◾ Based on the source of the faults, faults can be classified as:– Hardware faults, if the faults are caused by the failure of hardware components such as power outages, hard drive failures, bad memory chips, etc.– Software faults, if the faults are caused by software bugs such as race conditions and no-boundary-checks for arrays.– Operator faults, if the faults are caused by the operator of the system, for example, misconfiguration, wrong upgrade procedures, etc.

       ◾ Based on the intent of the faults, faults can be classified as:– Non-malicious faults, if the faults are not caused by a person with malicious intent. For example, the naturally occurred hardware faults and some remnant software bugs such as race conditions are non-malicious faults.– Malicious faults, if the faults are caused by a person with intent to harm the system, for example, to deny services to legitimate clients or to compromise the integrity of the service. Malicious faults are often referred to as commission faults, or Byzantine faults [5].

       ◾ Based on the duration of the faults, faults can be classified as:– Transient faults, if such a fault is activated momentarily and becomes dormant again. For example, the race condition might often show up as transient fault because if the threads stop accessing the shared variable concurrently, the fault appears to have disappeared.– Permanent faults, if once a fault is activated, the fault stays activated unless the faulty component is repaired or the source of the fault is addressed. For example, a power outage is considered a permanent fault because unless the power is restored, a computer system will remain powered off. A specific permanent fault is the (process) crash fault. A segmentation fault could result in the crash of a process.

       ◾ Based on how a fault in a component reveals to other components in the system, faults can be classified as:– Content faults, if the values passed on to other components are wrong due to the faults. A faulty component may always pass on the same wrong values to other components, or it may return different values to different components that it interacts with. The latter is specifically modeled as Byzantine faults [5].– Timing faults, if the faulty component either returns a reply too early, or too late alter receiving a request from another component. An extreme case is when the faulty component stops responding at all (i.e., it takes infinite amount of time to return a reply), e.g., when the component crashes, or hangs due to an infinite loop or a deadlock.

       ◾ Based on whether or not a fault is reproducible or deterministic, faults (primarily software faults) can be classified as:– Reproducible/deterministic faults. The fault happens deterministically and can be easily reproduced. Accessing a null pointer is an example of deterministic fault, which often would lead to the crash of the system. This type of faults can be easily identified and repaired.– Nondeterministic faults. The fault appears to happen nondeterministically and hard to reproduce. For example, if a fault is caused by a specific interleaving of several threads when they access some shared variable, it is going to be hard to reproduce such a fault. This type of software faults is also referred to as Heisenbugs to highlight their uncertainty.

       ◾ Given a number of faults within a system, we can classify them based on their relationship:– Independent faults, if there is no causal relationship between the faults, e.g., given fault A and fault B, B is not caused by A, and A is not caused by B.– Correlated faults, if the faults are causally related, e.g., given fault A and fault B, either B is caused by A, or A is caused by B. If multiple components fail due to a common reason, the failures are referred to as common mode failures.

       ◾ Mean time to failure (MTTF). It characterizes how long the system can run without a failure.

       ◾ Mean time to repair (MTTR). It characterizes how long the system can be repaired and recovered to be fully functional again.