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Getting started

Initializing EventFlow always starts with an EventFlowOptions.New as this performs the initial bootstrap and starts the fluent configuration API. The very minimum initialization of EventFlow can be done in a single line, but wouldn't serve any purpose as no domain has been configured.

var resolver = EventFlowOptions.New.CreateResolver();

The above line does configures several important defaults

  • Custom internal IoC container
  • In-memory :ref:`event store <eventstores>`
  • Console logger
  • A "null" snapshot store, that merely writes a warning if used (no need to do anything before going to production if you aren't planning to use snapshots)
  • And lastly, default implementations of all the internal parts of EventFlow


If you're using ASP.NET Core, you should install the *EventFlow.AspNetCore* package and invoke AddAspNetCoreMetadataProviders in Startup.

public void ConfigureServices(IServiceCollection services)
    services.AddEventFlow(ef =>


Before using EventFlow in a production environment, you should configure an alternative event store, an alternative IoC container and another logger that sends log messages to your production log store.

To start using EventFlow, a domain must be configured which consists of the following parts

In addition to the above, EventFlow provides several optional features. Whether or not these features are utilized depends on the application in which EventFlow is used.

Example application

The example application includes one of each of the required parts: aggregate, event, aggregate identity, command and a command handler. Further down we will go through each of the individual parts.


The example code provided here is located within the EventFlow code base exactly as shown, so if you would like to debug and step through the entire flow, checkout the code and execute the GettingStartedExample test.

All classes create for the example application are prefixed with Example.

.. literalinclude:: ../Source/EventFlow.Documentation/GettingStarted/ExampleTests.cs
  :dedent: 12
  :language: c#
  :lines: 38-75

The above example publishes the ExampleCommand to the aggregate with the exampleId identity with the magical value of 42. After the command has been published, the accompanying read model ExampleReadModel is fetched and we verify that the magical number has reached it.

During the execution of the example application, a single event is emitted and stored in the in-memory event store. The JSON for the event is shown here.

  "MagicNumber": 42

The event data itself is straightforward as it is merely the JSON serialization of an instance of the type ExampleEvent with the value we defined. A bit more interesting is the metadata that EventFlow stores alongside the event, which is used by the EventFlow event store.

  "timestamp": "2016-11-09T20:56:28.5019198+01:00",
  "aggregate_sequence_number": "1",
  "aggregate_name": "ExampleAggrenate",
  "aggregate_id": "example-c1d4a2b1-c75b-4c53-ae44-e67ee1ddfd79",
  "event_id": "event-d5622eaa-d1d3-5f57-8023-4b97fabace90",
  "timestamp_epoch": "1478721389",
  "batch_id": "52e9d7e9-3a98-44c5-926a-fc416e20556c",
  "source_id": "command-69176516-07b7-4142-beaf-dba82586152c",
  "event_name": "example",
  "event_version": "1"

All the built-in metadata is available on each instance of IDomainEvent<,,>, which is accessible from event handlers for e.g. read models or subscribers. It is also possible to create your own :ref:`metadata providers <metadata-providers-custom>` or add additional EventFlow built-in providers as needed.

Aggregate identity

The aggregate ID in EventFlow is represented as a value object that inherits from the IIdentity interface. You can provide your own implementation, but EventFlow provides a convenient implementation that will suit most needs. Be sure to read the section about the :ref:`Identity\<\> <identity>` class for details on how to use it.

For our example application we use the built-in class, which makes the implementation very simple.

.. literalinclude:: ../Source/EventFlow.Documentation/GettingStarted/ExampleId.cs
  :dedent: 4
  :language: c#
  :lines: 29-34


Now we'll take a look at the ExampleAggregate. It is rather simple as the only thing it can do is apply the magic number once.

.. literalinclude:: ../Source/EventFlow.Documentation/GettingStarted/ExampleAggregate.cs
  :dedent: 4
  :language: c#
  :lines: 30-55

Be sure to read the section on :ref:`aggregates <aggregates>` to get all the details right. For now the most important thing to note, is that the state of the aggregate (updating the _magicNumber variable) happens in the Apply(ExampleEvent) method. This is the event sourcing part of EventFlow in effect. As state changes are only saved as events, mutating the aggregate state must happen in such a way that the state changes are replayed the next time the aggregate is loaded. EventFlow has a :ref:`set of different approaches <aggregates_applying_events>` that you can select from. In this example we use the Apply methods as they are the simplest.


