SharePoint 2013 Workflows – High Availability (MSMQ)

Workflow Manager only supports a farm with 1 computer or a farm with 3 computers.
http://msdn.microsoft.com/en-us/library/jj193434(v=azure.10).aspx

There is a very good article by harbar.net  that you can read details .
“However this isn’t high availability, its horizontal scalability. It’s pretty sweet. The trouble is the perception that “Service Bus takes care of everything”. Which is not the Case…

When we create a connection to a Workflow Manager farm from a SharePoint farm with the Register-SPWorkflowService cmdlet we pass in a WorkflowHostUri parameter. This typically is the host name of a Workflow Manager host. If we have three Workflow Manager hosts, which host name should we use? Well we can use anyone we like, as long as it’s valid. This will work. But it’s not highly available. If that particular host is down for whatever reason, our Workflow Connection – which is a Service Application Proxy – will be broken and we cannot configure or execute any SharePoint 2013 workflows.”

http://www.harbar.net/articles/wfm1.aspx

“The servers should be configured with a software or hardware load balancer for proper load balancing, or can be accessed directly”
Load Balancing is a solution for this issue but it is not enough .
As again

“A load balancer generally does load balancing. Whether it’s NLB or ARR or an “intelligent”, “hardware” device from the usual suspects, it doesn’t make any difference. They all require configuration and scripting to truly function for HA. If all you need is a solution for when you reboot a host due to Windows Updates or similar scenarios you are good. But for everything else you need to tell the load balancer how to be “intelligent”, it will not happen out of the box.”
http://www.harbar.net/articles/wfm1.aspx

there is also one more resolution for high availability : MSMQ.
MSMQ is a Windows Server feature that you can enable on your SharePoint Server computer to allow asynchronous event messaging in SharePoint workflows. To support asynchronous event messaging, you must enable MSMQ on your SharePoint Server computer.
http://msdn.microsoft.com/en-us/library/office/dn467936(v=office.15).aspx

After MSMQ installed it can be easily activate by powershell:

$proxy = Get-SPWorkflowServiceApplicationProxy
$proxy.AllowQueue = $true;
$proxy.Update();

MSMQ provides Queue functionality  for SharePoint side , like Service Bus does  for Workflow Manager side. If we summarize
For High Availability:

1) Scale up your Workflow Manager Farm by 3 server.
2) Configure Workflow Manager host uri for your LB and DNS as correctly
3) And Use MSMQ !!! which is not OOB enabled for SharePoint 2013 servers.