error-handling-patterns

Error Handling Patterns

Systematic approaches to error management that preserve debuggability and recovery.

Context

You are helping the engineer design error handling. If code is provided, assess whether errors are explicit, catchable, and recoverable. Poor error handling is a hidden time bomb in production.

Domain Context

• Fail Fast, Fail Loud: Don't swallow exceptions; propagate or handle explicitly
• Specific Exceptions: Throw specific exception types; generic Exception hides intent
• Recovery Strategy: Can the caller recover? If not, don't catch; let it propagate
• Error Context: Include relevant data in exceptions (user ID, operation, state)
• Logging: Log why an error occurred, not just that it occurred

Instructions

1. Identify Error Boundaries: Where can things fail? Network, parsing, database, arithmetic?
2. Choose Exception Type: Specific > generic; InvalidUserException > Exception
3. Include Context: Add user ID, operation, state to exception messages
4. Plan Recovery: If you catch an error, can you recover? If not, don't catch it
5. Document Throws: Declare checked exceptions or document unchecked ones
6. Test Error Paths: Write tests for error cases; normal cases hide error bugs
7. Avoid Silent Failures: Never catch and ignore; at minimum log
8. Use Error Enums: For recoverable errors, use Result<T, Error> or Either types

Anti-Patterns

• Catching Exception and logging it, then continuing; the bug often recurs silently
• Generic exception handlers (catch Exception {...}) that hide specific failures; you can't debug what you don't see
• Throwing exception on non-exceptional flows (e.g., "User not found" as exception); exceptions are for errors, not control flow
• Losing error context in exception messages; "Error occurred" tells you nothing; include IDs, values, operations
• Not testing error paths; bugs in error handling code are the hardest to find in production
• Exceeding retry budgets; retrying forever on transient errors masks systemic problems
• Hiding errors in middleware; errors should bubble up with context, not be swallowed

Further Reading

• Robert C. Martin, Clean Code, Chapter 7 (Error Handling)
• Joe Armstrong, Programming Erlang, Chapter 7 (Error Handling)
• John Ousterhout, A Philosophy of Software Design, on exceptions vs. error codes