Node Server Feature Development

The Node server (server/src/) is the middle tier of HOT-Step CPP, a local AI music-generation app. It serves the React UI, owns the SQLite database, and orchestrates the C++ inference engine (ace-server.exe, spawned as a child process). This skill covers how to add or change a server feature the way this codebase does it.

Glossary (no prior context assumed):

• Engine / ace-server — the C++ binary doing actual AI inference, HTTP API on port 8085. The Node server spawns it and talks to it over HTTP.
• DiT — Diffusion Transformer, the engine's main music-synthesis model. LM — the language model that generates "audio codes" before synthesis. VAE — decodes latents to audio. Pipeline: LM → DiT → VAE.
• AceRequest — the canonical JSON request shape the engine accepts, defined in server/src/services/aceClient.ts:38-151.
• aceReq — the AceRequest instance built for a generation job (from UI params via translateParams).
• Adapter — a LoRA/LoKr fine-tune applied to the DiT at generation time.

UI (fetch /api/*) → Express :3001 → routes/*.ts → services/*.ts
                                        ├→ getDb()  (better-sqlite3, synchronous)
                                        └→ aceClient (HTTP → ace-server :8085)

When to use this skill

• Adding a new /api/* endpoint or a whole feature (route + service).
• Modifying SQLite schema or queries (server/src/db/database.ts).
• Adding a config value or a Settings-UI-exposed .env key (server/src/config.ts).
• Making the server call the engine (new AceRequest field, new engine endpoint).
• Debugging server-side failures (jobs stalling, engine timeouts, DB errors).

Golden rules (hard constraints — each prevents expensive damage)

1. Type-check with npx tsc --noEmit during dev; only npm run build before user testing. (Institutional rule, verbatim.) tsx watch runs TypeScript directly, so a build is wasted work mid-dev — but tsx also tolerates errors that tsc rejects, so type-check before committing.

   cd d:\Ace-Step-Latest\hot-step-cpp\server; npx tsc --noEmit
   

2. Node 18–22 only. Node 24+ breaks dependencies; server/package.json:5-7 enforces >=18.0.0 <24.0.0. better-sqlite3 is a native module — postinstall runs npm rebuild better-sqlite3 (package.json:14).
3. ESM everywhere — relative imports MUST end in .js even though sources are .ts. server/package.json:8 sets "type": "module". import { config } from '../config.js' — omitting .js may run under tsx but fails tsc. There is no __dirname; the pattern is const __dirname = path.dirname(fileURLToPath(import.meta.url)) (config.ts:7, index.ts:46).
4. Never block a request on long work. The engine is single-threaded; generations take minutes. Return a job id immediately and let the UI poll (see the worked example). The generation queue is deliberately serialized to one job at a time (generate.ts:1286-1292).
5. Never call getDb() at module import time. initDb() runs at index.ts:60; a query executed during module load throws Database not initialized. Call initDb() first. (database.ts:17-22). Query only inside handlers/functions.
6. Declare static routes before /:id params. Express matches in order — GET /recent after GET /:id returns a 404 for id "recent". See the explicit warning at songs.ts:47 and seeds.ts (/favorites, /random at lines 131/143 precede /:name at 156).
7. Git: all work on master, stage explicit paths (never git add -A, never git add -f on gitignored paths — data/, logs/, docs/plans/, checkpoints/ are gitignored on purpose), commit locally often, push only with explicit user approval. Pushing any v* tag triggers a full multi-platform CI release build.
8. PowerShell syntax: ; not && for command chaining on this machine.
9. Don't visually verify the UI with a browser agent — ask the human user; they provide screenshots. (Hitting API endpoints programmatically is fine.)
10. If your change touches C++ (engine/src/), rebuild via dev-rebuild.bat at repo root — never engine/build.cmd directly (you cannot reliably tell whether the app is running; the Node server auto-respawns ace-server on crash, index.ts:284-309 → infinite respawn + file-lock loop). Never cmake --clean-first (20+ min CUDA recompile); for stale .obj issues delete only engine/build/acestep-core.dir/ and engine/build/Release/acestep-core.lib.
11. Never delete generated audio or test outputs, even ones you predict are bad — the human verifies results by ear. Leave all artifacts in place unless the user explicitly asks for cleanup.

