Extremely slow parse times can cause the router to stop serving requests

[xuorig]

The version of apollo-compiler used by router has a perfomance issue in validation, which combined with large schemas and queries with a lot of fragment spreads can cause devastatingly slow parsing times (in seconds). I've opened an issue on apollo-compiler which contains more details about the problem.

Combined with the fact parsing is executed between await points and is blocking (for potentially seconds), this can cause the router's async runtime to completely fall apart and not being able to respond to any queries for minutes. I've opened a PR to run parsing through spawn_blocking instead of blocking a worker. This is especially concerning on schema reloads, where contention is high.

Both issues most likely need to be fixed to maintain good performance on schema reloads when it comes to parsing. Additionally, it might be worth thinking about how the implementers_map (apollo-compiler issue) can be cached through router reloads.