# ARCHITECTURE.md

> Last updated: 2026-08-12 · Revision: 3
>
> This document describes the high-level architecture of RustFS.
> If you want to familiarize yourself with the code base, you are in the right place!
>
> See also [CONTRIBUTING.md](CONTRIBUTING.md) for development workflow.
> See also [docs/architecture](docs/architecture/overview.md) for active
> architecture migration guardrails.

## Bird's Eye View

RustFS is a high-performance, S3-compatible distributed object storage system written
in Rust. It uses erasure coding for data durability, supports multi-tenancy through
IAM/STS, and provides a web-based admin console.

A running RustFS node exposes:

- **S3 API** (port 9000) — the primary data path for object CRUD
- **Admin API** (port 9000, `/minio/` prefix) — cluster management, IAM, metrics
- **Console** (port 9001) — web UI backed by the Admin API
- **Inter-node RPC** (gRPC/tonic) — cluster communication for distributed mode

The core data flow for a PUT request looks like:

```
HTTP request
  → server (TLS, auth, routing, compression)
    → app/object_usecase (validation, policy, lifecycle)
      → storage/ecfs (erasure coding, encryption, checksums)
        → ecstore (disk pool selection, data distribution)
          → rio (reader pipeline: encrypt → compress → hash → write)
            → io-core (buffer pool, storage profiling, admission control)
              → local disk / remote disk via RPC
```

## Code Map

The repository is a Cargo workspace with a flat `crates/` layout:

```
rustfs/                      # Workspace root (virtual manifest)
├── rustfs/                  # Main binary + library crate
│   └── src/
│       ├── main.rs          # Entry point, startup sequence
│       ├── lib.rs           # Module tree root
│       ├── server/          # HTTP server, TLS, routing, middleware
│       ├── admin/           # Admin API handlers and console
│       ├── app/             # Use-case layer (object, bucket, multipart)
│       ├── storage/         # Storage engine interface and implementation
│       ├── auth.rs          # S3 request authentication
│       ├── config/          # CLI args, config parsing, workload profiles
│       └── ...
├── crates/                  # library crates (authoritative list: Cargo.toml [workspace].members)
│   ├── ecstore/             # Erasure-coded storage engine
│   ├── rio/                 # Reader I/O pipeline (encrypt, compress, hash)
│   ├── io-core/             # Buffer pool, storage profiling, backpressure/deadlock policy, lock optimizer, operation progress
│   ├── io-metrics/          # I/O metrics collection
│   ├── common/              # Shared runtime state, globals, data usage types
│   ├── config/              # Configuration types and parsing
│   ├── utils/               # Pure utility functions
│   ├── ...                  # (see "Crate Reference" below)
│   └── e2e_test/            # End-to-end integration tests
└── docs/                    # Agent knowledge base: contracts, runbooks, testing rules (index: docs/architecture/README.md)
```

### Main Crate Layers (`rustfs/src/`)

The main crate is organized in layers, top to bottom:

| Layer | Directory | Responsibility |
|-------|-----------|----------------|
| **Server** | `server/` | HTTP listener, TLS, CORS, compression, middleware, graceful shutdown |
| **Admin** | `admin/` | Admin API routing, 30+ handler modules, web console |
| **App** | `app/` | Use-case orchestration: object (per-operation modules under `app/object/`, re-exported as `object_usecase`), bucket_usecase, multipart_usecase |
| **Storage** | `storage/` | S3 API translation, erasure-coded FS, SSE encryption, RPC, concurrency |
| **Auth** | `auth.rs` | S3 signature verification, credential validation |
| **Config** | `config/` | CLI parsing, config struct, workload profiles |

A request flows **downward** through the layers. No layer should reach upward
(e.g., storage must not import from admin).

### Crate Reference

`Cargo.toml` is the authoritative workspace membership and `cargo tree` is the
authoritative dependency graph. This overview deliberately avoids line-count
and dependency-depth snapshots because both quickly become stale during
refactors.