Dev loop

1. Start dev mode: d:\Ace-Step-Latest\hot-step-cpp\dev.bat — runs Vite (UI, :3000, HMR) plus server\restart-loop.cmd, which loops npx tsx watch src/index.ts (restart-loop.cmd:6). Prod mode is LAUNCH.bat (Node :3001 serving prebuilt ui/dist/).
2. Save any server/src/**/*.ts file → tsx watch restarts the whole Node process, which kills and respawns ace-server too (it's a child process). Expect a few seconds of engine downtime after every save; a generation in flight will die.
3. The restart loop re-enters if a .restart-requested marker file exists at repo root — this is how the in-app "restart server" works without killing Vite.
4. Type-check before committing: cd d:\Ace-Step-Latest\hot-step-cpp\server; npx tsc --noEmit.
5. Logs land in logs/<YYYY-MM-DD_HH-MM-SS>/ at repo root: node_console.log (server), ace_engine.log (engine), generations/gen_<jobId>_<task>.log (per generation). Newest folder = current session.

Procedure: add a new feature (route + service)

1. Create the route file server/src/routes/myFeature.ts. Convention: open with a comment block stating the mount point and a route table (best example: seeds.ts:1-20).

   // myFeature.ts — one-line purpose
   // Mounts at: /api/my-feature
   // Routes:
   //   GET  /api/my-feature        — list things
   import { Router } from 'express';
   import { config } from '../config.js';   // note the .js extension
   
   const router = Router();
   
   router.get('/', (req, res) => {
     try {
       res.json({ ok: true });
     } catch (err: any) {
       console.error('[MyFeature] list failed:', err.message);
       res.status(500).json({ error: err.message });
     }
   });
   
   export default router;
   

2. Register it in server/src/index.ts — two edits: add the import to the block at index.ts:21-44 (import myFeatureRoutes from './routes/myFeature.js';) and mount it in the block at index.ts:72-95 (app.use('/api/my-feature', myFeatureRoutes);). URL segments are kebab-case (/api/model-manager, /api/cover-art, /api/stem-studio). One legacy exception: songBuilder.ts mounts at /api/builder.
3. Put non-trivial logic in a service: server/src/services/myFeature.ts. When a feature grows, use a sub-folder — services/generation/ holds translateParams.ts, lmCache.ts, postProcessing.ts, sourceAudio.ts, etc., all extracted from the once-monolithic generate.ts.
4. Auth-scope if the data is per-user: import { getUserId } from './auth.js'; then

   const userId = getUserId(req);
   if (!userId) { res.status(401).json({ error: 'Unauthorized' }); return; }
   

   (exact pattern: songs.ts:19-20). Auth is deliberately lightweight: an in-memory Map<token, userId> that resets on restart, with a single auto-created local user "Producer" (auth.ts:13-26). getUserId is auth.ts:98-102.
5. Express 5 handler style: end early with res.status(...).json(...); return; — do not return res... from typed handlers. The SPA fallback uses Express-5 wildcard syntax app.get('/{*splat}', ...) (index.ts:136) — Express 4 '*' patterns do not work.
6. Log with a bracketed feature tag — console.log('[MyFeature] ...'). Existing tags: [Server], [DB], [Config], [Generate], [Settings], [Logger], [ace-server]. This convention is what makes node_console.log greppable. To also surface lines in the UI's live log panel, import { pushLog } from './logs.js'; pushLog('text', 'server');.
7. Type-check (npx tsc --noEmit), test via the running dev server, then commit explicit paths:

   cd d:\Ace-Step-Latest\hot-step-cpp; git add server/src/routes/myFeature.ts server/src/services/myFeature.ts server/src/index.ts; git commit -m "feat(server): my feature"
   

Not every feature needs SQLite. routes/seeds.ts stores JSON files under server/data/seeds/ on purpose (ComfyUI-compatible, import/export-friendly) with a filename-sanitization regex at seeds.ts:44. Use seeds.ts as the template for small self-contained features; use generate.ts for engine-orchestrating ones.

Procedure: SQLite schema change

All DB work lives in server/src/db/database.ts. One connection, synchronous better-sqlite3 API, module singleton (initDb() once at startup, getDb() everywhere else, closeDb() on shutdown at index.ts:564). Pragmas at open: WAL, synchronous = NORMAL, foreign_keys = ON (database.ts:32-34).

