Deterministic Constraints

Orchestration code must be deterministic—it must produce the same sequence of operations every time it runs with the same history. This is required because orchestrations are replayed to rebuild state after interruptions.

The Golden Rule

The same input must always produce the same sequence of durable operations.

Durable operations include:

• ScheduleTask / ScheduleWithRetry
• CreateTimer
• WaitForExternalEvent
• CreateSubOrchestrationInstance
• ContinueAsNew

What NOT to Do

❌ Don't Use Current Time Directly

// ❌ WRONG - Non-deterministic
if (DateTime.UtcNow > deadline)
{
    await context.ScheduleTask<string>(typeof(ExpiredActivity), input);
}

// ✅ CORRECT - Use orchestration time
if (context.CurrentUtcDateTime > deadline)
{
    await context.ScheduleTask<string>(typeof(ExpiredActivity), input);
}

❌ Don't Use Random Numbers

// ❌ WRONG - Different on replay
var random = new Random();
if (random.Next(100) > 50)
{
    await context.ScheduleTask<string>(typeof(ActivityA), input);
}

// ✅ CORRECT - Get random value from activity
var randomValue = await context.ScheduleTask<int>(typeof(GetRandomNumberActivity), 100);
if (randomValue > 50)
{
    await context.ScheduleTask<string>(typeof(ActivityA), input);
}

// ✅ OR use a fixed seed
var random = new Random(42);  // Fixed seed
if (random.Next(100) > 50)
{
    await context.ScheduleTask<string>(typeof(ActivityA), input);
}

❌ Don't Use GUIDs Directly

// ❌ WRONG - Different GUID on replay
var id = Guid.NewGuid().ToString();
await context.ScheduleTask<string>(typeof(ProcessActivity), id);

// ✅ CORRECT - Use orchestration's NewGuid
var id = context.NewGuid().ToString();
await context.ScheduleTask<string>(typeof(ProcessActivity), id);

// ✅ Also correct - Get from activity
var id = await context.ScheduleTask<string>(typeof(GenerateIdActivity), null);

❌ Don't Read Environment Variables

// ❌ WRONG - May change between replays
var endpoint = Environment.GetEnvironmentVariable("API_ENDPOINT");
await context.ScheduleTask<string>(typeof(CallApiActivity), endpoint);

// ✅ CORRECT - Pass as input or read in activity
// Option 1: Pass as orchestration input
await context.ScheduleTask<string>(typeof(CallApiActivity), input.ApiEndpoint);

// Option 2: Read in activity
await context.ScheduleTask<string>(typeof(CallApiWithConfigActivity), input);

❌ Don't Make Network Calls

// ❌ WRONG - Side effect, non-deterministic
var response = await httpClient.GetAsync("https://api.example.com/data");
var data = await response.Content.ReadAsStringAsync();

// ✅ CORRECT - Use activity for network calls
var data = await context.ScheduleTask<string>(typeof(FetchDataActivity), "https://api.example.com/data");

❌ Don't Access Databases

// ❌ WRONG - Data may change between replays
var user = await dbContext.Users.FindAsync(userId);

// ✅ CORRECT - Use activity
var user = await context.ScheduleTask<User>(typeof(GetUserActivity), userId);

❌ Don't Use Thread.Sleep

// ❌ WRONG - Blocks thread, doesn't persist
Thread.Sleep(TimeSpan.FromMinutes(5));
await Task.Delay(TimeSpan.FromMinutes(5));

// ✅ CORRECT - Use durable timer
await context.CreateTimer(context.CurrentUtcDateTime.AddMinutes(5), true);

❌ Don't Use Mutable Static Variables

// ❌ WRONG - State not preserved across replays
static int counter = 0;
counter++;
if (counter > 5) { ... }

// ✅ CORRECT - Use orchestration input/output for state
public override async Task<int> RunTask(OrchestrationContext context, int currentCount)
{
    currentCount++;
    if (currentCount > 5) { ... }
}

❌ Don't Use Non-Deterministic Collections

// ❌ WRONG - HashSet and Dictionary iteration order is not guaranteed
var items = new HashSet<string> { "a", "b", "c" };
foreach (var item in items)
{
    await context.ScheduleTask<string>(typeof(ProcessActivity), item);
}

