System Design Simple

How big apps really work, explained simply with diagrams. System design, AI coding tools (Claude, Codex) and web security. New short video + article every week.

[System Design Simple @SysDesignSimple]

In our simulation, jobs that needed at most 2 ms of work averaged 31 seconds, because the shop called ten services one by one and one of them was slow. Posting each order once to a topic brought the fast jobs down to 1.87 ms.

#PubSub #SystemDesign #Messaging

[System Design Simple @SysDesignSimple]

50K fans, 5K seats. Design it.

Performance is the latency of one request at low load; scalability is how throughput and latency behave as load and resources grow. Queueing delay rises like ρ/(1−ρ) in the M/M/1 model, so run near 70%…

https://t.co/fFy9f9eePH

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[System Design Simple @SysDesignSimple]

Service discovery: how 500 services find each other

Callers find live instances through a registry: instances heartbeat in, silent ones expire after a TTL, callers cache the list. Dead instances still get traffic for about TTL +…

https://t.co/6TGIyznETz

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[System Design Simple @SysDesignSimple]

How can an AI answer with no network? On-device AI explained

On-device AI can answer with no network because the model executes on your machine: its weights, tokenizer and config are files on the disk, and a runtime does the math on the…

https://t.co/IUOg5Ldf0P

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[System Design Simple @SysDesignSimple]

Publish/subscribe: why 2 ms jobs waited 31 seconds behind one slow service

When a publisher calls every subscriber in turn, its time per event is the sum of all the calls, so one slow subscriber sets the pace for everyone. In our…

https://t.co/2C2RhDJaeZ

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[System Design Simple @SysDesignSimple]

AI citation checks: the link opened, but the quote was missing

A link that opens proves a page exists at that address; it says nothing about the quoted words. In our lab all 60 cited local files existed and were readable while 30 planted…

https://t.co/IGSqNheCGq

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[System Design Simple @SysDesignSimple]

I searched car and keyword search found only 2 of 14 road-car sentences; the other 12 say vehicle, engine or hatchback. In our toy lab, an embedding built by counting shared sentences put 8 car sentences in its top 10, 6 of them never saying …

#AISystems #SystemDesign #AI

[System Design Simple @SysDesignSimple]

Our demo's 3 a.m. login rule raised 78.9 false alarms per 1,000 logins, and only 1.9% of its alerts were real. Giving each user a habit card of their own hours, country and devices cut that to 3.8 per 1,000 and caught 15 of 20 plant…

#Security #SystemDesign #CyberSecurity

[System Design Simple @SysDesignSimple]

1M rides a day. What do you do in the first 5 minutes?

Write functional requirements as user actions and non-functional ones as SLOs: an SLI (good ÷ valid events), a target and a window. 99.9% over 30 days leaves a 43.2 minute error…

https://t.co/ZkNjofG6nE

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[System Design Simple @SysDesignSimple]

Claude Code cloud sessions: how the setup script, cache and network access really work

Each Claude Code cloud session runs in a fresh Ubuntu 24.04 VM. A cloud environment decides its network access, variables and a setup script. The…

https://t.co/CsbDWjDGQ3

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[System Design Simple @SysDesignSimple]

Multi-tenancy: pool vs silo, tenant context, and noisy neighbor limits

Multi-tenancy serves several customers in one environment, with dedicated (silo) resources, shared (pool) resources, or a mix per layer. In pooled layers, a tenant ID…

https://t.co/JtsvJj7hii

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[System Design Simple @SysDesignSimple]

Prompt injection: why the AI obeyed a hidden line

Prompt injection works because a model reads the page you ask it to summarize in the same token stream as your instructions, so hidden text can steer it. Marking the page as data plus a…

https://t.co/igM52ql1A5

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[System Design Simple @SysDesignSimple]

Monolith vs microservices: fault isolation and its cost

Separate services can contain process-fatal faults; bulkheads contain resource exhaustion. Shared dependencies and synchronous calls can still propagate failures. In our simulation…

https://t.co/AyKtxypy9d

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[System Design Simple @SysDesignSimple]

AI agent permissions: would an agent delete my files?

Unrestricted destructive tools can cause unrecoverable loss without retained versions. In our scripted lab a careless 7-step tidy plan with full access lost 7 of 10 important files, 0…

https://t.co/tPkYEuF4j6

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[System Design Simple @SysDesignSimple]

Eventual consistency: stale likes and safe withdrawals

A like count can be a few behind because reads go to copies that lag, and that is cheap. For a balance, an atomic conditional debit prevents overdrafts under the stated assumptions:…

https://t.co/l57Cu5Swr4

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[System Design Simple @SysDesignSimple]

I clicked an AI answer's source and the quote wasn't there. In our lab with 10 synthetic docs, all 60 cited links opened, but 30 planted quotes were missing. A normalized exact match caught 30 of 30 fakes and passed every real quote; a loose …

#AISystems #SystemDesign #AI

[System Design Simple @SysDesignSimple]

47,999 rewrites, because I renamed one mug. Same SQLite engine, same 100,000 users: one document per user vs normalized tables.

#SystemDesign #Database #SQL

[System Design Simple @SysDesignSimple]

Design Instagram's feed: system design in five steps

System design is turning requirements and load into an API, a data model and an architecture whose costs you can compute, then measure. For a feed of 20 posts from 150 follows at an…

https://t.co/n4PgN9QEV6

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[System Design Simple @SysDesignSimple]

Claude Code sessions can message each other: how the hand-off works

Claude Code sessions can list the other sessions on the same machine and send them short messages. On one machine a message travels over a local socket owned by your…

https://t.co/wG8rG1CJ3f

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[System Design Simple @SysDesignSimple]

How a leaked password check works without sending the full password

A leaked password check hashes the password with SHA-1 and sends only the first 5 hex characters (a 20-bit prefix) to the Pwned Passwords range API. The service returns…

https://t.co/Rq4aelkMfI

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