Tag

system design

69 cards

Cards

The ReAct loop of think, act, observe and repeat feeding each observation back into the context, beside plan-and-execute and reflection, and the loop that fails by never converging

AI SystemsDay 63Oct 4, 2026

Think, Act, Look, Repeat

system-design ai-systems agents

A prompted JSON response wrapped in markdown and a cheerful preamble against constrained decoding that enforces the shape while generating, and the business validation a well-formed response still needs

AI SystemsDay 62Oct 3, 2026

Stop Asking Nicely For JSON

system-design ai-systems llm

Recall@k measuring whether the right passage reached the context and faithfulness measuring whether the answer was supported by it, and a chunking change that improved one number while breaking the other

AI SystemsDay 61Oct 2, 2026

We Can't Improve What We Only Eyeball

system-design ai-systems rag

Vector search ranking millions of separately embedded documents by distance in milliseconds, then a cross-encoder reranker reading the query and fifty candidates together and reordering the top five

AI SystemsDay 60Oct 1, 2026

The Top Five Aren't The Best Five

system-design ai-systems rag

A policy clause cut in half at 500 characters so neither piece retrieves, beside fixed-size, recursive, semantic and parent-document chunking, and the token ranges different content types actually want

AI SystemsDay 59Sep 30, 2026

Where We Cut Decides What We Find

system-design ai-systems rag

A p99 latency spike to 800ms caused by a stop-the-world garbage collection pause, beside generational and concurrent collectors that shrink the stop by spending CPU to avoid it

ConcurrencyDay 58Sep 29, 2026

The Pause Nobody Scheduled

system-design concurrency garbage-collection

Two threads reading 41 and both writing 42 so one increment vanishes, beside a lock that makes threads queue and a compare-and-swap instruction that retries on collision instead of blocking

ConcurrencyDay 57Sep 28, 2026

Don't Take The Lock. Ask Instead.

system-design concurrency locks

Four CPU-bound Python threads funnelled into a single lane by the GIL against I/O-bound threads that release it while waiting, with processes and C extensions as the routes to real parallelism

ConcurrencyDay 56Sep 27, 2026

Four Cores, One Lane

system-design concurrency python

A sequential async method compiled into a state machine that releases its thread at every await and keeps locals on the heap, beside C# rewriting the code and Java rewriting the runtime

ConcurrencyDay 55Sep 26, 2026

The Keyword That Rewrites Our Method

system-design concurrency async-await

Platform threads carrying a megabyte of OS stack each against virtual threads parked on the heap by the Java runtime, letting plain sequential blocking code scale to millions of concurrent requests

ConcurrencyDay 54Sep 25, 2026

Threads That Cost Almost Nothing

system-design concurrency java

Ten thousand threads holding a megabyte of stack each while asleep, against one event loop asking the kernel which sockets have data ready, and the rule that nothing in a handler may ever block

ConcurrencyDay 53Sep 24, 2026

One Thread, Ten Thousand Connections

system-design concurrency event-loop

A multi-tenant query missing its tenant filter and returning every customer's rows, the four isolation levels from shared table to cluster per tenant, and row-level security enforcing the filter in the database

SecurityDay 52Sep 23, 2026

One Missing WHERE Clause

system-design security multi-tenancy

Envelope encryption splitting one key into many short-lived data keys wrapped by a master key inside a hardware module, with every unwrap logged and rotation touching the small keys rather than every row

SecurityDay 51Sep 22, 2026

The Key That Guards The Keys

system-design security encryption

A user fired at 10am whose JWT stays valid until 11, the three parts of a self-contained token, and short-lived access tokens paired with a server-side refresh token that can actually be invalidated

SecurityDay 50Sep 21, 2026

The Token We Can't Take Back

system-design security jwt

Handing an app a password against granting it a scoped token, the five steps by which the app never sees the credential, and what OpenID Connect adds on top of OAuth

SecurityDay 49Sep 20, 2026

Let Them In Without Giving Them the Keys

system-design security oauth

Authentication answering who you are against authorization answering what you may do, the one-digit URL change that reaches another person's record, and enforcing ownership at the data layer

SecurityDay 48Sep 19, 2026

Logged In Is Not the Same as Allowed

system-design security authorization

The same architecture diagram seen from the builder's view and the attacker's view, the six STRIDE prompts applied to every arrow, and trust boundaries as the place to ask

SecurityDay 47Sep 18, 2026

Draw the System, Then Draw the Attacker

system-design security threat-modelling

A 2TB table moving onto four shards with no downtime through dual write, backfill, verify, gradual read shift and finish, set against the naive stop-and-copy plan

DatabasesDay 46Sep 17, 2026

Splitting a Table While People Are Using It

system-design databases sharding

A column rename breaking during a rolling deploy while both code versions are live, and the expand-migrate-contract sequence that makes each step safe alone and reversible

DatabasesDay 45Sep 16, 2026

Never Change Anything in One Step

system-design databases migrations

B-tree, GIN, BRIN, partial and covering indexes compared by the question each one answers, and a billion-row table where BRIN costs 300 KB against a B-tree's 5 GB

