Pluggable Engine Architecture

The idea

Every ModusBrain operation goes through BrainEngine. The engine is the contract between “what the brain can do” and “how it’s stored.” Swap the engine, keep everything else. v0 shipped PostgresEngine backed by Supabase. v0.7 adds PGLiteEngine — embedded Postgres 17.5 via WASM (@electric-sql/pglite), zero-config default. The interface is designed so a DuckDBEngine, TursoEngine, or any custom backend could slot in without touching the CLI, MCP server, skills, or any consumer code.

Why this matters

Different users have different constraints:

User	Needs	Best engine
Getting started	Zero-config, no accounts, no server	PGLiteEngine (default since v0.7)
Power user (you)	World-class search, 7K+ pages, zero-ops	PostgresEngine + Supabase
Open source hacker	Single file, no server, git-friendly	PGLiteEngine
Team/enterprise	Multi-user, RLS, audit trail	PostgresEngine + self-hosted
Researcher	Analytics, bulk exports, embeddings	DuckDBEngine (someday)
Edge/mobile	Offline-first, sync later	PGLiteEngine + sync (someday)

The engine interface means we don’t have to choose. PGLite is the zero-friction default. Supabase is the production scale path. modusbrain migrate --to supabase/pglite moves between them.

The interface

// src/core/engine.ts

export interface BrainEngine {
  // Lifecycle
  connect(config: EngineConfig): Promise<void>;
  disconnect(): Promise<void>;
  initSchema(): Promise<void>;
  transaction<T>(fn: (engine: BrainEngine) => Promise<T>): Promise<T>;

  // Pages CRUD
  getPage(slug: string): Promise<Page | null>;
  putPage(slug: string, page: PageInput): Promise<Page>;
  deletePage(slug: string): Promise<void>;
  listPages(filters: PageFilters): Promise<Page[]>;

  // Search
  searchKeyword(query: string, opts?: SearchOpts): Promise<SearchResult[]>;
  searchVector(embedding: Float32Array, opts?: SearchOpts): Promise<SearchResult[]>;

  // Chunks
  upsertChunks(slug: string, chunks: ChunkInput[]): Promise<void>;
  getChunks(slug: string): Promise<Chunk[]>;

  // Links
  addLink(from: string, to: string, context?: string, linkType?: string): Promise<void>;
  removeLink(from: string, to: string): Promise<void>;
  getLinks(slug: string): Promise<Link[]>;
  getBacklinks(slug: string): Promise<Link[]>;
  traverseGraph(slug: string, depth?: number): Promise<GraphNode[]>;

  // Tags
  addTag(slug: string, tag: string): Promise<void>;
  removeTag(slug: string, tag: string): Promise<void>;
  getTags(slug: string): Promise<string[]>;

  // Timeline
  addTimelineEntry(slug: string, entry: TimelineInput): Promise<void>;
  getTimeline(slug: string, opts?: TimelineOpts): Promise<TimelineEntry[]>;

  // Raw data
  putRawData(slug: string, source: string, data: object): Promise<void>;
  getRawData(slug: string, source?: string): Promise<RawData[]>;

  // Versions
  createVersion(slug: string): Promise<PageVersion>;
  getVersions(slug: string): Promise<PageVersion[]>;
  revertToVersion(slug: string, versionId: number): Promise<void>;

  // Stats + health
  getStats(): Promise<BrainStats>;
  getHealth(): Promise<BrainHealth>;

  // Ingest log
  logIngest(entry: IngestLogInput): Promise<void>;
  getIngestLog(opts?: IngestLogOpts): Promise<IngestLogEntry[]>;

  // Config
  getConfig(key: string): Promise<string | null>;
  setConfig(key: string, value: string): Promise<void>;

  // Migration + advanced (added v0.7)
  runMigration(sql: string): Promise<void>;
  getChunksWithEmbeddings(slug: string): Promise<ChunkWithEmbedding[]>;
}

Key design choices

Slug-based API, not ID-based. Every method takes slugs, not numeric IDs. The engine resolves slugs to IDs internally. This keeps the interface portable… slugs are strings, IDs are database-specific. Embedding is NOT in the engine. The engine stores embeddings and searches by vector, but it doesn’t generate embeddings. src/core/embedding.ts handles that. This is intentional: embedding is an external API call (OpenAI), not a storage concern. All engines share the same embedding service. Chunking is NOT in the engine. Same logic. src/core/chunkers/ handles chunking. The engine stores and retrieves chunks. All engines share the same chunkers. Search returns SearchResult[], not raw rows. The engine is responsible for its own search implementation (tsvector vs FTS5, pgvector vs sqlite-vss) but must return a uniform result type. RRF fusion and dedup happen above the engine, in src/core/search/hybrid.ts. traverseGraph exists but is engine-specific. Postgres uses recursive CTEs. SQLite would use a loop with depth tracking. The interface is the same: give me a slug and max depth, return the graph.

