Breakscale

Breakscale is a system design simulator. You place components, wire them, and raise the offered traffic until queues, timeouts, or shed requests show up. The numbers come from a discrete-event engine in src/sim, which has no React and no network calls. The same engine is what the website, the VS Code panel, and the MCP server run. Checked 2 November 2026 against main at 9c71585.

This is a different product from GonnaCrash. GonnaCrash is 200Lab's level game: a server grades a design against a budget. Breakscale is an open canvas with bundled examples and a local engine.

GitHub snapshot the same day: workersio/breakscale, MIT, TypeScript, created 23 August 2026, pushed 30 October 2026, 1,953 stars, 163 forks, 12 open issues, about 2 MB. xevrion/breakscale resolves to this repo. The npm author and the VS Code publisher are still xevrion. There are no GitHub releases. The homepage and https://docs.breakscale.tech both returned HTTP 200. This note did not run bun dev, did not run the test suite, and did not compare the published npm tarball with main.

Where it runs

main was pushed on 30 October 2026, after both the npm publish and the Marketplace update. The hosted server and the installed extension can lag the repo. The four tools registered in createServer are read_me, create_view, export_design, and open_design. create_view is the app tool: it builds a topology, runs it, and returns measured lines plus a breakscale.tech link. All four set readOnlyHint. open_design also sets openWorldHint because it fetches a share link.

Engine

Engine in src/sim/engine.ts advances a min-heap of events. One advance call clamps the wall-clock step to 100 ms and processes at most 60,000 events. A request deeper than 32 hops fails with reason depth.

Service time is not a fixed multiple of the mean. Rng in src/sim/random.ts is mulberry32, so one seed and one topology replay the same draws. serviceTime(mean, cv) returns the mean when cv is at most 0.01, and otherwise a gamma draw with shape 1/cv^2 (Marsaglia-Tsang, with a boost when the shape is below 1). cv of 1 is the exponential case.

A component's capacity is how many requests it can work on at once, and instances is how many copies exist. Work past instances × capacity waits in a FIFO queue. The engine comment on buffering nodes says the caller's chain resolves as a success at the buffer, and a detached copy is relayed onward, so a queue can ack while workers are still behind. Latency percentiles come from a ring buffer of completed samples. Past 512 samples the percentile pass strides the window (PERCENTILE_SAMPLE_CAP).

Chaos is FailureKind: crash, slow, errors, and partition.

Catalog

NodeKind in src/sim/types.ts has 33 members: client, lb, service, cache, db, queue, worker, autoscaler, region, cdn, ratelimiter, breaker, replica, shard, objectstore, searchindex, timeseriesdb, graphdb, coldstorage, vectordb, streambroker, pubsub, websocket, apigateway, sidecar, lambda, cron, bulkhead, retryqueue, transcoder, edgecompute, writebehind, loadshedder.

PRESETS has 23 entries. The first 16 are teaching topologies. The last seven are named after published systems. The README feature table names nine of the teaching presets and all seven company names. The other seven teaching presets are still in the array.

The README says the company presets are reconstructions from published engineering write-ups, and that each one states what it leaves out. This note did not check those descriptions against the papers.

src/sim/challenges.ts adds four briefs on top of presets: hold-the-line, more-machines, stop-the-storm, keep-it-warm. The file header says each shipped preset fails at the brief's load, a named change makes it pass, and the thresholds were measured and pinned by tests. Those tests were not re-run here.

What leaves the machine

A design can stay in browser storage or in the VS Code globalState key above. Sharing has two shapes, both described in src/share/crypto.ts.

An unstored link carries the design in the URL fragment. Browsers do not send the fragment to a server.

A stored link looks like https://breakscale.tech/?s=<id>#k=<key>. The key is a fresh AES-GCM 128-bit key per design. The 12-byte IV is prepended to the ciphertext. The Worker at links.breakscale.tech (worker/src/index.ts, KV binding LINKS) stores that blob and the id. The comment in the Worker says it never sees the key, so it cannot read the design. A GET of https://links.breakscale.tech/ returned 404, which matches a store that only answers by id. Limits in that file and in worker/wrangler.toml: blob from 28 bytes to 256 KiB, id of 10 random bytes, 20 writes per address per 60 seconds, and browser writes allowed from https://breakscale.tech and http://localhost:5173. seal needs WebCrypto. The crypto file says crypto.subtle is missing on an insecure origin, so plain HTTP on a LAN address cannot seal.

SECURITY.md says Breakscale has no backend, no accounts, and sends no data anywhere. That sentence is older than the rest of the repo. The website entry src/main.tsx mounts Vercel Analytics and Speed Insights, and the comment there says both no-op outside a Vercel deployment. The README calls the page-view counter cookie-free and says the extension and the MCP server have no analytics. A search of the repo found @vercel/analytics imported only from src/main.tsx. The share Worker is a backend, and it is limited to ciphertext.

The code license is MIT. The bundled Caveat webfont under public/fonts/Caveat/ is called out in the README as SIL Open Font License 1.1, copyright 2014 The Caveat Project Authors, with OFL.txt beside the file.

Local use

git clone https://github.com/workersio/breakscale.git
cd breakscale
bun install
bun dev

The engine can be driven without the UI. The README's script constructs Engine with a preset topology and a seed, calls advance(1000 / 60) in a loop, and prints snapshot().system.

See also: GonnaCrash · Sys0 · Syscraft · Designing For Distributed Failure · System design fundamentals · Retry And Circuit Breaker