Programming Patterns

Principle

Criteria for judgment

SOLID, DRY, YAGNI

Pattern

Solution structure for a recurring problem

Strategy, Adapter, Repository, Retry

Idiom

Natural expression per language

Python with, C++ RAII, Rust ?

Syntax

Forms permitted by the language

if, class, interface

Creational Pattern

Factory Method, Abstract Factory, Builder, Prototype, Singleton

How to create objects

Structural Pattern

Adapter, Decorator, Facade, Proxy, Bridge, Composite, Flyweight

How to assemble objects

Behavioral Pattern

Strategy, State, Observer, Command, Template Method, Iterator, Visitor

How objects collaborate

Policy

A decision rule that changes depending on the situation. Discount policy, authorization policy, shipping fee policy — anything that answers "how should this be decided?"

Strategy

A swappable algorithm or policy. In the Strategy Pattern, a policy is extracted into an object so it can be replaced.

Logging

Recording what happened during execution. Used for incident analysis, auditing, and debugging.

Caching

Temporarily storing frequently used values in a nearby storage location. It is fast, but introduces cache invalidation problems.

Validation

Checking whether input values or state satisfy a set of rules. Comes up frequently in API requests, form inputs, and domain commands.

Renderer

The role of transforming data into an output form such as a screen, image, HTML, or PDF.

Storage

A place that holds data, such as a file system, database, or cache. Depending on context, it may refer to a physical storage medium or a storage abstraction.

External SDK

Libraries provided by payment processors, SMS gateways, or cloud vendors. If they spread directly into internal code, replacing them becomes difficult.

Legacy API

An API from an older system. It often has a mismatched interface with new code, making it a good candidate to wrap with an Adapter.

Cross-Cutting Concern

A concern that cuts across multiple features. Examples include logging, authorization checks, caching, metrics, and retry.

Domain

The business area the program addresses. For an online store, the domain includes orders, payments, shipping, and products.

DDD Model

A domain-centric model as described in Domain-Driven Design. It prioritizes business concepts and rules over database tables.

Entity

A domain object with an identifier and a lifecycle. Two objects with the same values are still different objects if their IDs differ.

Value Object

An object whose meaning lies in its value rather than its identity. Examples include money, coordinates, time periods, and email addresses.

DTO

Data Transfer Object. A data shape used for API responses or for crossing process boundaries.

Aggregate

A grouping of related Entities and Value Objects into a single consistency boundary. It typically serves as the unit of persistence for a Repository.

API Handler

An entry point that receives an HTTP request, validates it, and forwards it to the appropriate service or command.

Command Handler

An object or function that processes commands that cause state changes, such as "create order" or "withdraw member."

Middleware

A layer that intercepts requests and responses to handle common processing. Logging, authentication, CORS, and compression are common examples.

ORM

Object-Relational Mapping. A tool that maps between objects and relational database tables. It does not, however, mean you can ignore SQL.

RPC

Remote Procedure Call. A communication style in which a function across a network is called as if it were local.

Lock

A mechanism that locks a resource that would be corrupted by concurrent access. Examples include DB row locks, mutexes, and distributed locks.

Idempotency

The property of ensuring that sending the same request multiple times produces the same result as sending it once. Often seen alongside Retry.

Backoff

A strategy of progressively increasing the interval between retries, used to avoid hammering a system that is already experiencing failures.

Fallback

An alternative path used when the primary path fails. Examples include returning a cached value, sending a default response, or calling a backup server.

TTL

Time To Live. The duration a cache entry or token remains valid.

Stale data

Data that is no longer up to date. Caches often trade accepting stale data for improved speed.

Execution plan

The plan a database generates for how it will execute a SQL query. This is where it decides whether to use an index or perform a full table scan.

Transaction

Grouping multiple database operations within a single consistency boundary. It commits on success and rolls back on failure.

Composition

Policies, renderers, storage, validators

Look here first when inheritance hierarchies grow too deep

Strategy Pattern

Discounts, shipping fees, authorization, sorting, validation

When an algorithm or policy changes frequently

Adapter Pattern

External SDKs, payment processors, file storage, legacy APIs

When preventing an external interface from leaking into internal code

Facade Pattern

Order creation, member registration, payment completion processing

When a simple entry point is needed in front of a complex subsystem

Decorator Pattern

Logging, caching, authorization checks, metrics

When adding extra behavior without modifying the original object

Guard Clause

API handlers, validation, command handlers

When reducing nested if blocks by handling failure conditions up front

Result Pattern

Parsing, validation, domain commands

When failure is part of the normal business flow rather than an exception

State Pattern

Order status, game state, approval workflows

When per-state behavior and transitions become complex

Repository Pattern

Aggregate storage for orders, members, products, etc.

When pushing DB details out of domain code

Query Facade

Admin screens, dashboards, list APIs

When read-only queries are forced to drag along the domain model unnecessarily

Model boundary separation: DTO / Entity / Value Object

Web APIs, external integrations, DDD models

When the rate of change differs among the database, domain, and API response

Retry

HTTP clients, DB commands, message processing

When the failure is transient and retrying may succeed

Timeout

External APIs, databases, RPCs, lock acquisition

When a slow dependency is holding up the entire system

Circuit Breaker

Payment processor APIs, auth servers, remote services

When a failing external dependency threatens to bring down your own server

Cache-Aside Pattern

Product detail pages, configuration values, authorization lists

When the same data is read repeatedly and slightly stale values are acceptable

Inheritance hierarchies grow too deep

Composition

A policy or algorithm changes frequently

Strategy Pattern

The shape of an external SDK spreads into internal code

Adapter Pattern

Subsystem call sequences become complex

Facade Pattern

You want to add extra behavior without modifying the original object

Decorator Pattern

Nested if blocks grow too deep

Guard Clause

The caller must handle failures directly

Result Pattern

Per-state behavior and transitions are complex

State Pattern

DB details are leaking into domain code

Repository Pattern

Screen queries are forced to drag along the domain model

Query Facade

External calls fail occasionally

Retry + Timeout

A failure in an external dependency is bringing down your system

Circuit Breaker

The same data is read repeatedly

Cache-Aside