Group Communication in Distributed Systems

Charateristics:

  • Fault tolerance based on replicated services: Client request multicast to a bunch of server.
  • Finding the discovery servers in spontaneous networking
  • Better performance through replicated data: when a value changes it is multicasted
  • Propagation of event notifications:

IP Multicast:

  • Sender is unaware of the identities of the recipients
  • Multicast group is a class D Internet address, first 4 bits are 1110
  • Membership is dynamic
  • Only available via UDP
  • When a packet arrives copies are delivered to each socket bound to the group
  • Multicast address permanent (224.0.0.1 to 224.0.0.255) or temporary.
  • Failure Model:  Same as UDP

Reliability and Ordering:

  • Fault tolerance based on replicated services: Bad, it can create inconsistent servers’ state
  • Finding the discovery servers in spontaneous networking: Ok, some drops doesn’t affect this one.
  • Better performance through replicated data:  same as above
  • Propagation of event notifications: same