Freakout

📤 Just a general outbox thing

What do you mean by "outbox"?

An outbox is a simple mechanism that allows you to store outbound messages in an atomic ☢️ way as part of your normal transaction.

It addresses the ubiquitous dilemma of which to do first: (1) commit your database transaction OR (2) whatever else you want to do.

The problem is that (1) is generally inherently risky (database deadlocks, key violations, etc etc), and (2) is also generally risky (writing to a file share, communicating with a message broker, calling an external web service, etc.).

Two risky things combined are very likely to fail! - which will leave your system in a state where either (1) or (2) was not carried out (depending on the order you chose to execute them in), because it failed.

🆘 This problem can be addressed in many ways, e.g. by retrying, using a middleman with higher availability (e.g. a message broker), etc. etc.

OR you could simply store information about which actions to perform in (2) as part of (1)!! 🤯 That's what the outbox does! 🤓

Until now, I have only seen outboxes built into messaging libraries - why now a separate library?

Why? Because "outbox" is closer to your chosen type of persistence (SQL Server, Postgres, etc.) than to anything else.

Which types of persistence does it support?

1. Microsoft SQL Server (for when you're working with the "Microsoft.Data.SqlClient" NuGet package and SqlConnection/SqlTransaction)
2. PostgreSQL (for when you're working with "Npgsql" NuGet package and NpgsqlConnection/NpgsqlTransaction)

and that's it for now. 😅

How?

First, pull in the relevant NuGet package for your chosen type of persistence - e.g. the "Freakout.MsSql" NuGet package.

Next, enable Freakout in your app:

services.AddFreakout(new MsSqlFreakoutConfiguration(connectionString));

It will register a couple of things, e.g. a background worker that will poll the outbox for pending commands. AddFreakout can only be called once.

Then, add your handlers:

services.AddCommandHandler<RecalculateClaimSummaryCommandHandler>();
services.AddCommandHandler<SendEmailCommandHandler>();
services.AddCommandHandler<PublishJournalEntryAddedCommandHandler>();

which will of course be resolved from the container, each in their own service scope.

Now it's fully configured - what's missing is putting something in the outbox.

Two ways of adding commands to the outbox

First way: By using IOutbox

This is the neat way to do it: You can manage your unit of work with your SqlConnection and SqlTransaction somewhere and then make them available to Freakout by using a FreakoutContextScope like this:

var context = new MsSqlFreakoutContext(connection, transaction);

using (new FreakoutContextScope(context))
{
	// there's an ambient context now! 🙂
}

Inside the scope, IOutbox can be resolved, which then provides a technology-agnostic way of putting commands in the outbox!

A cool place to create/dispose FreakoutContextScope would be in your ASP.NET Core request handler pipeline, e.g. like this:

app.Use(async (context, next) => {
	var provider = context.Request.RequestServices;
	
	// let's just assume we can get these from the request-scoped services:
	var connection = provider.GetRequiredService<SqlConnection>();
	var transaction = provider.GetRequiredService<SqlTransaction>();

	var freakoutContext = new MsSqlFreakoutContext(connection, transaction);

	using (new FreakoutContextScope(freakoutContext))
	{
		// there's an ambient context now! 🙂
		//
		// ASP.NET Core controllers can have IOutbox injected if they want!
		await next();
	}
});

Having IOutbox injected is pretty neat, because it allows you to put commands in the outbox simply by going:

await outbox.AddOutboxCommandAsync(command);

without having to bother with thinking about which type of persistence is being used.

Second way: Directly on the database transaction

This way is more involved, because it's closer to the metal.

Since this example is for SQL Server, and we're pretending to be working with Microsoft.Data.SqlClient and SqlConnection, it's natural to provide the outbox functionality as an extension method to DbTransaction. This way, your code can do stuff like this:

await using var connection = new SqlConnection(_connectionString);
await connection.OpenAsync();

await using var transaction = await connection.BeginTransactionAsync();

// do your own work with connection+transaction here
// (...)

// possibly call this bad boy a couple of times
await transaction.AddOutboxCommandAsync(serializer, "dbo", "OutboxCommands", new PublishJournalEntryAddedCommand(Id: journalEntryId));

// do more of your own work
// (...)

// commit it all atomically
await transaction.CommitAsync();

which in this case would result in publishing a couple of JournalEntryAdded events using Rebus.

What does a command handler look like?

Outbox commands are dispatched to handlers. Handlers are classes that implement ICommandHandler<TCommand> and are registered in the container using the AddCommandHandler extension method shown above.

A command handler to publish the aforementioned Rebus event could look like this (assuming Rebus has also been configured in the given container):

public class PublishJournalEntryAddedCommandHandler(IBus bus) : ICommandHandler<PublishJournalEntryAddedCommand>
{
	public async Task HandleAsync(PublishJournalEntryAddedCommand command, CancellationToken token)
	{
		await bus.Publish(new JournalEntryAdded(command.Id));
	}
}