System Design · core
Command & Chain
Command, undo, chain of responsibility.
Mental model
The command pattern turns an action into a small object you can store, queue, log, or reverse. Pair it with two stacks for undo/redo, with a queue for background jobs, or with a chain for middleware-style processing.
How to study Command & Chain
Begin by restating the mental model in your own words, then connect it to a concrete system you have built or operated. Name the mechanism, the constraint it addresses, and the trade-off it introduces. Use Refactoring Guru — Command to check details, but close the source before writing your explanation. Retrieval is the learning step; rereading is only preparation.
Next, compare Command & Chain with the neighboring concepts in its roadmap. Ask what changes in correctness, latency, resource use, operability, and failure recovery. Complete Undo / redo stack and preserve the command, input, output, and one failed attempt as evidence. Finish by explaining the idea without jargon to someone who has not studied the track.
Proof of understanding
- Explain the mechanism from first principles and identify the state it reads or changes.
- Give one situation where the concept is the right choice and one where it is not.
- Predict a realistic failure mode before running the drill, then compare the prediction with evidence.
- Connect the result to a roadmap or build artifact instead of treating the concept as isolated trivia.
Learn from primary sources
Practice and explain it back
Undo / redo stack
Design undo/redo for a text editor using the Command pattern. Each user action becomes a Command with execute/undo. Decide how the redo stack interacts with new edits.
Expected evidence: Command interface + 2-3 concrete commands + the two stacks and their invariants.
Open the interactive drill →Review prompts
- What must a command object carry to support undo, and where does undo stop working?
Build evidence
Use a roadmap capstone to turn this concept into working evidence.
Prerequisites
Related concepts
None assigned yet.