High-Level Design

S4 uses a three-layer architecture inspired by the Bitcask storage model.

System Layers

+------------------------------------------+
|  Layer 1: Network & Protocol (Axum)      |
|  HTTP/1.1, HTTP/2                        |
|  AWS Signature V4 Authentication         |
|  S3 API Compatibility                    |
+--------------------+---------------------+
                     |
+--------------------v---------------------+
|  Layer 2: Business Logic                 |
|  Deduplication Engine (CAS, SHA-256)     |
|  Atomic Directory Operations             |
|  IAM / ACL Policy Checks                |
+--------------------+---------------------+
                     |
+--------------------v---------------------+
|  Layer 3: Storage Engine (Bitcask-style) |
|  Hot Index: fjall (LSM, MVCC, LZ4)      |
|  Cold Data: Append-only volume files     |
|  Compactor (background GC)              |
+------------------------------------------+

Design Principles

• Single-node focus — optimized for maximum performance on one machine, no distributed consensus overhead
• Append-only writes — all data is written sequentially for maximum throughput
• ACID metadata — fjall provides crash-safe metadata storage with MVCC and atomic cross-keyspace batches
• Strict consistency — every write is fsynced before returning success
• Content-addressable — SHA-256 hashing enables automatic deduplication