How Youku Scaled to 8900 M Daily Users: Front‑End, DB & Caching Architecture

1. Website Basic Data Overview

According to 2010 statistics, Youku had a daily average of 89 million unique visitors (UV) and 1.7 billion page views (PV), making it the top-ranked Chinese video site on Google’s list. The hardware primarily consisted of Dell PowerEdge 1950 and 860 servers and Dell MD1000 storage arrays, with over 1,000 servers deployed nationwide by 2007.

2. Front‑End Framework

Youku built a custom CMS to handle page rendering, separating modules for high extensibility and simplifying development and maintenance. The module‑method‑params calling model is illustrated in the following diagrams.

3. Database Architecture

Youku’s database architecture evolved from a single MySQL instance to master‑slave replication, SSD optimization, vertical partitioning, and horizontal sharding.

3.1 Simple MySQL Master‑Slave Replication

Replication enabled read‑write separation and improved read performance.

The replication process is shown below:

However, this approach introduced bottlenecks such as write scalability limits, lack of write caching, replication lag, increased lock contention, and reduced cache hit rates.

3.2 MySQL Vertical Partitioning

By separating independent business domains onto different database servers, vertical partitioning improved load distribution and fault isolation. The resulting architecture is illustrated below.

Nevertheless, cross‑domain data (e.g., user information) still required a solution, leading to horizontal sharding.

3.3 MySQL Horizontal Sharding (Sharding)

Users are hashed by ID and assigned to specific shards, allowing the system to scale by adding servers. The sharding principle diagram is shown below.

To locate a user's shard, a lookup table maps user IDs to shard IDs, as depicted here.

Youku minimizes cross‑shard queries; when unavoidable, it employs multi‑dimensional indexes, distributed search engines, or distributed database queries, acknowledging the high complexity and performance cost.

4. Caching Strategy

Large systems heavily rely on caching, but Youku avoids in‑memory caches like Memcached to prevent memory copying, locking, and complications when removing content. It also found that Squid’s write() and Lighttpd’s asynchronous I/O introduced inefficiencies.

Instead, Youku leverages a robust Content Delivery Network (CDN) that directs users to the nearest video server, ensuring fast video loading compared to competitors.

Source: http://www.kuqin.com/system-analysis/20110918/264936.html