M13 Domain Error

---
name: m13-domain-error
description: "Use when designing domain error handling. Keywords: domain error, error categorization, recovery strategy, retry, fallback, domain error hierarchy, user-facing vs internal errors, error code design, circuit breaker, graceful degradation, resilience, error context, backoff, retry with backoff, error recovery, transient vs permanent error, 领域错误, 错误分类, 恢复策略, 重试, 熔断器, 优雅降级"
user-invocable: false
---

# Domain Error Strategy

> **Layer 2: Design Choices**

## Core Question

**Who needs to handle this error, and how should they recover?**

Before designing error types:
- Is this user-facing or internal?
- Is recovery possible?
- What context is needed for debugging?

---

## Error Categorization

| Error Type | Audience | Recovery | Example |
|------------|----------|----------|---------|
| User-facing | End users | Guide action | `InvalidEmail`, `NotFound` |
| Internal | Developers | Debug info | `DatabaseError`, `ParseError` |
| System | Ops/SRE | Monitor/alert | `ConnectionTimeout`, `RateLimited` |
| Transient | Automation | Retry | `NetworkError`, `ServiceUnavailable` |
| Permanent | Human | Investigate | `ConfigInvalid`, `DataCorrupted` |

---

## Thinking Prompt

Before designing error types:

1. **Who sees this error?**
   - End user → friendly message, actionable
   - Developer → detailed, debuggable
   - Ops → structured, alertable

2. **Can we recover?**
   - Transient → retry with backoff
   - Degradable → fallback value
   - Permanent → fail fast, alert

3. **What context is needed?**
   - Call chain → anyhow::Context
   - Request ID → structured logging
   - Input data → error payload

---

## Trace Up ↑

To domain constraints (Layer 3):

```
"How should I handle payment failures?"
    ↑ Ask: What are the business rules for retries?
    ↑ Check: domain-fintech (transaction requirements)
    ↑ Check: SLA (availability requirements)
```

| Question | Trace To | Ask |
|----------|----------|-----|
| Retry policy | domain-* | What's acceptable latency for retry? |
| User experience | domain-* | What message should users see? |
| Compliance | domain-* | What must be logged for audit? |

---

## Trace Down ↓

To implementation (Layer 1):

```
"Need typed errors"
    ↓ m06-error-handling: thiserror for library
    ↓ m04-zero-cost: Error enum design

"Need error context"
    ↓ m06-error-handling: anyhow::Context
    ↓ Logging: tracing with fields

"Need retry logic"
    ↓ m07-concurrency: async retry patterns
    ↓ Crates: tokio-retry, backoff
```

---

## Quick Reference

| Recovery Pattern | When | Implementation |
|------------------|------|----------------|
| Retry | Transient failures | exponential backoff |
| Fallback | Degraded mode | cached/default value |
| Circuit Breaker | Cascading failures | failsafe-rs |
| Timeout | Slow operations | `tokio::time::timeout` |
| Bulkhead | Isolation | separate thread pools |

## Error Hierarchy

```rust
#[derive(thiserror::Error, Debug)]
pub enum AppError {
    // User-facing
    #[error("Invalid input: {0}")]
    Validation(String),

    // Transient (retryable)
    #[error("Service temporarily unavailable")]
    ServiceUnavailable(#[source] reqwest::Error),

    // Internal (log details, show generic)
    #[error("Internal error")]
    Internal(#[source] anyhow::Error),
}

impl AppError {
    pub fn is_retryable(&self) -> bool {
        matches!(self, Self::ServiceUnavailable(_))
    }
}
```

## Retry Pattern

```rust
use tokio_retry::{Retry, strategy::ExponentialBackoff};

async fn with_retry<F, T, E>(f: F) -> Result<T, E>
where
    F: Fn() -> impl Future<Output = Result<T, E>>,
    E: std::fmt::Debug,
{
    let strategy = ExponentialBackoff::from_millis(100)
        .max_delay(Duration::from_secs(10))
        .take(5);

    Retry::spawn(strategy, || f()).await
}
```

---

## Common Mistakes

| Mistake | Why Wrong | Better |
|---------|-----------|--------|
| Same error for all | No actionability | Categorize by audience |
| Retry everything | Wasted resources | Only transient errors |
| Infinite retry | DoS self | Max attempts + backoff |
| Expose internal errors | Security risk | User-friendly messages |
| No context | Hard to debug | .context() everywhere |

---

## Anti-Patterns

| Anti-Pattern | Why Bad | Better |
|--------------|---------|--------|
| String errors | No structure | thiserror types |
| panic! for recoverable | Bad UX | Result with context |
| Ignore errors | Silent failures | Log or propagate |
| Box<dyn Error> everywhere | Lost type info | thiserror |
| Error in happy path | Performance | Early validation |

---

## Related Skills

| When | See |
|------|-----|
| Error handling basics | m06-error-handling |
| Retry implementation | m07-concurrency |
| Domain modeling | m09-domain |
| User-facing APIs | domain-* |