// ✅ CORRECT - Use ordered collection
var items = new List<string> { "a", "b", "c" };
foreach (var item in items)
{
    await context.ScheduleTask<string>(typeof(ProcessActivity), item);
}

❌ Don't Use Task.Run or Threading APIs

// ❌ WRONG - Background tasks are non-deterministic and may not complete before replay
await Task.Run(() => ProcessData(input));

// ❌ WRONG - Manual thread creation is non-deterministic
var thread = new Thread(() => DoWork());
thread.Start();

// ❌ WRONG - ThreadPool work is non-deterministic
ThreadPool.QueueUserWorkItem(_ => ProcessItem(input));

// ✅ CORRECT - Use activities for background work
var result = await context.ScheduleTask<string>(typeof(ProcessDataActivity), input);

// ✅ CORRECT - Use fan-out pattern for parallel work
var tasks = input.Items.Select(item => 
    context.ScheduleTask<string>(typeof(ProcessItemActivity), item));
var results = await Task.WhenAll(tasks);

What IS Safe

✅ Local Computation

// ✅ Safe - deterministic computation
var sum = input.Values.Sum();
var filtered = input.Items.Where(x => x.IsActive).ToList();
var formatted = $"Order {input.OrderId}: {input.Description}";

✅ Using Context Properties and Methods

// ✅ Safe - consistent across replays
var instanceId = context.OrchestrationInstance.InstanceId;
var currentTime = context.CurrentUtcDateTime;
var newId = context.NewGuid();

✅ Conditional Logic Based on Durable Results

// ✅ Safe - result comes from history during replay
var status = await context.ScheduleTask<OrderStatus>(typeof(GetStatusActivity), orderId);
if (status == OrderStatus.Approved)
{
    await context.ScheduleTask<string>(typeof(ProcessOrderActivity), orderId);
}

✅ Loops with Deterministic Bounds

// ✅ Safe - loop bounds are deterministic
for (int i = 0; i < input.Items.Count; i++)
{
    await context.ScheduleTask<string>(typeof(ProcessItemActivity), input.Items[i]);
}

✅ Parallel Execution

// ✅ Safe - Task.WhenAll is deterministic
var tasks = input.Items.Select(item => 
    context.ScheduleTask<string>(typeof(ProcessItemActivity), item));
var results = await Task.WhenAll(tasks);

Summary Table

Operation	Allowed in Orchestration?	Alternative
DateTime.UtcNow	❌ No	context.CurrentUtcDateTime
Guid.NewGuid()	❌ No	context.NewGuid()
Random.Next()	❌ No	Get from activity
Thread.Sleep() / Task.Delay()	❌ No	context.CreateTimer()
Task.Run()	❌ No	Use activity or fan-out
ThreadPool.QueueUserWorkItem()	❌ No	Use activity
Manual thread creation	❌ No	Use activity
HTTP calls	❌ No	Use activity
Database queries	❌ No	Use activity
File I/O	❌ No	Use activity
Environment variables	⚠️ Avoid	Pass as input or read in activity
Static mutable state	❌ No	Use orchestration state
HashSet or Dictionary iteration	⚠️ Avoid	Use List or sorted collection
Local computation	✅ Yes	—
String manipulation	✅ Yes	—
LINQ queries (on local data)	✅ Yes	—

Detecting Non-Determinism

Runtime Detection

Some non-deterministic issues cause runtime errors:

NonDeterministicOrchestrationException: The orchestration 'MyOrchestration' 
has a non-deterministic replay detected. The history expected 'TaskScheduled' 
for 'ActivityA' but got 'TaskScheduled' for 'ActivityB'.

Static Analysis

Consider using analyzers or code reviews to catch issues:

• Review all DateTime, Guid, Random usage
• Search for HTTP client usage
• Check for Thread.Sleep or Task.Delay
• Check for Task.Run, ThreadPool, or new Thread

Next Steps

• Replay and Durability — Why determinism matters
• Versioning — Safely updating orchestration code
• Error Handling — Handling failures deterministically