Repository: matfij/particle-simulator
Pull request #47: Add Download Lambda
Author: login: matfij
Created: 2025-05-25T13:49:32Z | Merged: 2025-05-25T14:25:25Z



<!-- This is an auto-generated comment: release notes by coderabbit.ai -->
## Summary by CodeRabbit

- **New Features**
  - Added support for secure simulation file downloads via a new API endpoint, providing pre-signed S3 URLs valid for 5 minutes.
- **Improvements**
  - Infrastructure now provisions a dedicated Lambda function for download operations and updates permissions for enhanced security.
  - Visual Studio solution and deployment configuration updated to support the new download functionality.
- **Changes**
  - Pre-signed S3 upload URLs now expire after 5 minutes instead of 15 minutes.
  - Error responses in upload and preview endpoints updated to return 500 Internal Server Error instead of 400 on unexpected failures.
- **Chores**
  - Minor file formatting improvements (added newlines).
<!-- end of auto-generated comment: release notes by coderabbit.ai -->


Diff:
diff --git a/SimulatorUI/Components/DownloadPage.xaml b/SimulatorUI/Components/DownloadPage.xaml
index f9cd225..d687a5b 100644
--- a/SimulatorUI/Components/DownloadPage.xaml
+++ b/SimulatorUI/Components/DownloadPage.xaml
@@ -75,4 +75,4 @@
             AutomationId="CancelButton" 
             SemanticProperties.Description="Cancel Download" />
     </VerticalStackLayout>
-</ContentPage>
\ No newline at end of file
+</ContentPage>
diff --git a/SimulatorUI/Components/DownloadPage.xaml.cs b/SimulatorUI/Components/DownloadPage.xaml.cs
index 60fb036..4d4244a 100644
--- a/SimulatorUI/Components/DownloadPage.xaml.cs
+++ b/SimulatorUI/Components/DownloadPage.xaml.cs
@@ -65,4 +65,4 @@ private async void OnCancel(object sender, EventArgs e)
     {
         await Navigation.PopModalAsync();
     }
-}
\ No newline at end of file
+}
diff --git a/backend/cdk.json b/backend/cdk.json
index 7a71b84..bbf2557 100644
--- a/backend/cdk.json
+++ b/backend/cdk.json
@@ -1,5 +1,5 @@
 {
-  "app": "dotnet run --project src/Backend/Backend.csproj",
+  "app": "dotnet run --project src/CfnStack/CfnStack.csproj",
   "watch": {
     "include": ["**"],
     "exclude": [
diff --git a/backend/src/Backend/BackendStack.cs b/backend/src/Backend/BackendStack.cs
deleted file mode 100644
index cace345..0000000
--- a/backend/src/Backend/BackendStack.cs
+++ /dev/null
@@ -1,80 +0,0 @@
-using Amazon.CDK;
-using Amazon.CDK.AWS.APIGateway;
-using Amazon.CDK.AWS.DynamoDB;
-using Amazon.CDK.AWS.Lambda;
-using Amazon.CDK.AWS.S3;
-using Constructs;
-
-namespace Backend
-{
-    public class BackendStack : Stack
-    {
-        internal BackendStack(Construct scope, string id, IStackProps props = null) : base(scope, id, props)
-        {
-            #region Upload Lambda
-            var uploadLambda = new Function(this, "upload-lambda", new FunctionProps
-            {
-                Runtime = Runtime.DOTNET_8,
-                Handler = "UploadLambda",
-                Code = Code.FromAsset("./src/UploadLambda/bin/Release/net8.0/UploadLambda.zip"),
-                Timeout = Duration.Seconds(30),
-            });
-            #endregion
-
-            #region Preview Lambda
-            var previewLambda = new Function(this, "preview-lambda", new FunctionProps
-            {
-                Runtime = Runtime.DOTNET_8,
-                Handler = "PreviewLambda",
-                Code = Code.FromAsset("./src/PreviewLambda/bin/Release/net8.0/PreviewLambda.zip"),
-                Timeout = Duration.Seconds(30),
-            });
-            #endregion
-
-            #region Simulation DynamoDB Table
-            var simulationTable = new Table(this, "simulation-table", new TableProps
-            {
-                TableName = "Simulation",
-                PartitionKey = new Attribute
-                {
-                    Name = "Id",
-                    Type = AttributeType.STRING,
-                },
-                RemovalPolicy = RemovalPolicy.DESTROY, // RETAIN in production
-                BillingMode = BillingMode.PAY_PER_REQUEST,
-            });
-
-            simulationTable.GrantWriteData(uploadLambda);
-            simulationTable.GrantReadData(previewLambda);
-            #endregion
-
-            #region Simulation S3 Bucket
-            var simulationBucketProps = new BucketProps
-            {
-                BucketName="particle-simulation-bucket",
-                RemovalPolicy= RemovalPolicy.DESTROY,
-                Encryption = BucketEncryption.S3_MANAGED,
-                BlockPublicAccess = BlockPublicAccess.BLOCK_ALL,
-                EnforceSSL = true,
-            };
-            var simulationBucket = new Bucket(this, "particle-simulation-bucket", simulationBucketProps);
-
-            simulationBucket.GrantWrite(uploadLambda);
-            #endregion
-
-            #region API Gateway
-            var api = new RestApi(this, "simulation-api", new RestApiProps
-            {
-                RestApiName = "Particle Simulator Gateway",
-                ApiKeySourceType = ApiKeySourceType.HEADER,
-            });
-
-            var uploadLambdaIntegration = new LambdaIntegration(uploadLambda);
-            api.Root.AddResource("upload").AddMethod("POST", uploadLambdaIntegration);
-
-            var previewLambdaIntegration = new LambdaIntegration(previewLambda);
-            api.Root.AddResource("preview").AddMethod("GET", previewLambdaIntegration);
-            #endregion
-        }
-    }
-}
diff --git a/backend/src/Backend/Program.cs b/backend/src/Backend/Program.cs
deleted file mode 100644
index b2bd763..0000000
--- a/backend/src/Backend/Program.cs
+++ /dev/null
@@ -1,14 +0,0 @@
-﻿using Amazon.CDK;
-
-namespace Backend
-{
-    static class Program
-    {
-        public static void Main(string[] args)
-        {
-            var app = new App();
-            new BackendStack(app, "ParticleSimulatorStack", new StackProps { });
-            app.Synth();
-        }
-    }
-}
diff --git a/backend/src/Backend.sln b/backend/src/CfnStack.sln
similarity index 79%
rename from backend/src/Backend.sln
rename to backend/src/CfnStack.sln
index 9efaa35..fccf862 100644
--- a/backend/src/Backend.sln
+++ b/backend/src/CfnStack.sln
@@ -3,7 +3,7 @@ Microsoft Visual Studio Solution File, Format Version 12.00
 # Visual Studio Version 17
 VisualStudioVersion = 17.12.35707.178
 MinimumVisualStudioVersion = 15.0.26124.0
-Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Backend", "Backend\Backend.csproj", "{C1E324D5-B497-4BF3-B06A-94B792834A5D}"
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "CfnStack", "CfnStack\CfnStack.csproj", "{C1E324D5-B497-4BF3-B06A-94B792834A5D}"
 EndProject
 Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "UploadLambda", "UploadLambda\UploadLambda.csproj", "{1C5B279A-AA15-4F3B-983C-460DB54B921B}"
 EndProject
@@ -11,6 +11,8 @@ Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "PreviewLambda", "PreviewLam
 EndProject
 Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "SharedLibrary", "SharedLibrary\SharedLibrary.csproj", "{998604FE-5BC2-427F-98C2-CC5CBADFD3A0}"
 EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "DownloadLambda", "DownloadLambda\DownloadLambda.csproj", "{5BA3D072-E2CD-4030-9962-9D658740EB9A}"
+EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
 		Debug|Any CPU = Debug|Any CPU
@@ -69,6 +71,18 @@ Global
 		{998604FE-5BC2-427F-98C2-CC5CBADFD3A0}.Release|x64.Build.0 = Release|Any CPU
 		{998604FE-5BC2-427F-98C2-CC5CBADFD3A0}.Release|x86.ActiveCfg = Release|Any CPU
 		{998604FE-5BC2-427F-98C2-CC5CBADFD3A0}.Release|x86.Build.0 = Release|Any CPU
+		{5BA3D072-E2CD-4030-9962-9D658740EB9A}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{5BA3D072-E2CD-4030-9962-9D658740EB9A}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{5BA3D072-E2CD-4030-9962-9D658740EB9A}.Debug|x64.ActiveCfg = Debug|Any CPU
+		{5BA3D072-E2CD-4030-9962-9D658740EB9A}.Debug|x64.Build.0 = Debug|Any CPU
+		{5BA3D072-E2CD-4030-9962-9D658740EB9A}.Debug|x86.ActiveCfg = Debug|Any CPU
+		{5BA3D072-E2CD-4030-9962-9D658740EB9A}.Debug|x86.Build.0 = Debug|Any CPU
+		{5BA3D072-E2CD-4030-9962-9D658740EB9A}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{5BA3D072-E2CD-4030-9962-9D658740EB9A}.Release|Any CPU.Build.0 = Release|Any CPU
+		{5BA3D072-E2CD-4030-9962-9D658740EB9A}.Release|x64.ActiveCfg = Release|Any CPU
+		{5BA3D072-E2CD-4030-9962-9D658740EB9A}.Release|x64.Build.0 = Release|Any CPU
+		{5BA3D072-E2CD-4030-9962-9D658740EB9A}.Release|x86.ActiveCfg = Release|Any CPU
+		{5BA3D072-E2CD-4030-9962-9D658740EB9A}.Release|x86.Build.0 = Release|Any CPU
 	EndGlobalSection
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE
diff --git a/backend/src/Backend/Backend.csproj b/backend/src/CfnStack/CfnStack.csproj
similarity index 100%
rename from backend/src/Backend/Backend.csproj
rename to backend/src/CfnStack/CfnStack.csproj
diff --git a/backend/src/CfnStack/MainStack.cs b/backend/src/CfnStack/MainStack.cs
new file mode 100644
index 0000000..5cd74d0
--- /dev/null
+++ b/backend/src/CfnStack/MainStack.cs
@@ -0,0 +1,97 @@
+using Amazon.CDK;
+using Amazon.CDK.AWS.APIGateway;
+using Amazon.CDK.AWS.DynamoDB;
+using Amazon.CDK.AWS.IAM;
+using Amazon.CDK.AWS.Lambda;
+using Amazon.CDK.AWS.S3;
+using Constructs;
+
+namespace CfnStack;
+
+public class MainStack : Stack
+{
+    internal MainStack(Construct scope, string id, IStackProps props = null) : base(scope, id, props)
+    {
+        #region Upload Lambda
+        var uploadLambda = new Function(this, "upload-lambda", new FunctionProps
+        {
+            Runtime = Runtime.DOTNET_8,
+            Handler = "UploadLambda",
+            Code = Code.FromAsset("./src/UploadLambda/bin/Release/net8.0/UploadLambda.zip"),
+            Timeout = Duration.Seconds(30),
+        });
+        #endregion
+
+        #region Preview Lambda
+        var previewLambda = new Function(this, "preview-lambda", new FunctionProps
+        {
+            Runtime = Runtime.DOTNET_8,
+            Handler = "PreviewLambda",
+            Code = Code.FromAsset("./src/PreviewLambda/bin/Release/net8.0/PreviewLambda.zip"),
+            Timeout = Duration.Seconds(30),
+        });
+        #endregion
+
+        #region Download Lambda
+        var downloadLambda = new Function(this, "download-lambda", new FunctionProps
+        {
+            Runtime = Runtime.DOTNET_8,
+            Handler = "DownloadLambda",
+            Code = Code.FromAsset("./src/DownloadLambda/bin/Release/net8.0/DownloadLambda.zip"),
+            Timeout = Duration.Seconds(30),
+        });
+        #endregion
+
+        #region Simulation DynamoDB Table
+        var simulationTable = new Table(this, "simulation-table", new TableProps
+        {
+            TableName = "Simulation",
+            PartitionKey = new Attribute
+            {
+                Name = "Id",
+                Type = AttributeType.STRING,
+            },
+            RemovalPolicy = RemovalPolicy.DESTROY, // RETAIN in production
+            BillingMode = BillingMode.PAY_PER_REQUEST,
+        });
+
+        simulationTable.GrantWriteData(uploadLambda);
+        simulationTable.GrantReadData(previewLambda);
+        simulationTable.GrantReadWriteData(downloadLambda);
+        #endregion
+
+        #region Simulation S3 Bucket
+        var simulationBucketProps = new BucketProps
+        {
+            BucketName = "particle-simulation-bucket",
+            RemovalPolicy = RemovalPolicy.DESTROY,
+            BlockPublicAccess = BlockPublicAccess.BLOCK_ALL,
+            EnforceSSL = true,
+        };
+        var simulationBucket = new Bucket(this, "particle-simulation-bucket", simulationBucketProps);
+
+        simulationBucket.GrantWrite(uploadLambda);
+        simulationBucket.GrantRead(downloadLambda);
+        #endregion
+
+        #region API Gateway
+        var api = new RestApi(this, "simulation-api", new RestApiProps
+        {
+            RestApiName = "Particle Simulator Gateway",
+            EndpointConfiguration = new EndpointConfiguration
+            {
+                Types = [EndpointType.REGIONAL]
+            }
+        });
+
+        var uploadLambdaIntegration = new LambdaIntegration(uploadLambda);
+        api.Root.AddResource("upload").AddMethod("POST", uploadLambdaIntegration);
+
+        var previewLambdaIntegration = new LambdaIntegration(previewLambda);
+        api.Root.AddResource("preview").AddMethod("GET", previewLambdaIntegration);
+
+        var downloadLambdaIntegration = new LambdaIntegration(downloadLambda);
+        api.Root.AddResource("download").AddMethod("POST", downloadLambdaIntegration);
+        #endregion
+    }
+}
diff --git a/backend/src/CfnStack/Program.cs b/backend/src/CfnStack/Program.cs
new file mode 100644
index 0000000..0eb371e
--- /dev/null
+++ b/backend/src/CfnStack/Program.cs
@@ -0,0 +1,13 @@
+﻿using Amazon.CDK;
+
+namespace CfnStack;
+
+static class Program
+{
+    public static void Main(string[] args)
+    {
+        var app = new App();
+        new MainStack(app, "ParticleSimulatorStack", new StackProps { });
+        app.Synth();
+    }
+}
diff --git a/backend/src/DownloadLambda/DownloadLambda.csproj b/backend/src/DownloadLambda/DownloadLambda.csproj
new file mode 100644
index 0000000..8476f8b
--- /dev/null
+++ b/backend/src/DownloadLambda/DownloadLambda.csproj
@@ -0,0 +1,24 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net8.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <GenerateRuntimeConfigurationFiles>true</GenerateRuntimeConfigurationFiles>
+    <AWSProjectType>Lambda</AWSProjectType>
+    <!-- This property makes the build directory similar to a publish directory and helps the AWS .NET Lambda Mock Test Tool find project dependencies. -->
+    <CopyLocalLockFileAssemblies>true</CopyLocalLockFileAssemblies>
+    <!-- Generate ready to run images during publishing to improve cold start time. -->
+    <PublishReadyToRun>true</PublishReadyToRun>
+  </PropertyGroup>
+  <ItemGroup>
+    <PackageReference Include="Amazon.Lambda.APIGatewayEvents" Version="2.7.1" />
+    <PackageReference Include="Amazon.Lambda.RuntimeSupport" Version="1.12.0" />
+    <PackageReference Include="Amazon.Lambda.Core" Version="2.5.0" />
+    <PackageReference Include="Amazon.Lambda.Serialization.SystemTextJson" Version="2.4.4" />
+    <PackageReference Include="AWSSDK.S3" Version="4.0.0.2" />
+  </ItemGroup>
+  <ItemGroup>
+    <ProjectReference Include="..\SharedLibrary\SharedLibrary.csproj" />
+  </ItemGroup>
+</Project>
diff --git a/backend/src/DownloadLambda/Function.cs b/backend/src/DownloadLambda/Function.cs
new file mode 100644
index 0000000..09bc263
--- /dev/null
+++ b/backend/src/DownloadLambda/Function.cs
@@ -0,0 +1,99 @@
+using System.Net;
+using System.Text.Json;
+using Amazon.Lambda.Core;
+using Amazon.Lambda.RuntimeSupport;
+using Amazon.Lambda.Serialization.SystemTextJson;
+using Amazon.S3;
+using SharedLibrary;
+using Amazon.Lambda.APIGatewayEvents;
+using Amazon.S3.Model;
+using Amazon.DynamoDBv2.DataModel;
+using Amazon.DynamoDBv2;
+
+var bucketName = "particle-simulation-bucket";
+var s3Client = new AmazonS3Client();
+
+var dbClient = new AmazonDynamoDBClient();
+var dbContext = new DynamoDBContext(dbClient);
+
+var httpHeaders = new Dictionary<string, string>
+{
+    { "Content-Type", "application/json" }
+};
+
+var handler = async (APIGatewayProxyRequest request, ILambdaContext context) =>
+{
+    try
+    {
+        SimulationDownloadRequest input;
+        try
+        {
+            input = JsonSerializer.Deserialize<SimulationDownloadRequest>(request.Body);
+        }
+        catch (Exception ex) when (ex is JsonException || ex is ArgumentNullException)
+        {
+            context.Logger.LogError($"Invalid request body: {ex}");
+            var error = new ApiError { Message = $"Invalid request body" };
+            return new APIGatewayProxyResponse
+            {
+                StatusCode = (int)HttpStatusCode.BadRequest,
+                Body = JsonSerializer.Serialize(error),
+                Headers = httpHeaders,
+            };
+        }
+
+        var simulation = await dbContext.LoadAsync<Simulation>(input.SimulationId);
+
+        if (simulation == null)
+        {
+            context.Logger.LogError($"Simulation not found: {input.SimulationId}");
+            var error = new ApiError { Message = $"Simulation not found: {input.SimulationId}" };
+            return new APIGatewayProxyResponse
+            {
+                StatusCode = (int)HttpStatusCode.NotFound,
+                Body = JsonSerializer.Serialize(error),
+                Headers = httpHeaders,
+            };
+        }
+
+        simulation.Downloads += 1;
+        await dbContext.SaveAsync(simulation);
+
+        var s3Request = new GetPreSignedUrlRequest
+        {
+            BucketName = bucketName,
+            Key = simulation.FileName,
+            Verb = HttpVerb.GET,
+            Expires = DateTime.UtcNow.AddMinutes(5),
+        };
+
+        var url = s3Client.GetPreSignedURL(s3Request);
+
+        var response = new SimulationDownloadResponse
+        {
+            DownloadUrl = url,
+        };
+
+        return new APIGatewayProxyResponse
+        {
+            StatusCode = (int)HttpStatusCode.OK,
+            Body = JsonSerializer.Serialize(response),
+            Headers = httpHeaders,
+        };
+    }
+    catch (Exception ex)
+    {
+        context.Logger.LogError($"Error processing request: {ex}");
+        var error = new ApiError { Message = $"Unexpected error occurred" };
+        return new APIGatewayProxyResponse
+        {
+            StatusCode = (int)HttpStatusCode.InternalServerError,
+            Body = JsonSerializer.Serialize(error),
+            Headers = httpHeaders,
+        };
+    }
+};
+
+await LambdaBootstrapBuilder.Create(handler, new DefaultLambdaJsonSerializer())
+        .Build()
+        .RunAsync();
diff --git a/backend/src/DownloadLambda/aws-lambda-tools-defaults.json b/backend/src/DownloadLambda/aws-lambda-tools-defaults.json
new file mode 100644
index 0000000..454d5a3
--- /dev/null
+++ b/backend/src/DownloadLambda/aws-lambda-tools-defaults.json
@@ -0,0 +1,9 @@
+{
+    "profile": "default",
+    "region": "eu-central-1",
+    "configuration": "Release",
+    "function-runtime": "dotnet8",
+    "function-memory-size": 512,
+    "function-timeout": 30,
+    "function-handler": "DownloadLambda"
+}
diff --git a/backend/src/PreviewLambda/Function.cs b/backend/src/PreviewLambda/Function.cs
index 6f7fb34..dc6713d 100644
--- a/backend/src/PreviewLambda/Function.cs
+++ b/backend/src/PreviewLambda/Function.cs
@@ -53,7 +53,7 @@
         var error = new ApiError { Message = $"Unexpected error occurred" };
         return new APIGatewayProxyResponse
         {
-            StatusCode = (int)HttpStatusCode.BadRequest,
+            StatusCode = (int)HttpStatusCode.InternalServerError,
             Body = JsonSerializer.Serialize(error),
             Headers = httpHeaders,
         };
diff --git a/backend/src/SharedLibrary/DTOs.cs b/backend/src/SharedLibrary/DTOs.cs
index c4809dc..ad91c2d 100644
--- a/backend/src/SharedLibrary/DTOs.cs
+++ b/backend/src/SharedLibrary/DTOs.cs
@@ -1,5 +1,15 @@
 ﻿namespace SharedLibrary;
 
