How get a data without polling?

Question

This is more of a theorical question.

Well, imagine that I have two programas that work simultaneously, the main one only do something when he receives a flag marked with true from a secondary program. So, this main program has a function that will keep asking to the secondary for the value of the flag, and when it gets true, it will do something.

What I learned at college is that the polling is the simplest way of doing that. But when I started working as an developer, coworkers told me that this method generate some overhead or it's waste of computation, by asking every certain amount of time for a value.

I tried to come up with some ideas for doing this in a different way, searched on the internet for something like this, but didn't found a useful way about how to do this.

I read about interruptions and passive ways that can cause the main program to get that data only if was informed by the secondary program. But how this happen? The main program will need a function to check for interruption right? So it will not end the same way as before?

What could I do differently?

Answer 1

There is no magic... no program will guess when it has new information to be read, what you can do is decide between two approaches,

A -> asks -> B  
A <- is informed <- B

whenever use each? it depends in many other factors like:

1- how fast you need the data be delivered from the moment it is generated? as far as possible? or keep a while and acumulate
2- how fast the data is generated?
3- how many simoultaneuos clients are requesting data at same server
4- what type of data you deal with? persistent? fast-changing?

If you are building something like a stocks analyzer where you need to ask the price of stocks everysecond (and it will change also everysecond) the approach you mentioned may be the best

if you are writing a chat based app like whatsapp where you need to check if there is some new message to the client and most of time wont... publish subscribe may be the best

but all of this is a very superficial look into a high impact architecture decision, it is not possible to get the best by just looking one factor

what i want to show is that

coworkers told me that this method generate some overhead or it's waste of computation

it is not a right statement, it may be in some particular scenario but overhead will always exist in distributed systems