GitHub - NikolayS/PGSimCity: An explorable 3D city that shows how Postgres actually works

An explorable 3D city that shows how PostgreSQL actually works. PGSimCity turns a PostgreSQL cluster into a city you can inspect, walk through, and break. It is for engineers who are good at their job and have never had to operate a database — the people who need to understand why a checkpoint spikes latency, why one forgotten transaction bloats a table forever, and what synchronous_commit is really charging them.

This project contains no SimCity code, assets, artwork, logos, characters, audio, or game content.

How much to trust this PGSimCity is still 0.x: early and moving. It is a model of PostgreSQL, not an emulator: no PostgreSQL source code runs here, and the numbers are scaled so a human can watch them. Three specialist review rounds checked PostgreSQL correctness against postgresql.org/docs and the source rather than memory; every finding was independently checked by a reviewer tasked with refuting it. A separate audit treated buildings, adjacencies, and animations as claims. The suite has 234 tests; CI fails the build on a red test. They pin the WAL trigger point as max_wal_size / (1 + checkpoint_completion_target) at every call site, cache hit ratio as blks_hit / (blks_hit + blks_read), and the clock-sweep usage_count cap at 5. Mistakes have been found and fixed throughout; the commit history records them. Known limitation: touch controls have been verified only in Chrome's mobile emulation. Corrections from people who know the engine are exactly what this needs: open an issue or send a pull request.

What you are looking at

District What it is

Client sky (north, above) Connections arriving from the application tier

Postmaster The supervisor. Forks one backend per connection and never touches your data

Backend row 16 backend processes. Their lighting is their state — including idle in transaction

Buffer pool (shared_buffers) A representative sample of 1,024 frames, beside wal_buffers, the ProcArray, lock table, CLOG and buffer mapping table

The excavation The data directory: where memory ends and storage begins

Storage (below) Heap files as fields of 8 KiB pages, B-trees as actual trees, TOAST, the FSM and visibility map, the OS page cache and the disks

WAL district (east) walwriter → pg_wal segments → archiver → walsender

Maintenance yard (west) Checkpointer, background writer, autovacuum launcher and its workers

Standby (south) walreceiver, the startup process replaying WAL, and the lag between them

Continuity quarter (outer east and south) WAL archive, base backups, point-in-time recovery, a second delayed standby, leader lease and rejoin machinery

Query lab (above the backends) Select a backend and its statement unfolds: parse → rewrite → plan → execute

Colour is semantic everywhere and never decorative: WAL is amber, dirty pages are red, clean pages are blue, vacuum is violet, checkpoints are pink, the background writer is teal, replication is orange, storage is green, indexes are aqua, locks are red.

Things worth trying

Press T for the 14-chapter guided tour. It follows one connection from the client through planning, caching, WAL, checkpoints, vacuum and replication. Press Enter to trace one statement. Pick Non-HOT UPDATE and slow playback exposes where it enters the buffer pool, creates WAL and waits to commit. Run Cache thrash from the Scenarios menu. It sets shared_buffers to 16 MiB — below the manual control's 128 MiB minimum — so the clock sweep races and backends write their own dirty victims before they can read another page. Turn on Long-running transaction. The xmin horizon blade sinks and goes red; autovacuum still travels to the tables, but reports zero removable rows while the sessions table keeps bloating. Release the transaction and cleanup can begin again. Run Checkpoint storm. Watch the checkpointer's flywheel spin up, the fsync phase shudder, and a wall of full-page writes flood the WAL district after each checkpoint begins. Set synchronous_commit to off and watch backends stop waiting in commit_wait. Then read what you just traded away. Turn on Slow replay and watch sent_lsn, write_lsn, flush_lsn and replay_lsn pull apart on the standby. Press G and walk through the city at eye level. A buffer frame that read as one tile from the establishing shot becomes a structure above your head.