#### By Domain

| Domain | Current workspace crates | Responsibility |
|--------|--------------------------|----------------|
| Foundation | `checksums`, `common`, `config`, `data-usage`, `heal-contracts`, `scanner-metrics`, `utils` | Shared configuration, data-usage models, heal domain contracts, scanner telemetry types, utilities, and checksums. |
| I/O and storage | `concurrency`, `ecstore`, `filemeta`, `heal`, `io-core`, `io-metrics`, `lifecycle`, `lock`, `object-capacity`, `object-data-cache`, `replication`, `rio`, `rio-v2`, `s3-client`, `scanner`, `storage-api` | Erasure-coded object storage, metadata, recovery, lifecycle, replication, locking, cache, I/O pipelines, and the engine-side S3 client for remote tier/transition targets. |
| Security and identity | `credentials`, `crypto`, `iam`, `keystone`, `kms`, `policy`, `security-governance`, `signer`, `tls-runtime`, `trusted-proxies` | Credentials, authentication, authorization, encryption, key management, TLS, and security contracts. |
| Protocols and contracts | `extension-schema`, `madmin`, `protos`, `protocols`, `s3-ops`, `s3-types`, `s3select-api`, `s3select-query` | Admin, inter-node, S3, S3 Select, and optional protocol contracts. |
| Operations and integration | `audit`, `notify`, `obs`, `targets`, `zip` | Auditing, observability, event delivery, notification targets, and archive support. |
| Test support | `e2e_test`, `test-utils` | End-to-end validation and shared test bootstrap utilities. |

The `rustfs` binary crate composes these libraries into the running server.
`ecstore` remains the storage engine at the architectural center; its internal
module split is tracked under `docs/architecture/`. `rio-v2` is the
feature-gated MinIO on-disk format compatibility I/O layer; it ships in no
default build (lifecycle:
[docs/architecture/minio-file-format-compat.md](docs/architecture/minio-file-format-compat.md)).

## Architecture Invariants

> These are rules that the codebase should follow. Some are currently violated
> (marked with ⚠️). Documenting them here makes the violations explicit and
> trackable.

1. **Layers flow downward.** Server → Admin/App → Storage → ecstore → rio/io-core.
   No upward imports.

2. **Leaf crates depend only on external crates, with adjudicated exceptions
   pinned by a guard.** `config`, `credentials`, and `crypto` take no internal
   dependency. `io-metrics` takes exactly `rustfs-s3-ops` (transitively
   `rustfs-s3-types`), a pure contract crate with no I/O and no global state —
   adjudicated in rustfs/backlog#1834. `madmin` left the leaf set when #6166 made
   it the SigV4-signed admin SDK client; its internal dependency surface is pinned
   to exactly `rustfs-signer`. Both pins live in the leaf allowlist in
   `scripts/check_architecture_migration_rules.sh`; any other internal dependency
   fails the guard ([crate boundaries](docs/architecture/crate-boundaries.md)).
   - ✅ RESOLVED: the historical `utils → config` and `common → filemeta`/`madmin`
     edges were removed; do not reintroduce them (see Known Structural Issues).

3. **Each type has exactly one definition.** Types shared across crates must be defined
   in one crate and re-exported or imported by others.
   - ⚠️ VIOLATED: `ReplicationStats` names three unrelated types
     (`crates/data-usage/src/data_usage.rs`,
     `crates/obs/src/metrics/collectors/replication.rs`,
     `crates/ecstore/src/bucket/replication/replication_state.rs`) — a naming
     collision, not copies; renaming is tracked in rustfs/backlog#1847.
   - `LastMinuteLatency` has two deliberately different implementations: the
     per-second bucketed accumulator in `crates/scanner-metrics/src/last_minute.rs` and
     the in-memory endpoint-health sample tracker in
     `crates/ecstore/src/bucket/bucket_target_sys.rs` (its doc comment explains
     why it stays local).
   - ✅ RESOLVED: `BackpressureConfig` and `DataUsageInfo` each have exactly one
     definition (`crates/io-core/src/backpressure.rs`,
     `crates/data-usage/src/data_usage.rs`). The zero-consumer
     `BackpressureSettings` copy that lingered in io-metrics was removed
     (rustfs/backlog#1833).