The Apply(ExampleEvent) is invoked by the Emit(...) method, so after the event has been emitted, the aggregate state has changed.

The ExampleAggregate exposes the SetMagicNumer(int) method, which is used to expose the business rules for changing the magic number. If the magic number hasn't been set before, the event ExampleEvent is emitted and the aggregate state is mutated.

If the magic numer was changed, we return a failed IExecutionResult with an error message. Returning a failed execution result will make EventFlow disregard any events the aggregate has emitted.

If you need to return something more useful than a bool in an execution result, merely create a new class that implements the IExecutionResult interface and specific the type as generic arguments for the command and command handler.


While possible, do not use the execution results as a method of reading values from the aggregate, that's what the IQueryProcessor and read models are for.


Next up is the event which represents something that has happened in our domain. In this example, it's merely that some magic number has been set. Normally these events should have a really, really good name and represent something in the ubiquitous language for the domain.

.. literalinclude:: ../Source/EventFlow.Documentation/GettingStarted/ExampleEvent.cs
  :dedent: 4
  :language: c#
  :lines: 30-41

We have applied the [EventVersion("example", 1)] to our event, marking it as the example event version 1, which directly corresponds to the event_name and event_version from the metadata store along side the event mentioned. The information is used by EventFlow to tie the name and version to a specific .NET type.


Even though the using the EventVersion attribute is optional, it is highly recommended. EventFlow will infer the information if it isn't provided, thus making it vulnerable to type renames among other things.


Once you have aggregates in your production environment that have emitted an event, you should never change the .NET implementation. You can deprecate it, but you should never change the type or the data stored in the event store.


Commands are the entry point to the domain and if you remember from the example application, they are published using the ICommandBus as shown here.

.. literalinclude:: ../Source/EventFlow.Documentation/GettingStarted/ExampleTests.cs
  :dedent: 16
  :language: c#
  :lines: 55-59

In EventFlow commands are simple value objects that merely house the arguments for the command execution. All commands implement the ICommand<,> interface, but EventFlow provides an easy-to-use base class that you can use.

.. literalinclude:: ../Source/EventFlow.Documentation/GettingStarted/ExampleCommand.cs
  :dedent: 4
  :language: c#
  :lines: 29-42

A command doesn't do anything without a command handler. In fact, EventFlow will throw an exception if a command doesn't have exactly one command handler registered.

Command handler

The command handler provides the glue between the command, the aggregate and the IoC container as it defines how a command is executed. Typically they are rather simple, but they could contain more complex logic. How much is up to you.

.. literalinclude:: ../Source/EventFlow.Documentation/GettingStarted/ExampleCommandHandler.cs
  :dedent: 4
  :language: c#
  :lines: 31-43

The ExampleCommandHandler in our case here merely invokes the SetMagicNumer on the aggregate and returns the execution result. Remember, if a command handler returns a failed execution result, EventFlow will disregard any events the aggregate has emitted.


Everything inside the ExecuteAsync(...) method of a command handler may be executed more than once if there's an optimistic concurrency exception, i.e., something else has happened to the aggregate since it as loaded from the event store and its therefor automatically reloaded by EventFlow. It is therefor essential that the command handler doesn't mutate anything other than the aggregate.

Read model

If you ever need to access the data in your aggregates efficiently, its important that :ref:`read models <read-stores>` are used. Loading aggregates from the event store takes time and its impossible to query for e.g. aggregates that have a specific value in its state.

In our example we merely use the built-in in-memory read model store. It is useful in many cases, e.g. executing automated domain tests in a CI build.

.. literalinclude:: ../Source/EventFlow.Documentation/GettingStarted/ExampleReadModel.cs
  :dedent: 4
  :language: c#
  :lines: 30-43

Notice the IDomainEvent<ExampleAggrenate, ExampleId, ExampleEvent> domainEvent argument. It's merely a wrapper around the specific event we implemented earlier. The IDomainEvent<,,> provides additional information, e.g. any metadata stored alongside the event.

The main difference between the event instance emitted in the aggregate and the instance wrapped here, is that the event has been committed to the event store.

Next steps

Although the implementation in this guide enables you to create a complete application, there are several topics that are recommended as next steps.