dotnet-resilience

name	dotnet-resilience
description	Adds fault tolerance. Polly v8 + MS.Extensions.Http.Resilience, retry/circuit breaker/timeout.
metadata	{"short-description":".NET skill guidance for architecture tasks"}

dotnet-resilience

Modern resilience patterns for .NET applications using Polly v8 and Microsoft.Extensions.Http.Resilience. Covers the standard resilience pipeline (rate limiter, total timeout, retry, circuit breaker, attempt timeout), custom pipeline configuration, and integration with the .NET dependency injection system.

Superseded package: Microsoft.Extensions.Http.Polly is superseded by Microsoft.Extensions.Http.Resilience. Do not use Microsoft.Extensions.Http.Polly for new projects. See the migration guide for upgrading existing code.

Scope

• Standard resilience pipeline (rate limiter, timeout, retry, circuit breaker)
• Custom resilience pipeline configuration with Polly v8
• DI integration via MS.Extensions.Http.Resilience
• Resilience telemetry and Polly metering

Out of scope

• DI container mechanics and service lifetimes -- see [skill:dotnet-csharp-dependency-injection]
• Async/await patterns and cancellation token propagation -- see [skill:dotnet-csharp-async-patterns]
• HTTP client factory patterns (typed clients, DelegatingHandlers) -- see [skill:dotnet-http-client]
• Testing resilience policies -- see [skill:dotnet-integration-testing] and [skill:dotnet-xunit]

Cross-references: [skill:dotnet-csharp-dependency-injection] for service registration, [skill:dotnet-csharp-async-patterns] for cancellation token propagation, [skill:dotnet-http-client] for applying resilience to HTTP clients.

---

Package Landscape

Package	Status	Purpose
Polly (v8+)	Current	Core resilience library -- strategies, pipelines, telemetry
Microsoft.Extensions.Resilience	Current	DI integration for non-HTTP resilience pipelines
Microsoft.Extensions.Http.Resilience	Current	DI integration for IHttpClientFactory resilience pipelines
Microsoft.Extensions.Http.Polly	Superseded	Legacy HTTP resilience -- migrate to Microsoft.Extensions.Http.Resilience
Polly (v7 and earlier)	Legacy	Older API -- migrate to v8

Install the modern stack:

<PackageReference Include="Microsoft.Extensions.Http.Resilience" Version="9.*" />
<!-- Transitively brings in Polly v8 and Microsoft.Extensions.Resilience -->

```xml

For non-HTTP scenarios only:

```xml

<PackageReference Include="Microsoft.Extensions.Resilience" Version="9.*" />

```xml

---

## Standard Resilience Pipeline

`Microsoft.Extensions.Http.Resilience` provides a standard resilience pipeline that follows the recommended order. The pipeline layers execute from outermost to innermost:

```text

Request
  --> Rate Limiter        (1. shed excess load)
    --> Total Timeout      (2. cap total wall-clock time)
      --> Retry             (3. retry transient failures)
        --> Circuit Breaker  (4. stop calling failing services)
          --> Attempt Timeout (5. cap individual attempt time)
            --> HTTP call

```text

### Why This Order Matters

- **Rate limiter first**: prevents retry storms from overwhelming downstream services
- **Total timeout wraps retry**: ensures the entire operation (including all retries) has a deadline
- **Retry wraps circuit breaker**: retries can try again after the breaker resets; a broken circuit counts as a retriable failure
- **Circuit breaker wraps attempt timeout**: timed-out attempts count toward the breaker's failure threshold
- **Attempt timeout innermost**: each individual HTTP call has its own deadline

### Standard Pipeline with Defaults

```csharp

builder.Services
    .AddHttpClient("catalog-api", client =>
    {
        client.BaseAddress = new Uri("https://catalog.internal");
    })
    .AddStandardResilienceHandler();

```text

This applies the standard pipeline with sensible defaults:
- **Rate limiter**: 1000 concurrent requests
- **Total timeout**: 30 seconds
- **Retry**: 3 attempts, exponential backoff (2s base), jitter
- **Circuit breaker**: 10% failure ratio, 100 sample size, 5s break duration
- **Attempt timeout**: 10 seconds

### Standard Pipeline with Custom Options

```csharp

builder.Services
    .AddHttpClient("catalog-api", client =>
    {
        client.BaseAddress = new Uri("https://catalog.internal");
    })
    .AddStandardResilienceHandler(options =>
    {
        // Total timeout for the entire operation including retries
        options.TotalRequestTimeout.Timeout = TimeSpan.FromSeconds(60);

