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Short note: Process resilience

Short answer

Process resilience masks failures by replicating processes into a group. Flat groups decide collectively without single point of failure; hierarchical groups use a coordinator for speed. For crash failures, K+1 members tolerate K faults; for Byzantine, 2K+1 for majority vote, 3K+1 for agreement. Example: three replicated database servers survive one crash.

Write a short note on process resilience (explain process resilience with example).

Process resilience means protecting a system against process failures by replicating a process into a process group that works as a single logical process: if a member fails, another takes over, so the service continues.

Group organization

  • Flat group: all processes are equal and decide collectively (e.g. by voting). There is no single point of failure, but decision making is more complex and has more overhead.
  • Hierarchical group: one coordinator decides and the workers carry out its instructions. It is simple and fast, but if the coordinator fails, the group must elect a new one.
Flat group        Hierarchical group
 P1--P2            Coordinator
  \  /              /   |   \
   P3--P4         W1   W2   W3

Failure masking and group size: a group that can survive K failures is K-fault tolerant. With crash failures, K+1 members are enough; with arbitrary (Byzantine) failures, 2K+1 members are needed (with voting), and 3K+1 to reach agreement.

Agreement in faulty systems: members must agree on decisions even when some members are faulty (consensus, Byzantine agreement). Membership changes need a group membership service that adds and removes members and detects failed ones (by pinging and timeouts).

Example: a replicated database group of 3 servers answers client requests; if one crashes, the other two keep serving.

✓ Answer checked by Claude Opus 5.5 (high effort)

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