HostLoom.Leadership.DependencyInjection 0.16.0

HostLoom.Leadership.DependencyInjection

This package registers HostLoom.Leadership electors with Microsoft.Extensions.DependencyInjection and starts them with the host:

services
    .AddHostLoomLocking(locking => locking.Namespace = "billing")
    .UseRedis();

services
    .AddHostLoomLeadership()
    .AddRole("scheduler")
    .AddRole("reconciler", leadership => leadership.Lease = TimeSpan.FromSeconds(30));

AddRole registers one LeaderElector per role over the IDistributedLock registered by AddHostLoomLocking, resolved as ILeadership and LeaderElector keyed by the role, with LeadershipOptions named by the role and validated when the host starts, including Leadership:Lease against the lock's Locking:MaxLease. When exactly one role is registered the unkeyed ILeadership resolves it too; with several, inject [FromKeyedServices("scheduler")] ILeadership leadership. A repeated role is refused at registration, and so is an ILeadership registered before AddHostLoomLeadership or one keyed by a role before AddRole, because it would have taken precedence over the elector; register a test double after the builder to replace the electors deliberately, or register it keyed by a role you do not add. The hosted service starts every elector with the host and stops each with the host, releasing the leases they hold so a graceful restart hands over at once.

A leader-only background loop is then:

public sealed class ReconcilerLoop(ILeadership leadership) : BackgroundService
{
    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        while (!stoppingToken.IsCancellationRequested)
        {
            await leadership.WaitForLeadershipAsync(stoppingToken);
            var leadershipToken = leadership.LeadershipToken;
            using var linked = CancellationTokenSource.CreateLinkedTokenSource(
                stoppingToken, leadershipToken);
            try
            {
                await RunWhileLeaderAsync(linked.Token);
            }
            catch (OperationCanceledException) when (leadershipToken.IsCancellationRequested)
            {
                // Lost this term; wait to lead again.
            }
        }
    }
}

See the HostLoom.Leadership package README for the guarantee statement and the elector's behaviour on loss, resignation, and stop.