Replication
Last updated
Last updated
This is an feature, and it is available since Orbeon Forms 2017.2.
We also have reports of this feature working with OpenLiberty and Hazelcast session replication.
The purpose of replication is to provide high-availability of Orbeon Forms with as little disruption as possible to users currently filling out forms. This is achieved by replicating state between servers.
Consider a simple scenario of load balancing with two servers, with sticky sessions (that is, a given user's requests always reach the same server). If one of the servers fails, new users will be assigned to the other server. So the system remains operational from that point of view. However, users with active sessions will have their current work lost, as the content will still in the failed server-memory.
Replication changes that by replicating state to one or more additional servers. So if a server goes down, the load balancer can redirect users with active sessions to other servers, and because state was replicated there, users can continue their work.
Orbeon Forms achieves replication by enabling the replication of servlet sessions and of caches. All current state in memory, whether in the session or relevant caches, is replicated so that work can be resumed on replica servers when needed.
Sessions are still sticky for performance reasons. Because Orbeon Forms stores a lot of information in memory, and there are data structures associated with that information, there is a cost to recreate all necessary data structures at each request. Therefore, requests for a given user must constantly reach the same server. However, if a server fails, then there is a one-time cost to recreating data structures on the new server for the given user (in fact, for a given form in use by that user).
A load balancer is required. It is in charge of proxying client requests to specific servers, detect which servers might have failed or are being brought back, and ensuring session affinity.
When to use the Ehcache configuration:
Ehcache is better suited for traditional on-premises deployments where servers are on the same network and can use multicast for automatic peer discovery.
Your servers can communicate via multicast (IP multicast address and port).
You are using version of Orbeon Forms before 2024.1.2.
When to use the Redis configuration:
You're deploying in cloud environments where multicast is typically not available.
You have Redis available as a managed service (common in cloud platforms).
You need a simpler configuration with fewer components to manage.
You're using Orbeon Forms 2024.1.2 or newer.
Orbeon Forms has a single property enabling replication. By default, it is set to false, because there is a cost to serializing the state of forms after each update in memory.
The Orbeon Forms ehcache.xml must be modified to include replication settings, which are turned off by default. This is similar to Tomcat session replication. To modify this file, extract it from the WEB-INF/lib/orbeon-core.jar, and copy it in the WEB-INF/resources/config directory. You can then modify the ehcache.xml in that directory, and your updated version will take precedence over the built-in version of that file found inside orbeon-core.jar.
NOTE: There isn't as single set of settings to replicate the Tomcat servlet session and Ehcache, as the two products use different libraries for replication. But the idea is that both configuration should behave as closely as possible from each other.
The keys to this configuration are:
for relevant caches
RMICacheReplicatorFactory as <cacheEventListenerFactory>
RMIBootstrapCacheLoaderFactory as <bootstrapCacheLoaderFactory>
global
RMICacheManagerPeerProviderFactory as <cacheManagerPeerProviderFactory>
RMICacheManagerPeerListenerFactory as <cacheManagerPeerListenerFactory>
Here is an example configuration:
When using a firewall:
The multicastGroupPort port might need an UDP firewall unlock.
If you don't specify ports for <cacheManagerPeerListenerFactory>, the ports are chosen at random and might be blocked by the firewall. You can specify explicit ports to address this:
The servlet container must be configured to replicate the session information.
With Tomcat, this is done in server.xml within the <Engine> element:
In that configuration, the following can be changed:
the IP multicast address, here address="228.0.0.4"
the IP multicast port, here port="45564"
This setup is best for cloud deployments where instances of Orbeon Forms typically can't use multicast for discovery, and Redis is provided as a service.
Extract orbeon-redis-jars.zip which is part of the Orbeon Forms distribution.
Copy the jar files it contains to Tomcat's lib directory.
Create redisson-jcache.yaml in Tomcat's conf directory, changing redis.example.com to point to your Redis server:
In properties-local.xml, add the following (modifying the path to redisson-jcache.yaml as needed):
Inside the <Context> for Orbeon Forms (typically found in the Tomcat server.xml or an orbeon.xml), add:
Consider a scenario where you have two servers with replication enabled, and one of them fails. This means that users from the failed server are redirected by the load balancer to the server which is still working. If, at the time of failure, both servers were nearing their full capacity, then suddenly the only remaining server will have to handle all the load.
This means that the load balanced servers should not be allowed to reach full capacity so that, in case of failure of a single server, the remaining server can handle all the load. Theoretically, this means that each server, in normal use, should be at under 50 % of total capacity.
Using more than 2 replicated servers allows using more of the available capacity of all servers in the case of a single server failure.
If using HAProxy, a simple configuration looks like this:
This configuration round-robins between two servers, s1 and s2, on two ports, 8888 and 8889.
In this example, the servers are accessed at address 127.0.0.1, but in practice they might be on different physical servers.
For testing, you can start HAProxy with the following command:
Uploaded files which are not yet saved to a database are currently not replicated. If a user is switched from one server to another, Form Runner:
checks all unsaved attachments
if there are any
clears the associated temporary file path
shows an alert to the user
This requires users with unsaved attachments to re-upload their attachments. This is not ideal, but it is likely that the user still have the attachment or attachments available.
The ehcache.xml configuration provided above attempts to minimize this kind of issues by adding replicateAsynchronously=false. NOTE: We have feedback from customers that if manual peer discovery (RMI TCP unicast) is enabled, setting replicateAsynchronously="true" works and helps reduce latency.
For details about the Tomcat configuration, see .
For details about the HAProxy configuration, see the .
If a server fails instantly before it had the chance to replicate the latest modifications to a form, and after an Ajax response has been sent to the client, then state might be lost. The user is redirected by the load balancer to another server, but state will be missing from that server. In such cases, the user will see an error, and won't be able to continue working with the form. Unsaved data will be lost. In such cases, enabling the can alleviate the issue.
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