Use the generic host

Use AddDrasi when your app already has a generic host (ASP.NET Core, a worker service, or Host.CreateApplicationBuilder). The core API does not require DI. This guide is only for hosted apps.

Register the engine

using System.Text.Json.Nodes;
using Drasi;
using Drasi.DependencyInjection;
using Microsoft.Extensions.Hosting;

var builder = Host.CreateApplicationBuilder(args);
builder.Services.AddDrasi("orders-app", drasi =>
{
    drasi.AddSource("orders");
    drasi.AddQuery(
        "open",
        "MATCH (o:Order) WHERE o.status = 'open' RETURN o.id AS id, o.total AS total",
        ["orders"]);
    drasi.AddReaction("watch", ["open"], evt =>
    {
        foreach (var diff in evt.Results)
        {
            Console.WriteLine($"{diff.Type} {diff.Data}");
        }
    });
});
await builder.Build().RunAsync();

AddDrasi registers Engine as a singleton and a DrasiHostedService. On start, the host:

1. Creates the engine (and picks up the host ILoggerFactory).
2. Loads any plugins you declared on DrasiBuilder.
3. Calls StartAsync.
4. Adds sources, queries, and reactions.
5. Waits for each auto-started query.
6. Pushes any Seed changes.

On stop, the host shuts the engine down. You do not add your own IHostedService for Drasi.

Call AddDrasi once per IServiceCollection. A second call throws InvalidOperationException.

Resolve Engine from the container

Push changes from the rest of the app with the registered Engine:

var engine = app.Services.GetRequiredService<Engine>();
await engine.PushChangeAsync("orders", change);

An overload AddDrasi(engineId, (drasi, sp) => { ... }) gives the configure callback the IServiceProvider so reactions can resolve other services.

Seed data at startup

Call Seed for changes that should fire after the topology is up:

drasi.Seed("orders", new SourceChange
{
    Op = ChangeOp.Insert,
    Id = "o1",
    Labels = ["Order"],
    Properties = new JsonObject
    {
        ["id"] = "o1",
        ["status"] = "open",
        ["total"] = 42,
    },
});

Health checks

builder.Services.AddHealthChecks().AddDrasiCheck();

AddDrasiCheck reports healthy when Engine.IsRunningAsync is true.

Configure from appsettings

builder.Services.AddDrasi(builder.Configuration.GetSection("Drasi"), drasi =>
{
    drasi.AddReaction("watch", ["open"], evt => { /* C# callback */ });
});

FromConfigAsync starts the sources and queries declared in JSON (in-process or plugin). Use the DrasiBuilder callback for in-process reactions and seeds. See Load a topology from configuration.

The Hosted sample is a complete worker that uses this pattern.