How search works across engines

                        +-------------------+
                        |  hybrid.ts        |
                        |  (RRF fusion +    |
                        |   dedup, shared)  |
                        +--------+----------+
                                 |
                    +------------+------------+
                    |                         |
           +--------v--------+       +--------v--------+
           | engine.search   |       | engine.search   |
           |   Keyword()     |       |   Vector()      |
           +-----------------+       +-----------------+
                    |                         |
        +-----------+-----------+   +---------+---------+
        |                       |   |                   |
+-------v-------+  +-------v---+   +-------v---+  +----v--------+
| Postgres:     |  | PGLite:   |   | Postgres: |  | PGLite:     |
| tsvector +    |  | tsvector +|   | pgvector  |  | pgvector    |
| ts_rank +     |  | ts_rank   |   | HNSW      |  | HNSW        |
| websearch_to_ |  | (same SQL)|   | cosine    |  | cosine      |
| tsquery       |  |           |   |           |  | (same SQL)  |
+---------------+  +-----------+   +-----------+  +-------------+

RRF fusion, multi-query expansion, and 4-layer dedup are engine-agnostic. They operate on SearchResult[] arrays. Only the raw keyword and vector searches are engine-specific.

PostgresEngine (v0, ships)

Dependencies: postgres (porsager/postgres), pgvector Postgres-specific features used:

tsvector + GIN index for full-text search with ts_rank weighting
pgvector HNSW index for cosine similarity vector search
pg_trgm + GIN for fuzzy slug resolution
Recursive CTEs for graph traversal
Trigger-based search_vector (spans pages + timeline_entries)
JSONB for frontmatter with GIN index
Connection pooling via Supabase Supavisor (port 6543)

Hosting: Supabase Pro ($25/mo). Zero-ops. Managed Postgres with pgvector built in. Why not self-hosted for v0: The brain should be infrastructure agents use, not something you maintain. Self-hosted Postgres with Docker is a welcome community PR, but v0 optimizes for zero ops.

PGLiteEngine (v0.7, ships)

Dependencies: @electric-sql/pglite (v0.4.4+) What it is: Embedded Postgres 17.5 compiled to WASM via ElectricSQL’s PGLite. Runs in-process, no server, no Docker, no accounts. Same SQL as PostgresEngine — not a separate dialect. All 37 BrainEngine methods implemented. PGLite-specific details:

Uses pglite-schema.ts for DDL (pgvector extension, pg_trgm, triggers, indexes)
Parameterized queries throughout (shared utilities in src/core/utils.ts)
hybridSearch keyword-only fallback when OPENAI_API_KEY is not set
Data stored at ~/.modusbrain/brain.db (configurable)
pgvector HNSW index for cosine similarity vector search (same as Postgres)
tsvector + ts_rank for full-text search (same as Postgres)
pg_trgm for fuzzy slug resolution (same as Postgres)

When to use PGLite vs Postgres:

Factor	PGLite	PostgresEngine + Supabase
Setup	`modusbrain init` (zero-config)	Account + connection string
Scale	Good for < 1,000 files	Production-proven at 10K+
Multi-device	Single machine only	Any device via remote MCP
Cost	Free	Supabase Pro ($25/mo)
Concurrency	Single process	Connection pooling
Backups	Manual (file copy)	Managed by Supabase

Migration: modusbrain migrate --to supabase exports everything (pages, chunks, embeddings, links, tags, timeline) and imports into Supabase. modusbrain migrate --to pglite goes the other direction. Bidirectional, lossless.

JSONB writes: never double-encode (the #2339 trap)

Writing a JS value into a jsonb column has exactly two correct forms. Get this wrong and the write succeeds on PGLite but stores a jsonb string scalar on real Postgres — col ->> 'k' returns NULL, jsonb_array_elements throws, and a jsonb_typeof = 'array' CHECK rejects the row (this aborted every sync in #2339).

Form	Verdict
Template tag: sql`... ${sql.json(obj)}` (postgres-engine only)	✅ native jsonb serialization
Positional raw call, raw object: `executeRawJsonb(engine, sql, scalars, [obj])`	✅ object reaches the wire as jsonb
Positional raw call, stringified: `executeRaw(\`… $N::text::jsonb`, [JSON.stringify(x)])`	✅ binds as text, the cast parses it
Positional raw call, BARE cast: `executeRaw(\`… $N::jsonb`, [JSON.stringify(x)])`	❌ double-encodes under postgres.js `.unsafe()`
Template literal interpolation: `... ${JSON.stringify(x)}::jsonb`	❌ double-encodes

Why: postgres.js .unsafe(sql, params) (the path behind executeRaw / executeRawDirect) binds a JS string as a text param. A bare $N::jsonb cast then wraps that already-JSON string into a jsonb scalar string instead of parsing it. Casting through $N::text::jsonb forces a text→jsonb parse. PGLite’s db.query parses text→jsonb natively, so it hides the bug — which is why a regression only shows up on Postgres (and why the parity test must run there). Two CI guards enforce this, both wired into scripts/check-jsonb-pattern.sh:

the template-tag grep (${JSON.stringify(x)}::jsonb), and
scripts/check-jsonb-params.mjs, an AST-lite scanner for the positional $N::jsonb + JSON.stringify form the grep misses. Sanctioned escapes: $N::text::jsonb, $N::text[], executeRawJsonb, sql.json, or an inline jsonb-guard-ok comment.

