# Salsa Advanced Plumbing > Low-level and "Level 4" Salsa patterns for specialized use cases. Use when you need to avoid passing &db everywhere, need project-wide singleton inputs, want to serialize your database to disk, or need to override tracked functions for specific instances. Covers specify, singleton, attach, persistence, and synthetic writes. Use when this capability is needed. - Skill: `tomevault-io/salsa-advanced-plumbing` (Agent Skill, multi-file: 2 files) - Install (CLI): `npx skillmds@latest add tomevault-io/salsa-advanced-plumbing` - Raw SKILL.md: https://api.skillmd.com/api/skills/tomevault-io/salsa-advanced-plumbing/raw - Safety review: pending (external: skill-scanner PASS, skillspector PASS) - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Coding & Dev Tools - Author: tomevault-io (https://skillmd.com/u/tomevault-io) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/tomevault-io/salsa-advanced-plumbing --- # Salsa Advanced Plumbing: Level 4 Patterns These patterns move beyond the standard `#[salsa::...]` macros and into the low-level "engine" of Salsa. They are used by projects like `rust-analyzer` and `Cairo` to solve niche architectural problems. ## 1. `specify`: Overriding Tracked Functions Sometimes you want a tracked function to have a "default" implementation but allow certain instances to have a hardcoded or "late-bound" value. ```rust #[salsa::tracked(specify)] fn type_of(db: &dyn Db, expr: Expr) -> Type { // default: look at AST } // Elsewhere, for a built-in: let builtin = Expr::new(db, ...); type_of::specify(db, builtin, Type::Int); ``` * **Use case:** Built-in functions in a compiler, or "filling in" data that is discovered in a later phase but belongs logically to an earlier struct. ## 2. `singleton` Inputs For project-wide state that should only ever have one instance. ```rust #[salsa::input(singleton)] pub struct ProjectConfig { pub python_path: PathBuf, } // Access without passing an ID: let config = ProjectConfig::get(db); ``` * **Use case:** `Cairo` uses this for its various "Group" layers to store per-layer global configuration. ## 3. `attach`: Thread-Local Database Passing `&db` through every single method in a large system can be noisy. `attach` allows you to store the database in a thread-local for a specific scope. ```rust // In your database impl: db.attach(|db| { // Inside here, code can call `salsa::with_attached_database` // to get access to the &db without it being passed in. }); ``` * **Use case:** Implementing `Display` or `Debug` for interned types where you need the database to look up the string values but the trait doesn't provide it. ## 4. Persistence (Serde Integration) With the `persistence` feature, Salsa can serialize the entire state of the database to disk. ```rust // Serialize let bytes = db.as_serialize(); // Deserialize into a fresh DB db.deserialize(deserializer); ``` * **Use case:** `rust-analyzer` uses similar mechanisms to provide fast cold-starts by loading index data from disk instead of re-parsing the entire standard library. ## 5. Synthetic Writes & Runtime Inspection For profiling and advanced telemetry. * **`db.synthetic_write(Durability::HIGH)`**: Forces a revision bump as if a high-durability input changed. Useful for benchmarking incrementality. * **`db.ingredient_debug_name(index)`**: Resolves a low-level `IngredientIndex` to a human-readable name. * **`db.memory_usage()`**: (Unstable) Returns a `DatabaseInfo` struct with per-query and per-struct memory breakdown. ## Summary: When to use Plumbing | Feature | Use it when... | | :--- | :--- | | **`specify`** | You have "special cases" like built-ins that skip normal logic. | | **`singleton`** | You have exactly one "Project" or "Global" config. | | **`attach`** | You are trapped in a third-party trait (like `Display`) and need a DB. | | **`persistence`** | Cold-start time for your project is > 5 seconds. | | **`synthetic_write`** | You are writing a benchmark or stress-test for Salsa. | --- > Converted and distributed by [TomeVault](https://tomevault.io/claim/joshuadavidthomas) — claim your Tome and manage your conversions.