Rules for Safe Garbage Collection

• Union Rule:

  • if some replica xi points to z, and some replica xj is reachable, then z is reachable

• Scanning Rule:

  • scanning an object causes the corresponding create.scion message to be sent immediately

• Comprehensive Tracing:

  • when process i sends a union or a delete.scion message, all replicas at i must be clean

• Clean Propagation:

  • when process i sends a propagate(x), xi must be clean

• Causal Delivery of GC Messages

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