+public readonly struct SimulationDownloadRequest
+{
+    public required string SimulationId { get; init; }
+}
+
+public readonly struct SimulationDownloadResponse
+{
+    public required string DownloadUrl { get; init; }
+}
+
 public readonly struct SimulationPreview
 {
     public IReadOnlyList<Simulation> Simulations { get; init; }
diff --git a/backend/src/UploadLambda/Function.cs b/backend/src/UploadLambda/Function.cs
index bb69db1..b1182d2 100644
--- a/backend/src/UploadLambda/Function.cs
+++ b/backend/src/UploadLambda/Function.cs
@@ -59,7 +59,7 @@
             BucketName = bucketName,
             Key = fileName,
             Verb = HttpVerb.PUT,
-            Expires = DateTime.UtcNow.AddMinutes(15),
+            Expires = DateTime.UtcNow.AddMinutes(5),
             ContentType = input.ContentType,
         };
 
@@ -84,7 +84,7 @@
         var error = new ApiError { Message = $"Unexpected error occurred" };
         return new APIGatewayProxyResponse
         {
-            StatusCode = (int)HttpStatusCode.BadRequest,
+            StatusCode = (int)HttpStatusCode.InternalServerError,
             Body = JsonSerializer.Serialize(error),
             Headers = httpHeaders,
         };


Reviews:
user: login: coderabbitai[bot]
body: **Actionable comments posted: 8**

<details>
<summary>📜 Review details</summary>

**Configuration used: CodeRabbit UI**
**Review profile: ASSERTIVE**
**Plan: Pro**


<details>
<summary>📥 Commits</summary>

Reviewing files that changed from the base of the PR and between ac85f674b7f638bdbeb24f541dd39c3208e8b0dd and d1a16237da8e9c3e217f9304b0882b74b5e41fe3.

