Consider two communication services for use in asynchronous distributed systems. In service A, messages may be lost, duplicated or delayed and checksums apply only to headers. In service B, messages may be lost. delayed or delivered too fast for the recipient to handle them, but those that are delivered arrive order and with the correct contents. Describe the classes of failure exhibited by each service. Classify their failures according to their effect on the properties of validity and integrity. Can service B be described as a reliable communication service?
What will be an ideal response?
Service A can have:
arbitrary failures:
– as checksums do not apply to message bodies, message bodies can corrupted.
– duplicated messages,
omission failures (lost messages).
Because the distributed system in which it is used is asynchronous, it cannot suffer from timing failures.
Validity - is denied by lost messages
Integrity - is denied by corrupted messages and duplicated messages.
Service B can have:
omission failures (lost messages, dropped messages).
Because the distributed system in which it is used is asynchronous, it cannot suffer from timing failures.
It passes the integrity test, but not the validity test, therefore it cannot be called reliable.
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