In distributed systems, protocols and algorithms are each designed with regards to a particular set of assumptions. One of these assumptions is the failure model of components of the system. For example, we might make assumptions about how processes fail, and others about how the message-passing system, the network, fails. These assumptions are critical as they provide us with knowledge of the capabilities of protocols as well as a means of comparing them. In this article, I present a survey of the different failure models used in distributed systems.
A Note on Lingo
In distributed systems, often we reduce the system mentally to a collection of nodes which represent processes and edges which represent the network link(s) between processes. In the real world, these entities are more complicated than lines and dots. However, we can think of them simply as such and charactize the complexity in terms of a failure model. Neat-O!
Throughout this piece I may refer to a run. A run is simply an execution of a protocol, from start to finish. While we could also make assumptions about how components behave outside of a run, here I am restricting the scope to the failure model during a run.
A message is delivered when it is given to the application for processing. Since a distributed system may serve as middleware, it is necessary to distinguish delivered messages from received messages. A received message has arrived at the destination, but not yet delivered to the application.
A typical distributed protocol is implemented as follows.
state = init() loop: broadcast(state) messages = receive() state = tick(state, messages)
Processing faults (process faults, node faults) occur at the nodes of the system, where the execution of the above code is performed. The failure model in which no faults occur is sometimes reffered to as the benign failure model. Put another way, processes can be said to be correct, honest, or benign if they operate according to the benign failure model. This is the strongest assumption, and also the least attainable in practice. Under the benign failure model, all processes operate exactly according to the protocol for the duration of the run.
The next kind of processing failure model is the crash failure model. The crash model is weaker than the benign model. Under the crash model, processes execute according to the protocol. At some point during the run, an evil gremlin comes along and sends
SIGKILL to the processes. For the rest of the run, the process does no work and does not send any messages. In other words, processes can die, but before they die they are perfect little angels that can do no wrong (only the good die young).
The final kind of processing failure model is the byzantine failure model. This model is the weakest of the three, but most closely echoes reality. Under the byzantine failure model, processes are allowed to execute arbitrarily. Processes can decide to follow the protocol, if they want to, but maybe they won’t if they’re having a bad day. Processes can be honest, or they can lie, cheat, collude, time travel, etc. In other words, no one can be trusted, and most likely processes are stealing your money for Facebook.
An inumerable amount of papers end their list of assumptions with, “we assume a reliable message-passing channel where messages are eventually delivered.” Another variant of this sentiment is “the adversary cannot delay messages indefinitiely.” For obvious reasons, authors restrict the scope of their paper to the details of the consensus protocol. However, any real implementation must dive deep into this assumption of a “reliable channel.” What does that mean? How will we cope?
Well, one way is to not! Like for processing, networking has a benign failure model. That is, messages are always delivered exactly once and in a timely manner. For a real implementation, this assumption can be realized by implementing on top of an existing reliable channel, for example TCP. However, implementations that rely on such components, while they can ignore message-level failures, must address channel-level failures. What happens if the channel is shut down? How long do you wait to timeout a re-connect? These questions must be answered in a correct implementation.
The other options is to build the protocol on top of an unreliable channel, for example UDP. To build a reliable channel one needs to cope with 4 classes of message failure: Delay, Duplication, Disorder, and Drop. In my opinion, these are ordered from least to most severe; I will explain.
Delay is characterized by a message incuring some extra time before arrival, independent of normal transmission and propagation delays. Importantly, messages under the delay model still arrive by the end of the run. Otherwise, the delay failure would actually be a drop failure. Delay failure, I argue, is the least severe because a system with delay failure can be transformed into a system with no delay failures but longer propagation delays. There are no extra steps necessary to cope with a network experiencing delay failures.
Duplication failure occurs when a message is sent twice. Imagine that the message is copied by the sender and sent two or more times. Duplication is more severe than delay, because the solution to cope with duplication requires some piece of state on the node and in the messages. Each message carries a sequence number. When a node receives a message, it accepts and delivers that message if the sequence number is the next that is expected. If the node’s current sequence number is equal to the message’s, then it has already received the message and can ignore it.
Disorder is the next most severe network failure. Under disorder failure, messages from one node to another may not arrive in FIFO order. The solution to this failure is simple, but requires more state per node than for duplication failure. A distributed service buffers messages between hosts. An application may call a function that retrieves the next message in FIFO order. If the service has message $x+1$ but not $x$, the service may block or deliver no message until the message arrives.
Drop failures are characterized by the loss (or non-delivery) of messages. Simply, messages are sent from the source but never arrive at their destination. This can alternatively be imagined as the message having a delay failure until after the end of the run (which may be infinitiely long). Drop failures are the most severe, because a possible solution to coping with drop failures is the most complex. In all previous network failures, we have not had to make re-transmissions. The method for coping with any previous failure can be resolved by making decisions about messages that exist. Now, we must make decisions about messages that may or may not exist. First, messages are marked by sequence number. Upon receipt, the sequence number is checked to verify the message is expected to be delivered next. If it is, deliver the message. If it is not, buffer the message; when the sender sends the missing message, deliver the messages in-order.