        // Retry strategy
        options.Retry.MaxRetryAttempts = 5;
        options.Retry.Delay = TimeSpan.FromSeconds(1);
        options.Retry.BackoffType = DelayBackoffType.Exponential;
        options.Retry.UseJitter = true;
        options.Retry.ShouldHandle = args => ValueTask.FromResult(
            args.Outcome.Result?.StatusCode is
                HttpStatusCode.RequestTimeout or
                HttpStatusCode.TooManyRequests or
                >= HttpStatusCode.InternalServerError);

        // Circuit breaker
        options.CircuitBreaker.SamplingDuration = TimeSpan.FromSeconds(30);
        options.CircuitBreaker.FailureRatio = 0.1;
        options.CircuitBreaker.MinimumThroughput = 20;
        options.CircuitBreaker.BreakDuration = TimeSpan.FromSeconds(10);

        // Per-attempt timeout
        options.AttemptTimeout.Timeout = TimeSpan.FromSeconds(5);
    });

```text

### Configuration via appsettings.json

Bind resilience options from configuration for environment-specific tuning:

```csharp

builder.Services
    .AddHttpClient("catalog-api", client =>
    {
        client.BaseAddress = new Uri("https://catalog.internal");
    })
    .AddStandardResilienceHandler(options =>
    {
        builder.Configuration
            .GetSection("Resilience:CatalogApi")
            .Bind(options);
    });

```text

```json

{
  "Resilience": {
    "CatalogApi": {
      "Retry": {
        "MaxRetryAttempts": 5,
        "Delay": "00:00:02",
        "BackoffType": "Exponential"
      },
      "CircuitBreaker": {
        "BreakDuration": "00:00:15"
      },
      "TotalRequestTimeout": {
        "Timeout": "00:01:00"
      }
    }
  }
}

```text

---

## Custom Resilience Pipelines

When the standard pipeline does not fit, build custom pipelines with Polly v8 directly.

### Retry Strategy

```csharp

builder.Services.AddResiliencePipeline("db-retry", pipelineBuilder =>
{
    pipelineBuilder.AddRetry(new RetryStrategyOptions
    {
        MaxRetryAttempts = 3,
        Delay = TimeSpan.FromMilliseconds(500),
        BackoffType = DelayBackoffType.Exponential,
        UseJitter = true,
        ShouldHandle = new PredicateBuilder()
            .Handle<DbUpdateConcurrencyException>()
            .Handle<TimeoutException>(),
        OnRetry = args =>
        {
            // Structured logging of retry attempts
            var logger = args.Context.Properties
                .GetValue(new ResiliencePropertyKey<ILogger>("logger"), null!);
            logger?.LogWarning(
                args.Outcome.Exception,
                "Retry attempt {AttemptNumber} after {Delay}ms",
                args.AttemptNumber,
                args.RetryDelay.TotalMilliseconds);
            return ValueTask.CompletedTask;
        }
    });
});

// Inject and use
public sealed class OrderRepository(
    [FromKeyedServices("db-retry")] ResiliencePipeline pipeline,
    AppDbContext db)
{
    public async Task<Order> UpdateAsync(Order order, CancellationToken ct)
    {
        return await pipeline.ExecuteAsync(async token =>
        {
            db.Orders.Update(order);
            await db.SaveChangesAsync(token);
            return order;
        }, ct);
    }
}

```text

### Circuit Breaker Strategy

```csharp

builder.Services.AddResiliencePipeline("payment-gateway", pipelineBuilder =>
{
    pipelineBuilder.AddCircuitBreaker(new CircuitBreakerStrategyOptions
    {
        SamplingDuration = TimeSpan.FromSeconds(30),
        FailureRatio = 0.25,           // Open after 25% failure rate
        MinimumThroughput = 10,        // Need at least 10 calls to evaluate
        BreakDuration = TimeSpan.FromSeconds(15),
        ShouldHandle = new PredicateBuilder()
            .Handle<HttpRequestException>()
            .Handle<TimeoutException>()
    });
});

```text

### Timeout Strategy

```csharp

builder.Services.AddResiliencePipeline("external-api", pipelineBuilder =>
{
    // Total timeout for the entire pipeline execution
    pipelineBuilder.AddTimeout(new TimeoutStrategyOptions
    {
        Timeout = TimeSpan.FromSeconds(30),
        OnTimeout = args =>
        {
            // Log timeout details for diagnostics
            return ValueTask.CompletedTask;
        }
    });
});

```text

### Composing Multiple Strategies

Build a composite pipeline by chaining strategies. Order matters -- outermost strategy is added first:

