", "Comments");
         c.RoutePrefix = "swagger";

  5. Run the application and then click the url shown https://localhost:5001/
  6. Append swagger to the URL and press enter
  7. If everything went well, you should see Swagger UI now

Connection to AWS DynamoDB

  1. Navigate to you project folder. We will add two NuGet packages. These are essential to make a connection to DynamoDB.
    dotnet add package AWSSDK.DynamoDBv2
    dotnet add package AWSSDK.Extensions.NETCore.Setup 

    AWSSDK.DynamoDBv2 - Adds the ability to interact with DynamoDB using AWS .NET SDK AWSSDK.Extensions.NETCore.Setup - has methods for configuration and registrations of AWS Services with dependency injection.

  2. Now we need to register Dynamo Service. In the Startup.cs file, add the below code in ConfigureServices method:
    services.AddCors(c =>
     c.AddPolicy("AllowOrigin", options => options.AllowAnyOrigin());

    Code Explanation

    • We add CORS policy as the origin we will be accessing the dynamoDb will be a different one. Hence, specifying AllowAnyOrigin
    • services.AddAWSService<IAmazonDynamoDB>(); - registering the Interface IAmazonDynamoDB
  3. Delete the below line from the method:

    Now, that we are connected, let’s read from database

    As we don’t have any data to read from the database, let’s add a record in the database manually to test our GET functionality.

Insert Record in DynamoDB

Open AWS CLI and run the below command

        aws dynamodb put-item \
            --table-name <your-table-name> \
            --item '{ 
                "ID": {"S": "b44da539-f696-4ed8-881d-29c569416114"}, 
                "comment": {"S": "This is a test comment"} , 
                "date": {"S": "9/19/2020 12:36:22 PM"},
                "postId": {"S": "test"},
                "username": {"S": ""}
            }' \
            --return-consumed-capacity TOTAL

The above should create a record in your DynamoDB table. Open AWS console and navigate to DynamoDB table to ensure it’s created.

Getting all Comments from AWS DynamoDB (By Post)

In this section, we will get all the comments by Post. So, each post displays only comments that related to it

After registering IAmazonDynamoDB in our application we are now in the position to insert it using Microsoft Dependency injection in our Controller.

  1. Delete the two controllers, S3ProxyController and ValuesController in the controllers folder
  2. Right click on Controllers folder > New C# Class
  3. Name the class CommentsController
  4. Make sure the class is derived from ControllerBase
  5. Add the below line above the class keywords
    [Route("api/[controller]")] // Annotates the route where this controller can be accessed
    [ApiController]             // Denotes that this class is a controller
    [EnableCors("AllowOrigin")] // Enables Cross Origin Resources Sharing throughout the controller

    If you get compilation error (red squiggly lines) on the above three lines, this means that you need to add (click the squiggly line, a light bulb will appear, click bulb and choose the imports below respectively) or simply add below using statements at the top of the page:

    using Microsoft.AspNetCore.Mvc;     // Enable MVC web framework
    using System.Threading.Tasks;       // Provides types and simplifies asynchronous code
    using Microsoft.AspNetCore.Cors;    // Enables Cross Region Resource Sharing
  6. Add below code inside the class:
    private const string TableName = "comments";
    private readonly IAmazonDynamoDB _amazonDynamoDb;

    public CommentsController(IAmazonDynamoDB amazonDynammoDb)
        _amazonDynamoDb = amazonDynammoDb;

    public async Task<ScanResponse> GetComments()
        var request = new ScanRequest
                TableName = TableName

        var response = await _amazonDynamoDb.ScanAsync(request);
        return response;

    public async Task<ActionResult<string>> Get(string url)
        var request = new GetItemRequest
            TableName = TableName,
            Key = new Dictionary<string, AttributeValue>
                    new AttributeValue
                        S = url

        var response = await _amazonDynamoDb.GetItemAsync(request);

        return response.Item["username"].S;

If you get red squiggly lines any where, add all the imports needed.

Inserting Comment into AWS DynamoDB

We will be creating a form which will ask for username and a comment. Then upon pressing submit button, it will write the data to DynamoDB.

  1. In your frontend, create a form (copy paste the below code)
     <div class="comments-view">
         <h4>Please Share Thoughts</h4>
         <input id="name" type="name" class="form-control" placeholder="Enter your email">
         <textarea id="comment" class="form-control" placeholder="Share thoughts here"></textarea>
         <button id="submit" class="btn btn-primary" >Comment</button>
         <div id="all-comments" class="show-comments"><!--Comments populated by script above--></div>
  2. Add the below javascript / Ajax code to communicate to ASP.NET Core Web API
     let allComments =  document.getElementById("all-comments");
     let $name = $("#name");
     let $comment = $("#comment");
     $("#submit").on('click', function(){
         //validation to prevent html tags being inputted input box
         var reg =/<(.|\n)*?>/g;
         if (reg.test($('#comment').val()) == true) {
             var errorText ='HTML Tags Not Allowed';
             var item =
                 username: $name.val(),
                 comment: $comment.val(),
                 postId: window.location.pathname,
                 type: "POST",
                 url: "[your-api-url-here]]",
                 data: JSON.stringify(item),
                 contentType: "application/json",
                 success : function(){
                     alert("Comment submitted successfully")
             error : function(err){
                 alert("error in post" + JSON.stringify(err))

    That is all we needed to do in the frontend.

  3. Now to back to your ASP.NET Core Web API project
  4. Create a folder called Model and in it, create new C# class called Comments
  5. Add the below code in it
    namespace CommentsAPI.Models
         public class Comments
             public string Username { get; set; }
             public string Comment { get; set; }
             public string PostId { get; set; }
  6. Navigate to the CommentsController and add the below code:
    public async Task Post([FromBody] Comments comment)
        Guid uuid = Guid.NewGuid();
        var item = new Dictionary<string, AttributeValue>()
            {"id", new AttributeValue{S = uuid.ToString()}},
            {"username", new AttributeValue{S = comment.Username}},
            {"comment", new AttributeValue{S = comment.Comment}},
            {"postId", new AttributeValue{S = comment.PostId}},
            {"date", new AttributeValue{S = DateTime.Now.ToString()}}

        PutItemRequest request = new PutItemRequest
            TableName = TableName,
            Item = item
        await _amazonDynamoDb.PutItemAsync(request);

Code Explanation

[HttpPost] - Annotation that the this method handles POST requests only

[FromBody] - Annotate thats take all the content from the body

This function is taking the user’s input and inserting it into the relevant column in the database asynchronously

Deleting Comment From DynamoDB

This can be done in two ways

  1. Manually by logging into AWS and deleting the record from the DynamoDB Table
  2. Click a button from the frontend.

As a blog owner, I believe users shouldn’t have the ability to delete comments. Hence, I didn’t implement this, However, if you want to implement it, please copy paste the below code in the CommentsController and do an AJAX request from the frontend on the click of a button provided that the comment that needs to be deleted is selected.

    public async Task Delete(string id)
        var request = new DeleteItemRequest
            TableName = TableName,
            Key = new Dictionary<string, AttributeValue>()
                {"id", new AttributeValue{S = id}}
        await _amazonDynamoDb.DeleteItemAsync(request);

You have successfully implemented your own commenting system for your website / blog etc. I will be implementing the feature of notifying the admin of a new message via email and will blog that too when done.

If any of the above didn’t work for you, please leave a comment below and I’ll contact you through email

Please Share Thoughts