WeChat is a popular messaging app in China with 1.67 billion monthly active users. It started with 491 concurrent users and scaled up to over 400 million concurrent users. WeChat's development principle was to release first and then optimize. They used an asynchronous message queue for message delivery and restricted the number of people in a group chat to 500. WeChat's architecture includes an Access Layer, Logic Layer, and Storage Layer. They use C++ for their servers and communicate between services using Svrkit. The storage layer includes databases like MySQL and SDB, with Memcached for data access efficiency. WeChat also implemented the KVSvr algorithm on top of MySQL and SDB. They have multiple data centers, including Shanghai, Hong Kong, and Canada, and synchronize data between them using a Quorum-based queue
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