P0 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

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

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

API Design

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

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

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

Idempotency

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

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

Protocol Choices

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

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

Data Modelling

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

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

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

Consistent Hashing

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

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

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

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

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

Retry with Backoff & Jitter

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

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