Azure Functions

Azure Functions for .NET

Trigger On

• working on Azure Functions in .NET
• migrating from the in-process model to the isolated worker model
• adding Durable Functions, bindings, or host configuration

Documentation

• Guide for running C# Azure Functions in an isolated worker process
• Differences between in-process and isolated worker process
• Migrate C# app from in-process to isolated worker model
• Durable Functions overview
• Durable Functions best practices and diagnostic tools

References

• Patterns - Isolated worker patterns, Durable Functions patterns, advanced binding patterns
• Anti-Patterns - Common Azure Functions mistakes and how to avoid them

Workflow

1. Use isolated worker model for all new work:

• In-process model reaches end of support on November 10, 2026
• Runtime v1.x ends support on September 14, 2026
• Target .NET 8+ for longest support window

2. Detect current project shape:

• Target framework and runtime version
• Worker model (isolated vs in-process)
• Binding packages and host configuration

3. Use standard .NET patterns in isolated model:

• Normal dependency injection
• Middleware pipeline
• IOptions<T> for configuration
• ILogger<T> for logging

4. For Durable Functions:

• Validate orchestration determinism constraints
• Handle replay behavior correctly
• Use typed activity patterns

5. Verify both local and deployment behavior.

Isolated Worker Model Setup

Basic Function with DI

// Program.cs
var host = new HostBuilder()
    .ConfigureFunctionsWebApplication()
    .ConfigureServices(services =>
    {
        services.AddApplicationInsightsTelemetryWorkerService();
        services.ConfigureFunctionsApplicationInsights();
        services.AddSingleton<IMyService, MyService>();
    })
    .Build();

host.Run();

HTTP Trigger Function

public class HttpFunctions(ILogger<HttpFunctions> logger, IMyService myService)
{
    [Function("GetItems")]
    public async Task<IActionResult> GetItems(
        [HttpTrigger(AuthorizationLevel.Function, "get", Route = "items")] HttpRequest req)
    {
        logger.LogInformation("Processing GetItems request");
        var items = await myService.GetItemsAsync();
        return new OkObjectResult(items);
    }
}

Queue Trigger with Options

public class QueueFunctions(ILogger<QueueFunctions> logger, IOptions<ProcessingOptions> options)
{
    [Function("ProcessMessage")]
    public async Task ProcessMessage(
        [QueueTrigger("myqueue", Connection = "AzureWebJobsStorage")] string message)
    {
        logger.LogInformation("Processing message: {Message}", message);
        // Process with retry policy from options
    }
}

Middleware Pattern

Custom Middleware

// Program.cs
var host = new HostBuilder()
    .ConfigureFunctionsWebApplication(builder =>
    {
        builder.UseMiddleware<ExceptionHandlingMiddleware>();
        builder.UseMiddleware<CorrelationIdMiddleware>();
    })
    .Build();

// CorrelationIdMiddleware.cs
public class CorrelationIdMiddleware : IFunctionsWorkerMiddleware
{
    public async Task Invoke(FunctionContext context, FunctionExecutionDelegate next)
    {
        var correlationId = context.Features.Get<IHttpRequestFeature>()?.Headers["X-Correlation-Id"]
            ?? Guid.NewGuid().ToString();

        context.Items["CorrelationId"] = correlationId;

        await next(context);
    }
}

Durable Functions Patterns

Function Chaining

[Function(nameof(ChainOrchestrator))]
public static async Task<string> ChainOrchestrator(
    [OrchestrationTrigger] TaskOrchestrationContext context)
{
    var result1 = await context.CallActivityAsync<string>(nameof(Step1), "input");
    var result2 = await context.CallActivityAsync<string>(nameof(Step2), result1);
    var result3 = await context.CallActivityAsync<string>(nameof(Step3), result2);
    return result3;
}

[Function(nameof(Step1))]
public static string Step1([ActivityTrigger] string input) => $"Step1({input})";

[Function(nameof(Step2))]
public static string Step2([ActivityTrigger] string input) => $"Step2({input})";

[Function(nameof(Step3))]
public static string Step3([ActivityTrigger] string input) => $"Step3({input})";

Fan-Out/Fan-In

[Function(nameof(FanOutFanInOrchestrator))]
public static async Task<int[]> FanOutFanInOrchestrator(
    [OrchestrationTrigger] TaskOrchestrationContext context)
{
    var workItems = await context.CallActivityAsync<string[]>(nameof(GetWorkItems), null);

    // Fan out - process all items in parallel
    var tasks = workItems.Select(item =>
        context.CallActivityAsync<int>(nameof(ProcessWorkItem), item));

    // Fan in - wait for all to complete
    var results = await Task.WhenAll(tasks);

    return results;
}

[Function(nameof(ProcessWorkItem))]
public static int ProcessWorkItem([ActivityTrigger] string item)
{
    // Process item and return result
    return item.Length;
}

Human Interaction Pattern

[Function(nameof(ApprovalOrchestrator))]
public static async Task<string> ApprovalOrchestrator(
    [OrchestrationTrigger] TaskOrchestrationContext context)
{
    var request = context.GetInput<ApprovalRequest>();

    // Send notification
    await context.CallActivityAsync(nameof(SendApprovalRequest), request);

