Multi-server with WP Synchro

TL;DR

- WP Synchro supports multi-server

- May require changes to hosting

- If you see HTTP 401 REST errors, changes are required

The solution is to make sure that the connections between the sites are sticky, aka only hitting the same server in the cluster every time.

Running a multi-server/cluster WordPress setup is not the most common scenario. It is often used by high traffic sites or sites requiring high availability.

The setup usually consist of at least two servers and a load balancer in front to distribute the traffic.
This kind of setup is supported by WP Synchro, as we have customers with this site type on AWS, DigitalOcean etc.

But it is important to notice, that WP Synchro can require changes to your hosting configuration, with this type of setup.

This is primarily due to a specific security mechanism in WP Synchro that helps prevent man-in-the-middle attacks. This security check verifies that the IP address of the request is the same as in the initiation phase.

That is exactly what causes problems sometimes, because the load balancer distributes connections to the backend servers in a non-sticky way. This way a request will end up at different servers and WP Synchro will generate errors and halt, as this server is deemed an outsider. In practice you will see HTTP 401 REST errors in the synchronization log file.

The solution to this problem, is to make sure that connections are sticky between the multi-server/cluster and the remote server. This can normally be configured in the load balancer. In some cases, it may not be supported by the load balancer and other creative method are required, such as exposing a specific internal webserver to the other outside server with a firewall rule.

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