Os Rust Backend

Open Software backend (Rust)

This skill captures the house style used by os-accounts and os-platform (the fellow codebase). Both repositories were built independently and converged on the same shape — that convergence is not coincidence, it is the recipe. Follow it when starting a new service or extending one of these two, and your code will look at home in either repo.

The recipe in one sentence: a Cargo workspace of small, pointed crates where the domain doesn't know about HTTP, services don't know about Postgres, the API doesn't know about Privy or Stripe, and the binary wires it all together.

Read this file, then open the reference that matches the work.

Route the request first

Mental model: the dependency cone

                              app/main.rs
                              │ (only the binary knows everyone)
        ┌─────────────────────┼─────────────────────┐
        ▼                     ▼                     ▼
     accounts-api      accounts-services     accounts-providers
        │                  │                       │
        └────────┬─────────┴──────────┬────────────┘
                 ▼                    ▼
          accounts-persistence    accounts-domain  ◀── traits live here
                 │                    ▲
                 └────────────────────┘
                          ▲
                 accounts-config (shared types only)

Read the arrows as "depends on". The shape gives you four properties:

1. domain is pure. No HTTP, no DB, no reqwest. It defines value types, IDs, errors, and the traits (IdentityProvider, PaymentGateway, …) — the ports the rest of the world plugs into.
2. persistence and providers know nothing about each other. Both depend on domain so they can implement domain traits and return domain types, but they don't talk through anything except domain. This means you can rewrite Stripe → Adyen without touching the database layer, and you can swap Postgres → CockroachDB without touching Privy.
3. services is the orchestrator. This is where business logic lives — the things that read from persistence, call providers, return domain results. The API layer should be thin; if a handler has branching business logic, that logic belongs in services.
4. app/main.rs is the only place that names concrete provider impls. accounts_providers::ConfiguredIdentityProvider::from_config(...), then Arc::new(...) as Arc<dyn IdentityProvider> into the router state. The rest of the code only sees the trait.

If you find yourself adding sqlx to accounts-providers, or reqwest to accounts-persistence, or axum to accounts-domain — stop. The shape is wrong; the change probably belongs in a different crate.

Golden rules (and why they matter)

1. One service-prefix, every crate. accounts-api, accounts-domain, etc. for an "accounts" service; fellow-api, fellow-domain, etc. for the os-platform service. The prefix is the service identity — it is how a reader recognizes which workspace they are in.
2. The binary crate's package name is the bare service noun. accounts, fellow. The library crates carry the prefix; the binary doesn't, because cargo install accounts reads more naturally than cargo install accounts-app.
3. Edition 2024 + pinned toolchain. The toolchain channel lives in rust-toolchain.toml; the MSRV (rust-version) lives in the workspace [workspace.package] in Cargo.toml and is mirrored by clippy.toml. Both repos pin 1.95.0 and let CI install exactly that. Don't let the toolchain float.
4. Workspace deps, not crate-local versions. Every external dep is declared in the root [workspace.dependencies] once. Member crates write serde.workspace = true. No version skew across crates, no diamond problems.
5. Workspace lints are pedantic + paranoid. clippy::pedantic at warn, unsafe_code = forbid, unwrap_used / expect_used / panic / todo / unimplemented / dbg_macro / print_stdout / print_stderr all at warn, -D warnings in CI. This is what keeps the codebase out of "Rust slop" territory — no .unwrap() sneaks in, no dbg! ships.
6. std::env::var is a clippy.toml disallowed-method. See spec/rust-config-over-env.md. Add the env knob to your *Config struct and let figment merge it.
7. Every endpoint returns ApiResponse<T>. No raw arrays at the top level, no Json<MyThing> returned directly. See api-envelope.md.
8. Error codes are application-level integers, banded by HTTP status. 1001–1999 → 404, 2001–2999 → 400, 3001–3999 → 403, 4001–4099 → 409, 4101–4199 → 410, 4201–4299 → 422, 4301–4399 → 402, 4401–4499 → 429, 5000–5099 → 500. The envelope sets the HTTP status from the band so clients can branch on either status or error_code.
9. tracing only — no println! / eprintln! / dbg!. Use structured fields: tracing::error!(%err, "connection failed"), not tracing::error!("connection failed: {}", err). The JSON formatter relies on the fields being typed.
10. Wire enums use lowercase / snake_case in both JSON and SQL. Rust enums stay PascalCase; #[serde(rename_all = "snake_case")] and #[sqlx(rename_all = "snake_case")] translate at the boundary. Every enum column gets a SQL CHECK constraint enumerating the allowed values — never an open TEXT column. See persistence.md.
11. Functions with 3+ parameters take a *Params struct. Call sites read like documentation, adding a field is non-breaking. The repos already enforce this — see spec/struct-params.md.
12. No I/O in unit tests. If your test needs Postgres, it's an integration test and lives in crates/<x>/tests/. Mock providers with wiremock, not hand-rolled fakes inside #[cfg(test)].

