I knew the stack-based undo pattern going in, so the core implementation wasn't the hard part.
Start by defining a Command interface with execute() and undo() methods, then implement an Invoker (executor) that holds a stack of executed commands and delegates calls. Emphasize how this design decouples the executor from concrete commands, enabling extensibility and testability. Walk through a simple example and discuss trade-offs like error handling and memory management.
Pro tip: Mention that undo operations should ideally be idempotent and that you might use a stack to track command history, but also discuss how to handle failures during undo (e.g., compensating actions) to show production-level thinking.
Ask clarifying questions about expected command types, concurrency needs, error handling, and whether undo should support multiple levels. This shows you think about real-world constraints before designing.
Propose an interface with execute() and undo() methods. Discuss whether to include additional methods like canUndo() or getDescription() for logging, and explain why keeping it minimal promotes flexibility.
Describe an executor class that maintains a history (e.g., a stack) of executed commands. Its executeCommand(Command) method calls command.execute() and pushes to history; undoLast() pops and calls undo(). Highlight that the executor only knows the Command interface.
Provide a simple example (e.g., a LightOnCommand) to illustrate how a concrete command encapsulates receiver logic. Show that adding new commands requires no changes to the executor.
Address topics like thread safety, memory usage of history, error handling during execute/undo, and possible extensions (e.g., macro commands, transaction support). This demonstrates depth and awareness of production concerns.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
This is where the interview actually got interesting.
Start by clarifying the current system's command pattern and constraints, then propose a design that layers redo, multi-step undo, batching, and persistence on top of a command log. Emphasize trade-offs between simplicity, performance, and reliability, and how each feature interacts with the others.
Pro tip: Highlight that redo and multi-step undo require a cursor in the command history, and batching should be atomic to avoid partial failures. Mention that persisting the command log enables replay for debugging and recovery, but consider log compaction to manage growth.
Ask about the current system architecture, expected scale, consistency needs, and whether commands are deterministic. This ensures your design fits the context.
Introduce a history list and a cursor index to support undo/redo. Undo moves the cursor back and applies inverse; redo moves it forward and reapplies.
Group multiple commands into a single transaction using a composite command that executes all or none. Ensure atomicity and proper undo/redo behavior for the batch.
Serialize commands to a durable log (e.g., append-only file or database). On restart, replay the log to rebuild state, and support replay for debugging or auditing.
Discuss log compaction, snapshotting, idempotency, and performance impacts. Consider how batching affects undo granularity and how persistence affects latency.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.