    // Wait for external event with timeout
    using var cts = new CancellationTokenSource();
    var approvalTask = context.WaitForExternalEvent<bool>("ApprovalEvent");
    var timeoutTask = context.CreateTimer(context.CurrentUtcDateTime.AddDays(7), cts.Token);

    var winner = await Task.WhenAny(approvalTask, timeoutTask);

    if (winner == approvalTask)
    {
        cts.Cancel();
        return approvalTask.Result ? "Approved" : "Rejected";
    }

    return "Timed out";
}

Best Practices

1. Use isolated worker model for new development - Full .NET ecosystem access, middleware support, and longer support lifecycle 2. Inject dependencies via constructor - Use ILogger<T> and service interfaces for testability 3. Keep orchestrator code deterministic - No I/O, random, DateTime.Now, or Guid.NewGuid() in orchestrators 4. Handle sensitive data in activities - Fetch secrets from Key Vault in activity functions, never in orchestrators 5. Use unique task hub names - Prevent accidental sharing when multiple apps use the same storage 6. Avoid large inputs/outputs - Serialize to blob storage for large payloads to prevent history bloat 7. Configure concurrency limits - Set appropriate limits in host.json for resource-intensive functions 8. Keep SDK and extensions updated - Latest versions include performance improvements and bug fixes

Anti-Patterns to Avoid

Deployment Considerations

Linux Consumption Plan Limitations

.NET 10+ apps cannot run on Linux Consumption plan.
Use Flex Consumption plan or App Service for .NET 10+.
.NET 9 is the last version supported on Linux Consumption.

host.json Configuration

{
  "version": "2.0",
  "extensions": {
    "durableTask": {
      "hubName": "MyUniqueTaskHub",
      "maxConcurrentActivityFunctions": 10,
      "maxConcurrentOrchestratorFunctions": 5
    }
  },
  "logging": {
    "applicationInsights": {
      "samplingSettings": {
        "isEnabled": true,
        "excludedTypes": "Request"
      }
    }
  }
}

Deliver

• correct Functions project setup for the isolated worker model
• clear binding and host configuration
• middleware for cross-cutting concerns
• Durable Functions with proper orchestration patterns
• migration-safe guidance when upgrading execution models

Validate

• execution model guidance is consistent (isolated only for new work)
• orchestrator code is deterministic
• bindings and host settings match the target runtime
• large payloads are externalized to blob storage
• retry policies are configured for transient failures
• local and deployment behavior are both verified

Azure Functions Anti-Patterns

Isolated Worker Model Anti-Patterns

Using In-Process APIs in Isolated Worker

// WRONG - Using in-process types in isolated worker
public class BrokenFunction
{
    [Function("BrokenHttp")]
    public async Task<IActionResult> Run(
        [HttpTrigger(AuthorizationLevel.Function, "get")] HttpRequestMessage req) // Wrong type
    {
        // HttpRequestMessage is in-process model
        return new OkResult();
    }
}

// CORRECT - Using isolated worker types
public class CorrectFunction
{
    [Function("CorrectHttp")]
    public async Task<IActionResult> Run(
        [HttpTrigger(AuthorizationLevel.Function, "get")] HttpRequest req) // Correct type
    {
        return new OkResult();
    }
}

Service Locator Instead of Constructor Injection

// WRONG - Using service locator anti-pattern
public class ServiceLocatorFunction
{
    [Function("BadDI")]
    public async Task Run(
        [QueueTrigger("myqueue")] string message,
        FunctionContext context)
    {
        // Anti-pattern: getting services from context
        var service = context.InstanceServices.GetRequiredService<IMyService>();
        await service.ProcessAsync(message);
    }
}

// CORRECT - Constructor injection
public class ConstructorInjectionFunction(IMyService myService, ILogger<ConstructorInjectionFunction> logger)
{
    [Function("GoodDI")]
    public async Task Run([QueueTrigger("myqueue")] string message)
    {
        logger.LogInformation("Processing message");
        await myService.ProcessAsync(message);
    }
}

Blocking Async Code

// WRONG - Blocking on async operations
public class BlockingFunction
{
    [Function("BlockingCall")]
    public string Run([HttpTrigger(AuthorizationLevel.Function, "get")] HttpRequest req)
    {
        // Anti-pattern: blocking on async
        var result = _httpClient.GetStringAsync("https://api.example.com/data").Result;
        return result;
    }
}

// CORRECT - Async all the way
public class AsyncFunction
{
    [Function("AsyncCall")]
    public async Task<string> Run([HttpTrigger(AuthorizationLevel.Function, "get")] HttpRequest req)
    {
        var result = await _httpClient.GetStringAsync("https://api.example.com/data");
        return result;
    }
}

Creating HttpClient Per Request

// WRONG - Creating new HttpClient per request
public class BadHttpClientFunction
{
    [Function("BadHttpClient")]
    public async Task<string> Run([HttpTrigger(AuthorizationLevel.Function, "get")] HttpRequest req)
    {
        // Anti-pattern: socket exhaustion risk
        using var client = new HttpClient();
        return await client.GetStringAsync("https://api.example.com/data");
    }
}

// CORRECT - Using IHttpClientFactory
public class GoodHttpClientFunction(IHttpClientFactory httpClientFactory)
{
    [Function("GoodHttpClient")]
    public async Task<string> Run([HttpTrigger(AuthorizationLevel.Function, "get")] HttpRequest req)
    {
        var client = httpClientFactory.CreateClient("ApiClient");
        return await client.GetStringAsync("https://api.example.com/data");
    }
}

// Program.cs registration
services.AddHttpClient("ApiClient", client =>
{
    client.BaseAddress = new Uri("https://api.example.com");
    client.Timeout = TimeSpan.FromSeconds(30);
});

Ignoring Cancellation Tokens

// WRONG - Ignoring cancellation
public class NoCancellationFunction
{
    [Function("NoCancellation")]
    public async Task Run(
        [QueueTrigger("longrunning")] string message)
    {
        // Anti-pattern: long operation without cancellation support
        await LongRunningOperation();
    }
}

