"If even more capacity is required, active servers can be added as needed to stripe and mirror your Network Attached Memory across an n-way server array."
Does this mean that If I use the open source edition of Terracotta DSO, I can run many active servers in the same cluster? Do I get to have heap that can be bigger than the heap available to one VM?
DSO doesn't have anything to do with open source vs. commercial. DSO is a technology for distributing shared data. If you need more than one active server in a cluster, you must use the commercial version. You can get a trial version here: