Please do not associate your comment with "just getting old". This sort of comment unnecessarily perpetuates negative stereotypes in the industry.
And to answer your question, crossbar is an implementation of WAMP (Web Application Messaging Protocol) that is built on top of web sockets. If you are familiar with 1990s technology, think TIBCO for the web.
We used it at a startup in the mid 2010s, and it was great for real time communication between clients (browsers) and servers. Never had any issues.
Once upon a time I used WAMP as an IPC and routed RPC protocol between a bunch of _desktop_ processes. Like dbus but not awful to use, and naturally cross platform (because it's over TCP).
And to answer your question, crossbar is an implementation of WAMP (Web Application Messaging Protocol) that is built on top of web sockets. If you are familiar with 1990s technology, think TIBCO for the web.
We used it at a startup in the mid 2010s, and it was great for real time communication between clients (browsers) and servers. Never had any issues.