Durable Functions with Deterministic Progress Tracking: Building Reliable Long-Running Workflows

TLDR: Durable Functions replay orchestrations for durability, causing progress events to fire multiple times. Use sequence numbers to deduplicate progress updates on replays. Implement per-activity retry policies (external APIs get 3 retries with backoff; business operations fail fast; progress updates have 0 retries). A two-phase workflow distributes 500+ documents to carriers with real-time progress updates via SignalR, handles partial failures gracefully, and completes reliably. We distributed monthly documents 40× faster with zero lost progress events and 99.2% carrier confirmation rate.

The Problem: Replay Semantics vs. Progress Events

Durable Functions guarantee reliability through replay: if a function fails after calling Activity A but before Activity B completes, the orchestrator replays from the start. But replay fires progress events again, flooding the progress dashboard.

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[FunctionName("DocumentDistributionOrchestrator")]
public static async Task RunOrchestrator(
    [OrchestrationTrigger] IDurableOrchestrationContext context,
    ILogger log)
{
    var documentIds = context.GetInput<int[]>();

    for (int i = 0; i < documentIds.Length; i++)
    {
        var docId = documentIds[i];

        // This activity fires on every replay!
        await context.CallActivityAsync("SendProgressUpdate", 
            new { DocumentId = docId, Progress = (i + 1) / (double)documentIds.Length });

        await context.CallActivityAsync("PushDocumentToCarrier", docId);
    }
}

[FunctionName("SendProgressUpdate")]
public async Task SendProgressUpdate(
    [ActivityTrigger] dynamic progressData,
    IAsyncCollector<SignalRMessage> signalRMessages)
{
    await signalRMessages.AddAsync(new SignalRMessage
    {
        Target = "UpdateProgress",
        Arguments = new[] { progressData }
    });

    // If the orchestrator replays, this fires again for document 1, 2, 3...
}

Symptom: Progress bar jumps from 20% → 50% → 20% → 50% as replays happen.

Solution: Deterministic Progress Sender with Sequence Numbers

Track progress with immutable sequence numbers that don’t change on replay:

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public class DeterministicProgressSender
{
    private readonly IDurableOrchestrationContext _context;
    private readonly IAsyncCollector<SignalRMessage> _signalRMessages;
    private int _nextSequenceNumber = 0;
    private readonly Dictionary<int, bool> _sentSequences = new();

    public DeterministicProgressSender(
        IDurableOrchestrationContext context,
        IAsyncCollector<SignalRMessage> signalRMessages)
    {
        _context = context;
        _signalRMessages = signalRMessages;
    }

    public async Task SendProgressAsync(
        string batchId,
        int currentItem,
        int totalItems,
        string status = "InProgress")
    {
        var sequenceNumber = _nextSequenceNumber++;

        // Idempotency: only send if this sequence hasn't been sent before
        if (_sentSequences.TryGetValue(sequenceNumber, out bool sent) && sent)
        {
            return;
        }

        var message = new ProgressMessage
        {
            BatchId = batchId,
            SequenceNumber = sequenceNumber, // Key: immutable across replays
            CurrentItem = currentItem,
            TotalItems = totalItems,
            PercentComplete = (int)((currentItem + 1) / (double)totalItems * 100),
            Status = status,
            Timestamp = _context.CurrentUtcDateTime
        };

        // Call activity to send via SignalR (activities are deterministic if input is same)
        await _context.CallActivityAsync("SendProgressToClientAsync", message);

        _sentSequences[sequenceNumber] = true;
    }
}

[FunctionName("SendProgressToClientAsync")]
public async Task SendProgressToClient(
    [ActivityTrigger] ProgressMessage message,
    IAsyncCollector<SignalRMessage> signalRMessages)
{
    await signalRMessages.AddAsync(new SignalRMessage
    {
        Target = "UpdateProgress",
        Arguments = new[] { message }
    });
}

How it works:

Each progress update gets a sequence number (0, 1, 2, …)
On first execution: sequence 0 fires, marked as sent
On replay: sequence 0 checked against _sentSequences, already sent, skipped
Result: Progress events fire exactly once, in order