// CORRECT - Respecting cancellation
public class CancellationAwareFunction
{
    [Function("WithCancellation")]
    public async Task Run(
        [QueueTrigger("longrunning")] string message,
        CancellationToken cancellationToken)
    {
        // Pass cancellation token to long operations
        await LongRunningOperation(cancellationToken);
    }

    private async Task LongRunningOperation(CancellationToken cancellationToken)
    {
        foreach (var item in items)
        {
            cancellationToken.ThrowIfCancellationRequested();
            await ProcessItem(item, cancellationToken);
        }
    }
}

Static State Across Invocations

// WRONG - Mutable static state
public class StaticStateFunction
{
    private static int _requestCount = 0; // Anti-pattern: race conditions
    private static List<string> _cache = new(); // Anti-pattern: memory growth

    [Function("StaticState")]
    public string Run([HttpTrigger(AuthorizationLevel.Function, "get")] HttpRequest req)
    {
        _requestCount++; // Race condition in concurrent execution
        _cache.Add(req.Path); // Unbounded memory growth
        return $"Count: {_requestCount}";
    }
}

// CORRECT - Use proper distributed state
public class DistributedStateFunction(IDistributedCache cache)
{
    [Function("DistributedState")]
    public async Task<string> Run([HttpTrigger(AuthorizationLevel.Function, "get")] HttpRequest req)
    {
        var count = await cache.GetStringAsync("request-count") ?? "0";
        var newCount = int.Parse(count) + 1;
        await cache.SetStringAsync("request-count", newCount.ToString());
        return $"Count: {newCount}";
    }
}

---

Durable Functions Anti-Patterns

Non-Deterministic Orchestrator Code

// WRONG - Non-deterministic operations in orchestrator
[Function(nameof(BadOrchestrator))]
public static async Task BadOrchestrator([OrchestrationTrigger] TaskOrchestrationContext context)
{
    // Anti-pattern: DateTime.Now changes on replay
    var timestamp = DateTime.Now;

    // Anti-pattern: Guid.NewGuid() changes on replay
    var correlationId = Guid.NewGuid();

    // Anti-pattern: Random values change on replay
    var random = new Random().Next();

    // Anti-pattern: Reading config/environment in orchestrator
    var setting = Environment.GetEnvironmentVariable("MySetting");

    // Anti-pattern: I/O in orchestrator
    var httpClient = new HttpClient();
    var result = await httpClient.GetStringAsync("https://api.example.com");
}

// CORRECT - Deterministic orchestrator
[Function(nameof(GoodOrchestrator))]
public static async Task GoodOrchestrator([OrchestrationTrigger] TaskOrchestrationContext context)
{
    // Use context for deterministic time
    var timestamp = context.CurrentUtcDateTime;

    // Use NewGuid from context or pass from activity
    var correlationId = context.NewGuid();

    // Move I/O and non-deterministic operations to activities
    var result = await context.CallActivityAsync<string>(nameof(FetchDataActivity), null);
    var config = await context.CallActivityAsync<ConfigData>(nameof(GetConfigActivity), null);
}

Large Orchestrator Input/Output

// WRONG - Large payloads in orchestrator history
[Function(nameof(LargePayloadOrchestrator))]
public static async Task<byte[]> LargePayloadOrchestrator(
    [OrchestrationTrigger] TaskOrchestrationContext context)
{
    // Anti-pattern: Large input stored in history
    var largeInput = context.GetInput<byte[]>(); // Could be MB of data

    // Anti-pattern: Large activity result stored in history
    var processedData = await context.CallActivityAsync<byte[]>(
        nameof(ProcessLargeFile),
        largeInput); // Each call adds to history

    return processedData; // Large output in history
}

// CORRECT - Use blob storage for large payloads
[Function(nameof(SmallPayloadOrchestrator))]
public static async Task<string> SmallPayloadOrchestrator(
    [OrchestrationTrigger] TaskOrchestrationContext context)
{
    // Pass blob reference instead of data
    var blobRef = context.GetInput<BlobReference>();

    // Activity processes blob and returns new reference
    var resultBlobRef = await context.CallActivityAsync<BlobReference>(
        nameof(ProcessLargeFileFromBlob),
        blobRef);

    return resultBlobRef.Url; // Small reference in history
}

public class BlobReference
{
    public string ContainerName { get; set; }
    public string BlobName { get; set; }
    public string Url => $"https://storage.blob.core.windows.net/{ContainerName}/{BlobName}";
}

Shared Task Hub Across Applications

// WRONG - Default or shared task hub name
// host.json
{
    "version": "2.0",
    "extensions": {
        "durableTask": {
            // Anti-pattern: Using default name or sharing across apps
            // "hubName": "DurableFunctionsHub" (default)
        }
    }
}

// CORRECT - Unique task hub per application
// host.json
{
    "version": "2.0",
    "extensions": {
        "durableTask": {
            "hubName": "OrderProcessingApp-Prod-Hub"
        }
    }
}

Secrets in Orchestrator History

// WRONG - Secrets passed through orchestrator
[Function(nameof(SecretsInHistoryOrchestrator))]
public static async Task SecretsInHistoryOrchestrator(
    [OrchestrationTrigger] TaskOrchestrationContext context)
{
    // Anti-pattern: API key in orchestrator input (stored in history)
    var input = context.GetInput<ProcessingInput>();
    var apiKey = input.ApiKey; // Visible in Durable Functions storage

    // Anti-pattern: Fetching secrets in orchestrator
    var secret = await context.CallActivityAsync<string>(nameof(GetSecret), "my-secret");

