How do you ensure data consistency across Go microservices without distributed transactions?

Distributed transactions (2PC) are generally avoided in microservices — they couple services and create availability issues. The alternative is eventual consistency through careful design: idempotent consumers, compensating transactions, and the Outbox pattern.

// Pattern: last-write-wins with optimistic locking (version field)
type User struct {
    ID      int
    Name    string
    Email   string
    Version int // incremented on each update
}

func (r *UserRepo) UpdateOptimistic(
    ctx context.Context, user *User,
) error {
    result, err := r.db.ExecContext(ctx,
        `UPDATE users
         SET name=$1, email=$2, version=version+1
         WHERE id=$3 AND version=$4`,
        user.Name, user.Email, user.ID, user.Version,
    )
    if err != nil { return err }
    n, _ := result.RowsAffected()
    if n == 0 {
        return ErrConflict // another update happened concurrently
    }
    user.Version++ // reflect new version
    return nil
}

// Pattern: idempotent event handler with deduplication
type EventHandler struct {
    db    *sql.DB
    dedup *DedupeCache
}

func (h *EventHandler) Handle(ctx context.Context, event Event) error {
    // Use event ID as idempotency key
    if already, _ := h.dedup.Check(ctx, event.ID); already {
        return nil // already processed — safe to ack
    }

    tx, err := h.db.BeginTx(ctx, nil)
    if err != nil { return err }
    defer tx.Rollback()

    // Process the event
    if err := h.applyEvent(ctx, tx, event); err != nil {
        return fmt.Errorf("apply event: %w", err)
    }

    // Mark event as processed within the same transaction
    tx.ExecContext(ctx,
        "INSERT INTO processed_events (id) VALUES ($1)", event.ID)

    if err := tx.Commit(); err != nil {
        return fmt.Errorf("commit: %w", err)
    }
    h.dedup.Set(ctx, event.ID) // populate cache
    return nil
}

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