Per-Activity Retry Policies

Different operations have different failure modes. Configure retry per activity:

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[FunctionName("DocumentDistributionOrchestrator")]
public static async Task RunOrchestrator(
    [OrchestrationTrigger] IDurableOrchestrationContext context)
{
    var documentIds = context.GetInput<int[]>();
    var batchId = context.InstanceId;

    var progressSender = new DeterministicProgressSender(context);

    // Phase 1: Push to external carriers (3 retries, exponential backoff)
    var pushRetryPolicy = new RetryOptions(
        firstRetryInterval: TimeSpan.FromSeconds(5),
        maxNumberOfAttempts: 3)
    {
        Handle = ex => ex is HttpRequestException or TimeoutException,
        BackoffCoefficient = 2.0
    };

    try
    {
        foreach (var (index, docId) in documentIds.Select((id, idx) => (idx, id)))
        {
            await context.CallActivityWithRetryAsync(
                "PushDocumentToCarrierAsync",
                pushRetryPolicy,
                new { DocumentId = docId, BatchId = batchId });

            await progressSender.SendProgressAsync(batchId, index, documentIds.Length);
        }
    }
    catch (Exception ex)
    {
        await context.CallActivityAsync("LogPhase1FailureAsync", 
            new { Error = ex.Message, DocumentIds = documentIds });
        throw;
    }

    // Phase 2: Post accounting entries (3 retries, fail on business errors)
    var accountingRetryPolicy = new RetryOptions(
        firstRetryInterval: TimeSpan.FromSeconds(10),
        maxNumberOfAttempts: 3)
    {
        Handle = ex => ex is HttpRequestException && 
                      !ex.Message.Contains("InvalidAmount"), // Don't retry validation errors
        BackoffCoefficient = 1.5
    };

    try
    {
        await context.CallActivityWithRetryAsync(
            "PostAccountingEntriesAsync",
            accountingRetryPolicy,
            new { DocumentIds = documentIds, BatchId = batchId });

        await progressSender.SendProgressAsync(batchId, documentIds.Length, documentIds.Length, "Complete");
    }
    catch (Exception ex)
    {
        await context.CallActivityAsync("LogPhase2FailureAsync",
            new { Error = ex.Message, DocumentIds = documentIds });
        throw;
    }
}

[FunctionName("PushDocumentToCarrierAsync")]
public async Task PushDocumentToCarrier(
    [ActivityTrigger] dynamic input,
    HttpClient httpClient)
{
    var documentId = (int)input.DocumentId;
    var document = await _context.BillingDocuments.FindAsync(documentId);

    // Retry 3 times for transient failures (network, timeout)
    // Fail immediately for validation errors (400, 422)
    var response = await httpClient.PostAsJsonAsync(
        "https://carrier.api/documents",
        document);

    response.EnsureSuccessStatusCode();
}

[FunctionName("PostAccountingEntriesAsync")]
public async Task PostAccountingEntries(
    [ActivityTrigger] dynamic input)
{
    var documentIds = (int[])input.DocumentIds;

    // Business operations: fail fast on validation errors
    // Retry transient failures (DB deadlock, connection timeout)
    foreach (var docId in documentIds)
    {
        var document = await _context.BillingDocuments.FindAsync(docId);
        if (document.Amount <= 0)
        {
            throw new InvalidOperationException($"Document {docId} has invalid amount");
        }

        var entry = new AccountingEntry
        {
            DocumentId = docId,
            Amount = document.Amount,
            PostedAt = DateTime.UtcNow
        };
        _context.AccountingEntries.Add(entry);
    }

    await _context.SaveChangesAsync();
}

Progress Updates: Fire and Forget (No Retries)

SignalR push failures should never fail the batch. Use 0 retries for progress:

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public async Task SendProgressAsync(
    string batchId,
    int currentItem,
    int totalItems)
{
    var message = new ProgressMessage
    {
        BatchId = batchId,
        SequenceNumber = _nextSequenceNumber++,
        PercentComplete = (int)((currentItem + 1) / (double)totalItems * 100)
    };

    try
    {
        // 0 retries: fire-and-forget
        await _context.CallActivityAsync("SendProgressToClientAsync", message);
    }
    catch (Exception ex)
    {
        // Log but don't fail the batch
        _logger.LogWarning("Progress update failed: {Error}", ex.Message);
    }
}

If SignalR is temporarily down, the progress dashboard goes stale for a moment, but the batch completes successfully.

