Marten as Event Store Edit on GitHub

Marten's Event Store functionality is a powerful way to utilize Postgresql in the event sourcing style of persistence in your application. Beyond simple event capture and access to the raw event stream data, Marten also helps you create "read side" views of the raw event data through its support for projections.

Event Store Documentation

• Event Store Schema Objects
• Stream Identity
• Appending Events
• Querying Event and Stream Data
• Projections
  • Async Projections Daemon
  • Custom Projections
  • Projecting by Event Type
• Events Versioning

Event Store Quickstart

There is not anything special you need to do to enable the event store functionality in Marten, and it obeys the same rules about automatic schema generation described in Marten and the Postgresql Schema. Marten is just a client library, and there's nothing to install other than the Marten nuget.

Because I’ve read way too much epic fantasy fiction, my sample problem domain is an application that records, analyses, and visualizes the status of quests. During a quest, you may want to record events like:

public class ArrivedAtLocation
{
    public int Day { get; set; }

    public string Location { get; set; }

    public override string ToString()
    {
        return $"Arrived at {Location} on Day {Day}";
    }
}

public class MembersJoined
{
    public MembersJoined()
    {
    }

    public MembersJoined(int day, string location, params string[] members)
    {
        Day = day;
        Location = location;
        Members = members;
    }

    public Guid QuestId { get; set; }

    public int Day { get; set; }

    public string Location { get; set; }

    public string[] Members { get; set; }

    public override string ToString()
    {
        return $"Members {Members.Join(", ")} joined at {Location} on Day {Day}";
    }
}

public class QuestStarted
{
    public string Name { get; set; }
    public Guid Id { get; set; }

    public override string ToString()
    {
        return $"Quest {Name} started";
    }
}

public class QuestEnded
{
    public string Name { get; set; }
    public Guid Id { get; set; }

    public override string ToString()
    {
        return $"Quest {Name} ended";
    }
}

public class MembersDeparted
{
    public Guid Id { get; set; }

    public Guid QuestId { get; set; }

    public int Day { get; set; }

    public string Location { get; set; }

    public string[] Members { get; set; }

    public override string ToString()
    {
        return $"Members {Members.Join(", ")} departed at {Location} on Day {Day}";
    }
}

public class MembersEscaped
{
    public Guid Id { get; set; }

    public Guid QuestId { get; set; }

    public string Location { get; set; }

    public string[] Members { get; set; }

    public override string ToString()
    {
        return $"Members {Members.Join(", ")} escaped from {Location}";
    }
}

Now, let's say that we're starting a new "quest" with the first couple of events, then appending a couple more as other quest party members join up:

using (var session = store.OpenSession())
{
    var started = new QuestStarted { Name = "Destroy the One Ring" };
    var joined1 = new MembersJoined(1, "Hobbiton", "Frodo", "Sam");

    // Start a brand new stream and commit the new events as
    // part of a transaction
    // no stream type will be stored in database
    session.Events.StartStream(typeof(Quest), questId, started, joined1);
}

// SAMPLE: event-store-start-stream-with-no-type
using (var session = store.OpenSession())
{
    var started = new QuestStarted { Name = "Destroy the One Ring" };
    var joined1 = new MembersJoined(1, "Hobbiton", "Frodo", "Sam");

    // Start a brand new stream and commit the new events as
    // part of a transaction
    // no stream type will be stored in database
    session.Events.StartStream(questId, started, joined1);
}

In addition to generic StartStream<T>, IEventStore has a non-generic StartStream overload that let you pass explicit type.

var store = DocumentStore.For(_ =>
{
    _.Connection(ConnectionSource.ConnectionString);
    _.Events.InlineProjections.AggregateStreamsWith<QuestParty>();
});

var questId = Guid.NewGuid();

using (var session = store.OpenSession())
{
    var started = new QuestStarted { Name = "Destroy the One Ring" };
    var joined1 = new MembersJoined(1, "Hobbiton", "Frodo", "Sam");

    // Start a brand new stream and commit the new events as
    // part of a transaction
    session.Events.StartStream<Quest>(questId, started, joined1);
    session.SaveChanges();

    // Append more events to the same stream
    var joined2 = new MembersJoined(3, "Buckland", "Merry", "Pippen");
    var joined3 = new MembersJoined(10, "Bree", "Aragorn");
    var arrived = new ArrivedAtLocation { Day = 15, Location = "Rivendell" };
    session.Events.Append(questId, joined2, joined3, arrived);
    session.SaveChanges();
}

It has also overload to create streams without associating them with aggregate type (stored in `mt_streams` table).

using (var session = store.OpenSession())
{
    var started = new QuestStarted { Name = "Destroy the One Ring" };
    var joined1 = new MembersJoined(1, "Hobbiton", "Frodo", "Sam");

    // Start a brand new stream and commit the new events as
    // part of a transaction
    // no stream type will be stored in database
    session.Events.StartStream(typeof(Quest), questId, started, joined1);
}

// SAMPLE: event-store-start-stream-with-no-type
using (var session = store.OpenSession())
{
    var started = new QuestStarted { Name = "Destroy the One Ring" };
    var joined1 = new MembersJoined(1, "Hobbiton", "Frodo", "Sam");

    // Start a brand new stream and commit the new events as
    // part of a transaction
    // no stream type will be stored in database
    session.Events.StartStream(questId, started, joined1);
}