Register a user

Register Alice, save the event in SQLite, and query her name. Each account can register once; later requests return the saved name. Jump to the runnable sample.

Define the messages

module Account =

    open FCQRS.Common
    open FCQRS.FSharp

    type RegisterUser = RegisterUser of name: string
    type UserRegistered = UserRegistered of name: string

using static FCQRS.Common;
using static FCQRS.CSharp;

public sealed record RegisterUser(string Name);
public sealed record UserRegistered(string Name);
public sealed record AccountState(string? Name = null);

RegisterUser is a command: a request to register a name. UserRegistered is an event: the recorded result. The account's state holds the registered name, starting with None in F# or null in C#.

Decide what to save

Account.fs / Account.cs contains the rule. FCQRS passes the command and current state to decide / HandleCommand:

    let decide (command: Command<RegisterUser>) (state: string option) =
        let (RegisterUser name) = command.CommandDetails
        persistIf state.IsNone (UserRegistered(defaultArg state name))

    let fold (event: Event<UserRegistered>) (_state: string option) =
        let (UserRegistered name) = event.EventDetails
        Some name

public sealed class Account : Aggregate<AccountState, RegisterUser, UserRegistered>
{
    public override string EntityName => "RegistrationCSharpAccount";
    public override AccountState InitialState => new();

    public override EventAction<UserRegistered> HandleCommand(
        Command<RegisterUser> command, AccountState state) =>
        EventActions.PersistConditionally(state.Name is null,
            new UserRegistered(state.Name ?? command.CommandDetails.Name));

    public override AccountState ApplyEvent(Event<UserRegistered> stored, AccountState state) =>
        new(stored.EventDetails.Name);
}

• First registration: the name is absent, so persistIf / PersistConditionally saves the event.
• Repeat registration: the condition is false, so FCQRS returns a deferred reply without saving another event. defaultArg state name / state.Name ?? command.CommandDetails.Name keeps the existing name.
• Apply the event: fold / ApplyEvent copies its name into state. FCQRS also runs this during recovery to rebuild the account from saved events.

This state and its rules form an aggregate. FCQRS processes one account's commands one at a time. RegisterUser and UserRegistered are your payloads; the Command<T> and Event<T> wrappers add FCQRS metadata such as the request ID and event version.

Send the request

Program.fs / Program.cs registers the aggregate and obtains accounts, the handle used to send it commands:

open FCQRS.Model.Data
open FCQRS.Common
open FCQRS.FSharp
open Account
let send (accounts: AggregateHandle<RegisterUser, UserRegistered>) (id: AggregateId) = async {

    let! reply = accounts.Send (Fcqrs.newCid ()) id (RegisterUser "Alice") (fun _ -> true)

    return reply
}

var accounts = host.Services.GetRequiredService<Handler<RegisterUser, UserRegistered>>();
var reply = await accounts(_ => true, Values.NewCID(), id, new RegisterUser("Alice"));

id selects the account alice. The correlation ID identifies this request; the predicate accepts its reply. Awaiting the call gives you the aggregate's result.

The sample also builds a query view from the saved event and waits for it before printing Query: Alice. The query page shows that handler.

Run it

With .NET 10 and Git installed:

git clone https://github.com/OnurGumus/FCQRS.git
cd FCQRS

Choose a language. The first run restores the NuGet packages.

dotnet run --project samples/registration-fsharp

dotnet run --project samples/registration-csharp

Registered: Alice (version 1)
Query: Alice

Run the same command again:

Already registered: Alice (version 1)
Query: Alice

The stored registration survived the restart. The repeated request left the version at 1. The event history, called the journal, is in bin/Debug/net10.0/registration.db inside the sample folder. This example registers a profile; it does not implement passwords or login sessions.

To start your own project, copy just Account, Program, and the project file (.fsproj / .csproj) from your chosen sample into an empty folder, then run dotnet run there.

Complete source: F# · C#. Next, change the name, then the account ID.