Extend

Write your own job handler

IJobHandler executes a durable job under one handler key — a stable string such as contoso.nightly-report. The key is the job’s identity: it is both how the job is classified and how the background worker picks the handler that runs it. Tracon’s own handlers live under the reserved tracon. prefix; you pick a prefix of your own.

public sealed class NightlyReportJobHandler(ILogger<NightlyReportJobHandler>? logger = null) : IJobHandler
{
    public async ValueTask ExecuteAsync(JobContext context, CancellationToken cancellationToken = default)
    {
        foreach (var item in context.Items)
        {
            // context.Items is UNFILTERED: a retry hands back items an
            // earlier, now-dead attempt already finished. Skip them.
            if (item.Status != JobItemStatus.Pending)
            {
                continue;
            }

            if (await context.IsCancelledAsync(cancellationToken))
            {
                break;
            }

            logger?.LogInformation("Nightly report generated for '{Subject}'.", item.Input);

            await context.ReportItemAsync(
                new JobItemResult { JobId = context.Job.Id, Seq = item.Seq, Status = JobItemStatus.Completed },
                cancellationToken);
        }
    }
}

builder.Services.AddJobHandler<NightlyReportJobHandler>("contoso.nightly-report");

The worker looks the key up by exact match, so this call may come before or after AddTracon() — registration order never decides which handler runs, and no handler can shadow another. The handler is registered scoped and resolved from a fresh scope for every execution, so it may take scoped dependencies in its constructor: two jobs running in parallel, and two attempts of the same job, never share an instance.

flowchart TD
    accTitle: How a job finds its handler
    accDescr: A registration adds a key and a handler type to the registry, which rejects a duplicate key, a reserved key and a malformed key at host start. When the worker leases a job it looks the job's handler key up in that registry; an unregistered key fails the job with a stable error code, and a registered one opens a fresh dependency-injection scope, resolves the handler from it and runs it.
    REG["AddJobHandler&lt;T&gt;(&quot;contoso.nightly-report&quot;)"] --> MAP["Handler registry:<br/>key to type"]
    MAP --> CHECK{"Duplicate, reserved,<br/>or malformed key?"}
    CHECK -- yes --> STOP["Host does not start"]
    CHECK -- no --> READY["Ready"]
    READY --> LEASE["Worker leases a job"]
    LEASE --> LOOK{"Is job.handlerKey<br/>in the registry?"}
    LOOK -- no --> FAIL["Job fails with<br/>UnknownHandlerKey"]
    LOOK -- yes --> SCOPE["New DI scope per execution"]
    SCOPE --> RUN["ExecuteAsync"]
    RUN --> DISPOSE["Scope disposed"]

Three mistakes stop the host from starting rather than failing quietly later:

Two handlers under one key. A key identifies exactly one handler.
A key inside the tracon. namespace. That prefix is reserved.
A malformed key. 1-128 characters: lowercase ASCII letters, digits, ., _, or -, starting with a letter or digit. Uppercase is rejected rather than normalized, so one key can never become two.

Queue work for your handler

IJobDispatcher is the supported way to create a job from your own code. It fills in the identifier, status and timestamps, and refuses a key no handler serves:

public sealed class ReportScheduler(IJobDispatcher jobs)
{
    public ValueTask<JobRecord> QueueNightlyAsync(string tenantId, IReadOnlyList<string> customers)
        => jobs.EnqueueAsync(new JobRequest
        {
            TenantId = tenantId,
            HandlerKey = "contoso.nightly-report",
            TargetName = "nightly-report",
            Items = customers,
        });
}

If a job is somehow queued for a key nobody registered — a stale row, a mistyped key — the worker fails that job with the stable code JobErrorCodes.UnknownHandlerKey instead of leaving it in the queue forever. The raw key is written to the log, not to the job’s errorMessage: that field is read back over HTTP, and an unregistered key may have come from an untrusted source.

PUT /api/schedules/{name} accepts only the keys listed in TraconSchedulingOptions.HttpSchedulableHandlerKeys, which defaults to Tracon’s own built-in keys. A handler key is a dispatch identity, so an unrestricted endpoint would turn every registered handler — including internal ones you registered for your own background work — into an externally callable surface. Name your key there only when you want it schedulable over HTTP.

A non-empty list replaces that default rather than extending it, so an allow-list can also narrow the surface. To keep the built-in keys as well, name them alongside your own:

builder.Services.UseScheduling(options =>
{
    foreach (var key in JobHandlerKeys.BuiltIn)
    {
        options.HttpSchedulableHandlerKeys.Add(key);
    }

    options.HttpSchedulableHandlerKeys.Add("contoso.nightly-report");
});

Runtime contract

Execution is at-least-once, not exactly-once. The same job — the same JobContext.Job, with the SAME item list — can reach ExecuteAsync more than once: a thrown exception is retried up to the job’s attempt limit (JobRetryException.RetryAfter controls the delay), and a worker that crashes or whose lease simply expires before it returns lets another worker (or the same one, on its next poll) re-lease the job and call ExecuteAsync again from scratch. Neither path resets item progress.

JobContext.Items reflects that: it carries every item, not only the Pending ones. On a retry, items already Completed or Failed from an earlier attempt are present too. A handler with a side effect (an email, a payment, a call to an external system) must therefore either be idempotent on its own, or — the pattern every built-in handler uses, shown above — check JobItemRecord.Status and skip anything that is not Pending.

IsCancelledAsync reflects a POST /api/jobs/{id}/cancel request; check it between items so a canceled job stops promptly instead of finishing every remaining item first. ReportItemAsync must be called for every item the handler actually processes — it updates the item’s persisted status and the job’s doneItems/failedItems counters, which the job list and detail endpoints read.

A handler that throws is retried by the worker itself; there is no need to catch and report a job-level failure by hand. Report per-item failures with JobItemStatus.Failed and continue the loop instead, if a single bad input should not stop the rest of the batch — a batch with both successful and failed items still completes as Completed, and only a batch where every item failed completes as Failed.

Verify your implementation

Add the contract package to your test project:

dotnet add package Tracon.Testing.Contracts.Xunit --prerelease

using Tracon.Testing.Contracts.Scheduling;

public sealed class NightlyReportJobHandlerTests : JobHandlerContract
{
    protected override string HandlerKey => "contoso.nightly-report";

    protected override ValueTask<IJobHandler> CreateHandlerAsync()
        => ValueTask.FromResult<IJobHandler>(new NightlyReportJobHandler());

    protected override JobItemRecord CreateItem(int sequence, JobItemStatus status)
        => new() { Id = Guid.NewGuid(), JobId = JobId, Seq = sequence, Input = $"customer-{sequence}", Status = status };
}

The inherited tests verify that a Completed item is not processed again, that a retry with mixed item statuses only processes the Pending ones, and that cancellation is observed between items. They do not test the queue itself — leasing, retry scheduling, or persistence; see Reliable runs for that. samples/Tracon.Samples.CustomJobHandler in the repository runs this same contract as a package consumer, and also boots a real host to prove the sample handler actually receives a queued job.

Write your own job handler

Queue work for your handler

Runtime contract

Verify your implementation

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