# pgGraph Contributor Guide

This guide documents the Rust internals of the `graph` extension: crate layout,
engine ownership, memory model, build pipeline, traversal algorithms,
persistence format, sync overlays, safety boundaries, and release validation.

It is based on the current `graph/` codebase. SQL usage and function signatures
belong in the [User Guide](/user_guide).

## Crate Shape

<FileTree root="graph/" items={[
  { name: 'Cargo.toml', desc: 'pgrx extension crate, features, lints, dependencies' },
  { name: 'sql/bootstrap.sql', desc: 'extension-owned SQL bootstrap objects and catalogs' },
  { name: 'src/lib.rs', desc: 'pgrx entry point, GUC init, facade module registration, engine TLS' },
  { name: 'src/engine.rs', desc: 'Engine orchestration and store ownership' },
  { name: 'src/node_store.rs', desc: 'node struct-of-arrays and mmap backing' },
  { name: 'src/edge_store.rs', desc: 'CSR edge storage and mmap backing' },
  { name: 'src/builder.rs', desc: 'SPI cursor ingestion and graph construction' },
  { name: 'src/persistence.rs', desc: '.pggraph writer/loader and format validation' },
  { name: 'src/filter_index.rs', desc: 'typed traversal filter storage' },
  { name: 'src/resolution_index.rs', desc: 'table+primary-key resolution index' },
  { name: 'src/sql_facade/', desc: 'pgrx SQL wrapper modules for user-facing functions' },
  { name: 'src/catalog/', desc: 'catalog read/write/validation helpers' },
  { name: 'tests/heavy/', desc: 'external boundary, release, packaging, and stress tests' },
  { name: 'fuzz/', desc: 'libFuzzer targets for parser and persistence boundaries' }
]} />

## Layer Diagram

<ProcessTree title="PostgreSQL backend process">
  <ProcessStep title="pgrx SQL facade">
    - validates SQL call shape
    - enforces admin/query boundaries
    - translates SQL values to engine requests
  </ProcessStep>
  <ProcessStep title="Engine thread-local">
    - NodeStore
    - EdgeStore + reverse EdgeStore
    - ResolutionIndex
    - FilterIndex
    - sync edge overlays
    - tenant membership bitmaps
  </ProcessStep>
  <ProcessStep title="PostgreSQL SPI">
    - source-row search
    - build cursor reads
    - hydration
    - catalog reads/writes
  </ProcessStep>
</ProcessTree>

## Design Constraints

| Constraint | Implementation consequence |
|---|---|
| Runs inside PostgreSQL backends | Avoid unbounded memory, I/O, panics, or hidden background work in query hot paths |
| Graph queries must stay SQL-native | SQL facade modules own user-facing call shapes and hydration |
| Source tables are authoritative | Build, maintenance, and search read source tables through SPI |
| Traversal must be bounded | Depth, node count, and frontier circuit breakers are first-class config |
| Typed persisted views need stable bytes | Each backend snapshots the artifact into an anonymous read-only mapping before validation |
| CSR base topology is immutable | Sync edge changes use overlays until rebuild |
| PostgreSQL ACLs must still apply | Query paths check source table privileges |
| Unsafe mmap reads need proof | An owning validated artifact gives each store Arc-owned aligned ranges; safe accessors cannot outlive the mapping |

## Core Ownership Model

The active graph is held in a backend-local thread-local:

```rust
thread_local! {
    static ENGINE: RefCell<Engine> = RefCell::new(Engine::new());
}
```

Each backend process has its own `Engine` value, immutable artifact snapshot,
Rust heap structures, and sync overlays. Persisted graph files are shared, but
loaded snapshots are backend-local so external same-inode changes cannot
invalidate typed views.

## Contributing Rules Of Thumb

1. Keep SQL argument parsing and PostgreSQL error reporting in the SQL facade.
2. Keep engine methods independent of direct SQL where possible.
3. Preserve store invariants before optimizing call sites.
4. Avoid adding hot-loop string work to traversal.
5. Treat persistence format changes as compatibility changes.
6. Add SQL tests for user-facing behavior and Rust tests for store/parser
   invariants.
7. Keep unsafe contracts in rustdoc and local `// SAFETY:` comments.

## Recommended Reading Order

1. [Architecture](/contributor_guide/architecture) for crate graph, ownership, lifetimes, data flow, and design decisions.
2. [Engine Internals](/contributor_guide/engine-internals) for `Engine` fields and traversal call paths.
3. [Memory Model](/contributor_guide/memory-model) for owned vs mmap-backed vs per-backend heap state.
4. [Persistence Format](/contributor_guide/persistence-format) before changing `.pggraph` layout or loader validation.
5. [Safety And Security](/contributor_guide/safety-security) before touching unsafe code, ACLs, SQLSTATEs, or dynamic SQL.
6. [Batched Graph Mutations](/contributor_guide/batched-graph-mutations) before implementing the proposed 1.1 batch-write surface.
7. [SQL Tests](/contributor_guide/sql-tests) before adding or changing SQL-facing behavior.
8. [Scripts](/contributor_guide/scripts) before using or changing repository automation.
9. [Benchmarking](/contributor_guide/benchmarking) before interpreting benchmark or release-evidence timing.
10. [Testing And Release](/contributor_guide/testing-release) before preparing a PR or release gate.

<Callout type="warning">
This crate already enables `missing_docs`, `broken_intra_doc_links`,
`bare_urls`, `unsafe_op_in_unsafe_fn`, and `clippy::undocumented_unsafe_blocks`
checks. New public Rust APIs need contract-quality docs.
</Callout>
