2/4: running [==================================================>] 3/4: running [==================================================>] 4/4: running [==================================================>] verify: Service converged admin@570e9473805e:~$ docker service ls ID NAME MODE REPLICAS IMAGE PORTS pwu25hj3wpww webserver replicated 4/4 nginx:latest *:80->80/tcp 

## 11.2.11 Considerations regarding the number of manager nodes 

It is vital to understand how swarm mode’s fault-tolerance feature works to prevent problems with your cluster service. Although it is possible to run a cluster with only one manager node, this approach is not ideal for organizations that have high-availability policies. If the single-node manager fails, the services continue to process the user requests. However, you must create a cluster to recover the manager node operations. 

Docker uses a consensus algorithm that is named raft  to achieve internal consistency across the entire swarm cluster and all containers that are running on it. Docker recommends that you implement an odd number, based to your high-availability requirements, according to the following rules: 

GLYPH<SM590000> A three-manager swarm tolerates a maximum loss of one manager. 

GLYPH<SM590000> A five-manager swarm tolerates a maximum simultaneous loss of two manager nodes. 

GLYPH<SM590000> An N-manager cluster tolerates the loss of at most (N-1)/2 managers. 

For more information about the cluster algorithm, see this web page. 

