Reference on An observer's log

API Gateway

Mon, 01 Jan 0001 00:00:00 +0000

🔴 P0 — appears in nearly every system design; the single entry point pattern

Consistency Models

Mon, 01 Jan 0001 00:00:00 +0000

🔴 P0 — the spectrum from linearisability to eventual consistency

Data Model Selection

Mon, 01 Jan 0001 00:00:00 +0000

🔴 P0 — the first data decision in every design; determines query flexibility, scaling, and consistency

Deployment Strategies

Mon, 01 Jan 0001 00:00:00 +0000

🟡 P2 — mentioned in designs but rarely the crux; know the trade-offs

Leader-Follower Replication

Mon, 01 Jan 0001 00:00:00 +0000

🔴 P0 — the default replication strategy for most production databases

Networking Essentials

Mon, 01 Jan 0001 00:00:00 +0000

🔴 P0 — foundation for reasoning about inter-service communication, latency, and failure modes

Real-time & Fan-out Patterns

Mon, 01 Jan 0001 00:00:00 +0000

🟠 P1 — how to push updates to many recipients; classic interview problem

Serialisation Trade-offs

Mon, 01 Jan 0001 00:00:00 +0000

🔴 P0 — DDIA Ch4 core content; underpins the gRPC proxy pattern and API versioning

Statelessness & State Management

Mon, 01 Jan 0001 00:00:00 +0000

🔴 P0 — appears in nearly every system design interview

The Three Pillars

Mon, 01 Jan 0001 00:00:00 +0000

🟠 P1 — the foundational framework for observability

API Design

Mon, 01 Jan 0001 00:00:00 +0000

🔴 P0 — the craft of designing APIs; shows engineering judgement, not just pattern knowledge

Backend for Frontend (BFF)

Mon, 01 Jan 0001 00:00:00 +0000

🟠 P1 — solves the “one API doesn’t fit all clients” problem

CAP Theorem & PACELC

Mon, 01 Jan 0001 00:00:00 +0000

🔴 P0 — the foundational impossibility result and its more nuanced successor

Exactly-Once Semantics

Mon, 01 Jan 0001 00:00:00 +0000

🔴 P0 — the holy grail of distributed message processing; critical for payments

Feature Flags

Mon, 01 Jan 0001 00:00:00 +0000

🟠 P1 — decouple deployment from release; essential for large systems

Idempotency

Mon, 01 Jan 0001 00:00:00 +0000

🔴 P0 — critical for payment systems, Stripe’s public API design is built on this

Index Trade-offs (B-Tree vs LSM)

Mon, 01 Jan 0001 00:00:00 +0000

🟠 P1 — DDIA Ch3 deep knowledge; shows understanding of storage engine internals

Leaderless Replication

Mon, 01 Jan 0001 00:00:00 +0000

🟠 P1 — Dynamo-style replication; no single leader, quorum-based

Protocol Choices

Mon, 01 Jan 0001 00:00:00 +0000

🔴 P0 — REST vs gRPC vs GraphQL is a standard interview question

RED & USE Methods

Mon, 01 Jan 0001 00:00:00 +0000

🟠 P1 — systematic approaches to choosing what to measure

Data Modelling

Mon, 01 Jan 0001 00:00:00 +0000

🔴 P0 — the first design decision that cascades into everything else: scaling, consistency, query performance

Distributed Tracing

Mon, 01 Jan 0001 00:00:00 +0000

🟠 P1 — following a request across service boundaries

Failure & Partial Failures

Mon, 01 Jan 0001 00:00:00 +0000

🔴 P0 — the defining characteristic of distributed systems

Idempotency Keys

Mon, 01 Jan 0001 00:00:00 +0000

🔴 P0 — Stripe’s public API is built on this; the implementation of idempotency for APIs

Quorum Reads & Writes

Mon, 01 Jan 0001 00:00:00 +0000

🔴 P0 — the mathematical foundation for consistency in replicated systems

Sharding Strategies

Mon, 01 Jan 0001 00:00:00 +0000

🔴 P0 — how to split data across multiple databases when one isn’t enough

Transport Protocols

Mon, 01 Jan 0001 00:00:00 +0000

🟠 P1 — understanding HTTP/2 is essential for gRPC; WebSocket vs SSE comes up in real-time designs

Web-gRPC Proxy

Mon, 01 Jan 0001 00:00:00 +0000

🟠 P1 — decoupling internal gRPC from external REST via a stateless proxy

Write-Ahead Logging

Mon, 01 Jan 0001 00:00:00 +0000

🟠 P1 — the durability mechanism underlying every serious database

Zero-Downtime Migrations

Mon, 01 Jan 0001 00:00:00 +0000

🟠 P1 — underrated staff-level signal; how to change schemas on a live system

Consistent Hashing

Mon, 01 Jan 0001 00:00:00 +0000

🔴 P0 — minimises data movement when nodes are added or removed

CQRS

Mon, 01 Jan 0001 00:00:00 +0000

🟠 P1 — Command Query Responsibility Segregation; separate read and write models

Double-Entry Ledger

Mon, 01 Jan 0001 00:00:00 +0000

🟠 P1 — the accounting pattern underlying every financial system

Fallacies of Distributed Computing

Mon, 01 Jan 0001 00:00:00 +0000

🟠 P1 — Peter Deutsch’s eight fallacies; useful as a shortcut reference in interviews

