For some of our more taxing parts of our applications, something able to handle high I/O load quickly and with fast processing is needed. Go has completely filled that gap, allowing us to break down walls that would've been completely impossible with other languages.
When it comes to provisioning tens to hundreds of servers, you need a tool that can handle the load, as well as being extremely customisable. Fortunately, Salt has held that gauntlet for us consistently through any kind of issue you can throw at it.
Slack is a lifesaver, not only for our day to day team communications and it's direct links into our other tools, but for Beta testing as well, with our custom Slack bot in our beta group being an invaluable asset to avoid giving our testers direct JIRA access.