error-handling-and-result-modeling

Error Handling and Result Modeling

1. Classify Before You Handle

Every failure belongs to exactly one category. The category drives the handling strategy.

Category	Meaning	Retry?	Expose to client?
Validation	Caller sent bad input	No	Yes — full detail
Not found	Requested resource absent	No	Yes — safe message
Conflict	State mismatch (optimistic lock, duplicate)	No	Yes — safe message
Unauthorized / Forbidden	Auth failure	No	Minimal detail
Downstream	Dependency failed or timed out	Yes (idempotent)	Opaque service_unavailable
Bug / unexpected	Unhandled code path	No	Opaque internal_error

2. Typed Result Envelope (the Pattern)

Avoid throwing exceptions across use-case/service boundaries. Return a typed result:

// TypeScript example — adapt to your language
type Ok<T> = { ok: true; value: T };
type Err<E> = { ok: false; error: E };
type Result<T, E> = Ok<T> | Err<E>;

# Python example using dataclasses
@dataclass
class Ok(Generic[T]):
    value: T

@dataclass
class Err(Generic[E]):
    error: E

Result = Ok[T] | Err[E]

Rules:

1. Use-case methods return Result<DomainValue, DomainError> — never throw for expected failures.
2. Reserve exceptions for truly unexpected paths (bugs, unrecoverable state).
3. Domain errors are value objects that carry enough context for the handler to decide — not raw exception messages.

3. Layer Boundary Translation

HTTP handler
    └── Use-case / service  (returns Result<T, DomainError>)
            └── Repository / client  (returns Result<T, InfraError>)

At each boundary, translate inward-facing errors to the layer's vocabulary:

InfraError.DbConnectionFailed → DomainError.ServiceUnavailable
InfraError.UniqueConstraintViolated → DomainError.Conflict(entity, key)

The HTTP handler is the only layer that maps DomainError → HTTP status + JSON body.

4. HTTP Status Mapping Table

DomainError category	HTTP status	When
Validation	400	Bad request body/params
Not found	404	Resource absent
Conflict	409	Duplicate / stale update
Unauthorized	401	Missing / invalid credential
Forbidden	403	Authenticated but not allowed
Downstream	503	Dependency unreachable
Bug / unexpected	500	Anything else

5. Client-Safe Error Body

{
  "error": {
    "code": "PAYMENT_METHOD_EXPIRED",
    "message": "The payment method has expired.",
    "details": [{ "field": "card.expiry", "issue": "past_expiry" }]
  }
}

Rules:

• code is a stable machine-readable slug — don't change it after release.
• message is English prose safe to display (no stack traces, no internal state).
• details is optional; present only for validation errors with field-level specifics.
• Never include stack traces, SQL errors, or internal service names in 4xx/5xx responses.

6. Logging Strategy

Error category	Log level	What to include
Validation / not found	DEBUG or omit	High-volume, expected — log at trace if needed
Downstream (retried, eventually ok)	WARN	Attempt count, dependency name, duration
Downstream (exhausted)	ERROR	Full context, trace ID, retries attempted
Bug / unexpected	ERROR + alert	Stack trace, request ID, all context

Pitfalls

• Throwing exceptions for validation failures — forces callers into try/catch chains and hides intent.
• Returning { error: null, data: null } — ambiguous; force the caller to pick a branch.
• Using HTTP 500 for validation errors — clients can't distinguish "your fault" from "our fault".
• Leaking psycopg2.OperationalError: ... or ECONNREFUSED in the response body — exposes topology.
• Re-throwing the same raw exception across layers — the HTTP handler ends up doing domain logic.
• Not logging the trace/correlation ID — makes cross-service debugging impossible.