Recover failed tserver/master
Suppose you have a cluster running and now a tserver or master process/node has failed. Here are the steps to take to get the cluster back to shape:
NoteIn this scenario we have a N-node setup, with replication factor (RF)=3.
It's a good idea to have a cron/systemd setup to ensure that the yb-master/yb-tserver process is restarted if it is not running. This handles transient failures (such as a node rebooting or process crash due to a bug/some unexpected behavior).
If a node is down, the system will automatically heal and continue to function with the remaining N-1 nodes. If the down node doesn't come back soon enough, and N-1 >= 3, then tablets which are now under-replicated will be re-replicated automatically to get back to RF=3 on the remaining N-1 nodes.
If the node failure is a permanent failure, for the yb-tserver, simply starting another yb-tserver process on the new node is sufficient. It’ll join the cluster and load-balancer will automatically take the new yb-tserver into consideration and start rebalancing tablets to it.
If a new yb-master needs to be started to replace a failed master, the master quorum needs to be updated.
Suppose, the original yb-masters were n1, n2, n3. And n3 needs to be replaced with a new yb-master n4. Then you'll use the
./bin/yb-admin -master_addresses n1:7100,n2:7100 change_master_config REMOVE_SERVER n3 7100 ./bin/yb-admin -master_addresses n1:7100,n2:7100 change_master_config ADD_SERVER n4 7100
The TServers' in-memory state will automatically learn of the new master after the
ADD_SERVER step, and will not need a restart.
But you should also update the config file of all yb-tserver processes which specifies the master addresses to reflect the new quorum (n1, n2, n4).
This is to handle the case if the
yb-tserver restarts at some point in the future.
Planned cluster changes
You might also be interested in how to perform planned cluster changes (such as moving the entire cluster to a brand new set of nodes -- say move from machines of type A to type B).
For that see changing cluster config.