Problem Recently I came across a scenario at work where I had to deal with real time updates of payment status in mobile which involved an asynchronous chain of events in the backend. Mobile would call the backend to initiate a payment request, backend would do basic validation - initialize the payment and return a token to the UI. Actual payment involved multiple API calls to some of the internal systems as well as third party systems.

Why Some ISPs block TCP 22? Recently I got to know that some ISP indeed block any outbound traffic for port 22 (tcp) which is a standard port for ssh connections. (I came to know about this as I was being unable to log into one of the compute instances from my terminal via gcloud in gcp. I prefer terminal logins instead of browser as it’s simply 100 times easier to work with.

Learning from the udacity course on client server communication course link: https://classroom.udacity.com/courses/ud897 Using netcat: nc google.com 80 <head/get/post/put> headers: OPTIONS: get a list of methods supported by the URL end point HEAD: just the header and not the body.. validate if the incoming data will fit the memory, cache validation xml-http-request: xhr, traditional way of doing ajax, new way is javascript promises which are chainable callbacks and are asynchronous How head is used for cache checking.