Scaffold checklist (new service)

If the user is starting a brand-new service in this style (not extending os-accounts or os-platform):

1. Pick the service-prefix. A short noun, lowercase, no hyphens: accounts, fellow, library, forum. Used as <prefix>-api, <prefix>-domain, etc., and as the binary's package name (bare, no prefix).
2. Create the workspace. Root Cargo.toml with resolver = "3", members = ["crates/*"], the workspace-package fields, the full [workspace.dependencies] block (copy from api/Cargo.toml in either repo and trim deps you don't need), and the workspace lints block. See workspace-layout.md for the exact template.
3. Create the seven crates under crates/: domain, services, persistence, providers, config, api, app. Only app is a [[bin]]; the others are libraries. Each crate's Cargo.toml is small — it inherits nearly everything from the workspace.
4. Add rust-toolchain.toml, rustfmt.toml, clippy.toml at the workspace root. Pin the toolchain (1.95.0 is the current default), turn on the same rustfmt knobs (edition = "2024", max_width = 100, hard_tabs = false, tab_spaces = 4), and list the disallowed env methods in clippy.toml.
5. Define the first domain type and the first trait. Don't start with the handler. Start with domain/src/lib.rs: declare the User (or whatever your first entity is), declare the first port (IdentityProvider etc.) as a #[async_trait] trait, declare a DomainError enum with thiserror.
6. Build *-config second. A single root struct (AccountsConfig, FellowConfig) with nested sections (server, database, ...), a pub fn load() -> Result<Self, ConfigError> that uses figment to merge defaults + config.toml + env, and a small test that loads it from a fixture.
7. Add the app binary stub. Parse Cli with clap (subcommands Serve, Migrate), call accounts_config::load(), init tracing_subscriber with a JSON formatter, wire axum::serve against a router from accounts-api. The binary is the only crate that depends on every other crate.
8. Add the /livez health endpoint and the ApiResponse type next. Get the envelope shape right before writing real endpoints — every handler will use it.
9. Iterate. Real endpoints, real tables, real providers — but always in the same crate boundaries. Each new endpoint: handler in api, business logic in services, queries in persistence, external calls in providers, types in domain.

Workspace deps quick-reference

These are the libraries every Open Software backend uses. Reach for these first before introducing alternatives — consistency is part of the recipe.

Common mistakes to avoid

• Cramming everything into one crate. "It's a small service, I'll just have a src/ directory." The crate split is the architectural review tool — you cannot accidentally use Stripe from inside a SQL query if *-persistence doesn't depend on *-providers. Don't skip it because it feels heavy at scale 1; you'll regret it at scale 5.
• Putting trait impls in the wrong crate. impl IdentityProvider for PrivyClient lives in *-providers, not in *-domain. The trait lives in domain; impls live with the technology they wrap.
• Returning Json<T> directly from a handler. That ships a raw payload and breaks every caller's success check. Always wrap in ApiResponse.
• Reading std::env::var inside a handler or service. Every config knob belongs in the *Config struct and flows in via ApiState. The disallowed-method lint will catch you in CI; don't paper over it with #[allow(...)].
• tracing::info!("user {} logged in", user_id) instead of tracing::info!(%user_id, "user logged in"). The first is a string — the JSON formatter can't index it. Use structured fields.
• Logging a secret. Action-token bearers (agts_…), refresh tokens, the App API key (osk_…), JWTs — none of these go in any log line, span attribute, error message, or response body. Types that hold a secret implement Debug manually and redact the field; never ?token in a tracing::*! call.
• Open TEXT enum columns. Add a CHECK constraint enumerating the values, keep the SQL value and the JSON wire value identical (lowercase / snake_case), and have the Rust enum translate via #[serde(rename_all)].
• One mega errors.rs shared by every crate. Each layer defines its own error enum (DomainError, PersistenceError, ProviderError, ServiceError) and converts at boundaries with #[from]. The API layer maps the top-level service error into an ApiResponse with the right error code.
• Arc<Mutex<DbPool>> or other "I'll just clone the world into state". sqlx::PgPool is already cheap to clone. Arc<dyn Trait> is the right shape for shared trait objects. Don't reach for Mutex unless you're guarding mutable shared in-memory state (rare — the rate-limiter is the only example in os-accounts, and it's bounded).
• Skipping integration tests because "the unit tests pass". The unit tests prove the pure logic; the integration tests prove the SQL is real, the migrations apply, the route is wired, the JSON shape is the envelope. You need both. See testing.md.
• Building a Docker image that runs as root or includes the build toolchain. Multi-stage with cargo-chef; runtime is debian:13-slim (or matching); useradd a non-root user and USER to it; copy only the release binary. See dockerfile.md.

After scaffolding: where to look next

• For wiring CI (format, clippy, tests, contract drift, GHCR image build, artifact promotion via re-tag), use the sibling skill os-rust-backend-ci. It ships templated workflows and composite actions that match this recipe. Deployment (how artifacts get to a running environment) is out of scope for both skills — that's owned by whoever runs the service.
• For integrating apps that depend on this backend (Login with Open Software, metering, etc.), see the os-accounts-integration skill.
• For the actual deployed topology (domains, BFF), see docs/deployment.md in either repo.
• For the live decisions that shaped this recipe, read docs/adr/ in either repo — particularly ADR-0001 (platform shape).