Explanation
= Distributing seven managers across three datacenters or availability zones as 3-2-2 is not a good way to ensure high availability and fault tolerance. This is because a swarm cluster requires a majority of managers (more than half) to be available and able to communicate with each other in order to maintain the swarm state and avoid a split-brain scenario1. If one of the datacenters or availability zones with three managers goes down, the remaining four managers will not have a quorum and the swarm will stop functioning. A better way to distribute seven managers across three datacenters oravailability zones is 3-3-1 or 3-2-1-1, which will allow the swarm to survive the loss of one or two datacenters or availability zones, respectively2. References:
* Administer and maintain a swarm of Docker Engines | Docker Docs
* How to Create a Cluster of Docker Containers with Docker Swarm and DigitalOcean on Ubuntu 16.04 | DigitalOcean