DatabasesDay 44Sep 15, 2026

Not Every Index Is a Tree

system-design databases indexes

Why every Postgres connection is a separate OS process costing megabytes, how 500 connections on 16 cores collapses throughput, where the queue forms under a small pool versus a large one, and a pooler as the fix

DatabasesDay 43Sep 14, 2026

A Connection Costs More Than You Think

system-design databases connection-pooling

Index lookup against sequential scan for the same query, the row-count threshold where the planner switches, three ways statistics mislead it, and ANALYZE before reaching for a new index

DatabasesDay 42Sep 12, 2026

Why It Ignored Your Index

system-design databases query-planner

The write-ahead log ordering — transaction, append to WAL, fsync, modify in memory, flush data pages later — with crash recovery replaying committed changes, and the same log driving replication, PITR and audit

DatabasesDay 41Sep 10, 2026

Write It Down Before You Do It

system-design databases write-ahead-log

How an insert, update and delete leave live and dead row versions in the same Postgres table, the bloat after a million rows updated ten times each, and VACUUM marking dead space reusable

DatabasesDay 40Sep 9, 2026

Your Database Doesn't Delete Anything

system-design databases mvcc

Four hundred identical refund-policy questions each billed in full, beside prompt caching at the model provider and semantic caching by embedding similarity, and the 0.92 match that returns the wrong answer

AI SystemsDay 39Sep 8, 2026

The Same Question, Paid For Twice

system-design ai-systems semantic-caching

ObservabilityDay 38Sep 7, 2026

Which Of Your Forty Services Was Slow?

system-design observability distributed-tracing

A producer outpacing a consumer with the queue between them grown to two million stale messages, buffering and backpressure compared as curves, and four ways to signal a producer to slow down

ReliabilityDay 37Sep 6, 2026

Tell Them To Slow Down

system-design reliability backpressure

Circuit breaker states — closed, open and half-open — beside the bulkhead pattern giving each dependency its own connection pool, and a timeline of thread exhaustion without either

ReliabilityDay 36Sep 5, 2026

Stop Calling The Thing That's Down

system-design reliability circuit-breaker

Cassandra's leaderless ring of tokens with 256 per node by default, what virtual nodes buy when adding or losing a node, and the gossip and repair cost that comes with them

Case StudyDay 35Sep 4, 2026

Cassandra Calls Them Tokens

system-design distributed-data cassandra

Three servers on a hash ring owning wildly uneven shares, and the fix — hashing each server two hundred times so six hundred small slices interleave around the ring

Distributed DataDay 35Sep 4, 2026

One Server, Two Hundred Places

system-design distributed-data consistent-hashing

Ten shards holding ten percent of the data each while one hot key sends sixty percent of traffic to shard four, and the three fixes — cache in front, split the key artificially, replicate the hot ones

Distributed DataDay 34Sep 3, 2026

Evenly Distributed Isn't Even

system-design distributed-data partitioning

Partitioning by range against partitioning by hash, and the real question underneath both — what a single query has to touch, and whether the key keeps related rows on one shard

Distributed DataDay 33Sep 2, 2026

The Decision You Can't Undo

system-design distributed-data partitioning

The consistency menu — strong, causal and eventual models compared with the cost of each, and a list of what can safely be eventual against what cannot

Distributed DataDay 32Sep 1, 2026

What "Latest" Actually Means

system-design distributed-data consistency

Quorum reads and writes across three replicas — W plus R greater than N, the overlapping sets that guarantee one node in every read holds the newest value

Distributed DataDay 31Aug 31, 2026

How Many Copies Have To Say Yes?

system-design distributed-data quorum

Leader and follower replication — synchronous waiting for confirmation against asynchronous replying immediately, and a timeline showing followers landing the same write at different moments

Distributed DataDay 30Aug 30, 2026

Why Copies Disagree

system-design distributed-data replication

Password hashing compared — SHA-256 at a billion guesses a second against bcrypt and argon2, plus per-user salts that stop one cracked password revealing the next

SecurityDay 29Aug 29, 2026

The Algorithm That's Fast Is The Problem

system-design security password-hashing

B-tree versus LSM tree — updating a page in place against appending to a memtable and flushing to sorted files, with compaction as the hidden cost

Storage EnginesDay 28Aug 28, 2026

Two Ways To Write A Number Down

system-design storage-engines databases

The Rewrite That Never Ships — system design card, day 26, microservices: a proxy in front of a legacy system routes one endpoint at a time to new services until the old system serves nothing

MicroservicesDay 26Aug 27, 2026

The Rewrite That Never Ships

microservices system-design strangler-fig

The Search That Can't Spell — system design card, day 27, AI systems: BM25 keyword results and vector results merged by reciprocal rank fusion so an exact citation surfaces above essays about the concept

AI SystemsDay 27Aug 27, 2026

The Search That Can't Spell

ai-systems system-design rag

The Celebrity Who Breaks Your Feed — system design card, day 24, case study

Case StudyDay 24Aug 24, 2026

The Celebrity Who Breaks Your Feed

case-study scale system-design

Add One Server, Move Everything — system design card, day 25, networking: modulo hashing moves 80% of keys, consistent hashing moves 20%