Controls Press ? in the city for the complete input map and colour legend. Camera

Input Action

Left-drag Pan in orbit mode — grab the ground and move it, the way a map does

Shift-left-drag or Ctrl/Cmd-left-drag Orbit around the city

Middle-drag Pan in orbit mode

Right-click or touch long-press Open the context menu

Wheel Zoom towards the cursor in orbit mode · adjust movement speed in fly mode

1 finger Pan in orbit mode

2 fingers Pinch to zoom · twist to orbit · drag both up/down to tilt

First-person touch Left thumb moves · right thumb looks · buttons jump and crouch (rise and dive while swimming)

Click Select a building · in fly or walk mode, capture the mouse for looking

W A S D or the arrow keys Move

Space or E · C or Q Rise · descend in fly mode; Space jumps and C crouches in walk mode

PageUp / PageDown Change altitude in orbit or fly mode

Shift · Alt Boost · precision in orbit or fly mode; Shift runs in walk mode

Esc Leave pointer lock

Keys

Key Action

F Toggle fly / orbit camera

G Get down and walk the city on foot, 1.7 m tall

H Back to the establishing shot

Home Back to the default establishing shot

O Straight-down overview of the whole plate

T Guided tour — the whole city in 14 chapters

Enter Open Run a Query

/ or Ctrl/Cmd-K Command palette — search every component, setting and scenario

? Keyboard map and colour legend

L Toggle the floating labels

N Toggle daylight / night

M Toggle sound

K or P Pause / resume

, . Slower / faster (0.1× – 5×)

R Reset to the default settings

Esc Close the topmost overlay

1 – 8 Jump to a district: clients, backends, buffer pool, WAL, storage, query lab, maintenance, standby

How it is built src/ core/ shared contracts, event bus, registry, themes and utilities sim/ the PostgreSQL simulation world/ the city geometry, one module per district engine/ renderer, camera, flows, labels, picking, collision and audio ui/ controls, inspector, tour, search and written explanations observability/ a separate diagnostic interface over the same simulation

Three rules hold it together:

world/layout.ts is the single source of truth for geography. Anchors, table definitions and the route network live there. No district hard-codes a coordinate another district needs. The simulation never imports three.js, and the world never mutates the simulation. They meet at SimState. Rendering carries meaning differently by theme. At night structure is matte and meaning is neon; in daylight hue and value carry meaning without relying on bloom.

Stack: three.js r185, TypeScript, Vite. three.js is the 3D application's only bundled runtime dependency. The separate 2D Query flow may lazy-load PGlite after reader opt-in. There is no framework, and Plausible analytics is the sole external service. window.PGSIMCITY in the browser console includes sim, registry, bus, rig, gfx and flows if you would rather drive the city from the outside. For the accuracy boundary and review status, see How much to trust this above. Each inspector names material simplifications at the point where they matter. A possible future The accuracy boundary described above makes internals such as the clock sweep's frame-by-frame victim choice observable. The separate Query flow offers an opt-in PGlite mode: real PostgreSQL supplies parsing, plans, catalogs, buffer counters, errors and results, while its plan drives the closest modelled interior path. The page labels the two sources separately because PostgreSQL exposes the former and not the latter.

Run it locally You need Node.js 20 or newer and a browser with WebGL2. npm install npm run dev # http://localhost:5173 npm test npm run typecheck npm run build # static bundle in dist/ npm run preview # http://localhost:4173 There is no application server. The result is a static bundle. The 3D city and Diagnose surface make only the analytics requests described below. Query flow may, after an explicit click, load the same-origin PGlite JavaScript, data and WebAssembly assets and run an in-memory PostgreSQL in the browser. The modelled query flow continues to work when analytics or PGlite is blocked. Analytics and privacy. PGSimCity uses Plausible for aggregate, cookie-free analytics on the city and observability pages. It records pageviews, unique visitors, referring sites, bounce rate, visit duration and interactions such as starting the tour, changing playback, opening a panel, tracing a statement, selecting a building or following an outbound link. PGSimCity sends no names, email addresses, free-form input, browser fingerprint or application-supplied personal data, and creates no analytics cookies, analytics local storage, advertising identifier or session recording. Blocking plausible.io stops measurement without affecting the application.

Roadmap What is being worked on, what is known to be wrong, and what is deliberately not being done: ROADMAP.md. Licence Apache-2.0. Copyright 2026 Nikolay Samokhvalov. See NOTICE. PostgreSQL is a trademark of the PostgreSQL Community Association of Canada. PGSimCity is an independent educational project and is not affiliated with, sponsored by, or endorsed by the PostgreSQL project.