    // Anti-pattern: Passing secret as activity input
    await context.CallActivityAsync(nameof(CallApi), new { ApiKey = secret });
}

// CORRECT - Activities fetch their own secrets
[Function(nameof(SecureOrchestrator))]
public static async Task SecureOrchestrator(
    [OrchestrationTrigger] TaskOrchestrationContext context)
{
    var input = context.GetInput<ProcessingInput>();

    // Pass only non-sensitive identifiers
    await context.CallActivityAsync(nameof(CallApiSecurely), new {
        SecretName = "api-key-name", // Just the name, not the value
        Endpoint = input.Endpoint
    });
}

[Function(nameof(CallApiSecurely))]
public static async Task CallApiSecurely(
    [ActivityTrigger] ApiCallInput input,
    FunctionContext context)
{
    // Fetch secret in activity (not stored in orchestrator history)
    var secretClient = new SecretClient(new Uri("https://myvault.vault.azure.net"), new DefaultAzureCredential());
    var apiKey = await secretClient.GetSecretAsync(input.SecretName);

    // Use secret directly
    await MakeApiCall(input.Endpoint, apiKey.Value.Value);
}

Infinite Orchestration Without ContinueAsNew

// WRONG - Unbounded history growth
[Function(nameof(InfiniteLoopOrchestrator))]
public static async Task InfiniteLoopOrchestrator(
    [OrchestrationTrigger] TaskOrchestrationContext context)
{
    while (true)
    {
        // Anti-pattern: History grows unbounded
        await context.CallActivityAsync(nameof(DoWork), null);
        await context.CreateTimer(context.CurrentUtcDateTime.AddMinutes(5), CancellationToken.None);
        // Eventually: OutOfMemory, slow replays, storage bloat
    }
}

// CORRECT - Use ContinueAsNew to reset history
[Function(nameof(EternalOrchestrator))]
public static async Task EternalOrchestrator(
    [OrchestrationTrigger] TaskOrchestrationContext context)
{
    var state = context.GetInput<OrchestratorState>() ?? new OrchestratorState();

    // Do bounded amount of work
    for (int i = 0; i < 10; i++)
    {
        await context.CallActivityAsync(nameof(DoWork), state.LastProcessedId);
        state.IterationCount++;
        state.LastProcessedId++;
    }

    await context.CreateTimer(context.CurrentUtcDateTime.AddMinutes(5), CancellationToken.None);

    // Reset history and continue with current state
    context.ContinueAsNew(state);
}

Awaiting Non-Durable Tasks

// WRONG - Awaiting non-durable async operations
[Function(nameof(NonDurableAwaitOrchestrator))]
public static async Task NonDurableAwaitOrchestrator(
    [OrchestrationTrigger] TaskOrchestrationContext context)
{
    // Anti-pattern: Direct HTTP call in orchestrator
    var client = new HttpClient();
    var result = await client.GetStringAsync("https://api.example.com"); // Not replayed correctly

    // Anti-pattern: Direct database call
    using var db = new MyDbContext();
    var data = await db.Items.ToListAsync(); // Not replayed correctly

    // Anti-pattern: Reading from file system
    var content = await File.ReadAllTextAsync("data.json"); // Not replayed correctly
}

// CORRECT - Use activities for all I/O
[Function(nameof(DurableAwaitOrchestrator))]
public static async Task DurableAwaitOrchestrator(
    [OrchestrationTrigger] TaskOrchestrationContext context)
{
    // All I/O through activities
    var apiResult = await context.CallActivityAsync<string>(nameof(CallApiActivity), null);
    var dbData = await context.CallActivityAsync<List<Item>>(nameof(GetDatabaseItemsActivity), null);
    var fileContent = await context.CallActivityAsync<string>(nameof(ReadFileActivity), "data.json");
}

Missing Retry Policies for External Calls

// WRONG - No retry policy for unreliable operations
[Function(nameof(NoRetryOrchestrator))]
public static async Task NoRetryOrchestrator(
    [OrchestrationTrigger] TaskOrchestrationContext context)
{
    // Anti-pattern: External call without retry
    var result = await context.CallActivityAsync<string>(nameof(CallExternalApi), null);
    // First transient failure = orchestration failure
}

// CORRECT - Configure appropriate retry policies
[Function(nameof(RetryOrchestrator))]
public static async Task RetryOrchestrator(
    [OrchestrationTrigger] TaskOrchestrationContext context)
{
    var retryOptions = new TaskOptions
    {
        Retry = new RetryPolicy(
            maxNumberOfAttempts: 5,
            firstRetryInterval: TimeSpan.FromSeconds(1),
            backoffCoefficient: 2.0,
            maxRetryInterval: TimeSpan.FromMinutes(1),
            retryTimeout: TimeSpan.FromMinutes(5))
    };

    var result = await context.CallActivityAsync<string>(
        nameof(CallExternalApi),
        null,
        retryOptions);
}

---

Configuration and Deployment Anti-Patterns

Hardcoded Connection Strings

// WRONG - Hardcoded secrets
public class HardcodedSecretsFunction
{
    [Function("HardcodedSecrets")]
    public async Task Run([TimerTrigger("0 */5 * * * *")] TimerInfo timer)
    {
        // Anti-pattern: Secrets in code
        var connectionString = "AccountName=myaccount;AccountKey=abc123...";
        var apiKey = "sk-live-abcdefg";
    }
}

// CORRECT - Use configuration and Key Vault
// Program.cs
var host = new HostBuilder()
    .ConfigureAppConfiguration((context, config) =>
    {
        var builtConfig = config.Build();
        var keyVaultUri = builtConfig["KeyVaultUri"];
        config.AddAzureKeyVault(new Uri(keyVaultUri), new DefaultAzureCredential());
    })
    .Build();