1. New table: add a CREATE TABLE IF NOT EXISTS block inside the big db.exec(...) at database.ts:37-209. Schema is re-run idempotently at every startup — there is no migration framework and no version table.
2. New column on an existing table: add an entry to the checked-migration list (the songsMigrations array pattern, database.ts:213-259):

   {
     check: `SELECT COUNT(*) as c FROM pragma_table_info('songs') WHERE name='my_col'`,
     alter: `ALTER TABLE songs ADD COLUMN my_col TEXT DEFAULT ''`,
   },
   

   The loop runs ALTER only when the column is absent and logs [DB] Migration: .... Use this idiom for new columns. (A second, older idiom — try { db.exec("ALTER TABLE ...") } catch {} — exists for the lireek tables at database.ts:262-278; don't add to it.)
3. Column conventions: TEXT primary keys are UUIDs (uuidv4()) for core tables; lireek (Lyric Studio) tables use INTEGER PRIMARY KEY AUTOINCREMENT; timestamps TEXT DEFAULT (datetime('now')); booleans are INTEGER 0/1, re-hydrated as !!row.is_public (songs.ts:38); complex values are JSON-in-TEXT parsed with JSON.parse(s.tags || '[]').
4. Query style: inline getDb().prepare(sql).get/all/run(...params) with ? placeholders — no statement-caching layer. Dynamic filters append to the SQL string and a params array (songs.ts:24-32), including JSON queries like json_extract(generation_params, '$.source') = ?.
5. Don't imitate the two-database pattern. db/lireekDb.ts is a query-helper module over the same connection; its initLireekDb/closeLireekDb are deprecated no-ops (lireekDb.ts:12-23). The old separate lireek.db was consolidated into server/data/hotstep.db (one-time ATTACH DATABASE migration at database.ts:293-386, a good reference if you ever need bulk data migration).

Procedure: add a config / .env setting

server/src/config.ts builds one exported config object (config.ts:96-257) grouping aceServer (engine exe autodetected from 4 candidate paths at config.ts:46-52; port default 8085), server (port 3001), data (getter dbPath → server/data/hotstep.db; DATA_DIR in .env resolves relative to server/, NOT the repo root — config.ts:192. A stale legacy data/ dir may exist at repo root with an outdated hotstep.db; ignore it), lireek (LLM keys/endpoints), vst, whisper, essentia. .env lives at repo root and is auto-bootstrapped from .env.example on first launch (config.ts:22-30).

To expose a new env setting in the Settings UI, you must touch three places in config.ts or it silently won't work:

1. Add the key to the EXPOSED_ENV_KEYS whitelist (config.ts:265-286) — "nothing else leaks" to the UI.
2. If it only takes effect at process/engine spawn time, also add it to RESTART_REQUIRED_KEYS (config.ts:289-294) so the UI shows "restart required".
3. Add an apply('MY_KEY', setter, getter) line inside reloadEnvConfig() (config.ts:300-395) — without this the value saves to .env but never hot-patches the live config.

The Settings route (POST /api/settings/env, settings.ts:109-174 — note: the header comment says PUT, the code is POST; code wins) rewrites .env preserving comments and line endings, calls reloadEnvConfig(), and returns restartRequired.

Procedure: call the engine (aceClient)

server/src/services/aceClient.ts is the typed wrapper over ace-server's HTTP API (BASE = config.aceServer.url, i.e. http://127.0.0.1:8085 by default).

Critical constraint (aceClient.ts:6-8, verbatim comment): "ace-server uses single-threaded httplib. During heavy compute (DiT generation, adapter merge, VAE decode) it cannot respond to HTTP requests." Hence three timeout tiers via AbortSignal.timeout (aceClient.ts:17-19) — pick one deliberately for any new call:

Tier	Value	For
TIMEOUT_QUICK	15 s	health, props, job submit
TIMEOUT_POLL	30 s	job polling — fail fast, let the watchdog decide
TIMEOUT_RESULT	300 s	fetching large audio bodies