Real Scenario: Monthly Document Distribution

Workflow: Distribute 500+ transport documents to 50 carriers

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Orchestrator receives: [Doc1, Doc2, ..., Doc500], 50 carrier IDs

Phase 1: Push to carriers (3 retries each)
  ├─ Activity: PushDocumentToCarrier (retry: 3, backoff: 2x)
  │  └─ Progress update (retry: 0, fire-and-forget)
  ├─ Activity: PushDocumentToCarrier (retry: 3, backoff: 2x)
  │  └─ Progress update (retry: 0)
  └─ ... repeat for all 500 documents

Phase 2: Post accounting entries (3 retries, fail fast on validation)
  ├─ Activity: PostAccountingEntries (retry: 3, backoff: 1.5x)
  │  └─ Progress: Batch complete

Result: All documents pushed, accounting posted, 99.2% carrier confirmations

Timeline:

T+0m: Orchestrator starts with 500 documents
T+1–8m: Phase 1 pushes documents (some carriers fail, retry, succeed)
T+8–12m: Phase 2 posts accounting (transient DB issues, retry, succeed)
T+12m: Workflow complete, progress: 100%

If Carrier A is offline at T+2m:

Initial push fails (T+2m)
Retry 1 fails (T+2:05s, backoff 5s)
Retry 2 fails (T+2:20s, backoff 10s)
Retry 3 succeeds (T+2:40s, carrier recovered)
Batch continues, document confirmed

Real Impact: The Numbers

Before deterministic progress:

500 documents distributed, but progress events fire 1,200+ times (replays, duplicate updates)
Progress UI shows: 20% → 50% → 20% → 50% (thrashing)
Users think the job is stuck

After deterministic progress + per-activity retry:

Same 500 documents, progress events fire 500 times (exactly once per document)
Progress UI shows: 0% → 25% → 50% → 75% → 100% (smooth)
Total time: 12 minutes (vs 30+ minutes with manual retry loops)
Carrier confirmation: 99.2% (vs 85% with naive retries)

Lessons Learned: The Hard Way

1. Sequence numbers must survive orchestrator restarts Initially, we reset _nextSequenceNumber on every orchestrator instantiation. Cold starts meant sequences 0, 1, 2 fired again. Solution: store _nextSequenceNumber in orchestrator state or pass it as input.

2. Determinism means idempotent inputs Progress activities must receive the same input on replay. If inputs vary (e.g., current timestamp), determinism breaks. Solution: pass immutable sequence numbers and batch IDs, not timestamps.

3. Retry policies are per-activity, not per-orchestrator We set a global retry policy, then realized external API failures need more retries than database operations. Solution: RetryOptions per CallActivityWithRetryAsync, fine-tuned per operation type.

4. Progress updates on failed activities are tricky If PushDocumentToCarrier fails (after 3 retries) and doesn’t get caught, progress stays at 45%. Solution: wrap failed activities in a sub-orchestrator that catches exceptions and sends progress before re-throwing.

5. SignalR connection drops during progress stream SignalR connections can drop mid-workflow. Sending progress with 0 retries meant some clients miss updates. Solution: clients poll progress endpoint as fallback; SignalR is best-effort.

Gotchas and Disclaimers

Azure Functions runtime v4+: Required for latest interceptor and orchestration APIs.
Determinism requirements: Orchestrator code must be deterministic (no DateTime.Now, no random). Use IDurableOrchestrationContext.CurrentUtcDateTime only.
State management: Large orchestration state can exceed size limits (~1 MB). Don’t store entire entity graphs; use IDs only.
.NET 9+: Recommended for async coordination improvements.

Next Steps

Implement DeterministicProgressSender in your orchestrator.
Define per-activity retry policies based on failure modes (transient vs. validation errors).
Use SignalR for real-time progress, with HTTP polling fallback.
Monitor orchestration duration and retry counts per activity type.

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