Cursor Develops Continuity, a Git Storage Architecture Using S3-Backed WAL for Scalability

Continuity Architecture Overview

Cursor has introduced Continuity, a new Git storage architecture designed to scale Git operations. Unlike traditional systems that rely on replica coordination for consistency, Continuity uses an S3-backed write-ahead log (WAL) as the primary source of truth. This fundamental shift aims to improve scalability and performance for Git hosting.

Performance and Scalability

Continuity demonstrates linear read scaling, supporting up to 100 replicas in synthetic tests. Cursor reports achieving more than 300 pushes per second when utilizing S3 Express One Zone. This performance targets both large repositories with intensive continuous integration (CI) workloads and the numerous smaller repositories generated by coding agents.

Technical Implementation

In the Continuity model, pushed data is stored in S3, and corresponding reference updates are recorded in the WAL. A push is acknowledged only after the necessary data has been durably persisted. Local NVMe repositories function as warm caches, with the S3-backed WAL remaining the authoritative copy. UDP gossip propagates WAL updates, and conditional S3 reads verify replica state, with these reads averaging under 10 milliseconds.

Comparison to Existing Solutions

Existing Git hosting solutions, such as GitHub's Spokes architecture, use multiple NVMe replicas and a three-phase commit for reference updates. While this keeps replicas synchronized, coordination overhead increases with more replicas. Continuity's approach of treating Git storage like a database, with S3 as the durable source, offers a different model for handling consistency and scalability.

Industry Reception

The Continuity architecture has drawn comparisons to database systems due to its focus on push consistency, force-push transactions, and point-in-time recovery. Casey Lee, CTO at Liatrio, noted the architectural shift of using S3 as the source of truth and local NVMe as a cache, though he also mentioned that Cursor's performance figures have not been independently verified.