When Master server goes down, either you need to promote one of slave node or configure HA mechanism so that in case of master server goes down it will automatically promote the slave server as new master.
Patroni HA tool would be good option for your requirement.
What is the difference between High Availability and Replication?
The former is a goal, the later is a technique.
Perhaps more specifically: Replication is simply Replicating - or copying - the “master” database to one or more “slave” databases, generally in real-time such that the slave database clusters are replicas of the master. This is good when the master goes down, because you’ll still have one or more copies of it available, but by itself it doesn’t keep there from being an outage if/when the master goes down.
High Availability layers on top of replication to provide some means of ensuring that the database is HIGHLY available, such as an automatic failover system or load balancer. Many different options that work in many different ways are available to help meet this goal.
You said "This is good when the master goes down, because you’ll still have one or more copies of it available, but by itself it doesn’t keep there from being an outage if/when the master goes down.". What does "goes down" mean?
Exactly what I said - it goes down. Not functioning. Offline. Inaccessible. It is not up and running, therefore, it is down.
In Replication mode, if the primary server is shut down, then the data will also be lost?
No. As I said - and you quoted - “When the master goes down…you’ll still have one or more copies of it available”. So no, the data will NOT be lost.