</details>

<details>
<summary>📒 Files selected for processing (13)</summary>

* `SimulatorUI/Components/DownloadPage.xaml` (1 hunks)
* `SimulatorUI/Components/DownloadPage.xaml.cs` (1 hunks)
* `backend/cdk.json` (1 hunks)
* `backend/src/Backend/BackendStack.cs` (0 hunks)
* `backend/src/Backend/Program.cs` (0 hunks)
* `backend/src/CfnStack.sln` (2 hunks)
* `backend/src/CfnStack/MainStack.cs` (1 hunks)
* `backend/src/CfnStack/Program.cs` (1 hunks)
* `backend/src/DownloadLambda/DownloadLambda.csproj` (1 hunks)
* `backend/src/DownloadLambda/Function.cs` (1 hunks)
* `backend/src/DownloadLambda/aws-lambda-tools-defaults.json` (1 hunks)
* `backend/src/SharedLibrary/DTOs.cs` (1 hunks)
* `backend/src/UploadLambda/Function.cs` (1 hunks)

</details>

<details>
<summary>💤 Files with no reviewable changes (2)</summary>

* backend/src/Backend/Program.cs
* backend/src/Backend/BackendStack.cs

</details>

<details>
<summary>🧰 Additional context used</summary>

<details>
<summary>🧠 Learnings (1)</summary>

<details>
<summary>backend/src/UploadLambda/Function.cs (1)</summary>

```
Learnt from: matfij
PR: matfij/particle-simulator#34
File: backend/src/FileUploadLambda/aws-lambda-tools-defaults.json:1-9
Timestamp: 2025-04-28T19:38:26.290Z
Learning: The FileUploadLambda function in the particle-simulator project is responsible for generating pre-signed URLs for S3 bucket uploads, not for streaming files. It should have an appropriate timeout setting (30-60 seconds) rather than the initial 600 seconds.
```

</details>

</details><details>
<summary>🧬 Code Graph Analysis (2)</summary>

<details>
<summary>backend/src/CfnStack/Program.cs (1)</summary><blockquote>

<details>
<summary>backend/src/CfnStack/MainStack.cs (2)</summary>

* `MainStack` (11-97)
* `MainStack` (13-96)

</details>

</blockquote></details>
<details>
<summary>backend/src/DownloadLambda/Function.cs (1)</summary><blockquote>

<details>
<summary>SimulatorUI/Api/ApiModels.cs (1)</summary>

* `ApiError` (29-32)

</details>

</blockquote></details>

</details>

</details>

<details>
<summary>⏰ Context from checks skipped due to timeout of 90000ms (1)</summary>

* GitHub Check: Build and analyze

</details>

<details>
<summary>🔇 Additional comments (12)</summary><blockquote>

<details>
<summary>backend/src/SharedLibrary/DTOs.cs (1)</summary>

`3-11`: **LGTM! Well-structured DTOs following established patterns.**

The new `SimulationDownloadRequest` and `SimulationDownloadResponse` structs follow the same design patterns as existing DTOs in the file:
- `readonly struct` design for immutability
- `required` properties with `init`-only setters
- Consistent naming conventions
- Appropriate property types

</details>
<details>
<summary>SimulatorUI/Components/DownloadPage.xaml.cs (1)</summary>

`68-68`: **Minor formatting improvement.**

Good practice to ensure files end with a newline character.

</details>
<details>
<summary>SimulatorUI/Components/DownloadPage.xaml (1)</summary>

`78-78`: **Minor formatting improvement.**

Good practice to ensure files end with a newline character.

</details>
<details>
<summary>backend/src/UploadLambda/Function.cs (1)</summary>

`62-62`: **Good security improvement - reducing pre-signed URL expiry time.**

Reducing the pre-signed URL expiry from 15 minutes to 5 minutes improves security by limiting the window of validity for upload URLs while still providing sufficient time for users to complete their uploads.

</details>
<details>
<summary>backend/cdk.json (1)</summary>

`2-2`: **Updated CDK app command**  
The `app` entry now points to `src/CfnStack/CfnStack.csproj` instead of the old Backend project. Ensure that any CI/CD scripts, documentation, and local developer workflows are updated to use this new path.

</details>
<details>
<summary>backend/src/DownloadLambda/aws-lambda-tools-defaults.json (1)</summary>

`1-9`: **Validate AWS Lambda tool defaults**  
This file configures the `DownloadLambda` deployment. Please confirm that:
- The `function-handler` follows the correct .NET Lambda handler format (e.g., `Assembly::Namespace.Class::Method`).  
- The chosen region (`eu-central-1`), runtime (`dotnet8`), memory (512 MB), and timeout (30 s) align with what's expected in your CDK stack and operational requirements.

</details>
<details>
<summary>backend/src/CfnStack.sln (3)</summary>

`6-6`: **Rename project entry to CfnStack**  
The old `Backend` project has been replaced with `CfnStack`. Verify that all references to the former `Backend` project have been removed and that the solution correctly loads and builds the `CfnStack` project.

---

`14-15`: **Add DownloadLambda project to solution**  
A new `DownloadLambda` project entry and its GUID have been added. Ensure the path `DownloadLambda\DownloadLambda.csproj` matches the folder structure and that the GUID is consistent across all solution configurations.

---

`74-85`: **Configure build settings for DownloadLambda**  
Debug/Release configurations for the `DownloadLambda` project have been appended. Confirm that these lines correctly enable builds for all specified platforms (`Any CPU`, `x64`, `x86`) and that the solution builds without errors.

</details>
<details>
<summary>backend/src/DownloadLambda/DownloadLambda.csproj (1)</summary>

`1-24`: **Review project file and package versions**  
The overall project structure and AWS Lambda settings look correct. Please verify that the NuGet package `AWSSDK.S3` version `4.0.0.2` is valid and consistent with other Lambda projects in the solution to avoid runtime mismatches.

</details>
<details>
<summary>backend/src/DownloadLambda/Function.cs (1)</summary>

`28-43`: **LGTM!**

Good error handling for request deserialization with appropriate logging and error response.

</details>
<details>
<summary>backend/src/CfnStack/MainStack.cs (1)</summary>

`15-43`: **LGTM!**

Lambda function configurations are well-structured with appropriate timeouts and runtime settings.

</details>

</blockquote></details>

</details>

<!-- This is an auto-generated comment by CodeRabbit for review status -->
state: COMMENTED
author_association: NONE
submitted_at: 2025-05-25T13:53:53Z
user: login: matfij
state: COMMENTED
author_association: OWNER
submitted_at: 2025-05-25T14:13:23Z
user: login: coderabbitai[bot]
state: COMMENTED
author_association: NONE
submitted_at: 2025-05-25T14:14:18Z
user: login: matfij
state: COMMENTED
author_association: OWNER
submitted_at: 2025-05-25T14:15:10Z
user: login: coderabbitai[bot]
state: COMMENTED
author_association: NONE
submitted_at: 2025-05-25T14:15:35Z

Review comments:
diff_hunk: @@ -0,0 +1,13 @@
+﻿using Amazon.CDK;
+
+namespace CfnStack;
+
+static class Program
+{
+    public static void Main(string[] args)
+    {
+        var app = new App();
+        new MainStack(app, "ParticleSimulatorStack", new StackProps { });
+        app.Synth();
+    }
+}
path: backend/src/CfnStack/Program.cs
user: login: coderabbitai[bot]
body: _🧹 Nitpick (assertive)_

**Explicitly specify deployment environment**  
Currently `MainStack` is instantiated with empty `StackProps`. Consider supplying an `env: new Environment { Account = "<ACCOUNT_ID>", Region = "<REGION>" }` in the `StackProps` constructor to bind this stack to a specific AWS account/region for reproducible deployments.

<details>
<summary>🤖 Prompt for AI Agents</summary>

```
In backend/src/CfnStack/Program.cs around lines 1 to 13, the MainStack is
created with empty StackProps, lacking explicit environment specification.
Update the StackProps constructor to include an env property with a new
Environment object specifying the AWS Account ID and Region to bind the stack to
a specific deployment environment for consistent and reproducible deployments.
```

</details>

<!-- This is an auto-generated comment by CodeRabbit -->
created_at: 2025-05-25T13:53:51Z
author_association: NONE
diff_hunk: @@ -0,0 +1,99 @@
+using System.Net;
+using System.Text.Json;
+using Amazon.Lambda.Core;
+using Amazon.Lambda.RuntimeSupport;
+using Amazon.Lambda.Serialization.SystemTextJson;
+using Amazon.S3;
+using SharedLibrary;
+using Amazon.Lambda.APIGatewayEvents;
+using Amazon.S3.Model;
+using Amazon.DynamoDBv2.DataModel;
+using Amazon.DynamoDBv2;
+
+var bucketName = "particle-simulation-bucket";
+var s3Client = new AmazonS3Client();
+
+var dbClient = new AmazonDynamoDBClient();
+var dbContext = new DynamoDBContext(dbClient);
+
+var httpHeaders = new Dictionary<string, string>
+{
+    { "Content-Type", "application/json" }
+};
+
+var handler = async (APIGatewayProxyRequest request, ILambdaContext context) =>
+{
+    try
+    {
+        SimulationDownloadRequest input;
+        try
+        {
+            input = JsonSerializer.Deserialize<SimulationDownloadRequest>(request.Body);
+        }
+        catch (Exception ex) when (ex is JsonException || ex is ArgumentNullException)
+        {
+            context.Logger.LogError($"Invalid request body: {ex}");
+            var error = new ApiError { Message = $"Invalid request body" };
+            return new APIGatewayProxyResponse
+            {
+                StatusCode = (int)HttpStatusCode.BadRequest,
+                Body = JsonSerializer.Serialize(error),
+                Headers = httpHeaders,
+            };
+        }
+
+        var simulation = await dbContext.LoadAsync<Simulation>(input.SimulationId);
+
+        if (simulation == null)
+        {
+            context.Logger.LogError($"Simulation not found: {input.SimulationId}");
+            var error = new ApiError { Message = $"Simulation not found: {input.SimulationId}" };
+            return new APIGatewayProxyResponse
+            {
+                StatusCode = (int)HttpStatusCode.BadRequest,
+                Body = JsonSerializer.Serialize(error),
+                Headers = httpHeaders,
+            };
+        }
+
+        simulation.Downloads += 1;
+        await dbContext.SaveAsync(simulation);
+
+        var s3Request = new GetPreSignedUrlRequest
+        {
+            BucketName = bucketName,
+            Key = simulation.FileName,
+            Verb = HttpVerb.GET,
+            Expires = DateTime.UtcNow.AddMinutes(5),
+        };
+
+        var url = s3Client.GetPreSignedURL(s3Request);
+
+        var response = new SimulationDownloadResponse
+        {
+            DownloadUrl = url,
+        };
+
+        return new APIGatewayProxyResponse
+        {
+            StatusCode = (int)HttpStatusCode.OK,
+            Body = JsonSerializer.Serialize(response),
+            Headers = httpHeaders,
+        };
+    }
+    catch (Exception ex)
+    {
+        context.Logger.LogError($"Error processing request: {ex}");
+        var error = new ApiError { Message = $"Unexpected error occurred" };
+        return new APIGatewayProxyResponse
+        {
+            StatusCode = (int)HttpStatusCode.BadRequest,
+            Body = JsonSerializer.Serialize(error),
+            Headers = httpHeaders,
+        };
+    }
path: backend/src/DownloadLambda/Function.cs
user: login: coderabbitai[bot]
body: _⚠️ Potential issue_

**Return appropriate HTTP status code for server errors.**

The generic exception handler returns 400 (Bad Request) for all errors, including potential server-side issues. Use 500 (Internal Server Error) for unexpected exceptions.


```diff
     catch (Exception ex)
     {
         context.Logger.LogError($"Error processing request: {ex}");
         var error = new ApiError { Message = $"Unexpected error occurred" };
         return new APIGatewayProxyResponse
         {
-            StatusCode = (int)HttpStatusCode.BadRequest,
+            StatusCode = (int)HttpStatusCode.InternalServerError,
             Body = JsonSerializer.Serialize(error),
             Headers = httpHeaders,
         };
     }
```


<!-- suggestion_start -->

<details>
<summary>📝 Committable suggestion</summary>

> ‼️ **IMPORTANT**
> Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.

```suggestion
    catch (Exception ex)
    {
        context.Logger.LogError($"Error processing request: {ex}");
        var error = new ApiError { Message = $"Unexpected error occurred" };
        return new APIGatewayProxyResponse
        {
            StatusCode = (int)HttpStatusCode.InternalServerError,
            Body = JsonSerializer.Serialize(error),
            Headers = httpHeaders,
        };
    }
```