Raft Consensus

Mon, 01 Jan 0001 00:00:00 +0000

🟠 P1 — the understandable consensus algorithm; know conceptually, not implementation details

Strangler Fig (API Migration)

Mon, 01 Jan 0001 00:00:00 +0000

🟡 P2 — Martin Fowler’s pattern for incremental migration from monolith to microservices

API Versioning Strategies

Mon, 01 Jan 0001 00:00:00 +0000

🟠 P1 — how you evolve public APIs without breaking consumers

Back-of-Envelope Estimation

Mon, 01 Jan 0001 00:00:00 +0000

🔴 P0 — expected in every system design interview; grounds design decisions in numbers

Cache Patterns

Mon, 01 Jan 0001 00:00:00 +0000

🔴 P0 — the primary tool for read scaling; multiple patterns with different consistency guarantees

Event Sourcing

Mon, 01 Jan 0001 00:00:00 +0000

🟠 P1 — store events, not state; the append-only log as source of truth

Multi-tenancy Patterns

Mon, 01 Jan 0001 00:00:00 +0000

🟠 P1 — directly relevant to Stripe (multi-tenant SaaS platform)

Two-Phase Commit (2PC)

Mon, 01 Jan 0001 00:00:00 +0000

🔴 P0 — the classical distributed transaction protocol and why it’s limited

Cache Invalidation

Mon, 01 Jan 0001 00:00:00 +0000

🔴 P0 — “There are only two hard things in CS: cache invalidation and naming things”

Saga Pattern

Mon, 01 Jan 0001 00:00:00 +0000

🔴 P0 — the distributed transaction pattern for microservices; critical for Stripe payment flows

Sync vs Async Communication

Mon, 01 Jan 0001 00:00:00 +0000

🔴 P0 — the most fundamental inter-service communication decision

Message Queue Patterns

Mon, 01 Jan 0001 00:00:00 +0000

🔴 P0 — queues are the backbone of async architectures

Rate Limiting & Backpressure

Mon, 01 Jan 0001 00:00:00 +0000

🔴 P0 — definitely Stripe territory; protecting services from overload

Timeout Strategies

Mon, 01 Jan 0001 00:00:00 +0000

🔴 P0 — the most fundamental reliability mechanism; every network call needs one

Load Balancing Patterns

Mon, 01 Jan 0001 00:00:00 +0000

🟠 P1 — distributing traffic across service instances

Batch & Stream Processing

Mon, 01 Jan 0001 00:00:00 +0000

🟠 P1 — two paradigms for processing large volumes of data

Choreography vs Orchestration

Mon, 01 Jan 0001 00:00:00 +0000

🔴 P0 — staff-level framing question: how do multi-service workflows coordinate?

Circuit Breaker

Mon, 01 Jan 0001 00:00:00 +0000

🔴 P0 — prevents cascading failures by stopping calls to a failing service

Outbox Pattern

Mon, 01 Jan 0001 00:00:00 +0000

🟠 P1 — solves the dual-write problem; critical for exactly-once in event-driven systems

Retry with Backoff & Jitter

Mon, 01 Jan 0001 00:00:00 +0000

🔴 P0 — the default error recovery mechanism; Stripe definitely asks about this

Bulkhead Isolation

Mon, 01 Jan 0001 00:00:00 +0000

🟠 P1 — isolating failures so one component’s problems don’t sink the ship

Ordering & Causality

Mon, 01 Jan 0001 00:00:00 +0000

🟠 P1 — how distributed systems reason about “what happened before what”

Communication & API Design

Mon, 01 Jan 0001 00:00:00 +0000

Core Concepts

Mon, 01 Jan 0001 00:00:00 +0000

Technology-agnostic fundamentals that underpin every system design. These are the building blocks: how networks behave, how to model data, how to design APIs. Patterns build on these; without them, pattern-level answers lack grounding (see Patterns Taxonomy).

Data Storage & Retrieval

Mon, 01 Jan 0001 00:00:00 +0000

Deployment & Evolution

Mon, 01 Jan 0001 00:00:00 +0000

Domain-Specific Patterns

Mon, 01 Jan 0001 00:00:00 +0000

Encoding & Data Formats

Mon, 01 Jan 0001 00:00:00 +0000

Fundamental Concepts

Mon, 01 Jan 0001 00:00:00 +0000

Observability

Mon, 01 Jan 0001 00:00:00 +0000

Reliability, Consistency & Synchronisation

Mon, 01 Jan 0001 00:00:00 +0000

Scaling & Performance

Mon, 01 Jan 0001 00:00:00 +0000