Jades
Java distributed executor service library
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The only requirement is that all machines have SSH and a compatible JRE (>= 6). More precisely, when a node is added, the application's jar is automatically copied (scp) to the node and a new server started (ssh).
Handling of access restrictions is left to the user (ie you can type your password every time or use authentication keys).
Nodes can be added dynamically. When a node is added, workers are created according to the node's processor count. Eg, a cluster of 4 quadcores will give 16 workers.