• Async-job pattern: submitLm / submitSynth / submitUnderstand / warm POST and return a job id string; poll with pollJob(id) → { status: 'running'|'done'|'failed'|'cancelled', phase?, phase_step?, phase_total? } (phases like adapter_precompute, dit_inference — aceClient.ts:165-180); fetch the result with getJobResult(id) (GET /job?id=N&result=1); cancel with cancelJob(id). Sync endpoints (/spectral-lifter, /pp-vae-reencode) return processed WAV bytes directly.
• A poll timeout is NOT a failure. The engine may simply be busy mid-DiT-step. Catch transient poll errors and retry; only treat explicit failed/cancelled statuses as terminal (see generate.ts:158-166).
• New engine parameter? Add the field to the AceRequest interface first (aceClient.ts:38-151), then plumb it through services/generation/translateParams.ts (UI params → AceRequest). Known gotcha (institutional, from the LM-echo bug): server-only fields do not survive the engine's /lm round trip — synth requests are rebuilt from the original aceReq plus only the LM-generated fields (audio_codes, caption, lyrics, bpm, duration, keyscale, timesignature — generate.ts:237-246). Never assume a field you sent to /lm comes back.
• File upload to the engine: multipart is hand-rolled with a manual boundary because the engine's parser expects a request JSON part plus named binary parts (audio, ref_audio, src_latents, ref_latents, seed_latents). The multipart path takes a single JSON object, not an array (aceClient.ts:374-378). Copy submitSynthMultipart (aceClient.ts:347-424).
• Error convention: non-OK responses throw new Error('ace-server POST /x failed (status): body') (aceClient.ts:266-270); soft-fail helpers return null/false (getJobLatent, isReachable).
• Readiness gate: routes that hit the engine should return 503 while the engine is booting. import { engineReady, engineBootStatus } from '../engineState.js'; — this module exists solely to break a circular import between index.ts and routes (engineState.ts:1-5). Pattern: generate.ts:1356-1363.

Worked example (dissected): the generation feature

routes/generate.ts (~1600 lines) is the canonical "server orchestrates engine" feature. Copy its skeleton for anything long-running.

1. In-memory job map — interface GenerationJob (id = uuid, userId, status pending|lm_running|synth_running|saving|succeeded|failed|cancelled, stage/progress for the UI, aceJobId, acePhase, result, error, params) in const jobs = new Map() (generate.ts:41-79). A TTL sweeper prunes terminal jobs older than 1 h every 10 min, with .unref() so the interval never blocks process exit (generate.ts:83-94).
2. POST /api/generate (generate.ts:1355-1388): 503 if !engineReady; 401 if no user; build the job object from req.body; jobs.set(...); enqueueGeneration(job); respond { jobId, status } immediately.
3. Serialized queue + retry (generate.ts:1286-1352): a plain closure array + generationRunning boolean — one generation at a time (the in-code comment explains why: subscribeLines() is a global log pub/sub with no job tagging, so concurrent jobs would cross-contaminate progress). One retry on transient failure with a fresh random seed; Cancelled/Unauthorized are non-retryable.
4. Pipeline (runGeneration, generate.ts:173 onward): translateParams(job.params) → aceReq; write the resolved seed back into job.params so the DB stores the actual seed used (reproducibility, generate.ts:193-197); decide whether to skip the LM phase (explicit skipLm, audio_codes already present, or a cover-type task — generate.ts:207-210); startGenerationLog(job.id, taskType) + logGenerationParams(job.id, aceReq); check the LM cache (services/generation/lmCache.ts, keyed by seed+params); submit via aceClient.submitSynth or, when any source/reference audio or latents are present, submitSynthMultipart (generate.ts:814-827); stage timings via a small timed() helper.
5. Watchdog polling (pollUntilDone, generate.ts:102-170): 500 ms interval; honors an AbortController for cancellation; stall detection — no stage/progress change for 120 s → cancel the engine job and throw (generate.ts:126-134); wall-clock timeout clamped to 5–120 min, default 45 (generate.ts:104-106); transient poll errors are logged and retried.
6. Persist: on success, INSERT INTO songs (...) with a fresh uuid, generation_params = JSON.stringify(job.params) (generate.ts:1150-1161); audio files go under config.data.audioDir and are served statically at /audio/* (index.ts:98).
7. Companion endpoints: GET /status/:id returns { jobId, status, stage, progress, result, error, ace_job_id, ace_phase, ace_phase_progress } (generate.ts:1391-1406); POST /cancel/:id sets status, cancels the engine job, fires the AbortController; plus /cancel-all, /queue, /reset-queue, and GET /stream/:id (SSE previews).

