Description:
With a 3 node cluster, when asking for the status, we get "Cluster is ONLINE and can tolerate up to ONE failure."
From this we can assume that an operational cluster is at least two nodes, and a cluster with at least three nodes can tolerate failures.
So, I try with 4 nodes:
{
"clusterName": "testcluster",
"defaultReplicaSet": {
"name": "default",
"primary": "mysqlgr1:3306",
"status": "OK",
"statusText": "Cluster is ONLINE and can tolerate up to ONE failure.",
"topology": {
"mysqlgr1:3306": {
"address": "mysqlgr1:3306",
"mode": "R/W",
"readReplicas": {},
"role": "HA",
"status": "ONLINE"
},
"mysqlgr2:3306": {
"address": "mysqlgr2:3306",
"mode": "R/O",
"readReplicas": {},
"role": "HA",
"status": "ONLINE"
},
"mysqlgr3:3306": {
"address": "mysqlgr3:3306",
"mode": "R/O",
"readReplicas": {},
"role": "HA",
"status": "ONLINE"
},
"mysqlgr4:3306": {
"address": "mysqlgr4:3306",
"mode": "R/O",
"readReplicas": {},
"role": "HA",
"status": "ONLINE"
}
}
}
}
It says the same as for 3 nodes. But is I kill the primary node, wait for a new one to be appointed, and then kill that one, I can see that there the cluster could withstand TWO failures.
Similarly, for five nodes, It says that it can tolerate 2 failures, but I am able to kill the primary three times, and operations for R/W and R/O through the router keep working
{
"clusterName": "testcluster",
"defaultReplicaSet": {
"name": "default",
"primary": "mysqlgr1:3306",
"status": "OK",
"statusText": "Cluster is ONLINE and can tolerate up to 2 failures.",
"topology": {
"mysqlgr1:3306": {
"address": "mysqlgr1:3306",
"mode": "R/W",
"readReplicas": {},
"role": "HA",
"status": "ONLINE"
},
"mysqlgr2:3306": {
"address": "mysqlgr2:3306",
"mode": "R/O",
"readReplicas": {},
"role": "HA",
"status": "ONLINE"
},
"mysqlgr3:3306": {
"address": "mysqlgr3:3306",
"mode": "R/O",
"readReplicas": {},
"role": "HA",
"status": "ONLINE"
},
"mysqlgr4:3306": {
"address": "mysqlgr4:3306",
"mode": "R/O",
"readReplicas": {},
"role": "HA",
"status": "ONLINE"
},
"mysqlgr5:3306": {
"address": "mysqlgr5:3306",
"mode": "R/O",
"readReplicas": {},
"role": "HA",
"status": "ONLINE"
}
}
}
}
How to repeat:
Create a three node cluster and a four node cluster.
Run getCluster().status()
Description: With a 3 node cluster, when asking for the status, we get "Cluster is ONLINE and can tolerate up to ONE failure." From this we can assume that an operational cluster is at least two nodes, and a cluster with at least three nodes can tolerate failures. So, I try with 4 nodes: { "clusterName": "testcluster", "defaultReplicaSet": { "name": "default", "primary": "mysqlgr1:3306", "status": "OK", "statusText": "Cluster is ONLINE and can tolerate up to ONE failure.", "topology": { "mysqlgr1:3306": { "address": "mysqlgr1:3306", "mode": "R/W", "readReplicas": {}, "role": "HA", "status": "ONLINE" }, "mysqlgr2:3306": { "address": "mysqlgr2:3306", "mode": "R/O", "readReplicas": {}, "role": "HA", "status": "ONLINE" }, "mysqlgr3:3306": { "address": "mysqlgr3:3306", "mode": "R/O", "readReplicas": {}, "role": "HA", "status": "ONLINE" }, "mysqlgr4:3306": { "address": "mysqlgr4:3306", "mode": "R/O", "readReplicas": {}, "role": "HA", "status": "ONLINE" } } } } It says the same as for 3 nodes. But is I kill the primary node, wait for a new one to be appointed, and then kill that one, I can see that there the cluster could withstand TWO failures. Similarly, for five nodes, It says that it can tolerate 2 failures, but I am able to kill the primary three times, and operations for R/W and R/O through the router keep working { "clusterName": "testcluster", "defaultReplicaSet": { "name": "default", "primary": "mysqlgr1:3306", "status": "OK", "statusText": "Cluster is ONLINE and can tolerate up to 2 failures.", "topology": { "mysqlgr1:3306": { "address": "mysqlgr1:3306", "mode": "R/W", "readReplicas": {}, "role": "HA", "status": "ONLINE" }, "mysqlgr2:3306": { "address": "mysqlgr2:3306", "mode": "R/O", "readReplicas": {}, "role": "HA", "status": "ONLINE" }, "mysqlgr3:3306": { "address": "mysqlgr3:3306", "mode": "R/O", "readReplicas": {}, "role": "HA", "status": "ONLINE" }, "mysqlgr4:3306": { "address": "mysqlgr4:3306", "mode": "R/O", "readReplicas": {}, "role": "HA", "status": "ONLINE" }, "mysqlgr5:3306": { "address": "mysqlgr5:3306", "mode": "R/O", "readReplicas": {}, "role": "HA", "status": "ONLINE" } } } } How to repeat: Create a three node cluster and a four node cluster. Run getCluster().status()