4. **ecstore does not *serve* HTTP or the S3 wire protocol.** It operates on
   storage-level abstractions (objects, buckets, disks, pools) and holds no
   wire or DTO types of the serving surface. *Consuming* remote S3-compatible
   endpoints (ILM tier warm backends, transition targets) is a legitimate
   engine capability, but it lives in the dedicated `rustfs-s3-client` crate
   (`crates/s3-client`, extracted from the formerly embedded
   `crates/ecstore/src/client/` by rustfs/backlog#1842), not inside ecstore.
   - ⚠️ PARTIALLY VIOLATED: serving-side `s3s` references remain in ecstore
     (bucket metadata/replication/lifecycle DTOs and error mapping). The
     count is ratcheted shrink-only by `scripts/check_s3s_footprint.sh`
     (`S3S_ECSTORE_FILES_BASELINE`; the `object_lock` module was converted to
     storage-level types as the first ratchet step). Target state: the
     baseline reaches zero and ecstore's `Cargo.toml` drops `s3s`.

5. **The `rustfs` binary crate is the only place that wires everything together.**
   Individual crates should be testable in isolation.

6. **Error types use `thiserror` with descriptive names** (e.g., `StorageError`,
   not bare `Error`).
   - ✅ RESOLVED (strategy): `snafu` is gone from source
     (`rg -l snafu crates/ rustfs/` is empty) and library code no longer uses
     `anyhow` (remaining hits are test code and the `e2e_test` crate; `heal`
     uses `thiserror`).
   - ⚠️ VIOLATED (naming): 6 crates still export a bare `pub enum Error`:
     `crypto`, `filemeta`, `heal`, `iam`, `policy`, and `replication`
     (`src/resync.rs`) — all `thiserror`-derived.

## Known Structural Issues

> This section documents known problems in the current architecture.
> It exists so the team can track and address them deliberately.

### Critical

- **scanner/data-usage duplicate `.usage-cache.bin` serialization types.** The
  original finding ("common/scanner code duplication, ~3K lines") is resolved:
  `scanner` imports the shared data-usage types from `rustfs-data-usage` (see
  the `pub use rustfs_data_usage::…` re-exports at the top of
  `crates/scanner/src/data_usage_define.rs`). What remains: `scanner` and
  `data-usage` each hold their own serialization types for the scanner cache
  file (`DataUsageCacheInfo`/`DataUsageEntryInfo` in
  `crates/scanner/src/data_usage_define.rs` vs
  `DataUsageCacheInfo`/`DataUsageEntry` in
  `crates/data-usage/src/data_usage.rs`); convergence is tracked in
  rustfs/backlog#1828.

- **ecstore is a monolith (265 files, ~288K lines — roughly half is inline
  `#[cfg(test)]` code).** Measured with
  `find crates/ecstore/src -name '*.rs' | xargs wc -l`. It contains disk
  management, bucket management, erasure coding, replication, lifecycle, RPC,
  and configuration — all in one crate. It should be decomposed along its
  existing subdirectories; the split plan lives in
  [docs/architecture/ecstore-module-split-plan.md](docs/architecture/ecstore-module-split-plan.md).

### High

- **Dependency inversions.** Historical `utils → config` and
  `common → filemeta/madmin` edges must stay removed so leaf/helper crates do
  not regain upward dependencies.

- **Three-layer backpressure/deadlock policy bridging** across io-core,
  concurrency, and `rustfs/src/storage`. The config types are no longer
  duplicated (`BackpressureConfig` and `DeadlockDetectorConfig` are each
  defined once, in io-core). Storage policies expose and consume explicit
  projections into the concurrency/io-core policy shapes, and workload
  admission snapshots are composed through provider registries; later work
  should use those bridges before deleting compatibility wrappers.

### Medium

- **Bare `Error` naming.** Error-handling strategy has converged on `thiserror`
  (no `snafu`, no `anyhow` in library code); the remaining inconsistency is the
  bare `pub enum Error` naming in the 6 crates listed under Invariant 6.

- **`common` is mostly parked domain code, not shared utilities.** Of its
  6,724 lines, ~83% is scanner/heal domain code stranded there to break
  dependency cycles (`metrics.rs`, ~4,810 lines of scanner-domain metrics;
  `heal_channel.rs`, ~776 lines of heal-domain channel types). The
  "common vs utils" naming ambiguity is secondary to moving that code to its
  domain owners.

