Note that all publicly available information in the Internet about ICQ protocol describing legacy binary protocol called OSCAR. Our custom build system created C-functions which pack/unpack OSCAR messages Ĩ) Huge portion of core code-base was dedicated to create very scalable distributed server architecture

Here are some quick facts about ICQ backend infrastructure:ġ) ICQ servers has over 2'700'000 lines of code which is written in C and C++ Ģ) ICQ has its own TCP/IP implementation which works in the user space ģ) Concurrency based on event-driven model (epoll), each instance handles many requests semi-simultaneously Ĥ) About 60 different interconnected internal services ĥ) Each kind of backend service is a cluster of more than a hundred of instances Ħ) C core-code is written in old-school style and I liked that ħ) OSCAR protocol is defined in TCL files. I was enjoyed working with ICQ backend code-base written by AOL (although to be honest not all code was high quality). I started to work at Mail.Ru in 2010, when Mail.Ru bought ICQ from AOL. I'm former ICQ backend developer (2010 - 2014).

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