Two nodes HA cluster

abogdanov37 · March 23, 2020, 10:44am

Hi everyone
Can someone help me to understand how two nodes cluster work internally?
For example, I have 2 nodes cluster. All works fine
Then one node die.
What will happen after that?
Will cluster be available?

sgotti · March 23, 2020, 1:49pm

@abogdanov37 The purpose of stolon is postgres high availability. So if you have two nodes and the primary dies, the other node will be elected as primary. Please take a look at the examples and the docs in the stolon github repo: GitHub - sorintlab/stolon: PostgreSQL cloud native High Availability and more.

abogdanov37 · March 23, 2020, 2:41pm

Thank you for reply.
If I understand right only etcd should have 3 or 5 or more nodes?
Another components like stolon-keeper, stolon-sentinel, stolon-proxy do not take part in leader election?

sgotti · March 23, 2020, 3:48pm

@abogdanov37 Yes, etcd or consul requires a quorum so at least 3 nodes and should leave outside the stolon nodes (or at least don’t share the same I/O paths). For some deployment examples see:

github.com

sorintlab/stolon/blob/master/doc/twonodes.md

# Two (or more) nodes setup example

If you're going to run stolon on an IaaS or any kind of two nodes (baremetal, VM) this document provides some example configurations. These examples provide a two nodes setup that can be extended to a number greater than two nodes.

The related store should live outside the instances hosting the stolon cluster for multiple reasons:

* Stores like etcd/consul uses the raft consensus protocol that requires at least 3 nodes to provides high availability (it can survive to the death of (N/2)-1 members)
* Disk I/O of postgres must not impact the etcd/consul I/O
* You can run multiple stolon clusters using the same store

# Two nodes with load balancer

![Two nodes with a load balancer](twonodes_lb_small.png)

Every nodes will run one keeper, one sentinel and one proxy.

The load balancer will balance between the stolon proxies.

Clients will connect to the load balancer VIP.

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abogdanov37 · March 23, 2020, 5:07pm

Thank you for fast and meaningful reply. I should make sure that I understand the stolon correctly.