```csharp

builder.Services.AddResiliencePipeline("composed", pipelineBuilder =>
{
    // 1. Total timeout (outermost -- caps entire operation)
    pipelineBuilder.AddTimeout(new TimeoutStrategyOptions
    {
        Timeout = TimeSpan.FromSeconds(45)
    });

    // 2. Retry (retries on transient failures)
    pipelineBuilder.AddRetry(new RetryStrategyOptions
    {
        MaxRetryAttempts = 3,
        Delay = TimeSpan.FromSeconds(1),
        BackoffType = DelayBackoffType.Exponential,
        UseJitter = true,
        ShouldHandle = new PredicateBuilder()
            .Handle<HttpRequestException>()
            .Handle<TimeoutException>()
    });

    // 3. Circuit breaker (stops calling failing services)
    pipelineBuilder.AddCircuitBreaker(new CircuitBreakerStrategyOptions
    {
        FailureRatio = 0.1,
        MinimumThroughput = 20,
        SamplingDuration = TimeSpan.FromSeconds(30),
        BreakDuration = TimeSpan.FromSeconds(10),
        ShouldHandle = new PredicateBuilder()
            .Handle<HttpRequestException>()
            .Handle<TimeoutException>()
    });

    // 4. Attempt timeout (innermost -- caps single attempt)
    pipelineBuilder.AddTimeout(new TimeoutStrategyOptions
    {
        Timeout = TimeSpan.FromSeconds(10)
    });
});

```text

---

## Typed Resilience Pipelines

For result-bearing operations, use `ResiliencePipeline<T>`:

```csharp

builder.Services.AddResiliencePipeline<string, HttpResponseMessage>(
    "typed-http",
    pipelineBuilder =>
    {
        pipelineBuilder.AddRetry(new RetryStrategyOptions<HttpResponseMessage>
        {
            MaxRetryAttempts = 3,
            Delay = TimeSpan.FromSeconds(1),
            BackoffType = DelayBackoffType.Exponential,
            ShouldHandle = new PredicateBuilder<HttpResponseMessage>()
                .Handle<HttpRequestException>()
                .HandleResult(r => r.StatusCode >= HttpStatusCode.InternalServerError)
        });
    });

```text

---

## Hedging Strategy

Send parallel requests to reduce tail latency. The hedging strategy dispatches additional attempts if the initial request is slow:

```csharp

builder.Services
    .AddHttpClient("search-api")
    .AddStandardHedgingHandler(options =>
    {
        options.Hedging.MaxHedgedAttempts = 2;
        options.Hedging.Delay = TimeSpan.FromMilliseconds(500);
        // Hedging sends a parallel request if the first hasn't
        // responded within 500ms
    });

```text

**Use hedging when:**
- Operations are idempotent (GET requests, read-only queries)
- Tail latency reduction matters more than extra load

**Do not use hedging when:**
- Operations have side effects (POST, PUT, DELETE)
- Downstream services cannot handle increased load

---

## Telemetry and Observability

Polly v8 emits metrics and traces via `System.Diagnostics` out of the box when using the DI integration.

### Built-in Metrics

The `Microsoft.Extensions.Resilience` package automatically reports:

| Metric | Description |
|--------|-------------|
| `polly.strategy.attempt.duration` | Duration of each attempt |
| `polly.strategy.pipeline.duration` | Duration of the entire pipeline execution |
| `polly.strategy.attempt.count` | Count of attempts (including retries) |

These integrate with OpenTelemetry automatically when the OpenTelemetry SDK is configured in your application -- see [skill:dotnet-observability] for collector setup.

### Enabling Telemetry

Resilience telemetry is enabled automatically when using the DI-based registration (`AddResiliencePipeline`, `AddStandardResilienceHandler`). The `Microsoft.Extensions.Resilience` package registers a `MeteringEnricher` and `LoggingEnricher` that emit structured logs and metrics through the standard `ILoggerFactory` and `IMeterFactory` from DI:

```csharp

// Telemetry is automatic -- no extra configuration needed.
// Structured logs appear via ILogger; metrics via IMeter.
builder.Services
    .AddHttpClient("catalog-api")
    .AddStandardResilienceHandler();

// To see resilience logs, set the Polly category to Information:
// appsettings.json
// {
//   "Logging": {
//     "LogLevel": {
//       "Polly": "Information"
//     }
//   }
// }

```text

---

## Migrating from Microsoft.Extensions.Http.Polly

If upgrading from the superseded `Microsoft.Extensions.Http.Polly` package:

### Before (Legacy)