NetworkingDay 25Aug 24, 2026

Add One Server, Move Everything

networking system-design consistent-hashing

The Consumer That Fell Behind Forever — system design card, day 23, kafka

KafkaDay 23Aug 23, 2026

The Consumer That Fell Behind Forever

consumer-lag kafka system-design

The Lock That Outlived Its Owner — system design card, day 22, distributed systems

Distributed SystemsDay 22Aug 22, 2026

The Lock That Outlived Its Owner

distributed-locks distributed-systems system-design

The Limit That Punishes Your Best Users — system design card, day 21, apis

APIsDay 21Aug 21, 2026

The Limit That Punishes Your Best Users

apis rate-limiting system-design

You Posted It. Then It Vanished. — system design card, day 20, databases

DatabasesDay 20Aug 20, 2026

You Posted It. Then It Vanished.

databases postgresql system-design

Say No Before You Have To — system design card, day 19, reliability

ReliabilityDay 19Aug 19, 2026

Say No Before You Have To

load-shedding reliability system-design

The Index You Added Made Writes Slower — system design card, day 18, databases

DatabasesDay 18Aug 18, 2026

The Index You Added Made Writes Slower

databases postgresql system-design

Your Agent Is A Confused Deputy — system design card, day 17, ai systems

AI SystemsDay 17Aug 17, 2026

Your Agent Is A Confused Deputy

ai ai-systems security

Everyone Asks At The Same Second — system design card, day 16, caching

CachingDay 16Aug 16, 2026

Everyone Asks At The Same Second

caching redis system-design

The Average User Doesn't Exist — system design card, day 15, foundations

FoundationsDay 15Aug 15, 2026

The Average User Doesn't Exist

foundations performance system-design

More Threads, Slower System — system design card, day 14, concurrency

ConcurrencyDay 14Aug 14, 2026

More Threads, Slower System

concurrency context-switching system-design

You Didn't Split It. You Stretched It. — system design card, day 13, microservices

MicroservicesDay 13Aug 13, 2026

You Didn't Split It. You Stretched It.

distributed-monolith microservices system-design

Why The First Request Is Always Slow — system design card, day 12, networking

NetworkingDay 12Aug 12, 2026

Why The First Request Is Always Slow

dns networking system-design

The Two Writes That Must Be One — system design card, day 11, kafka

KafkaDay 11Aug 11, 2026

The Two Writes That Must Be One

kafka system-design transactional-outbox

Why Uber Chose Hexagons — system design card, day 10, case study

Case StudyDay 10Aug 10, 2026

Why Uber Chose Hexagons

case-study system-design uber

Turning The Earth Into Words — system design card, day 10, distributed systems

Distributed SystemsDay 10Aug 10, 2026

Turning The Earth Into Words

distributed-systems geospatial system-design

The Retry That Charges Twice — system design card, day 9, apis

APIsDay 9Aug 9, 2026

The Retry That Charges Twice

apis idempotency system-design

The Outage That Starts After You're Saved — system design card, day 8, case study

Case StudyDay 8Aug 8, 2026

The Outage That Starts After You're Saved

case-study resilience system-design

Why 80% Full Is Already Too Full — system design card, day 8, reliability

ReliabilityDay 8Aug 8, 2026

Why 80% Full Is Already Too Full

performance reliability system-design

Your RAG Isn't Hallucinating — system design card, day 7, ai systems

AI SystemsDay 7Aug 7, 2026

Your RAG Isn't Hallucinating

ai ai-systems rag

The Bug That Passes Every Test — system design card, day 6, databases

DatabasesDay 6Aug 6, 2026

The Bug That Passes Every Test

databases postgresql system-design

Netflix Doesn't Stream From The Cloud — system design card, day 5, case study

Case StudyDay 5Aug 5, 2026

Netflix Doesn't Stream From The Cloud

case-study netflix system-design

Ship It Before They Ask — system design card, day 5, caching

CachingDay 5Aug 5, 2026

Ship It Before They Ask

cache-warming caching system-design

Order Is A Choice, Not A Gift — system design card, day 4, kafka

KafkaDay 4Aug 4, 2026

Order Is A Choice, Not A Gift

kafka partitions system-design

The Only Numbers You Must Memorise — system design card, day 3, foundations

FoundationsDay 3Aug 3, 2026

The Only Numbers You Must Memorise

foundations latency system-design

Amazon's Honest Answer — system design card, day 2, case study

Case StudyDay 2Aug 2, 2026

Amazon's Honest Answer

availability case-study system-design

Clocks That Count, Not Tick — system design card, day 2, distributed systems

Distributed SystemsDay 2Aug 2, 2026

Clocks That Count, Not Tick

distributed-systems system-design vector-clocks

The Cart That Refuses To Say No — system design card, day 1, case study

Case StudyDay 1Aug 1, 2026

The Cart That Refuses To Say No

availability case-study system-design

There Is No Best Design — system design card, day 1, foundations

FoundationsDay 1Aug 1, 2026

There Is No Best Design

foundations system-design tradeoffs