The real backstop is test/e2e/op-checkpoint-jsonb-parity.test.ts + test/e2e/jsonb-roundtrip.test.ts, which round-trip writes through real Postgres and assert jsonb_typeof — the assertion PGLite cannot make.

Adding a new engine

Create src/core/<name>-engine.ts implementing BrainEngine

Add to engine factory in src/core/engine-factory.ts:

export function createEngine(type: string): BrainEngine {
  switch (type) {
    case 'pglite': return new PGLiteEngine();
    case 'postgres': return new PostgresEngine();
    case 'myengine': return new MyEngine();
    default: throw new Error(`Unknown engine: ${type}`);
  }
}

The factory uses dynamic imports so engines are only loaded when selected.

Store engine type in ~/.modusbrain/config.json: { "engine": "myengine", ... }
Add tests. The test suite should be engine-agnostic where possible… same test cases, different engine constructor.
Document in this file + add a design doc in docs/

What you DON’T need to touch

src/cli.ts (dispatches to engine, doesn’t know which one)
src/mcp/server.ts (same)
src/core/chunkers/* (shared across engines)
src/core/embedding.ts (shared across engines)
src/core/search/hybrid.ts, expansion.ts, dedup.ts (shared, operate on SearchResult[])
skills/* (fat markdown, engine-agnostic)

What you DO need to implement

Every method in BrainEngine. The full interface. No optional methods, no feature flags. If your engine can’t do vector search (e.g., a pure-text engine), implement searchVector to return [] and document the limitation.

Capability matrix

Capability	PostgresEngine	PGLiteEngine	Notes
CRUD	Full	Full	Same SQL
Keyword search	tsvector + ts_rank	tsvector + ts_rank	Identical (real Postgres)
Vector search	pgvector HNSW	pgvector HNSW	Identical (real Postgres)
Fuzzy slug	pg_trgm	pg_trgm	Identical (real Postgres)
Graph traversal	Recursive CTE	Recursive CTE	Same SQL
Transactions	Full ACID	Full ACID	Both support this
JSONB queries	GIN index	GIN index	Identical
Concurrent access	Connection pooling	Single process	PGLite limitation
Hosting	Supabase, self-hosted, Docker	Local file
Migration methods	runMigration, getChunksWithEmbeddings	Same	Added v0.7

Future engine ideas

TursoEngine. libSQL (SQLite fork) with embedded replicas and HTTP edge access. Would give SQLite’s simplicity with cloud sync. Interesting for mobile/edge use cases. DuckDBEngine. Analytical workloads. Bulk exports, embedding analysis, brain-wide statistics. Not for OLTP. Could be a secondary engine for analytics alongside Postgres for operations. Custom/Remote. The interface is clean enough that someone could build an engine backed by any storage: Firestore, DynamoDB, a REST API, even a flat file system. The interface doesn’t assume SQL. Note: The original SQLite engine plan (docs/SQLITE_ENGINE.md) was superseded by PGLite. PGLite uses the same SQL as Postgres, eliminating the need for a separate SQLite dialect with FTS5/sqlite-vss translation.

CLI & Core Reference

Evals & Metrics

Database & Schemas

Schema Development

AI Providers

Incidents & Issues

Migrations & Plans

Internal Designs

Ethos & Origin

ENGINES

Pluggable Engine Architecture

The idea

Why this matters

The interface

Key design choices

How search works across engines

PostgresEngine (v0, ships)

PGLiteEngine (v0.7, ships)

JSONB writes: never double-encode (the #2339 trap)

Adding a new engine

What you DON’T need to touch

What you DO need to implement

Capability matrix

Future engine ideas

​Pluggable Engine Architecture

​The idea

​Why this matters

​The interface

​Key design choices

​How search works across engines

​PostgresEngine (v0, ships)

​PGLiteEngine (v0.7, ships)

​JSONB writes: never double-encode (the #2339 trap)

​Adding a new engine

​What you DON’T need to touch

​What you DO need to implement

​Capability matrix

​Future engine ideas

Pluggable Engine Architecture

The idea

Why this matters

The interface

Key design choices

How search works across engines

PostgresEngine (v0, ships)

PGLiteEngine (v0.7, ships)

JSONB writes: never double-encode (the #2339 trap)

Adding a new engine

What you DON’T need to touch

What you DO need to implement

Capability matrix

Future engine ideas