// Function
public class SecureFunction(IConfiguration configuration)
{
    [Function("SecureConfig")]
    public async Task Run([TimerTrigger("0 */5 * * * *")] TimerInfo timer)
    {
        var connectionString = configuration["StorageConnectionString"]; // From Key Vault
    }
}

Missing Logging Correlation

// WRONG - Basic logging without correlation
public class NoCorrelationFunction
{
    private readonly ILogger _logger;

    [Function("NoCorrelation")]
    public async Task Run([QueueTrigger("myqueue")] string message)
    {
        // Anti-pattern: No correlation across function calls
        _logger.LogInformation("Processing message");
        await CallDownstreamService();
        _logger.LogInformation("Done"); // Can't correlate with downstream
    }
}

// CORRECT - Correlation via middleware and structured logging
// Middleware
public class CorrelationMiddleware : IFunctionsWorkerMiddleware
{
    public async Task Invoke(FunctionContext context, FunctionExecutionDelegate next)
    {
        var correlationId = context.TraceContext?.TraceParent
            ?? Guid.NewGuid().ToString();

        using (_logger.BeginScope(new Dictionary<string, object>
        {
            ["CorrelationId"] = correlationId,
            ["FunctionName"] = context.FunctionDefinition.Name
        }))
        {
            await next(context);
        }
    }
}

Over-Provisioned or Under-Provisioned Concurrency

// WRONG - Default concurrency for all workloads
{
    "version": "2.0"
    // No concurrency configuration - defaults may not fit workload
}

// WRONG - Aggressive concurrency for CPU-bound work
{
    "version": "2.0",
    "extensions": {
        "queues": {
            "batchSize": 32,
            "newBatchThreshold": 16
        }
    }
    // CPU-bound functions will compete for threads
}

// CORRECT - Tuned for workload type
// For I/O-bound queue processing
{
    "version": "2.0",
    "extensions": {
        "queues": {
            "batchSize": 16,
            "newBatchThreshold": 8,
            "maxDequeueCount": 5,
            "visibilityTimeout": "00:05:00"
        }
    }
}

// For CPU-intensive Durable Functions
{
    "version": "2.0",
    "extensions": {
        "durableTask": {
            "maxConcurrentActivityFunctions": 4,
            "maxConcurrentOrchestratorFunctions": 2
        }
    }
}

Ignoring Cold Start Impact

// WRONG - Heavy initialization in function code
public class HeavyInitFunction
{
    [Function("HeavyInit")]
    public async Task<string> Run([HttpTrigger(AuthorizationLevel.Function, "get")] HttpRequest req)
    {
        // Anti-pattern: Initialization on every cold start
        var bigCache = LoadLargeDataset(); // 5 seconds
        var mlModel = LoadMachineLearningModel(); // 10 seconds

        return "Ready";
    }
}

// CORRECT - Lazy initialization or warm-up
public class OptimizedInitFunction
{
    private static readonly Lazy<BigDataset> _cache = new(() => LoadLargeDataset());

    [Function("OptimizedInit")]
    public async Task<string> Run([HttpTrigger(AuthorizationLevel.Function, "get")] HttpRequest req)
    {
        // Lazy loaded once, reused across invocations
        var data = _cache.Value;
        return "Ready";
    }
}

// Program.cs - Pre-warm in startup
services.AddSingleton<IModelService>(sp =>
{
    var service = new ModelService();
    // Warm up during startup, not first request
    _ = service.EnsureModelLoadedAsync();
    return service;
});

Missing Health Checks for Dependencies

// WRONG - No health checks, silent failures
public class NoHealthCheckFunction(IMyService service)
{
    [Function("NoHealthCheck")]
    public async Task<IActionResult> Run(
        [HttpTrigger(AuthorizationLevel.Function, "get")] HttpRequest req)
    {
        // Fails with cryptic errors if database is down
        var result = await service.GetDataAsync();
        return new OkObjectResult(result);
    }
}

// CORRECT - Health endpoint for monitoring
public class HealthCheckFunction(
    IMyService service,
    IHealthCheck[] healthChecks)
{
    [Function("HealthCheck")]
    public async Task<IActionResult> Run(
        [HttpTrigger(AuthorizationLevel.Anonymous, "get", Route = "health")] HttpRequest req)
    {
        var results = new Dictionary<string, string>();
        var isHealthy = true;

        foreach (var check in healthChecks)
        {
            try
            {
                var result = await check.CheckHealthAsync(new HealthCheckContext());
                results[check.GetType().Name] = result.Status.ToString();
                if (result.Status != HealthStatus.Healthy)
                {
                    isHealthy = false;
                }
            }
            catch (Exception ex)
            {
                results[check.GetType().Name] = $"Failed: {ex.Message}";
                isHealthy = false;
            }
        }

        return isHealthy
            ? new OkObjectResult(new { status = "Healthy", checks = results })
            : new ObjectResult(new { status = "Unhealthy", checks = results })
              { StatusCode = 503 };
    }
}

Azure Functions Patterns

Isolated Worker Model Patterns

Dependency Injection with Scoped Services

// Program.cs - Register services with appropriate lifetimes
var host = new HostBuilder()
    .ConfigureFunctionsWebApplication()
    .ConfigureServices(services =>
    {
        // Singleton - shared across all function invocations
        services.AddSingleton<ICacheService, RedisCacheService>();

        // Scoped - one instance per function invocation
        services.AddScoped<IUnitOfWork, UnitOfWork>();

