---
title: Error Handling
description: Sentinel errors and how Relay surfaces problems.
---

Relay defines a set of sentinel errors in the root package. Use `errors.Is` to check for specific conditions.

## Sentinel errors

```go
var (
    ErrNoStore                 = errors.New("relay: store is required")
    ErrEndpointNotFound        = errors.New("relay: endpoint not found")
    ErrEventTypeNotFound       = errors.New("relay: event type not found")
    ErrEventTypeDeprecated     = errors.New("relay: event type is deprecated")
    ErrPayloadValidationFailed = errors.New("relay: payload validation failed")
    ErrDuplicateIdempotencyKey = errors.New("relay: duplicate idempotency key")
    ErrEndpointDisabled        = errors.New("relay: endpoint is disabled")
    ErrStoreClosed             = errors.New("relay: store is closed")
    ErrMigrationFailed         = errors.New("relay: migration failed")
    ErrDLQNotFound             = errors.New("relay: dlq entry not found")
    ErrDeliveryNotFound        = errors.New("relay: delivery not found")
    ErrEventNotFound           = errors.New("relay: event not found")
)
```

## Checking errors

```go
err := r.Send(ctx, evt)
if errors.Is(err, relay.ErrEventTypeNotFound) {
    // event type not registered in the catalog
}
```

## Wrapping convention

Subsystems wrap the sentinel with `fmt.Errorf` and `%w`:

```go
return fmt.Errorf("%w: %s", ErrEventTypeNotFound, evt.Type)
```

## Validation errors

The endpoint service returns `*endpoint.ValidationError` for invalid input:

```go
var valErr *endpoint.ValidationError
if errors.As(err, &valErr) {
    log.Printf("invalid %s: %s", valErr.Field, valErr.Message)
}
```

## Idempotency

Duplicate idempotency keys are not treated as errors. When `Send()` encounters a duplicate, it returns `nil` (no-op success).