16 Failure Modes
Crash-Stop Failure
Server may crash any time
- Stops taking any forward steps in execution
- Stops sending any messages over the network
- Stops receiving and/or responding to messages
if machine crashes, remain crashed forever
Omission Failure
Basically crash-stop. In addition, at any point in time a server may stop sending or receiving messages, but may continue to run.
Crash-Recover Failure
Server may crash anytime, but we can recover
- Restarts from some initial or “correct” state
- State stored in volatile memory may be lost
- Messages may be omitted during recovery
- Persisted state is recovered assuming that stable storage (e.g., hard disk) survives the “crash”
Persistent state - still there after shut-down
Volatile state - RAM
Byzantine Failure
Everything above, but crashed system may act arbitrarily
- Send corrupted messages
- Updates its state in ways not specified by the protocol
- Act maliciously, trying to attack the system
