You can use https://github.com/ConradIrwin/pry-rescue#rspec pry-rescues rspec integration to automatically fall into pry when a test fails

> Implications: if you run a query in production that doesn't hit an index, all traffic stops

Even if Mongo did have a global write lock, which it doesn't as has already been covered, it yields on page faults which means that other queries are minimally impacted. See: http://docs.mongodb.org/manual/faq/concurrency/#does-a-read-...

Mongo does have a global write lock. A per-db lock does me very little good when I only have one DB.

As to your linked doc, emphasis added:

> In some situations, read and write operations can yield their locks.

> Long running read and write operations, such as queries, updates, and deletes, yield under many conditions.

In practice, I've been bitten hard by this. A new feature rolls out, and users can't log in anymore, because a query is taking 2 minutes to run.

That's solves the contention problem, but just by virtue of yields on page faults being a big feature you can see where Mongo does have some scaling issue.