New answers tagged server
This sort of requirement would be better suited to a document storage database, such as MongoDB. You will have a database connection layer that will be able to govern access control and it removes the need for username/password that you were concerned about. You'll find that most sites which provide this sort of functionality will use a document storage DB.
1.1) you can't think in term of one Thread by user. Depending of machine configuration but you could load thousands of threads but it will not scale and lose a lot of time in Thread context switch. Better think NIO Netty like framework with few incoming and outcoming Thread pool executor that perform incoming messages under milli second execution. 1.2) ...
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