Write isolation

Sparingly, we could use $isolated to isolate writes to multiple documents from other writers or readers to these documents. In the previous example, we could use $isolated to update multiple documents and make sure that we update both balances before anyone else gets the chance to double-spend to drain the source account from its funds.

What this won't give us though, is atomicity, the all-or-nothing approach. So if the update only partially modifies both accounts, we still need to detect and unroll any modifications made in the pending state.

$isolated uses an exclusive lock in the entire collection no matter the storage engine used. This means a severe speed penalty when using it, especially for WiredTiger document level locking semantics.

$isolated does not work with sharded clusters, which may be an issue when we decide to go from replica sets to sharded deployment.