</details>

<!-- suggestion_end -->

<details>
<summary>🤖 Prompt for AI Agents</summary>

```
In backend/src/DownloadLambda/Function.cs around lines 84 to 94, the catch block
for generic exceptions returns a 400 Bad Request status code, which is incorrect
for unexpected server errors. Change the StatusCode in the
APIGatewayProxyResponse to 500 to indicate an Internal Server Error for these
exceptions.
```

</details>

<!-- This is an auto-generated reply by CodeRabbit -->
created_at: 2025-05-25T13:53:52Z
author_association: NONE
diff_hunk: @@ -0,0 +1,99 @@
+using System.Net;
+using System.Text.Json;
+using Amazon.Lambda.Core;
+using Amazon.Lambda.RuntimeSupport;
+using Amazon.Lambda.Serialization.SystemTextJson;
+using Amazon.S3;
+using SharedLibrary;
+using Amazon.Lambda.APIGatewayEvents;
+using Amazon.S3.Model;
+using Amazon.DynamoDBv2.DataModel;
+using Amazon.DynamoDBv2;
+
+var bucketName = "particle-simulation-bucket";
+var s3Client = new AmazonS3Client();
path: backend/src/DownloadLambda/Function.cs
user: login: coderabbitai[bot]
body: _🛠️ Refactor suggestion_

**Make the bucket name configurable via environment variables.**

Hardcoding the bucket name makes it difficult to deploy to different environments or regions. Consider using environment variables for better flexibility and maintainability.


```diff
-var bucketName = "particle-simulation-bucket";
+var bucketName = Environment.GetEnvironmentVariable("S3_BUCKET_NAME") ?? throw new InvalidOperationException("S3_BUCKET_NAME environment variable is not set");
```


<!-- suggestion_start -->

<details>
<summary>📝 Committable suggestion</summary>

> ‼️ **IMPORTANT**
> Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.

```suggestion
var bucketName = Environment.GetEnvironmentVariable("S3_BUCKET_NAME") 
    ?? throw new InvalidOperationException("S3_BUCKET_NAME environment variable is not set");
var s3Client = new AmazonS3Client();
```

</details>

<!-- suggestion_end -->

<details>
<summary>🤖 Prompt for AI Agents</summary>

```
In backend/src/DownloadLambda/Function.cs around lines 13 to 14, the bucket name
is hardcoded as a string literal, which reduces flexibility. Modify the code to
read the bucket name from an environment variable instead, using a method like
Environment.GetEnvironmentVariable. This change will allow configuring the
bucket name externally without code changes, improving maintainability and
deployment flexibility.
```

</details>

<!-- This is an auto-generated comment by CodeRabbit -->
created_at: 2025-05-25T13:53:52Z
author_association: NONE
diff_hunk: @@ -0,0 +1,99 @@
+using System.Net;
+using System.Text.Json;
+using Amazon.Lambda.Core;
+using Amazon.Lambda.RuntimeSupport;
+using Amazon.Lambda.Serialization.SystemTextJson;
+using Amazon.S3;
+using SharedLibrary;
+using Amazon.Lambda.APIGatewayEvents;
+using Amazon.S3.Model;
+using Amazon.DynamoDBv2.DataModel;
+using Amazon.DynamoDBv2;
+
+var bucketName = "particle-simulation-bucket";
+var s3Client = new AmazonS3Client();
+
+var dbClient = new AmazonDynamoDBClient();
+var dbContext = new DynamoDBContext(dbClient);
+
+var httpHeaders = new Dictionary<string, string>
+{
+    { "Content-Type", "application/json" }
+};
+
+var handler = async (APIGatewayProxyRequest request, ILambdaContext context) =>
+{
+    try
+    {
+        SimulationDownloadRequest input;
+        try
+        {
+            input = JsonSerializer.Deserialize<SimulationDownloadRequest>(request.Body);
+        }
+        catch (Exception ex) when (ex is JsonException || ex is ArgumentNullException)
+        {
+            context.Logger.LogError($"Invalid request body: {ex}");
+            var error = new ApiError { Message = $"Invalid request body" };
+            return new APIGatewayProxyResponse
+            {
+                StatusCode = (int)HttpStatusCode.BadRequest,
+                Body = JsonSerializer.Serialize(error),
+                Headers = httpHeaders,
+            };
+        }
+
+        var simulation = await dbContext.LoadAsync<Simulation>(input.SimulationId);
+
+        if (simulation == null)
+        {
+            context.Logger.LogError($"Simulation not found: {input.SimulationId}");
+            var error = new ApiError { Message = $"Simulation not found: {input.SimulationId}" };
+            return new APIGatewayProxyResponse
+            {
+                StatusCode = (int)HttpStatusCode.BadRequest,
+                Body = JsonSerializer.Serialize(error),
+                Headers = httpHeaders,
+            };
+        }
path: backend/src/DownloadLambda/Function.cs
user: login: coderabbitai[bot]
body: _⚠️ Potential issue_

**Use HTTP 404 and avoid exposing simulation IDs in error messages.**

When a simulation is not found, HTTP 404 (Not Found) is more semantically correct than 400 (Bad Request). Also, consider using a generic error message to avoid exposing potentially sensitive IDs.


```diff
 if (simulation == null)
 {
     context.Logger.LogError($"Simulation not found: {input.SimulationId}");
-    var error = new ApiError { Message = $"Simulation not found: {input.SimulationId}" };
+    var error = new ApiError { Message = "Simulation not found" };
     return new APIGatewayProxyResponse
     {
-        StatusCode = (int)HttpStatusCode.BadRequest,
+        StatusCode = (int)HttpStatusCode.NotFound,
         Body = JsonSerializer.Serialize(error),
         Headers = httpHeaders,
     };
 }
```


<!-- suggestion_start -->

<details>
<summary>📝 Committable suggestion</summary>

> ‼️ **IMPORTANT**
> Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.

```suggestion
        if (simulation == null)
        {
            context.Logger.LogError($"Simulation not found: {input.SimulationId}");
            var error = new ApiError { Message = "Simulation not found" };
            return new APIGatewayProxyResponse
            {
                StatusCode = (int)HttpStatusCode.NotFound,
                Body = JsonSerializer.Serialize(error),
                Headers = httpHeaders,
            };
        }
```

</details>

<!-- suggestion_end -->

<details>
<summary>🤖 Prompt for AI Agents</summary>

```
In backend/src/DownloadLambda/Function.cs around lines 47 to 57, change the HTTP
status code from 400 (Bad Request) to 404 (Not Found) when the simulation is not
found. Also, replace the error message to a generic one that does not expose the
simulation ID, such as "Simulation not found," to avoid revealing sensitive
information.
```

</details>

<!-- This is an auto-generated comment by CodeRabbit -->
created_at: 2025-05-25T13:53:52Z
author_association: NONE
diff_hunk: @@ -0,0 +1,99 @@
+using System.Net;
+using System.Text.Json;
+using Amazon.Lambda.Core;
+using Amazon.Lambda.RuntimeSupport;
+using Amazon.Lambda.Serialization.SystemTextJson;
+using Amazon.S3;
+using SharedLibrary;
+using Amazon.Lambda.APIGatewayEvents;
+using Amazon.S3.Model;
+using Amazon.DynamoDBv2.DataModel;
+using Amazon.DynamoDBv2;
+
+var bucketName = "particle-simulation-bucket";
+var s3Client = new AmazonS3Client();
+
+var dbClient = new AmazonDynamoDBClient();
+var dbContext = new DynamoDBContext(dbClient);
+
+var httpHeaders = new Dictionary<string, string>
+{
+    { "Content-Type", "application/json" }
+};
+
+var handler = async (APIGatewayProxyRequest request, ILambdaContext context) =>
+{
+    try
+    {
+        SimulationDownloadRequest input;
+        try
+        {
+            input = JsonSerializer.Deserialize<SimulationDownloadRequest>(request.Body);
+        }
+        catch (Exception ex) when (ex is JsonException || ex is ArgumentNullException)
+        {
+            context.Logger.LogError($"Invalid request body: {ex}");
+            var error = new ApiError { Message = $"Invalid request body" };
+            return new APIGatewayProxyResponse
+            {
+                StatusCode = (int)HttpStatusCode.BadRequest,
+                Body = JsonSerializer.Serialize(error),
+                Headers = httpHeaders,
+            };
+        }
+
+        var simulation = await dbContext.LoadAsync<Simulation>(input.SimulationId);
+
+        if (simulation == null)
+        {
+            context.Logger.LogError($"Simulation not found: {input.SimulationId}");
+            var error = new ApiError { Message = $"Simulation not found: {input.SimulationId}" };
+            return new APIGatewayProxyResponse
+            {
+                StatusCode = (int)HttpStatusCode.BadRequest,
+                Body = JsonSerializer.Serialize(error),
+                Headers = httpHeaders,
+            };
+        }
+
+        simulation.Downloads += 1;
+        await dbContext.SaveAsync(simulation);
+
+        var s3Request = new GetPreSignedUrlRequest
+        {
+            BucketName = bucketName,
+            Key = simulation.FileName,
+            Verb = HttpVerb.GET,
+            Expires = DateTime.UtcNow.AddMinutes(5),
+        };
+
+        var url = s3Client.GetPreSignedURL(s3Request);
+
+        var response = new SimulationDownloadResponse
+        {
+            DownloadUrl = url,
+        };
path: backend/src/DownloadLambda/Function.cs
user: login: coderabbitai[bot]
body: _⚠️ Potential issue_

**Verify S3 object exists before incrementing download count.**

The current implementation increments the download count before verifying if the S3 object exists. If the file was deleted from S3 but the DynamoDB record remains, users would get invalid download URLs while the count keeps increasing.


Consider checking if the S3 object exists first:

```diff
+        // Verify the S3 object exists
+        try
+        {
+            var headRequest = new Amazon.S3.Model.GetObjectMetadataRequest
+            {
+                BucketName = bucketName,
+                Key = simulation.FileName
+            };
+            await s3Client.GetObjectMetadataAsync(headRequest);
+        }
+        catch (Amazon.S3.AmazonS3Exception ex) when (ex.StatusCode == HttpStatusCode.NotFound)
+        {
+            context.Logger.LogError($"S3 object not found for simulation: {input.SimulationId}");
+            var error = new ApiError { Message = "Simulation file not found" };
+            return new APIGatewayProxyResponse
+            {
+                StatusCode = (int)HttpStatusCode.NotFound,
+                Body = JsonSerializer.Serialize(error),
+                Headers = httpHeaders,
+            };
+        }
+
         simulation.Downloads += 1;
         await dbContext.SaveAsync(simulation);
```


<!-- suggestion_start -->

<details>
<summary>📝 Committable suggestion</summary>

> ‼️ **IMPORTANT**
> Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.

```suggestion
        // Verify the S3 object exists
        try
        {
            var headRequest = new Amazon.S3.Model.GetObjectMetadataRequest
            {
                BucketName = bucketName,
                Key = simulation.FileName
            };
            await s3Client.GetObjectMetadataAsync(headRequest);
        }
        catch (Amazon.S3.AmazonS3Exception ex) when (ex.StatusCode == HttpStatusCode.NotFound)
        {
            context.Logger.LogError($"S3 object not found for simulation: {input.SimulationId}");
            var error = new ApiError { Message = "Simulation file not found" };
            return new APIGatewayProxyResponse
            {
                StatusCode = (int)HttpStatusCode.NotFound,
                Body = JsonSerializer.Serialize(error),
                Headers = httpHeaders,
            };
        }

        simulation.Downloads += 1;
        await dbContext.SaveAsync(simulation);

        var s3Request = new GetPreSignedUrlRequest
        {
            BucketName = bucketName,
            Key = simulation.FileName,
            Verb = HttpVerb.GET,
            Expires = DateTime.UtcNow.AddMinutes(5),
        };

        var url = s3Client.GetPreSignedURL(s3Request);

        var response = new SimulationDownloadResponse
        {
            DownloadUrl = url,
        };
```