```csharp

// Using Microsoft.Extensions.Http.Polly (superseded)
builder.Services
    .AddHttpClient("catalog-api")
    .AddTransientHttpErrorPolicy(p =>
        p.WaitAndRetryAsync(3, attempt =>
            TimeSpan.FromSeconds(Math.Pow(2, attempt))))
    .AddTransientHttpErrorPolicy(p =>
        p.CircuitBreakerAsync(5, TimeSpan.FromSeconds(30)));

```text

### After (Modern)

```csharp

// Using Microsoft.Extensions.Http.Resilience (current)
builder.Services
    .AddHttpClient("catalog-api")
    .AddStandardResilienceHandler(options =>
    {
        options.Retry.MaxRetryAttempts = 3;
        options.Retry.Delay = TimeSpan.FromSeconds(2);
        options.Retry.BackoffType = DelayBackoffType.Exponential;

        options.CircuitBreaker.MinimumThroughput = 5;
        options.CircuitBreaker.BreakDuration = TimeSpan.FromSeconds(30);
    });

```text

### Migration Steps

1. Replace `Microsoft.Extensions.Http.Polly` NuGet reference with `Microsoft.Extensions.Http.Resilience`
2. Replace `AddTransientHttpErrorPolicy` calls with `AddStandardResilienceHandler` or custom pipeline
3. Translate Polly v7 policy configuration to v8 strategy options
4. Remove explicit `IAsyncPolicy<HttpResponseMessage>` type references
5. Verify retry/circuit breaker behavior matches previous configuration

---

## Key Principles

- **Use the standard pipeline for HTTP** -- `AddStandardResilienceHandler()` covers the most common case with battle-tested defaults
- **Customize via options, not custom code** -- the standard handler options cover retry count, backoff, circuit breaker thresholds, and timeouts
- **Use `AddResiliencePipeline` for non-HTTP** -- database calls, message queues, file I/O
- **Always add jitter to retries** -- prevents thundering herd when multiple clients retry simultaneously
- **Configure via appsettings.json** -- different environments need different thresholds; avoid hardcoded values in production
- **Do not suppress `TimeoutRejectedException`** -- let it propagate through the pipeline so outer strategies (retry, circuit breaker) can react
- **Do not use `Microsoft.Extensions.Http.Polly`** for new projects -- it wraps Polly v7 and is superseded

---

## Agent Gotchas

1. **Do not mix Polly v7 and v8 APIs** -- v8 uses `ResiliencePipeline` and strategy options; v7 uses `IAsyncPolicy`. They are not interchangeable.
2. **Do not add `AddStandardResilienceHandler` twice** -- it composes a full pipeline; adding it twice doubles every strategy layer.
3. **Do not set attempt timeout higher than total timeout** -- the attempt timeout must be shorter than the total timeout or it has no effect.
4. **Do not retry non-idempotent operations** -- only retry operations that are safe to repeat (GETs, idempotent writes with idempotency keys).
5. **Do not use hardcoded retry delays in production** -- always use `UseJitter = true` and configure delays via appsettings.

---

## References

- [Polly v8 documentation](https://www.pollydocs.org/)
- [Microsoft.Extensions.Http.Resilience](https://learn.microsoft.com/en-us/dotnet/core/resilience/http-resilience)
- [Microsoft.Extensions.Resilience](https://learn.microsoft.com/en-us/dotnet/core/resilience/)
- [Migration from Http.Polly to Http.Resilience](https://learn.microsoft.com/en-us/dotnet/fundamentals/networking/resilience/migration-guide)
- [Standard resilience pipeline](https://learn.microsoft.com/en-us/dotnet/core/resilience/http-resilience#standard-resilience-handler)
- [Polly v8 strategy options](https://www.pollydocs.org/strategies/)

Code Navigation (Serena MCP)

Primary approach: Use Serena symbol operations for efficient code navigation:

1. Find definitions: serena_find_symbol instead of text search
2. Understand structure: serena_get_symbols_overview for file organization
3. Track references: serena_find_referencing_symbols for impact analysis
4. Precise edits: serena_replace_symbol_body for clean modifications

When to use Serena vs traditional tools:

• Use Serena: Navigation, refactoring, dependency analysis, precise edits
• Use Read/Grep: Reading full files, pattern matching, simple text operations
• Fallback: If Serena unavailable, traditional tools work fine

Example workflow:

# Instead of:
Read: src/Services/OrderService.cs
Grep: "public void ProcessOrder"

# Use:
serena_find_symbol: "OrderService/ProcessOrder"
serena_get_symbols_overview: "src/Services/OrderService.cs"