Aspire Initialisers

This is a proof of concept for doing initialisers in Aspire.

Initialisers run after all the health checks for a resource has passed, but before dependencies using WaitFor will start. This makes it perfect for any kind of initialisation logic such as

• Schema Migrations
• Sample Data Population

This is somewhat a follow on from dotnet/aspire#6117 following David Fowler's suggestion of using Health Checks. Whilst using Health Checks works for these initialisers, the UX is a bit funky in places, including

• Resources show as "Unhealthy" whilst they are being initialised
• Resources do not go healthy immediately after all initialisers are complete - needs to wait for the next health check poll interval (p to 5 seconds).

Operators could be a better way to plumb this in over the longer term.

Examples

Basic example:

var sql = builder.AddSqlServer("sql");
var database = sql.AddDatabase("database");

database.WithInitialiser(async ctx =>
{
    var connectionString = await database.Resource.ConnectionStringExpression.GetValueAsync(ctx.CancellationToken);
    //Do something with the connection string
    await Task.Delay(TimeSpan.FromSeconds(5));
});

Rather than custom code, you can wait for completion of any other resource

var sql = builder.AddSqlServer("sql");
var database = sql.AddDatabase("database");

database.WithInitialiser(async ctx =>
{
    var connectionString = await database.Resource.ConnectionStringExpression.GetValueAsync(ctx.CancellationToken);
    //Do something with the connection string
    await Task.Delay(TimeSpan.FromSeconds(5));
});

// Or use another resource:
var migrator = builder.AddExecutable("db-migrations", "dotnet-ef", "..")
    .WithArgs("database",
        "update",
        "--connection", database.Resource.ConnectionStringExpression,
        "--project", new Projects.AspireInitialisation_DbMigrations().ProjectPath,
        "--no-build",
        "-v");

database.AddInitialiser("migrator", migrator);

When using a resource as an initialiser, do not configure any WaitFor between the resources.

Known Issues

• Resources show as Unhealthy whilst initialisers are running
• There can be up to 5 seconds delay between initialisers completing and any dependencies starting. This is due to Health Checks only being polled every 5 seconds. During this time, UX is a bit odd as the resource has progressed from Initialising to Running, but still shows as unhealthy. Not much can be done about this unless Operators are introduced.
• Initialisers won't re-run if a resource is restarted.