There is an interesting tidbit you can learn about Google’s web rendering service (WRS) in the latest JavaScript SEO hangout with Martin Splitt of Google where Martin said he is “spilling the beans a little bit” by sharing that there is a layer on top of WRS that handles “prioritization, error retry, quality of service guarantees, caching
and it also interacts with a crawling subsystem to actually fetch things.”

To be clear, Martin seemed to know what this was at a high level and did not go into too much detail on this layer. He said “I can’t say the name, but there’s like an…
it’s larger than the WRS.” He added “the WRS really is just the rendering service and then there’s like a slightly bigger layer on top of it and that actually has prioritization, error retry, quality of service guarantees, caching and it also interacts with a crawling subsystem to actually fetch things.”

He mentioned this at the 16:30 mark into the video when he was asked “Are we talking about dynamics serving looking at UA WRS caches resources and that’s okay but do you cache url using UA as a key?”

Here is the embed at the start time followed by the transcript:

Forum discussion at YouTube Community.