Is discoveryutil mdnsflushcache synchronous or asynchronous?

I am trying to flush my cache with:

sudo discoveryutil mdnsflushcache

sudo discoveryutil udnsflushcache

And then immediately run a curl requesting a file from the web. I've had to add a sleep statement after the cache flushes in order for curl to pickup (request) the correct DNS information.

Is that correct? Or is something else going on here?