</details>

<!-- suggestion_end -->

<details>
<summary>🤖 Prompt for AI Agents</summary>

```
In backend/src/DownloadLambda/Function.cs around lines 59 to 75, the code
increments the download count before verifying the existence of the S3 object,
which can lead to inaccurate counts if the file is missing. Modify the code to
first check if the S3 object exists using an appropriate S3 API call before
incrementing the download count and saving the simulation. Only increment and
save after confirming the object is present.
```

</details>

<!-- This is an auto-generated comment by CodeRabbit -->
created_at: 2025-05-25T13:53:52Z
author_association: NONE
diff_hunk: @@ -0,0 +1,97 @@
+using Amazon.CDK;
+using Amazon.CDK.AWS.APIGateway;
+using Amazon.CDK.AWS.DynamoDB;
+using Amazon.CDK.AWS.IAM;
+using Amazon.CDK.AWS.Lambda;
+using Amazon.CDK.AWS.S3;
+using Constructs;
+
+namespace CfnStack;
+
+public class MainStack : Stack
+{
+    internal MainStack(Construct scope, string id, IStackProps props = null) : base(scope, id, props)
+    {
+        #region Upload Lambda
+        var uploadLambda = new Function(this, "upload-lambda", new FunctionProps
+        {
+            Runtime = Runtime.DOTNET_8,
+            Handler = "UploadLambda",
+            Code = Code.FromAsset("./src/UploadLambda/bin/Release/net8.0/UploadLambda.zip"),
+            Timeout = Duration.Seconds(30),
+        });
+        #endregion
+
+        #region Preview Lambda
+        var previewLambda = new Function(this, "preview-lambda", new FunctionProps
+        {
+            Runtime = Runtime.DOTNET_8,
+            Handler = "PreviewLambda",
+            Code = Code.FromAsset("./src/PreviewLambda/bin/Release/net8.0/PreviewLambda.zip"),
+            Timeout = Duration.Seconds(30),
+        });
+        #endregion
+
+        #region Download Lambda
+        var downloadLambda = new Function(this, "download-lambda", new FunctionProps
+        {
+            Runtime = Runtime.DOTNET_8,
+            Handler = "DownloadLambda",
+            Code = Code.FromAsset("./src/DownloadLambda/bin/Release/net8.0/DownloadLambda.zip"),
+            Timeout = Duration.Seconds(30),
+        });
+        #endregion
+
+        #region Simulation DynamoDB Table
+        var simulationTable = new Table(this, "simulation-table", new TableProps
+        {
+            TableName = "Simulation",
+            PartitionKey = new Attribute
+            {
+                Name = "Id",
+                Type = AttributeType.STRING,
+            },
+            RemovalPolicy = RemovalPolicy.DESTROY, // RETAIN in production
+            BillingMode = BillingMode.PAY_PER_REQUEST,
+        });
+
+        simulationTable.GrantWriteData(uploadLambda);
+        simulationTable.GrantReadData(previewLambda);
+        simulationTable.GrantReadWriteData(downloadLambda);
+        #endregion
+
+        #region Simulation S3 Bucket
+        var simulationBucketProps = new BucketProps
+        {
+            BucketName = "particle-simulation-bucket",
+            RemovalPolicy = RemovalPolicy.DESTROY,
path: backend/src/CfnStack/MainStack.cs
user: login: coderabbitai[bot]
body: _🛠️ Refactor suggestion_

**Parameterize bucket name and use environment-based removal policy.**

The hardcoded bucket name can cause deployment conflicts, and the DESTROY policy risks data loss in production.


```diff
+        var isProd = Environment.GetEnvironmentVariable("ENVIRONMENT") == "production";
+        var stackId = Stack.Of(this).StackId;
         var simulationBucketProps = new BucketProps
         {
-            BucketName = "particle-simulation-bucket",
-            RemovalPolicy = RemovalPolicy.DESTROY,
+            BucketName = $"particle-simulation-{stackId}",
+            RemovalPolicy = isProd ? RemovalPolicy.RETAIN : RemovalPolicy.DESTROY,
             BlockPublicAccess = BlockPublicAccess.BLOCK_ALL,
             EnforceSSL = true,
         };
```

Also, pass the bucket name to the Lambda functions as an environment variable:

```csharp
downloadLambda.AddEnvironment("S3_BUCKET_NAME", simulationBucket.BucketName);
```

<details>
<summary>🤖 Prompt for AI Agents</summary>

```
In backend/src/CfnStack/MainStack.cs around lines 66 to 67, replace the
hardcoded bucket name with a parameter or configuration value to avoid
deployment conflicts. Adjust the RemovalPolicy to use a conditional approach
based on the environment, such as retaining data in production and allowing
destruction only in development. Additionally, pass the bucket name to the
Lambda functions by adding it as an environment variable using
AddEnvironment("S3_BUCKET_NAME", simulationBucket.BucketName).
```

</details>

<!-- This is an auto-generated comment by CodeRabbit -->
created_at: 2025-05-25T13:53:52Z
author_association: NONE
diff_hunk: @@ -0,0 +1,97 @@
+using Amazon.CDK;
+using Amazon.CDK.AWS.APIGateway;
+using Amazon.CDK.AWS.DynamoDB;
+using Amazon.CDK.AWS.IAM;
+using Amazon.CDK.AWS.Lambda;
+using Amazon.CDK.AWS.S3;
+using Constructs;
+
+namespace CfnStack;
+
+public class MainStack : Stack
+{
+    internal MainStack(Construct scope, string id, IStackProps props = null) : base(scope, id, props)
+    {
+        #region Upload Lambda
+        var uploadLambda = new Function(this, "upload-lambda", new FunctionProps
+        {
+            Runtime = Runtime.DOTNET_8,
+            Handler = "UploadLambda",
+            Code = Code.FromAsset("./src/UploadLambda/bin/Release/net8.0/UploadLambda.zip"),
+            Timeout = Duration.Seconds(30),
+        });
+        #endregion
+
+        #region Preview Lambda
+        var previewLambda = new Function(this, "preview-lambda", new FunctionProps
+        {
+            Runtime = Runtime.DOTNET_8,
+            Handler = "PreviewLambda",
+            Code = Code.FromAsset("./src/PreviewLambda/bin/Release/net8.0/PreviewLambda.zip"),
+            Timeout = Duration.Seconds(30),
+        });
+        #endregion
+
+        #region Download Lambda
+        var downloadLambda = new Function(this, "download-lambda", new FunctionProps
+        {
+            Runtime = Runtime.DOTNET_8,
+            Handler = "DownloadLambda",
+            Code = Code.FromAsset("./src/DownloadLambda/bin/Release/net8.0/DownloadLambda.zip"),
+            Timeout = Duration.Seconds(30),
+        });
+        #endregion
+
+        #region Simulation DynamoDB Table
+        var simulationTable = new Table(this, "simulation-table", new TableProps
+        {
+            TableName = "Simulation",
+            PartitionKey = new Attribute
+            {
+                Name = "Id",
+                Type = AttributeType.STRING,
+            },
+            RemovalPolicy = RemovalPolicy.DESTROY, // RETAIN in production
+            BillingMode = BillingMode.PAY_PER_REQUEST,
+        });
+
+        simulationTable.GrantWriteData(uploadLambda);
+        simulationTable.GrantReadData(previewLambda);
+        simulationTable.GrantReadWriteData(downloadLambda);
+        #endregion
+
+        #region Simulation S3 Bucket
+        var simulationBucketProps = new BucketProps
+        {
+            BucketName = "particle-simulation-bucket",
+            RemovalPolicy = RemovalPolicy.DESTROY,
+            BlockPublicAccess = BlockPublicAccess.BLOCK_ALL,
+            EnforceSSL = true,
+        };
+        var simulationBucket = new Bucket(this, "particle-simulation-bucket", simulationBucketProps);
+
+        simulationBucket.GrantWrite(uploadLambda);
+        simulationBucket.GrantRead(downloadLambda);
+        #endregion
+
+        #region API Gateway
+        var api = new RestApi(this, "simulation-api", new RestApiProps
+        {
+            RestApiName = "Particle Simulator Gateway",
+            EndpointConfiguration = new EndpointConfiguration
+            {
+                Types = [EndpointType.REGIONAL]
+            }
+        });
+
+        var uploadLambdaIntegration = new LambdaIntegration(uploadLambda);
+        api.Root.AddResource("upload").AddMethod("POST", uploadLambdaIntegration);
+
+        var previewLambdaIntegration = new LambdaIntegration(previewLambda);
+        api.Root.AddResource("preview").AddMethod("GET", previewLambdaIntegration);
+
+        var downloadLambdaIntegration = new LambdaIntegration(downloadLambda);
+        api.Root.AddResource("download").AddMethod("POST", downloadLambdaIntegration);
+        #endregion
path: backend/src/CfnStack/MainStack.cs
user: login: coderabbitai[bot]
body: _🧹 Nitpick (assertive)_

**Consider adding CORS configuration and authentication.**

The API Gateway setup lacks CORS configuration (which may be needed for web clients) and authentication mechanisms.


For CORS support, add default CORS configuration:

```diff
 var api = new RestApi(this, "simulation-api", new RestApiProps
 {
     RestApiName = "Particle Simulator Gateway",
     EndpointConfiguration = new EndpointConfiguration
     {
         Types = [EndpointType.REGIONAL]
-    }
+    },
+    DefaultCorsPreflightOptions = new CorsOptions
+    {
+        AllowOrigins = Cors.ALL_ORIGINS,
+        AllowMethods = Cors.ALL_METHODS,
+        AllowHeaders = ["Content-Type", "Authorization"]
+    }
 });
```

For production, consider adding authentication using API keys, IAM, or Cognito User Pools to protect these endpoints.


<!-- suggestion_start -->

<details>
<summary>📝 Committable suggestion</summary>

> ‼️ **IMPORTANT**
> Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.

```suggestion
        #region API Gateway
        var api = new RestApi(this, "simulation-api", new RestApiProps
        {
            RestApiName = "Particle Simulator Gateway",
            EndpointConfiguration = new EndpointConfiguration
            {
                Types = [EndpointType.REGIONAL]
            },
            DefaultCorsPreflightOptions = new CorsOptions
            {
                AllowOrigins = Cors.ALL_ORIGINS,
                AllowMethods = Cors.ALL_METHODS,
                AllowHeaders = ["Content-Type", "Authorization"]
            }
        });

        var uploadLambdaIntegration = new LambdaIntegration(uploadLambda);
        api.Root.AddResource("upload").AddMethod("POST", uploadLambdaIntegration);

        var previewLambdaIntegration = new LambdaIntegration(previewLambda);
        api.Root.AddResource("preview").AddMethod("GET", previewLambdaIntegration);

        var downloadLambdaIntegration = new LambdaIntegration(downloadLambda);
        api.Root.AddResource("download").AddMethod("POST", downloadLambdaIntegration);
        #endregion
```

</details>

<!-- suggestion_end -->

<details>
<summary>🤖 Prompt for AI Agents</summary>

```
In backend/src/CfnStack/MainStack.cs around lines 77 to 95, the API Gateway
setup is missing CORS configuration and authentication. Add a CORS configuration
block to the RestApiProps to enable default CORS settings for the API resources.
Additionally, implement authentication by configuring API keys, IAM roles, or
Cognito User Pools on the API methods to secure the endpoints for production
use.
```

