discovery cache: java.io.NotSerializableException

[peacekeeper]

We cache discovery results like this:

this.registryCache.put(new Element(discoveryCacheKey, value))

The value is a MessagingResponse, which implements Serializable. But MessagingResponse may contain a MemoryGraph, which is not Serializable. So we get the following error:

ERROR [2015-09-23 20:22:22,155] net.sf.ehcache.store.disk.DiskStorageFactory: Disk Write of 

DiscoveryCacheKey [query==joshsrnaccount3, xdiEndpointUri=http://54.88.185.78:3081/registry, endpointUriTypes=null] failed:
! java.io.NotSerializableException: xdi2.core.impl.memory.MemoryGraphFactory
! at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1183) ~[na:1.7.0_79]
! at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1547) ~[na:1.7.0_79]
! at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1508) ~[na:1.7.0_79]
! at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1431) ~[na:1.7.0_79]
! at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1177) ~[na:1.7.0_79]
! at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1547) ~[na:1.7.0_79]
! at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1508) ~[na:1.7.0_79]
! at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1431) ~[na:1.7.0_79]
! at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1177) ~[na:1.7.0_79]
! at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1547) ~[na:1.7.0_79]
! at java.io.ObjectOutputStream.defaultWriteObject(ObjectOutputStream.java:440) ~[na:1.7.0_79]
! at net.sf.ehcache.Element.writeObject(Element.java:867) ~[personal-cloud-service-application-1.0.jar:1.0]

Probably Graph and other interfaces/classes should not implement Serializable, and instead we should cache a string-serialized graph instead of the object itself.