ProcessBus

This is a simple abstraction layer over LanguageExt.Process to wrangle servicebus functionality out of it. It started as an excuse to see how writing Haskell in C# felt, using LanguageExt.

Usage

Configuration

We start with a routing configuration. Because we define our routing topology in YAML, it is quite easy to do some quite complex wiring up of various components of your architecture.

Transports:

# Events
- Path: UserJoined
  ForwardTo:
  - Email
- Path: Purchase
  ForwardTo:
  - Email
- Path: PrivateMessage
  ForwardTo:
  - WebSockets

# Endpoints
- Path: Email
- Path: WebSockets

# Logging
- Path: Logging
  ForwardFrom:
  - UserJoined
  - Purchase
  - PrivateMessage

Let's say a user joins your site. All your backend has to do is raise an event, UserJoined. This configuration will have the UserJoined message automatically forwarded to your email endpoint, where you could handle it to send a welcome email, for example. We can decouple our services very easily if our event raisers don't have to know where the messages should be delivered.

Similarly, the PrivateMessage is set up to automatically forward to the WebSockets endpoint, but if we wanted to send emails on private messages as well, we would only have to add a forward to the Email endpoint, and set up our email service to handle it (see Receiving section):

- Path: PrivateMessage
  ForwardTo:
  - WebSockets
  - Email
 ```

Note that the logging endpoint is configured to receive *from* events, so we don't have to clutter up every event with a forward to the logging endpoint.

### Flexibility
Let's say we want to add a monitoring dashboard. It doesn't take long to modify the configuration to handle this in an elegant way:

```Yaml
Transports:

# Events
- Path: UserJoined
  ForwardTo:
  - Email
- Path: Purchase
  ForwardTo:
  - Email
- Path: PrivateMessage
  ForwardTo:
  - WebSockets

 # Forward all events to this transport
- Path: Events 
  ForwardFrom:
  - UserJoined
  - Purchase
  - PrivateMessage

# Endpoints
- Path: Email
- Path: WebSockets

# Logging
- Path: Logging
  ForwardFrom:
  - Events

# Monitoring
- Path: Monitor
  ForwardFrom:
  - Events
```

We just add a new `Events` path that grabs all the events and groups them under one name. This means that instead of listing out every event for the `Logging` and `Monitor` endpoints, we can just get them to listen to the grouped `Event` endpoint.

### Validation
There's quite a bit of validation on the configuration, for example:

#### Typos
```Yaml
Transports:
- Path: EventA
- Path: EventB
  ForwardTo:
  - EvetnC
- Path: EventC  
```

gives:

```> Configuration specifies forward to non-existent transport "EvetnC".```

#### Cyclic forwarding 

```Yaml
Transports:
- Path: EventA
  ForwardTo:
  - EventB
- Path: EventB
  ForwardTo:
  - EventC
- Path: EventC  
  ForwardTo:
  - EventA
```
gives:

```> Messaging topologies with cycles are not supported.```

### Messages
It's recommended that you create a class library that just contains your messages, so they can be referenced by every project. Messages are just POCOs:

```C#
public class UserJoinedMessage 
{
	public User User { get; }
	public DateTime JoinedDate { get; }
}
```

### Sending
If you're using C#6, you can just import the `SendMessage` class:

```C#
using static ProcessBus.SendMessage;
```

Let's say the user calls an API method to join. You can just add the following line to send a `UserJoinedMessage` to the `UserJoined` endpoint:

```C#
public async Task<Response> Join(UserInfo userInfo) 
{
	var userJoinedResult = await joinUser(userInfo);
	if (userJoinedResult.Success) Send("UserJoined", user);
}
```

If you're not using C#6, you'll have to call `SendMessage.Send`.

### Receiving
Receiving messages is quite simple. Here's how you would implement a simple monitor for user joined messages:

```C#
using static ProcessBus.Subscriber;

Subscribe("Monitor")
	.Handle<UserJoinedMessage>(m => {
		Console.WriteLine($"User {m.User.Name} joined.")
	});
```

### Router
Of course, none of this will work if you don't have a router controlling where all the messages are sent.

```C#
using static ProcessBus.RouterFunctions;
using static ProcessBus.YamlParser;

var config = ParseFromFile("config.yaml");
SpawnRouterFromConfig(config);
```