</details>

<!-- This is an auto-generated reply by CodeRabbit -->
created_at: 2025-05-25T13:53:52Z
author_association: NONE
diff_hunk: @@ -0,0 +1,97 @@
+using Amazon.CDK;
+using Amazon.CDK.AWS.APIGateway;
+using Amazon.CDK.AWS.DynamoDB;
+using Amazon.CDK.AWS.IAM;
+using Amazon.CDK.AWS.Lambda;
+using Amazon.CDK.AWS.S3;
+using Constructs;
+
+namespace CfnStack;
+
+public class MainStack : Stack
+{
+    internal MainStack(Construct scope, string id, IStackProps props = null) : base(scope, id, props)
+    {
+        #region Upload Lambda
+        var uploadLambda = new Function(this, "upload-lambda", new FunctionProps
+        {
+            Runtime = Runtime.DOTNET_8,
+            Handler = "UploadLambda",
+            Code = Code.FromAsset("./src/UploadLambda/bin/Release/net8.0/UploadLambda.zip"),
+            Timeout = Duration.Seconds(30),
+        });
+        #endregion
+
+        #region Preview Lambda
+        var previewLambda = new Function(this, "preview-lambda", new FunctionProps
+        {
+            Runtime = Runtime.DOTNET_8,
+            Handler = "PreviewLambda",
+            Code = Code.FromAsset("./src/PreviewLambda/bin/Release/net8.0/PreviewLambda.zip"),
+            Timeout = Duration.Seconds(30),
+        });
+        #endregion
+
+        #region Download Lambda
+        var downloadLambda = new Function(this, "download-lambda", new FunctionProps
+        {
+            Runtime = Runtime.DOTNET_8,
+            Handler = "DownloadLambda",
+            Code = Code.FromAsset("./src/DownloadLambda/bin/Release/net8.0/DownloadLambda.zip"),
+            Timeout = Duration.Seconds(30),
+        });
+        #endregion
+
+        #region Simulation DynamoDB Table
+        var simulationTable = new Table(this, "simulation-table", new TableProps
+        {
+            TableName = "Simulation",
+            PartitionKey = new Attribute
+            {
+                Name = "Id",
+                Type = AttributeType.STRING,
+            },
+            RemovalPolicy = RemovalPolicy.DESTROY, // RETAIN in production
path: backend/src/CfnStack/MainStack.cs
user: login: coderabbitai[bot]
body: _🛠️ Refactor suggestion_

**Use environment-based removal policy for production safety.**

The current implementation uses `DESTROY` with only a comment suggesting `RETAIN` for production. This creates a risk of accidental data loss in production environments.


Consider using environment-based configuration:

```diff
+        var isProd = Environment.GetEnvironmentVariable("ENVIRONMENT") == "production";
         var simulationTable = new Table(this, "simulation-table", new TableProps
         {
             TableName = "Simulation",
             PartitionKey = new Attribute
             {
                 Name = "Id",
                 Type = AttributeType.STRING,
             },
-            RemovalPolicy = RemovalPolicy.DESTROY, // RETAIN in production
+            RemovalPolicy = isProd ? RemovalPolicy.RETAIN : RemovalPolicy.DESTROY,
             BillingMode = BillingMode.PAY_PER_REQUEST,
         });
```


<!-- suggestion_start -->

<details>
<summary>📝 Committable suggestion</summary>

> ‼️ **IMPORTANT**
> Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.

```suggestion
            var isProd = Environment.GetEnvironmentVariable("ENVIRONMENT") == "production";
            var simulationTable = new Table(this, "simulation-table", new TableProps
            {
                TableName = "Simulation",
                PartitionKey = new Attribute
                {
                    Name = "Id",
                    Type = AttributeType.STRING,
                },
                RemovalPolicy = isProd ? RemovalPolicy.RETAIN : RemovalPolicy.DESTROY,
                BillingMode = BillingMode.PAY_PER_REQUEST,
            });
```

</details>

<!-- suggestion_end -->

<details>
<summary>🤖 Prompt for AI Agents</summary>

```
In backend/src/CfnStack/MainStack.cs at line 54, the RemovalPolicy is hardcoded
to DESTROY, which risks accidental data loss in production. Modify the code to
set RemovalPolicy conditionally based on the environment, using RETAIN for
production and DESTROY for non-production environments. Implement an environment
check (e.g., using a configuration variable or environment flag) to apply the
appropriate RemovalPolicy dynamically.
```

</details>

<!-- This is an auto-generated comment by CodeRabbit -->
created_at: 2025-05-25T13:53:52Z
author_association: NONE
diff_hunk: @@ -0,0 +1,99 @@
+using System.Net;
+using System.Text.Json;
+using Amazon.Lambda.Core;
+using Amazon.Lambda.RuntimeSupport;
+using Amazon.Lambda.Serialization.SystemTextJson;
+using Amazon.S3;
+using SharedLibrary;
+using Amazon.Lambda.APIGatewayEvents;
+using Amazon.S3.Model;
+using Amazon.DynamoDBv2.DataModel;
+using Amazon.DynamoDBv2;
+
+var bucketName = "particle-simulation-bucket";
+var s3Client = new AmazonS3Client();
+
+var dbClient = new AmazonDynamoDBClient();
+var dbContext = new DynamoDBContext(dbClient);
+
+var httpHeaders = new Dictionary<string, string>
+{
+    { "Content-Type", "application/json" }
+};
+
+var handler = async (APIGatewayProxyRequest request, ILambdaContext context) =>
+{
+    try
+    {
+        SimulationDownloadRequest input;
+        try
+        {
+            input = JsonSerializer.Deserialize<SimulationDownloadRequest>(request.Body);
+        }
+        catch (Exception ex) when (ex is JsonException || ex is ArgumentNullException)
+        {
+            context.Logger.LogError($"Invalid request body: {ex}");
+            var error = new ApiError { Message = $"Invalid request body" };
+            return new APIGatewayProxyResponse
+            {
+                StatusCode = (int)HttpStatusCode.BadRequest,
+                Body = JsonSerializer.Serialize(error),
+                Headers = httpHeaders,
+            };
+        }
+
+        var simulation = await dbContext.LoadAsync<Simulation>(input.SimulationId);
+
+        if (simulation == null)
+        {
+            context.Logger.LogError($"Simulation not found: {input.SimulationId}");
+            var error = new ApiError { Message = $"Simulation not found: {input.SimulationId}" };
+            return new APIGatewayProxyResponse
+            {
+                StatusCode = (int)HttpStatusCode.BadRequest,
+                Body = JsonSerializer.Serialize(error),
+                Headers = httpHeaders,
+            };
+        }
+
+        simulation.Downloads += 1;
+        await dbContext.SaveAsync(simulation);
+
+        var s3Request = new GetPreSignedUrlRequest
+        {
+            BucketName = bucketName,
+            Key = simulation.FileName,
+            Verb = HttpVerb.GET,
+            Expires = DateTime.UtcNow.AddMinutes(5),
+        };
+
+        var url = s3Client.GetPreSignedURL(s3Request);
+
+        var response = new SimulationDownloadResponse
+        {
+            DownloadUrl = url,
+        };
+
+        return new APIGatewayProxyResponse
+        {
+            StatusCode = (int)HttpStatusCode.OK,
+            Body = JsonSerializer.Serialize(response),
+            Headers = httpHeaders,
+        };
+    }
+    catch (Exception ex)
+    {
+        context.Logger.LogError($"Error processing request: {ex}");
+        var error = new ApiError { Message = $"Unexpected error occurred" };
+        return new APIGatewayProxyResponse
+        {
+            StatusCode = (int)HttpStatusCode.BadRequest,
+            Body = JsonSerializer.Serialize(error),
+            Headers = httpHeaders,
+        };
+    }
path: backend/src/DownloadLambda/Function.cs
user: login: matfij
body: Good catch, will update for every lambda.
created_at: 2025-05-25T14:13:23Z
author_association: OWNER
diff_hunk: @@ -0,0 +1,99 @@
+using System.Net;
+using System.Text.Json;
+using Amazon.Lambda.Core;
+using Amazon.Lambda.RuntimeSupport;
+using Amazon.Lambda.Serialization.SystemTextJson;
+using Amazon.S3;
+using SharedLibrary;
+using Amazon.Lambda.APIGatewayEvents;
+using Amazon.S3.Model;
+using Amazon.DynamoDBv2.DataModel;
+using Amazon.DynamoDBv2;
+
+var bucketName = "particle-simulation-bucket";
+var s3Client = new AmazonS3Client();
+
+var dbClient = new AmazonDynamoDBClient();
+var dbContext = new DynamoDBContext(dbClient);
+
+var httpHeaders = new Dictionary<string, string>
+{
+    { "Content-Type", "application/json" }
+};
+
+var handler = async (APIGatewayProxyRequest request, ILambdaContext context) =>
+{
+    try
+    {
+        SimulationDownloadRequest input;
+        try
+        {
+            input = JsonSerializer.Deserialize<SimulationDownloadRequest>(request.Body);
+        }
+        catch (Exception ex) when (ex is JsonException || ex is ArgumentNullException)
+        {
+            context.Logger.LogError($"Invalid request body: {ex}");
+            var error = new ApiError { Message = $"Invalid request body" };
+            return new APIGatewayProxyResponse
+            {
+                StatusCode = (int)HttpStatusCode.BadRequest,
+                Body = JsonSerializer.Serialize(error),
+                Headers = httpHeaders,
+            };
+        }
+
+        var simulation = await dbContext.LoadAsync<Simulation>(input.SimulationId);
+
+        if (simulation == null)
+        {
+            context.Logger.LogError($"Simulation not found: {input.SimulationId}");
+            var error = new ApiError { Message = $"Simulation not found: {input.SimulationId}" };
+            return new APIGatewayProxyResponse
+            {
+                StatusCode = (int)HttpStatusCode.BadRequest,
+                Body = JsonSerializer.Serialize(error),
+                Headers = httpHeaders,
+            };
+        }
+
+        simulation.Downloads += 1;
+        await dbContext.SaveAsync(simulation);
+
+        var s3Request = new GetPreSignedUrlRequest
+        {
+            BucketName = bucketName,
+            Key = simulation.FileName,
+            Verb = HttpVerb.GET,
+            Expires = DateTime.UtcNow.AddMinutes(5),
+        };
+
+        var url = s3Client.GetPreSignedURL(s3Request);
+
+        var response = new SimulationDownloadResponse
+        {
+            DownloadUrl = url,
+        };
+
+        return new APIGatewayProxyResponse
+        {
+            StatusCode = (int)HttpStatusCode.OK,
+            Body = JsonSerializer.Serialize(response),
+            Headers = httpHeaders,
+        };
+    }
+    catch (Exception ex)
+    {
+        context.Logger.LogError($"Error processing request: {ex}");
+        var error = new ApiError { Message = $"Unexpected error occurred" };
+        return new APIGatewayProxyResponse
+        {
+            StatusCode = (int)HttpStatusCode.BadRequest,
+            Body = JsonSerializer.Serialize(error),
+            Headers = httpHeaders,
+        };
+    }
path: backend/src/DownloadLambda/Function.cs
user: login: coderabbitai[bot]
body: <details>
<summary>🧩 Analysis chain</summary>

@matfij Great to hear! Let me help you identify all the locations where this pattern needs to be updated.




This will help us identify all the Lambda functions that need the same HTTP status code fix! 🐰

---

🏁 Script executed:
```
#!/bin/bash
# Search for similar error handling patterns in Lambda functions that return 400 for generic exceptions
echo "=== Searching for Lambda functions with similar error handling patterns ==="
echo ""

# Find all Lambda function files
echo "Found Lambda function files:"
fd -e cs . backend/src/ | grep -i lambda

echo ""
echo "=== Checking for BadRequest status codes in catch blocks ==="
# Search for catch blocks that return BadRequest status codes
rg -A 10 -B 2 "catch.*Exception.*ex\)" backend/src/ | rg -A 8 -B 2 "BadRequest"

echo ""
echo "=== Checking for hardcoded 400 status codes in exception handlers ==="
# Also search for hardcoded 400 status codes in exception handling
rg -A 5 -B 5 "StatusCode.*400" backend/src/
```

Length of output: 4060

---



</details>

Perfect! I found the pattern across all your Lambda functions. Here are the locations where the generic exception handlers incorrectly return `400 Bad Request` instead of `500 Internal Server Error`:

## Files that need updates:

### 1. **DownloadLambda/Function.cs** ✅ (already identified)
- Generic exception handler around line 84-94

### 2. **PreviewLambda/Function.cs** 
- Generic exception handler (similar pattern)

### 3. **UploadLambda/Function.cs**
- Generic exception handler (similar pattern)

All three Lambda functions follow the same pattern:
- ✅ **Specific JSON exception handling** (correctly returns 400 - should stay as is)
- ❌ **Generic exception handling** (incorrectly returns 400 - should be 500)

The specific JSON deserialization errors should remain as `400 Bad Request` since those are client-side errors (malformed request body), but the generic `catch (Exception ex)` blocks should return `500 Internal Server Error` for unexpected server-side issues.

