Pattern Selection & Anti-Patterns

Choosing patterns under interview pressure

Patterns are tools, not goals. The right process: (1) identify the axis of variation, (2) check if a simple interface suffices, (3) name the pattern only if it clarifies communication.

Smell	Likely pattern	Simpler alternative
switch on type growing	Strategy or Factory	Map of handlers if < 5 variants
Behavior stacks (logging + cache + auth)	Decorator	Middleware chain / interceptors
Complex object with 10+ fields	Builder	Constructor with required fields only
Object changes mode frequently	State	Enum + methods if ≤ 4 states
Notify many listeners on change	Observer	Callback list on one interface

Common anti-patterns in LLD interviews

Pattern stacking — AbstractSingletonFactoryProxy before requirements are clear. Premature abstraction — interface per class when there's one implementation. Anemic domain model — entities are data bags, all logic in services (acceptable briefly, but note the trade-off). God class — one SystemManager owns everything. Copy-paste polymorphism — three classes with identical methods differing only in one line (extract Strategy).

The YAGNI handshake

Senior signal = naming what you'd defer: "I'll use a concrete TokenBucketLimiter today; if we add sliding window, I'll extract RateLimitAlgorithm then." Interviewers prefer honest scoping over speculative frameworks.

Interview recovery move

If you've over-patterned, pause: "Let me simplify — we only need one implementation today, so I'll collapse these three interfaces into a single PaymentService and extract when the second provider appears." Recovering gracefully scores well.

Senior-level signal

Practice the phrase: "I'd start with the simplest design that meets stated requirements, then isolate the first extension point behind an interface." If challenged, compare two designs in one sentence: "Strategy adds one interface and N classes; a switch is fine for two cases but breaks OCP at three."

Where this goes next

Running an LLD Round applies these principles to the full 45-minute interview flow — where patterns fit in the timeline and when to stop designing.