> Is Scala used exclusively for systems programming in Twitter or are there other languages in the mix?
Java and C++ were also used.
> The majority of other big software players seem to be not using it for various reasons. Is that a concern?
Not really.
> - How is the learning curve for new hires? How long before they become productive? How long before they write great code in Scala (as opposed to still figuring things out but maybe being productive)?
Took me about four months. It would be easier these days because the related tools (mostly IDE and build) suck less.
> - Isn't the JVM too big of a performance compromise for large scale software where every bit of performance matters?
It's mostly fine. It's just the very highest throughput systems (load balancers where you expect to saturate the pipe and not pause for 5ms) where you wish you had C.
>- Is it easier to hire people for Scala positions? Harder? The same?
You mostly train into it.
>- How do engineers in Twitter with a lot of experience in other platforms and languages feel about Scala after having used it for a while?
> Is Scala used exclusively for systems programming in Twitter or are there other languages in the mix?
Java and C++ were also used.
> The majority of other big software players seem to be not using it for various reasons. Is that a concern?
Not really.
> - How is the learning curve for new hires? How long before they become productive? How long before they write great code in Scala (as opposed to still figuring things out but maybe being productive)?
Took me about four months. It would be easier these days because the related tools (mostly IDE and build) suck less.
> - Isn't the JVM too big of a performance compromise for large scale software where every bit of performance matters?
It's mostly fine. It's just the very highest throughput systems (load balancers where you expect to saturate the pipe and not pause for 5ms) where you wish you had C.
>- Is it easier to hire people for Scala positions? Harder? The same?
You mostly train into it.
>- How do engineers in Twitter with a lot of experience in other platforms and languages feel about Scala after having used it for a while?
Mixed bag. Some love it, some hate it.