This consistency across all Lambda functions shows good architectural patterns - just needs this one HTTP status code fix applied uniformly! 

(◕‿◕)♡ Happy to help identify all the spots that need updating!

---

<details>
<summary>🧬 Code Graph Analysis Results</summary>

### Possibly relevant snippets from `SimulatorUI/Api/ApiModels.cs` (lines 29-32)
```csharp
public class ApiError
{
    public required string Message { get; init; }
}
```
- This class represents the error response model used in the Lambda function to return error messages in JSON format. It has a single required property `Message` which is a string describing the error. This is serialized and sent back in the API Gateway response body when errors occur.

</details>


<!-- <agentComponentType>chat-answer</agentComponentType> -->

<!-- <rawResChunk><answer><details>
<summary>🧩 Analysis chain</summary>

@matfij Great to hear! Let me help you identify all the locations where this pattern needs to be updated.




This will help us identify all the Lambda functions that need the same HTTP status code fix! 🐰

---

🏁 Script executed:
```
#!/bin/bash
# Search for similar error handling patterns in Lambda functions that return 400 for generic exceptions
echo "=== Searching for Lambda functions with similar error handling patterns ==="
echo ""

# Find all Lambda function files
echo "Found Lambda function files:"
fd -e cs . backend/src/ | grep -i lambda

echo ""
echo "=== Checking for BadRequest status codes in catch blocks ==="
# Search for catch blocks that return BadRequest status codes
rg -A 10 -B 2 "catch.*Exception.*ex\)" backend/src/ | rg -A 8 -B 2 "BadRequest"

echo ""
echo "=== Checking for hardcoded 400 status codes in exception handlers ==="
# Also search for hardcoded 400 status codes in exception handling
rg -A 5 -B 5 "StatusCode.*400" backend/src/
```

Length of output: 4060

---



</details>

Perfect! I found the pattern across all your Lambda functions. Here are the locations where the generic exception handlers incorrectly return `400 Bad Request` instead of `500 Internal Server Error`:

## Files that need updates:

### 1. **DownloadLambda/Function.cs** ✅ (already identified)
- Generic exception handler around line 84-94

### 2. **PreviewLambda/Function.cs** 
- Generic exception handler (similar pattern)

### 3. **UploadLambda/Function.cs**
- Generic exception handler (similar pattern)

All three Lambda functions follow the same pattern:
- ✅ **Specific JSON exception handling** (correctly returns 400 - should stay as is)
- ❌ **Generic exception handling** (incorrectly returns 400 - should be 500)

The specific JSON deserialization errors should remain as `400 Bad Request` since those are client-side errors (malformed request body), but the generic `catch (Exception ex)` blocks should return `500 Internal Server Error` for unexpected server-side issues.

This consistency across all Lambda functions shows good architectural patterns - just needs this one HTTP status code fix applied uniformly! 

(◕‿◕)♡ Happy to help identify all the spots that need updating!

---

<details>
<summary>🧬 Code Graph Analysis Results</summary>

### Possibly relevant snippets from `SimulatorUI/Api/ApiModels.cs` (lines 29-32)
```csharp
public class ApiError
{
    public required string Message { get; init; }
}
```
- This class represents the error response model used in the Lambda function to return error messages in JSON format. It has a single required property `Message` which is a string describing the error. This is serialized and sent back in the API Gateway response body when errors occur.

</details>

</answer></rawResChunk> -->

<!-- This is an auto-generated reply by CodeRabbit -->
created_at: 2025-05-25T14:14:18Z
author_association: NONE
diff_hunk: @@ -0,0 +1,97 @@
+using Amazon.CDK;
+using Amazon.CDK.AWS.APIGateway;
+using Amazon.CDK.AWS.DynamoDB;
+using Amazon.CDK.AWS.IAM;
+using Amazon.CDK.AWS.Lambda;
+using Amazon.CDK.AWS.S3;
+using Constructs;
+
+namespace CfnStack;
+
+public class MainStack : Stack
+{
+    internal MainStack(Construct scope, string id, IStackProps props = null) : base(scope, id, props)
+    {
+        #region Upload Lambda
+        var uploadLambda = new Function(this, "upload-lambda", new FunctionProps
+        {
+            Runtime = Runtime.DOTNET_8,
+            Handler = "UploadLambda",
+            Code = Code.FromAsset("./src/UploadLambda/bin/Release/net8.0/UploadLambda.zip"),
+            Timeout = Duration.Seconds(30),
+        });
+        #endregion
+
+        #region Preview Lambda
+        var previewLambda = new Function(this, "preview-lambda", new FunctionProps
+        {
+            Runtime = Runtime.DOTNET_8,
+            Handler = "PreviewLambda",
+            Code = Code.FromAsset("./src/PreviewLambda/bin/Release/net8.0/PreviewLambda.zip"),
+            Timeout = Duration.Seconds(30),
+        });
+        #endregion
+
+        #region Download Lambda
+        var downloadLambda = new Function(this, "download-lambda", new FunctionProps
+        {
+            Runtime = Runtime.DOTNET_8,
+            Handler = "DownloadLambda",
+            Code = Code.FromAsset("./src/DownloadLambda/bin/Release/net8.0/DownloadLambda.zip"),
+            Timeout = Duration.Seconds(30),
+        });
+        #endregion
+
+        #region Simulation DynamoDB Table
+        var simulationTable = new Table(this, "simulation-table", new TableProps
+        {
+            TableName = "Simulation",
+            PartitionKey = new Attribute
+            {
+                Name = "Id",
+                Type = AttributeType.STRING,
+            },
+            RemovalPolicy = RemovalPolicy.DESTROY, // RETAIN in production
+            BillingMode = BillingMode.PAY_PER_REQUEST,
+        });
+
+        simulationTable.GrantWriteData(uploadLambda);
+        simulationTable.GrantReadData(previewLambda);
+        simulationTable.GrantReadWriteData(downloadLambda);
+        #endregion
+
+        #region Simulation S3 Bucket
+        var simulationBucketProps = new BucketProps
+        {
+            BucketName = "particle-simulation-bucket",
+            RemovalPolicy = RemovalPolicy.DESTROY,
+            BlockPublicAccess = BlockPublicAccess.BLOCK_ALL,
+            EnforceSSL = true,
+        };
+        var simulationBucket = new Bucket(this, "particle-simulation-bucket", simulationBucketProps);
+
+        simulationBucket.GrantWrite(uploadLambda);
+        simulationBucket.GrantRead(downloadLambda);
+        #endregion
+
+        #region API Gateway
+        var api = new RestApi(this, "simulation-api", new RestApiProps
+        {
+            RestApiName = "Particle Simulator Gateway",
+            EndpointConfiguration = new EndpointConfiguration
+            {
+                Types = [EndpointType.REGIONAL]
+            }
+        });
+
+        var uploadLambdaIntegration = new LambdaIntegration(uploadLambda);
+        api.Root.AddResource("upload").AddMethod("POST", uploadLambdaIntegration);
+
+        var previewLambdaIntegration = new LambdaIntegration(previewLambda);
+        api.Root.AddResource("preview").AddMethod("GET", previewLambdaIntegration);
+
+        var downloadLambdaIntegration = new LambdaIntegration(downloadLambda);
+        api.Root.AddResource("download").AddMethod("POST", downloadLambdaIntegration);
+        #endregion
path: backend/src/CfnStack/MainStack.cs
user: login: matfij
body: Is it required for desktop .Net clients?
created_at: 2025-05-25T14:15:10Z
author_association: OWNER
diff_hunk: @@ -0,0 +1,97 @@
+using Amazon.CDK;
+using Amazon.CDK.AWS.APIGateway;
+using Amazon.CDK.AWS.DynamoDB;
+using Amazon.CDK.AWS.IAM;
+using Amazon.CDK.AWS.Lambda;
+using Amazon.CDK.AWS.S3;
+using Constructs;
+
+namespace CfnStack;
+
+public class MainStack : Stack
+{
+    internal MainStack(Construct scope, string id, IStackProps props = null) : base(scope, id, props)
+    {
+        #region Upload Lambda
+        var uploadLambda = new Function(this, "upload-lambda", new FunctionProps
+        {
+            Runtime = Runtime.DOTNET_8,
+            Handler = "UploadLambda",
+            Code = Code.FromAsset("./src/UploadLambda/bin/Release/net8.0/UploadLambda.zip"),
+            Timeout = Duration.Seconds(30),
+        });
+        #endregion
+
+        #region Preview Lambda
+        var previewLambda = new Function(this, "preview-lambda", new FunctionProps
+        {
+            Runtime = Runtime.DOTNET_8,
+            Handler = "PreviewLambda",
+            Code = Code.FromAsset("./src/PreviewLambda/bin/Release/net8.0/PreviewLambda.zip"),
+            Timeout = Duration.Seconds(30),
+        });
+        #endregion
+
+        #region Download Lambda
+        var downloadLambda = new Function(this, "download-lambda", new FunctionProps
+        {
+            Runtime = Runtime.DOTNET_8,
+            Handler = "DownloadLambda",
+            Code = Code.FromAsset("./src/DownloadLambda/bin/Release/net8.0/DownloadLambda.zip"),
+            Timeout = Duration.Seconds(30),
+        });
+        #endregion
+
+        #region Simulation DynamoDB Table
+        var simulationTable = new Table(this, "simulation-table", new TableProps
+        {
+            TableName = "Simulation",
+            PartitionKey = new Attribute
+            {
+                Name = "Id",
+                Type = AttributeType.STRING,
+            },
+            RemovalPolicy = RemovalPolicy.DESTROY, // RETAIN in production
+            BillingMode = BillingMode.PAY_PER_REQUEST,
+        });
+
+        simulationTable.GrantWriteData(uploadLambda);
+        simulationTable.GrantReadData(previewLambda);
+        simulationTable.GrantReadWriteData(downloadLambda);
+        #endregion
+
+        #region Simulation S3 Bucket
+        var simulationBucketProps = new BucketProps
+        {
+            BucketName = "particle-simulation-bucket",
+            RemovalPolicy = RemovalPolicy.DESTROY,
+            BlockPublicAccess = BlockPublicAccess.BLOCK_ALL,
+            EnforceSSL = true,
+        };
+        var simulationBucket = new Bucket(this, "particle-simulation-bucket", simulationBucketProps);
+
+        simulationBucket.GrantWrite(uploadLambda);
+        simulationBucket.GrantRead(downloadLambda);
+        #endregion
+
+        #region API Gateway
+        var api = new RestApi(this, "simulation-api", new RestApiProps
+        {
+            RestApiName = "Particle Simulator Gateway",
+            EndpointConfiguration = new EndpointConfiguration
+            {
+                Types = [EndpointType.REGIONAL]
+            }
+        });
+
+        var uploadLambdaIntegration = new LambdaIntegration(uploadLambda);
+        api.Root.AddResource("upload").AddMethod("POST", uploadLambdaIntegration);
+
+        var previewLambdaIntegration = new LambdaIntegration(previewLambda);
+        api.Root.AddResource("preview").AddMethod("GET", previewLambdaIntegration);
+
+        var downloadLambdaIntegration = new LambdaIntegration(downloadLambda);
+        api.Root.AddResource("download").AddMethod("POST", downloadLambdaIntegration);
+        #endregion
path: backend/src/CfnStack/MainStack.cs
user: login: coderabbitai[bot]
body: @matfij You're absolutely right! CORS configuration is **not required** for desktop .NET clients.

