Triggers and Bindings — Timer, Queue, Blob, Service Bus

HTTP Trigger:
  → Invoked by an HTTP request
  → Use for: lightweight APIs, webhooks, FHIR callbacks
  → Cold start: first invocation after idle may be slow

Timer Trigger:
  → Invoked on a CRON schedule
  → Use for: nightly report generation, prescription expiry jobs, data sync
  → Does NOT fire missed runs by default — use WEBSITE_TIME_ZONE for local time

Service Bus Trigger:
  → Invoked when a message arrives on a queue or topic subscription
  → Use for: processing integration events, outbox relay, notification dispatch
  → At-least-once delivery — your handler must be idempotent

Blob Trigger:
  → Invoked when a blob is created or modified in a container
  → Use for: processing uploaded patient documents, PDF generation
  → Note: can lag up to 10 minutes for large containers — use Event Grid trigger for real-time

// Isolated Worker model (.NET 8) — preferred over in-process
// NuGet: Microsoft.Azure.Functions.Worker, Microsoft.Azure.Functions.Worker.Extensions.Http.AspNetCore

public class PatientWebhookFunction
{
    private readonly IMediator _mediator;

    public PatientWebhookFunction(IMediator mediator) => _mediator = mediator;

    [Function("PatientAdmissionWebhook")]
    public async Task<IActionResult> RunAsync(
        [HttpTrigger(AuthorizationLevel.Function, "post", Route = "webhooks/patient-admission")]
        HttpRequest req,
        CancellationToken ct)
    {
        var body = await req.ReadFromJsonAsync<PatientAdmissionWebhookPayload>(ct);
        if (body is null) return new BadRequestResult();

        var command = new ProcessPatientAdmissionCommand(
            body.PatientId, body.WardId, body.AdmittedAt);

        var result = await _mediator.Send(command, ct);

        return result.IsSuccess
            ? new OkResult()
            : new BadRequestObjectResult(result.Error.Message);
    }
}

// Expire pending prescriptions that have been waiting more than 48 hours
public class ExpirePrescriptionsFunction
{
    private readonly IPrescriptionRepository _repository;
    private readonly IClock                  _clock;

    public ExpirePrescriptionsFunction(
        IPrescriptionRepository repository, IClock clock)
    {
        _repository = repository;
        _clock      = clock;
    }

    // CRON expression: "0 */6 * * *" = every 6 hours
    // "0 0 2 * * *" = daily at 2:00 AM UTC
    [Function("ExpirePrescriptions")]
    public async Task RunAsync(
        [TimerTrigger("0 */6 * * *")] TimerInfo timerInfo,
        CancellationToken ct)
    {
        if (timerInfo.IsPastDue)
        {
            // The function missed a scheduled invocation (e.g., function was stopped)
            // Decide: process anyway or skip
        }

        var cutoff = _clock.UtcNow.AddHours(-48);
        var expiring = await _repository.GetPendingOlderThanAsync(cutoff, ct);

        foreach (var prescription in expiring)
        {
            prescription.Expire();
            await _repository.SaveAsync(prescription, ct);
        }
    }
}

// Process PrescriptionCreated integration events from Service Bus
public class PrescriptionCreatedFunction
{
    private readonly IPharmacyDispenseService _dispenseService;

    public PrescriptionCreatedFunction(IPharmacyDispenseService dispenseService) =>
        _dispenseService = dispenseService;

    [Function("ProcessPrescriptionCreated")]
    public async Task RunAsync(
        [ServiceBusTrigger(
            topicName:        "prescriptioncreated-events",
            subscriptionName: "pharmacy-service",
            Connection:       "ServiceBus")]
        ServiceBusReceivedMessage message,
        ServiceBusMessageActions  messageActions,
        CancellationToken         ct)
    {
        var @event = message.Body.ToObjectFromJson<PrescriptionCreatedIntegrationEvent>();

        try
        {
            await _dispenseService.CreateDispenseTaskAsync(@event.PrescriptionId, ct);
            await messageActions.CompleteMessageAsync(message, ct);
        }
        catch (Exception ex) when (ex is not OperationCanceledException)
        {
            // On failure: dead-letter instead of letting Service Bus retry
            // Include reason for dead-lettering (aids investigation)
            await messageActions.DeadLetterMessageAsync(
                message,
                deadLetterReason:           "ProcessingFailed",
                deadLetterErrorDescription: ex.Message,
                ct);
        }
    }
}

// Process uploaded patient documents (e.g., scan and index content)
public class PatientDocumentProcessorFunction
{
    private readonly IDocumentIndexingService _indexing;

    public PatientDocumentProcessorFunction(IDocumentIndexingService indexing) =>
        _indexing = indexing;

    [Function("ProcessPatientDocument")]
    public async Task RunAsync(
        [BlobTrigger("patient-documents/{patientId}/{name}", Connection = "AzureStorage")]
        Stream documentStream,
        string patientId,
        string name,
        CancellationToken ct)
    {
        await _indexing.IndexDocumentAsync(
            patientId:    Guid.Parse(patientId),
            documentName: name,
            content:      documentStream,
            ct:           ct);
    }
}

// Note on Blob Trigger latency:
// If real-time processing is required (under 1 minute), use:
// → Event Grid trigger: notified immediately when blob is uploaded
// Blob trigger polled by Azure Functions every ~10 minutes for large containers

// Azure Functions Isolated Worker host setup
var host = new HostBuilder()
    .ConfigureFunctionsWebApplication()
    .ConfigureAppConfiguration(config =>
    {
        config.AddEnvironmentVariables();
        // Add Key Vault in production:
        if (!string.IsNullOrEmpty(Environment.GetEnvironmentVariable("KeyVaultUri")))
            config.AddAzureKeyVault(
                new Uri(Environment.GetEnvironmentVariable("KeyVaultUri")!),
                new DefaultAzureCredential());
    })
    .ConfigureServices((context, services) =>
    {
        var config = context.Configuration;

        services.AddDbContext<PrescriptionsDbContext>(opts =>
            opts.UseSqlServer(config.GetConnectionString("Clinical")));

        services.AddMediatR(cfg =>
            cfg.RegisterServicesFromAssembly(typeof(ProcessPatientAdmissionCommand).Assembly));

        services.AddSingleton<IClock, SystemClock>();
        services.AddScoped<IPrescriptionRepository, PrescriptionRepository>();
    })
    .Build();

await host.RunAsync();