Per-generation logging (services/logger.ts): startGenerationLog(jobId, taskType) → logGeneration(jobId, 'INFO'|'DEBUG'|'WARNING'|'ERROR', msg) / logGenerationParams(jobId, obj) → finishGenerationLog(jobId, taskType) or failGenerationLog(jobId, error, taskType). Lines are buffered in memory and flushed to logs/<session>/generations/gen_<jobId>_<task>.log only on completion (logger.ts:95-178) — a crash before finish loses the buffer, but every line is also streamed live via pushLog.

Key files

Path	Role
server/src/index.ts	App entry: route registration (imports :21-44, mounts :72-95), engine spawn/respawn (:158-316), graceful shutdown (:538-572)
server/src/config.ts	All configuration; .env hot-reload (EXPOSED_ENV_KEYS / RESTART_REQUIRED_KEYS / reloadEnvConfig)
server/src/db/database.ts	SQLite singleton, schema (CREATE TABLE IF NOT EXISTS), column migrations
server/src/db/lireekDb.ts	Lyric-Studio query helpers over the same connection (init/close are deprecated no-ops)
server/src/services/aceClient.ts	Typed HTTP client for the engine; AceRequest; timeout tiers; multipart
server/src/engineState.ts	engineReady / engineBootStatus flags (circular-import breaker)
server/src/routes/generate.ts	Worked example: job map, queue, watchdog, persistence
server/src/services/generation/translateParams.ts	UI params → AceRequest translation
server/src/routes/seeds.ts	Template for a small self-contained feature (filesystem storage, route-table header)
server/src/routes/songs.ts	Template for auth-scoped SQLite CRUD (incl. the static-before-/:id warning)
server/src/routes/auth.ts	Token-lite auth; getUserId(req) helper
server/src/services/logger.ts	Session log dirs, logEngine, per-generation buffered logs
server/src/routes/logs.ts	2000-line ring buffer + SSE at GET /api/logs; pushLog / subscribeLines
server/src/routes/settings.ts	.env read/rewrite + hot-reload endpoint (POST /api/settings/env)
server/restart-loop.cmd	Dev-mode tsx watch wrapper with .restart-requested re-entry
server/package.json	Scripts (dev/typecheck), "type": "module", Node <24 engines pin

Failure signatures

Symptom	Cause	Fix
Database not initialized. Call initDb() first.	A module ran a DB query at import time, before index.ts:60	Move queries inside handlers/functions
Engine requests time out during generation	Normal — single-threaded httplib engine can't respond mid-compute	Use the right timeout tier; retry transient poll errors; never treat a poll timeout as job failure
Generation stalled — no progress for Ns	The 120 s watchdog fired and cancelled a wedged engine job	Check gen_*.log + ace_engine.log in the newest logs/ session; the queue is unblocked automatically
[ace-server] Crashed 3 times within 30s — giving up	Missing engine DLL / broken build; crash limiter halts respawn and sets engine not-ready (index.ts:296-301)	Fix the engine build (via dev-rebuild.bat); check ace_engine.log
New env setting saves but never takes effect	Key missing from the apply(...) list in reloadEnvConfig() or from EXPOSED_ENV_KEYS	Touch all three places in config.ts (see procedure above)
Setting saved, hot-reload reports OK, engine ignores it	Key is spawn-time (in RESTART_REQUIRED_KEYS)	Restart the app so the engine child respawns with new args
GET /api/things/recent → 404 with id "recent"	Static route declared after /:id	Reorder — static routes first (songs.ts:47)
better-sqlite3 native/ABI error after npm install or Node switch	Node 24+ (unsupported) or skipped native rebuild	Use Node 18–22; npm rebuild better-sqlite3
tsc errors on imports that run fine under tsx	Missing .js extension on a relative ESM import	Add .js to the import path
Spawned engine can't find DLLs despite a PATH edit	Spreading process.env on Windows creates a case-sensitive duplicate (Path vs PATH) that shadows	Find the real key case-insensitively before prepending — pattern at index.ts:221-244
Multipart submit to /synth fails with a parse error	Sent a JSON array as the multipart request part	Engine multipart expects a single object (aceClient.ts:374-378)

Institutional knowledge