ARCHITECTURE.md

Last updated: 2026-07-02 · Revision: 2

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 for development workflow. See also docs/architecture 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 (zero-copy I/O, buffer pool, direct I/O)
              → 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/             # Zero-copy I/O, scheduling, buffer pool
│   ├── 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/                    # Design documents and analysis

Main Crate Layers (rustfs/src/)

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

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

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/.

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 have zero internal dependencies. config, credentials, crypto, io-metrics, and madmin should depend only on external crates.

   • ✅ 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 (4 copies), LastMinuteLatency (3 copies), BackpressureConfig (3 copies), DataUsageInfo (2 copies).

4. ecstore does not know about HTTP or S3 protocol details. It operates on storage-level abstractions (objects, buckets, disks, pools).

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).

   • ⚠️ VIOLATED: 6 crates use pub enum Error; 2 crates use snafu; heal use anyhow in library code.

Known Structural Issues

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

Critical

• common/scanner code duplication (~3K lines). scanner depends on common but maintains its own copies of DataUsageInfo, LastMinuteLatency, and related types instead of importing them.

• ecstore is a monolith (87K lines, 163 files). 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.

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 BackpressureConfig/DeadlockConfig duplication across io-core, concurrency, and rustfs/src/storage. Storage policies now 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

• Inconsistent error handling. Three strategies (thiserror/snafu/anyhow) and mixed naming (bare Error vs descriptive names).

• Ambiguous common vs utils boundary. Both described as "utilities and data structures." Need clear ownership rules.

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."

// 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, 75K lines)
                            │  main   │
                            └────┬────┘
                                 │
                 ┌───────────────┼───────────────┐
                 │               │               │
            ┌────▼────┐     ┌────▼────┐   ┌──────▼─────┐
            │ server  │     │  admin  │   │    app     │
            │ (HTTP)  │     │(console)│   │(use-cases) │
            └────┬────┘     └────┬────┘   └──────┬─────┘
                 │               │               │
                 └───────────────┼───────────────┘
                                 │
                          ┌──────▼──────┐
                          │   storage   │
                          │ (ecfs, SSE, │
                          │  RPC, ACL)  │
                          └──────┬──────┘
                                 │
              ┌──────────────────┼──────────────────┐
              │                  │                  │
        ┌─────▼──────┐    ┌──────▼──────┐    ┌──────▼──────┐
        │  ecstore   │    │     rio     │    │   io-core   │
        │ (87K,core) │    │  (readers)  │    │ (zero-copy) │
        └─────┬──────┘    └─────────────┘    └─────────────┘
              │
     ┌─────┬──┼──┬─────┬──────┐
     │     │  │  │     │      │
 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 writes

• "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, 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 and rust-analyzer's architecture.md.