## Cross-Cutting Concerns

### Error Handling

The project convention is `thiserror` for typed errors with descriptive names.
See `AGENTS.md`: "Prefer thiserror for library-facing error types."

```rust
// GOOD
#[derive(Debug, thiserror::Error)]
pub enum StorageError {
    #[error("disk not found: {0}")]
    DiskNotFound(String),
}

// AVOID
pub enum Error { ... }        // too generic
anyhow::Result<T>             // in library code (OK in tests/CLI)
```

### Logging & Tracing

- Use `tracing` crate (`info!`, `warn!`, `error!`, `debug!`, `trace!`)
- Structured fields: `tracing::info!(bucket = %name, "created bucket")`
- Spans for request-scoped context

### Metrics

- Prometheus-style metrics via `rustfs-obs` runtime and schema
- I/O-specific counters via `rustfs-io-metrics`
- Registration happens at crate level, collection/reporting in `rustfs-obs`

### Testing

- Unit tests: `#[cfg(test)] mod tests` in the same file
- Integration tests: inside respective crates (not top-level `tests/`)
- E2E tests: `crates/e2e_test/` — tests against a running server
- Run all: `make test` or `cargo nextest run`

## Startup Sequence

The binary (`main.rs`) boots in this order:

1. Environment variable compatibility (`MINIO_*` → `RUSTFS_*`)
2. Tokio runtime construction
3. CLI argument parsing
4. License, observability, TLS, trusted proxies initialization
5. Config parsing, server address resolution
6. Credentials, endpoints, local disks, lock client initialization
7. Capacity management initialization
8. HTTP server start (S3 API + optional console)
9. ECStore initialization (erasure coding storage engine)
10. Global config, background replication, KMS
11. Optional: FTP/FTPS/WebDAV servers
12. Event notifier, audit system, deadlock detector
13. Bucket metadata, IAM, Keystone, OIDC
14. Scanner and heal manager
15. Metrics system, mark `FullReady`
16. Wait for shutdown signal → graceful shutdown

## Dependency Diagram (Simplified)

```
                            ┌─────────┐
                            │  rustfs │  (binary + lib)
                            │  main   │
                            └────┬────┘
                                 │
                 ┌───────────────┼───────────────┐
                 │               │               │
            ┌────▼────┐     ┌────▼────┐   ┌──────▼─────┐
            │ server  │     │  admin  │   │    app     │
            │ (HTTP)  │     │(console)│   │(use-cases) │
            └────┬────┘     └────┬────┘   └──────┬─────┘
                 │               │               │
                 └───────────────┼───────────────┘
                                 │
                          ┌──────▼──────┐
                          │   storage   │
                          │ (ecfs, SSE, │
                          │  RPC, ACL)  │
                          └──────┬──────┘
                                 │
              ┌──────────────────┼──────────────────┐
              │                  │                  │
        ┌─────▼──────┐    ┌──────▼──────┐    ┌──────▼──────┐
        │  ecstore   │    │     rio     │    │   io-core   │
        │   (core)   │    │  (readers)  │    │ (buffers)   │
        └─────┬──────┘    └─────────────┘    └─────────────┘
              │
     ┌─────┬──┼──┬─────┬──────┐
     │     │  │  │     │      │
 common utils config policy filemeta ...
```

## How to Navigate

- **"Where does S3 PutObject go?"**
  `server/` routes → `app/object_usecase` validates → `storage/ecfs` encodes →
  `ecstore` distributes → `rio` encrypts/compresses → `io-core` supplies buffers

- **"Where are bucket policies enforced?"**
  `app/bucket_usecase` calls into `crates/policy/`

- **"Where is replication configured?"**
  `admin/handlers/replication.rs` and `admin/handlers/site_replication.rs` for API,
  `rustfs/src/site_replication/` for the site-replication service subsystem
  (state, peer transport, retry queue, repair, hooks),
  `ecstore/src/bucket/replication/` for engine

- **"Where do I add a new admin endpoint?"**
  Add handler in `admin/handlers/`, register in `admin/router.rs`

- **"Where do I add a new metric?"**
  Define descriptor/collector in `crates/obs/src/metrics/`, expose via `/minio/v2/metrics`

---

*Inspired by [matklad's ARCHITECTURE.md](https://matklad.github.io/2021/02/06/ARCHITECTURE.md.html)
and [rust-analyzer's architecture.md](https://github.com/rust-analyzer/rust-analyzer/blob/master/docs/book/src/contributing/architecture.md).*