        // Transient - new instance each time requested
        services.AddTransient<IEmailBuilder, EmailBuilder>();

        // Options pattern for configuration
        services.Configure<StorageOptions>(
            hostContext.Configuration.GetSection("Storage"));
    })
    .Build();

Request/Response Pipeline with Middleware

// Full middleware pipeline setup
var host = new HostBuilder()
    .ConfigureFunctionsWebApplication(builder =>
    {
        // Order matters - executed in registration order
        builder.UseMiddleware<ExceptionHandlingMiddleware>();
        builder.UseMiddleware<RequestLoggingMiddleware>();
        builder.UseMiddleware<AuthenticationMiddleware>();
        builder.UseMiddleware<CorrelationIdMiddleware>();
    })
    .Build();

// Exception handling middleware
public class ExceptionHandlingMiddleware : IFunctionsWorkerMiddleware
{
    private readonly ILogger<ExceptionHandlingMiddleware> _logger;

    public ExceptionHandlingMiddleware(ILogger<ExceptionHandlingMiddleware> logger)
    {
        _logger = logger;
    }

    public async Task Invoke(FunctionContext context, FunctionExecutionDelegate next)
    {
        try
        {
            await next(context);
        }
        catch (ValidationException ex)
        {
            _logger.LogWarning(ex, "Validation error in {Function}", context.FunctionDefinition.Name);
            await WriteErrorResponse(context, StatusCodes.Status400BadRequest, ex.Message);
        }
        catch (Exception ex)
        {
            _logger.LogError(ex, "Unhandled exception in {Function}", context.FunctionDefinition.Name);
            await WriteErrorResponse(context, StatusCodes.Status500InternalServerError, "Internal server error");
        }
    }

    private static async Task WriteErrorResponse(FunctionContext context, int statusCode, string message)
    {
        var httpReqData = await context.GetHttpRequestDataAsync();
        if (httpReqData != null)
        {
            var response = httpReqData.CreateResponse((HttpStatusCode)statusCode);
            await response.WriteAsJsonAsync(new { error = message });
            context.GetInvocationResult().Value = response;
        }
    }
}

Output Bindings with Multiple Outputs

public class MultiOutputFunctions
{
    [Function("ProcessOrder")]
    public async Task<MultiOutput> ProcessOrder(
        [QueueTrigger("orders", Connection = "StorageConnection")] Order order,
        FunctionContext context)
    {
        var logger = context.GetLogger<MultiOutputFunctions>();
        logger.LogInformation("Processing order {OrderId}", order.Id);

        var confirmation = new OrderConfirmation
        {
            OrderId = order.Id,
            ProcessedAt = DateTime.UtcNow
        };

        return new MultiOutput
        {
            Confirmation = confirmation,
            NotificationMessage = $"Order {order.Id} processed"
        };
    }
}

public class MultiOutput
{
    [QueueOutput("confirmations", Connection = "StorageConnection")]
    public OrderConfirmation Confirmation { get; set; }

    [QueueOutput("notifications", Connection = "StorageConnection")]
    public string NotificationMessage { get; set; }
}

Typed Configuration with Validation

// Program.cs
services.AddOptions<ServiceBusOptions>()
    .Bind(configuration.GetSection("ServiceBus"))
    .ValidateDataAnnotations()
    .ValidateOnStart();

// Options class with validation
public class ServiceBusOptions
{
    [Required]
    public string ConnectionString { get; set; } = string.Empty;

    [Required]
    public string QueueName { get; set; } = string.Empty;

    [Range(1, 100)]
    public int MaxConcurrentCalls { get; set; } = 10;
}

// Function using validated options
public class ServiceBusFunctions(IOptions<ServiceBusOptions> options, ILogger<ServiceBusFunctions> logger)
{
    [Function("ProcessServiceBusMessage")]
    public async Task ProcessMessage(
        [ServiceBusTrigger("%ServiceBus:QueueName%", Connection = "ServiceBusConnection")]
        ServiceBusReceivedMessage message)
    {
        logger.LogInformation("Processing message with max concurrency: {Max}",
            options.Value.MaxConcurrentCalls);
    }
}

---

Durable Functions Patterns

Sub-Orchestrations for Modularity

[Function(nameof(MainOrchestrator))]
public static async Task<OrderResult> MainOrchestrator(
    [OrchestrationTrigger] TaskOrchestrationContext context)
{
    var order = context.GetInput<Order>();

    // Validate order in sub-orchestration
    var isValid = await context.CallSubOrchestratorAsync<bool>(
        nameof(ValidationOrchestrator),
        order);

    if (!isValid)
    {
        return new OrderResult { Status = "ValidationFailed" };
    }

    // Process payment in sub-orchestration with retry
    var paymentResult = await context.CallSubOrchestratorAsync<PaymentResult>(
        nameof(PaymentOrchestrator),
        order.Payment,
        new TaskOptions { Retry = new RetryPolicy(3, TimeSpan.FromSeconds(5)) });

    // Ship order in sub-orchestration
    var shipmentResult = await context.CallSubOrchestratorAsync<ShipmentResult>(
        nameof(ShippingOrchestrator),
        new ShippingRequest { OrderId = order.Id, Address = order.ShippingAddress });

    return new OrderResult
    {
        Status = "Completed",
        PaymentId = paymentResult.TransactionId,
        TrackingNumber = shipmentResult.TrackingNumber
    };
}

[Function(nameof(ValidationOrchestrator))]
public static async Task<bool> ValidationOrchestrator(
    [OrchestrationTrigger] TaskOrchestrationContext context)
{
    var order = context.GetInput<Order>();

