[WIP] A voting application based on dapr

Note: This example may not follow best practices. It's a playground for me, to get comfortable with the .NET platform and dapr. This may change in the feature. Please don't hesitate to share your suggestions via GitHub issues.

An example application that uses different dapr concepts like Actors, Service-to-Service Invocation, State Management and Publish and Subscribe.

The application consists of three services:

Service	Description	Language
votes	Handles the voting	F#
subscriptions	Handles the subscriptions	F#
notifications	Allows to subscribe for updates	C#
frontend	The react frontend application	TypeScript

Architectural decisions

Using actors to manipulate state

The update process with Dapr's state store, isn't an atomic operation. We must first retrieve the data from the store, manipulate the data and store it again with the updated data. To not lose data because of race conditions, we use actors to handle the state manipulation. Due to it's nature an actor processes it's mail box sequentially. Therefore we don't need to implement locking or synchronization in our services.

Implement a gRPC API and call it from another service

The Dapr.NET SDK documentation is a bit vague when it comes to describing the implementation of a gRPC API and how they will be called from another service. After some googling, I found out that an application must implement the AppCallback service based on the appcallback.proto. The AppCallback service defines an OnInvoke method which must be implemented. This method can then be called with the DaprClient.InvokeMethodAsync method. In our case, we have implemented this method as a proxy that calls our gRPC API. The concrete implementation can be found in the notifications/Services/DaprService file.

All proto files are located in a separate proto project, so that it can be referenced by the other projects.

Run the votes service

dapr run --app-id votes --app-port 3000 -- dotnet run --project "./votes/votes.fsproj"

Run the subscriptions service

dapr run --app-id subscriptions --app-port 3001 -- dotnet run --project "./subscriptions/subscriptions.fsproj"

Run the notifications service

The notification service uses a gRPC instead of a REST API. This must be activated by the --protocol grpc argument.

dapr run --app-id notifications --app-port 3002 --app-protocol grpc -- dotnet run --project "./notifications/notifications.csproj"

Run a local mail server

The notification service sends emails to subscribed users. To run a local email server for development, the maildev package can be used.

npm install -g maildev
maildev --smtp 1025 --web 8080

This starts a local mail server on port 1025. The incoming mails can be viewed on http://localhost:8080