Here are a few hints on how to write scalable Eliom applications, running on multiple cores or servers.
Use an external load balancer that will route all the requests of a same user agent to the same server (using cookies for example).
Be careful with all global data (references, dynamically created services ...).
Use a distributed backend for Ocsipersist (postgresql for example), and disable persistent GC for all but one (by setting <persistentsessiongcfrequency value="infinity" />).
If you are using Eliom_notif or Os_notif, broadcast your notifications to all the servers.