    // Parallel validation checks
    var inventoryTask = context.CallActivityAsync<bool>(nameof(CheckInventory), order.Items);
    var fraudTask = context.CallActivityAsync<bool>(nameof(CheckFraud), order.Customer);
    var creditTask = context.CallActivityAsync<bool>(nameof(CheckCredit), order.Payment);

    var results = await Task.WhenAll(inventoryTask, fraudTask, creditTask);
    return results.All(r => r);
}

Eternal Orchestrations for Long-Running Processes

[Function(nameof(MonitorOrchestrator))]
public static async Task MonitorOrchestrator(
    [OrchestrationTrigger] TaskOrchestrationContext context)
{
    var config = context.GetInput<MonitorConfig>();

    // Check status
    var status = await context.CallActivityAsync<ServiceStatus>(
        nameof(CheckServiceStatus),
        config.ServiceEndpoint);

    if (status.IsHealthy)
    {
        // Wait before next check
        await context.CreateTimer(
            context.CurrentUtcDateTime.AddMinutes(config.CheckIntervalMinutes),
            CancellationToken.None);
    }
    else
    {
        // Alert and wait shorter interval
        await context.CallActivityAsync(nameof(SendAlert), new Alert
        {
            Service = config.ServiceName,
            Status = status,
            Timestamp = context.CurrentUtcDateTime
        });

        await context.CreateTimer(
            context.CurrentUtcDateTime.AddMinutes(1),
            CancellationToken.None);
    }

    // Continue as new to prevent history growth
    context.ContinueAsNew(config);
}

Aggregator Pattern with Entity Functions

// Entity definition
[Function(nameof(CounterEntity))]
public static Task CounterEntity(
    [EntityTrigger] TaskEntityDispatcher dispatcher)
{
    return dispatcher.DispatchAsync<Counter>();
}

public class Counter
{
    public int Value { get; set; }

    public void Add(int amount) => Value += amount;
    public void Reset() => Value = 0;
    public int Get() => Value;
}

// Using entity from orchestrator
[Function(nameof(AggregatorOrchestrator))]
public static async Task<int> AggregatorOrchestrator(
    [OrchestrationTrigger] TaskOrchestrationContext context)
{
    var entityId = new EntityInstanceId(nameof(CounterEntity), "global-counter");

    // Signal entity (fire and forget)
    await context.Entities.SignalEntityAsync(entityId, "Add", 5);

    // Call entity and get result
    var currentValue = await context.Entities.CallEntityAsync<int>(entityId, "Get");

    return currentValue;
}

Saga Pattern with Compensation

[Function(nameof(SagaOrchestrator))]
public static async Task<SagaResult> SagaOrchestrator(
    [OrchestrationTrigger] TaskOrchestrationContext context)
{
    var request = context.GetInput<BookingRequest>();
    var compensations = new Stack<Func<Task>>();

    try
    {
        // Step 1: Reserve flight
        var flightReservation = await context.CallActivityAsync<FlightReservation>(
            nameof(ReserveFlight), request.Flight);
        compensations.Push(() => context.CallActivityAsync(nameof(CancelFlight), flightReservation.Id));

        // Step 2: Reserve hotel
        var hotelReservation = await context.CallActivityAsync<HotelReservation>(
            nameof(ReserveHotel), request.Hotel);
        compensations.Push(() => context.CallActivityAsync(nameof(CancelHotel), hotelReservation.Id));

        // Step 3: Reserve car
        var carReservation = await context.CallActivityAsync<CarReservation>(
            nameof(ReserveCar), request.Car);
        compensations.Push(() => context.CallActivityAsync(nameof(CancelCar), carReservation.Id));

        // Step 4: Charge payment
        var payment = await context.CallActivityAsync<PaymentConfirmation>(
            nameof(ProcessPayment), new PaymentRequest
            {
                Amount = flightReservation.Price + hotelReservation.Price + carReservation.Price,
                CustomerId = request.CustomerId
            });

        return new SagaResult
        {
            Success = true,
            FlightConfirmation = flightReservation.ConfirmationNumber,
            HotelConfirmation = hotelReservation.ConfirmationNumber,
            CarConfirmation = carReservation.ConfirmationNumber
        };
    }
    catch (Exception ex)
    {
        // Compensate in reverse order
        while (compensations.Count > 0)
        {
            var compensation = compensations.Pop();
            try
            {
                await compensation();
            }
            catch (Exception compEx)
            {
                // Log compensation failure but continue
                context.SetCustomStatus($"Compensation failed: {compEx.Message}");
            }
        }

        return new SagaResult
        {
            Success = false,
            FailureReason = ex.Message
        };
    }
}

Retry Policies and Error Handling

[Function(nameof(ResilientOrchestrator))]
public static async Task<ProcessingResult> ResilientOrchestrator(
    [OrchestrationTrigger] TaskOrchestrationContext context)
{
    var input = context.GetInput<ProcessingInput>();

    // Retry policy for transient failures
    var retryPolicy = new RetryPolicy(
        maxNumberOfAttempts: 5,
        firstRetryInterval: TimeSpan.FromSeconds(1),
        backoffCoefficient: 2.0,
        maxRetryInterval: TimeSpan.FromMinutes(1));

    try
    {
        // Call with retry
        var result = await context.CallActivityAsync<string>(
            nameof(UnreliableActivity),
            input.Data,
            new TaskOptions { Retry = retryPolicy });

        return new ProcessingResult { Success = true, Data = result };
    }
    catch (TaskFailedException ex) when (ex.FailureDetails?.ErrorType == "TransientException")
    {
        // Handle specific transient failure after retries exhausted
        await context.CallActivityAsync(nameof(NotifyOperations), new OperationsAlert
        {
            Message = "Activity failed after retries",
            Exception = ex.Message
        });

        return new ProcessingResult { Success = false, Error = "Service temporarily unavailable" };
    }
    catch (TaskFailedException ex)
    {
        // Handle permanent failure
        return new ProcessingResult { Success = false, Error = ex.Message };
    }
}