CORS (Cross-Origin Resource Sharing) is specifically a browser security mechanism that restricts cross-origin requests made from JavaScript in web applications. Desktop .NET applications using `HttpClient` don't run in a browser context and therefore don't have same-origin policy restrictions - they can make requests to any API endpoint without CORS considerations.

You would only need CORS configuration if:
- You plan to access this API from a web frontend (JavaScript/browser-based)
- You're building a web application that calls this API from the client-side

For desktop .NET clients, the API Gateway configuration you have is perfectly adequate. Authentication would be a separate consideration based on your security requirements, but CORS specifically is not needed for your use case.

---

<details>
<summary>✏️ Learnings added</summary>

```
Learnt from: matfij
PR: matfij/particle-simulator#47
File: backend/src/CfnStack/MainStack.cs:77-95
Timestamp: 2025-05-25T14:15:34.374Z
Learning: CORS configuration in API Gateway is not required for desktop .NET clients since they don't run in browser contexts and aren't subject to same-origin policy restrictions.
```

> Note: Learnings are effective only in the context of similar code segments. To apply general coding guidelines, please configure [review instructions](https://docs.coderabbit.ai/guides/review-instructions). You can manage existing learnings in the [app](https://app.coderabbit.ai/learnings).

</details>


<!-- <agentComponentType>chat-answer</agentComponentType> -->

<!-- <rawResChunk><answer>@matfij You're absolutely right! CORS configuration is **not required** for desktop .NET clients.

CORS (Cross-Origin Resource Sharing) is specifically a browser security mechanism that restricts cross-origin requests made from JavaScript in web applications. Desktop .NET applications using `HttpClient` don't run in a browser context and therefore don't have same-origin policy restrictions - they can make requests to any API endpoint without CORS considerations.

You would only need CORS configuration if:
- You plan to access this API from a web frontend (JavaScript/browser-based)
- You're building a web application that calls this API from the client-side

For desktop .NET clients, the API Gateway configuration you have is perfectly adequate. Authentication would be a separate consideration based on your security requirements, but CORS specifically is not needed for your use case.

---

<details>
<summary>✏️ Learnings added</summary>

```
Learnt from: matfij
PR: matfij/particle-simulator#47
File: backend/src/CfnStack/MainStack.cs:77-95
Timestamp: 2025-05-25T14:15:34.374Z
Learning: CORS configuration in API Gateway is not required for desktop .NET clients since they don't run in browser contexts and aren't subject to same-origin policy restrictions.
```

> Note: Learnings are effective only in the context of similar code segments. To apply general coding guidelines, please configure [review instructions](https://docs.coderabbit.ai/guides/review-instructions). You can manage existing learnings in the [app](https://app.coderabbit.ai/learnings).

</details>

</answer></rawResChunk> -->

<!-- This is an auto-generated reply by CodeRabbit -->
created_at: 2025-05-25T14:15:35Z
author_association: NONE

PR comments:
user: login: coderabbitai[bot]
created_at: 2025-05-25T13:49:37Z
body: <!-- This is an auto-generated comment: summarize by coderabbit.ai -->
<!-- walkthrough_start -->

## Walkthrough

This update restructures the backend AWS infrastructure by replacing the original "Backend" CDK stack with a new "CfnStack" stack, introducing a new Lambda function for simulation downloads, and updating supporting DTOs. The upload Lambda's pre-signed URL expiry is shortened, and solution/project files are updated to reflect these changes. Minor formatting changes are made to UI files. Error handling status codes in upload and preview Lambdas are adjusted to use 500 Internal Server Error on failures.

## Changes

| File(s)                                                                                   | Change Summary                                                                                                                     |
|-------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------|
| SimulatorUI/Components/DownloadPage.xaml, DownloadPage.xaml.cs                            | Added newline at end of file; no functional or structural changes.                                                                 |
| backend/cdk.json                                                                          | Changed CDK "app" command to point from Backend project to CfnStack project.                                                       |
| backend/src/Backend/BackendStack.cs, Program.cs                                           | Deleted original backend CDK stack and entry point.                                                                                |
| backend/src/CfnStack.sln                                                                  | Renamed project from "Backend" to "CfnStack" and added "DownloadLambda" project to solution.                                       |
| backend/src/CfnStack/MainStack.cs                                                         | Added new CDK stack class provisioning upload, preview, and download Lambdas, DynamoDB, S3 bucket, and API Gateway resources.      |
| backend/src/CfnStack/Program.cs                                                           | Added new program entry point for synthesizing the new CDK stack.                                                                  |
| backend/src/DownloadLambda/DownloadLambda.csproj, aws-lambda-tools-defaults.json           | Added new .NET project and Lambda deployment config for DownloadLambda.                                                            |
| backend/src/DownloadLambda/Function.cs                                                    | Added new Lambda function to generate pre-signed S3 download URLs and update download counts in DynamoDB.                          |
| backend/src/SharedLibrary/DTOs.cs                                                         | Added DTOs: `SimulationDownloadRequest` and `SimulationDownloadResponse` for download Lambda.                                      |
| backend/src/UploadLambda/Function.cs                                                      | Reduced pre-signed S3 upload URL expiry from 15 to 5 minutes; changed error response status code from 400 to 500.                  |
| backend/src/PreviewLambda/Function.cs                                                     | Changed error response status code from 400 to 500 in catch block.                                                                 |

## Sequence Diagram(s)

```mermaid
sequenceDiagram
    participant Client
    participant API Gateway
    participant DownloadLambda
    participant DynamoDB
    participant S3

    Client->>API Gateway: POST /download (SimulationId)
    API Gateway->>DownloadLambda: Invoke Lambda with SimulationId
    DownloadLambda->>DynamoDB: GetItem(SimulationId)
    DynamoDB-->>DownloadLambda: Simulation record
    DownloadLambda->>DynamoDB: UpdateItem (increment download count)
    DownloadLambda->>S3: Generate pre-signed download URL
    S3-->>DownloadLambda: Pre-signed URL
    DownloadLambda-->>API Gateway: 200 OK (DownloadUrl)
    API Gateway-->>Client: 200 OK (DownloadUrl)
```

```mermaid
sequenceDiagram
    participant Client
    participant API Gateway
    participant UploadLambda
    participant S3

    Client->>API Gateway: POST /upload
    API Gateway->>UploadLambda: Invoke Lambda
    UploadLambda->>S3: Generate pre-signed upload URL (5 min expiry)
    S3-->>UploadLambda: Pre-signed URL
    UploadLambda-->>API Gateway: 200 OK (UploadUrl)
    API Gateway-->>Client: 200 OK (UploadUrl)
```

<!-- walkthrough_end -->


---

<details>
<summary>📜 Recent review details</summary>

**Configuration used: CodeRabbit UI**
**Review profile: ASSERTIVE**
**Plan: Pro**


<details>
<summary>📥 Commits</summary>

Reviewing files that changed from the base of the PR and between d1a16237da8e9c3e217f9304b0882b74b5e41fe3 and ef3a879cb0cc883e7e82883edfbf1a5ccb8e36f1.

</details>

<details>
<summary>📒 Files selected for processing (3)</summary>

* `backend/src/DownloadLambda/Function.cs` (1 hunks)
* `backend/src/PreviewLambda/Function.cs` (1 hunks)
* `backend/src/UploadLambda/Function.cs` (2 hunks)

</details>

<details>
<summary>🧰 Additional context used</summary>

<details>
<summary>🧠 Learnings (1)</summary>

<details>
<summary>backend/src/UploadLambda/Function.cs (1)</summary>

```
Learnt from: matfij
PR: matfij/particle-simulator#34
File: backend/src/FileUploadLambda/aws-lambda-tools-defaults.json:1-9
Timestamp: 2025-04-28T19:38:26.290Z
Learning: The FileUploadLambda function in the particle-simulator project is responsible for generating pre-signed URLs for S3 bucket uploads, not for streaming files. It should have an appropriate timeout setting (30-60 seconds) rather than the initial 600 seconds.
```

</details>

</details><details>
<summary>🧬 Code Graph Analysis (1)</summary>

<details>
<summary>backend/src/DownloadLambda/Function.cs (1)</summary><blockquote>

<details>
<summary>SimulatorUI/Api/ApiModels.cs (1)</summary>

* `ApiError` (29-32)

</details>

</blockquote></details>

</details>

</details>

<details>
<summary>⏰ Context from checks skipped due to timeout of 90000ms (1)</summary>

* GitHub Check: Build and analyze

</details>

<details>
<summary>🔇 Additional comments (6)</summary><blockquote>

<details>
<summary>backend/src/UploadLambda/Function.cs (2)</summary>

`62-62`: **Good security improvement reducing URL expiration time.**

Reducing the pre-signed URL expiration from 15 to 5 minutes improves security by minimizing the exposure window for temporary access URLs.

---

`87-87`: **Correct HTTP status code for server errors.**

Good fix! Changing from 400 (Bad Request) to 500 (Internal Server Error) is appropriate for unexpected exceptions, as these represent server-side failures rather than client errors.

</details>
<details>
<summary>backend/src/PreviewLambda/Function.cs (1)</summary>

`56-56`: **Consistent error handling improvement.**

Excellent! This change aligns with the same fix applied to other Lambda functions, ensuring consistent error handling across the codebase where unexpected exceptions return 500 (Internal Server Error).

</details>
<details>
<summary>backend/src/DownloadLambda/Function.cs (3)</summary>

`90-90`: **HTTP status code fix implemented correctly.**

Good! The generic exception handler now correctly returns 500 (Internal Server Error) instead of 400 (Bad Request) for unexpected server-side failures.

---

`53-53`: **HTTP 404 status code correctly implemented.**

Good! Using 404 (Not Found) is semantically correct when a simulation record doesn't exist in the database.

---

`67-67`: **Consistent URL expiration time across Lambda functions.**

Good consistency! The 5-minute expiration time matches the updated UploadLambda, providing a balanced approach between security and usability.

</details>

</blockquote></details>

</details>
<!-- internal state start -->


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-->

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<details open="true">
<summary>✨ Finishing Touches</summary>

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author_association: NONE
user: login: sonarqubecloud[bot]
created_at: 2025-05-25T14:24:13Z
body: ## [![Quality Gate Passed](https://sonarsource.github.io/sonarcloud-github-static-resources/v2/checks/QualityGateBadge/qg-passed-20px.png 'Quality Gate Passed')](https://sonarcloud.io/dashboard?id=matfij_particle-simulator&pullRequest=47) **Quality Gate passed**  
Issues  
![](https://sonarsource.github.io/sonarcloud-github-static-resources/v2/common/passed-16px.png '') [0 New issues](https://sonarcloud.io/project/issues?id=matfij_particle-simulator&pullRequest=47&issueStatuses=OPEN,CONFIRMED&sinceLeakPeriod=true)  
![](https://sonarsource.github.io/sonarcloud-github-static-resources/v2/common/accepted-16px.png '') [0 Accepted issues](https://sonarcloud.io/project/issues?id=matfij_particle-simulator&pullRequest=47&issueStatuses=ACCEPTED)

Measures  
![](https://sonarsource.github.io/sonarcloud-github-static-resources/v2/common/passed-16px.png '') [0 Security Hotspots](https://sonarcloud.io/project/security_hotspots?id=matfij_particle-simulator&pullRequest=47&issueStatuses=OPEN,CONFIRMED&sinceLeakPeriod=true)  
![](https://sonarsource.github.io/sonarcloud-github-static-resources/v2/common/passed-16px.png '') [0.0% Coverage on New Code](https://sonarcloud.io/component_measures?id=matfij_particle-simulator&pullRequest=47&metric=new_coverage&view=list)  
![](https://sonarsource.github.io/sonarcloud-github-static-resources/v2/common/passed-16px.png '') [0.0% Duplication on New Code](https://sonarcloud.io/component_measures?id=matfij_particle-simulator&pullRequest=47&metric=new_duplicated_lines_density&view=list)  
  
[See analysis details on SonarQube Cloud](https://sonarcloud.io/dashboard?id=matfij_particle-simulator&pullRequest=47)


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