Architecture

                       browser
                          |
                          |  HTML, then a websocket subscription
                          v
   +---------------------------------------------+
   |  apps/web (Next.js, port 3000)               |
   |    server components: fetchQuery             |
   |    client components: useQuery               |
   |    Better Auth routes under /api/auth        |
   +----------+--------------------------+-------+
              |                          |
   Drizzle    |                          |  Convex client, JWT from Better Auth
              v                          v
   +---------------------+     +-------------------------------------+
   |  Postgres           |     |  Convex backend (self-hosted)       |
   |    moj_auth         |     |    3210 cloud origin: queries and   |
   |    moj_dev          |     |         mutations from the browser  |
   +---------------------+     |    3211 site origin: HTTP actions   |
                               |         /judge/*, /api/problems/*   |
                               |    scheduler, crons, aggregates     |
                               +-------------------+-----------------+
                                                   ^
                                                   |  HTTPS, judge polls for work
                                                   |
                               +-------------------+-----------------+
                               |  apps/judge (Docker, DMOJ           |
                               |  judge-server with moj_packet.py)   |
                               |    /problems volume, sandbox        |
                               +-------------------------------------+

• apps/web — Next.js App Router on React 19. A page is a server component that fetchQuerys for the first render and hands the result to a client component that subscribes with useQuery; long lists use usePaginatedQuery.
• Convex — database, query layer and scheduler in one self-hosted process, storing its data in Postgres. The cloud origin serves the browser; the site origin serves the HTTP actions in convex/http/.
• Better Auth — sessions, passwords, two-factor secrets, passkeys and API keys, with Drizzle over Postgres. Convex stores no credentials: it trusts an RS256 JWT carrying {sub, username, isStaff}, registered by convex/auth.config.ts as a customJwt provider. It also verifies Django's pbkdf2_sha256$<iterations>$<salt>$<hash>, rewriting a legacy hash on the first sign-in.
• packages/core — pure functions: permissions, contest formats, ratings, points, verdicts, freeze logic.
• packages/content — remark and rehype with DMOJ's sanitiser presets, KaTeX, Shiki, markdown to Typst.
• apps/judge — DMOJ's judge-server as a git subtree, with dmoj/moj_packet.py and dmoj/moj_data.py added and a MojJudge selected when MOJ_URL is set.

What replaced what

DMOJ	MOJ
Django and gunicorn	Next.js App Router on React 19
MySQL through the Django ORM	Convex, self-hosted, with Postgres underneath
The bridge daemon on TCP 9999	The pull protocol over HTTPS
The websocket event daemon	Convex subscriptions
Celery workers	The jobs table and the Convex scheduler
pdfoid, mathoid, texoid	The Typst binary with vendored packages
Redis	Nothing; Convex holds the queue and the aggregates
Django auth with Fernet columns	Better Auth with Drizzle on Postgres
Pygments	Shiki
The Django admin	The staff console at /admin/

The judge protocol

Every call carries {judgeName, judgeKey}; the server compares sha256(key) against judges.authKeyHash.

Call	Interval	What it does
POST /judge/handshake	once, retried with backoff to 60 s	Reports problem codes and executors; marks the judge online.
POST /judge/heartbeat	10 s	Current load.
POST /judge/claim	500 ms while idle	Asks for one submission; null when there is none.
GET /judge/data	on demand	The archive, with X-Moj-Data-Hash and X-Moj-Data-Size. 404 for nothing stored, 409 for a stale hash.
POST /judge/event	per packet	grading-begin, batch-begin, test-case-status, batch-end, grading-end, compile-error, compile-message, internal-error, submission-terminated.
GET /judge/abort	1 s while grading	Whether the submission was aborted.
POST /judge/disconnect	on shutdown	Clean shutdown.

Candidates are QU submissions ordered by priority then date, priority being 0 in contest, 1 normal, 2 rejudge, 3 batch rejudge. Only judges in the lowest online tier claim; a judge claims only a submission whose language it reported and whose problem it reported or the site holds data for; judgePin sends one to a named judge; and rejudge priorities are skipped while the tier is busy.

problemDataHash on the claim is the sha256 of the archive the site holds, or null. Non-null means the judge grades the site's copy, fetching it once, verifying it, and caching it under MOJ_DATA_CACHE (/judge-data-cache, capped by MOJ_DATA_MAX_GB, 20 by default, evicted least-recently-used). The site's copy wins wherever both exist. A cron requeues submissions in P or G whose judge has not sent a heartbeat for 60 seconds, or which have made no case progress for 15 minutes, once; a second failure is an internal error.

Jobs and crons

jobs.create(type, args) inserts a row with a progress object and schedules the first chunk; each chunk does a bounded amount of work and schedules the next. The types are rejudge, rescore, rescoreContest, rateContest, rejudgeContestProblem, moss, userExport, pdf and sitemap; the last two are marked done immediately, because the web app renders them on request.

Four crons run: judge recovery and offline marking every minute, clearing stale contest mode every five minutes, and refreshing the language statistics every quarter hour.

Counts and ranks that would mean scanning a table use @convex-dev/aggregate, which has no triggers, so every write that moves a profile's points maintains them by hand and a bulk load is followed by rankings:rebuildAggregates. Rate limits use @convex-dev/rate-limiter.