[Function(nameof(UnreliableActivity))]
public static async Task<string> UnreliableActivity(
    [ActivityTrigger] string input,
    FunctionContext context)
{
    var logger = context.GetLogger(nameof(UnreliableActivity));

    // Simulate transient failure
    if (Random.Shared.NextDouble() < 0.3)
    {
        throw new TransientException("Temporary service unavailable");
    }

    return await ProcessData(input);
}

Timer-Based Scheduling in Orchestrations

[Function(nameof(ScheduledOrchestrator))]
public static async Task ScheduledOrchestrator(
    [OrchestrationTrigger] TaskOrchestrationContext context)
{
    var schedule = context.GetInput<ProcessingSchedule>();

    foreach (var scheduledTime in schedule.Times)
    {
        // Wait until scheduled time
        var fireAt = scheduledTime;
        if (fireAt > context.CurrentUtcDateTime)
        {
            await context.CreateTimer(fireAt, CancellationToken.None);
        }

        // Execute scheduled work
        await context.CallActivityAsync(nameof(ScheduledWork), new WorkItem
        {
            ScheduledFor = scheduledTime,
            ActualStart = context.CurrentUtcDateTime
        });
    }
}

[Function(nameof(BatchProcessingOrchestrator))]
public static async Task BatchProcessingOrchestrator(
    [OrchestrationTrigger] TaskOrchestrationContext context)
{
    var config = context.GetInput<BatchConfig>();
    var processedCount = 0;

    while (processedCount < config.TotalItems)
    {
        // Process batch
        var batchSize = Math.Min(config.BatchSize, config.TotalItems - processedCount);
        await context.CallActivityAsync(nameof(ProcessBatch), new BatchRequest
        {
            StartIndex = processedCount,
            Count = batchSize
        });

        processedCount += batchSize;

        // Delay between batches to avoid throttling
        if (processedCount < config.TotalItems)
        {
            await context.CreateTimer(
                context.CurrentUtcDateTime.AddSeconds(config.DelayBetweenBatchesSeconds),
                CancellationToken.None);
        }
    }
}

---

Advanced Binding Patterns

Blob Input/Output with Metadata

public class BlobFunctions(ILogger<BlobFunctions> logger)
{
    [Function("ProcessBlob")]
    [BlobOutput("processed/{name}", Connection = "StorageConnection")]
    public async Task<byte[]> ProcessBlob(
        [BlobTrigger("uploads/{name}", Connection = "StorageConnection")]
        BlobClient blobClient,
        string name,
        FunctionContext context)
    {
        logger.LogInformation("Processing blob: {Name}", name);

        // Read blob content
        var downloadResult = await blobClient.DownloadContentAsync();
        var content = downloadResult.Value.Content.ToArray();

        // Get blob properties
        var properties = await blobClient.GetPropertiesAsync();
        logger.LogInformation("Blob size: {Size}, Content-Type: {ContentType}",
            properties.Value.ContentLength,
            properties.Value.ContentType);

        // Process and return for output binding
        return TransformContent(content);
    }
}

Cosmos DB with Partition Key Routing

public class CosmosDbFunctions(ILogger<CosmosDbFunctions> logger)
{
    [Function("ProcessCosmosDocument")]
    [CosmosDBOutput(
        databaseName: "MyDatabase",
        containerName: "ProcessedItems",
        Connection = "CosmosDBConnection",
        PartitionKey = "/category")]
    public ProcessedItem ProcessDocument(
        [CosmosDBTrigger(
            databaseName: "MyDatabase",
            containerName: "Items",
            Connection = "CosmosDBConnection",
            LeaseContainerName = "leases",
            CreateLeaseContainerIfNotExists = true)]
        IReadOnlyList<MyDocument> documents)
    {
        logger.LogInformation("Processing {Count} documents", documents.Count);

        var processed = documents.Select(doc => new ProcessedItem
        {
            Id = Guid.NewGuid().ToString(),
            OriginalId = doc.Id,
            Category = doc.Category, // Partition key
            ProcessedAt = DateTime.UtcNow,
            Data = TransformData(doc)
        });

        // Return for output binding
        return processed.First();
    }
}

Event Grid with CloudEvents

public class EventGridFunctions(ILogger<EventGridFunctions> logger)
{
    [Function("HandleCloudEvent")]
    public async Task HandleCloudEvent(
        [EventGridTrigger] CloudEvent cloudEvent)
    {
        logger.LogInformation("Received event: Type={Type}, Source={Source}",
            cloudEvent.Type,
            cloudEvent.Source);

        // Deserialize event data
        var data = cloudEvent.Data?.ToObjectFromJson<MyEventData>();

        if (data != null)
        {
            await ProcessEventData(data);
        }
    }

    [Function("PublishEvent")]
    [EventGridOutput(TopicEndpointUri = "EventGridTopicUri", TopicKeySetting = "EventGridTopicKey")]
    public EventGridEvent PublishEvent(
        [HttpTrigger(AuthorizationLevel.Function, "post")] HttpRequest req)
    {
        return new EventGridEvent(
            subject: "myapp/items/created",
            eventType: "ItemCreated",
            dataVersion: "1.0",
            data: new { ItemId = Guid.NewGuid(), CreatedAt = DateTime.UtcNow });
    }
}