Though from 1993, in its time this paper sparked some controversy, provoking an impassioned response. We wanted to understand the debate about the question of providing ordering guarantees as part of the network.
What is CATOCS?
CATOCS stands for “causally and totally ordered communication.” It means that messages are delivered in the order they are sent, as specified by a happens-before relationship. A synonym for happens-before is causally-precedes. The following is the definition of happens-before:
happens-before if there exists a such that is sent or received at before sends .
Under causal ordering, concurrent writes may be seen in different orders at different participants. Total ordering is a stronger property; it ensures that messages are delivered to all participants in the same order.
What is wrong with CATOCS?
Cheriton and Skeen make the point that ensuring CATOCS in the network is prohibitively expensive, and since most applications need something stronger than CATOCS anyway (such as transactional consistency), there is no point in doing so. They claim that CATOCS violates the End To End principle, which states that application-specific functionality should reside at the end nodes of a network, instead of the intermediary nodes. It is worth noting that this principle is frequently misapplied.
They identify the following limitations in CATOCS systems:
Can’t say “for sure”
There are almost always hidden channels in a group of nodes, or methods of communication not captured by the network. For example, processes might all write to a shared database, and writes seen at that database might not preserve CATOCS. Similarly, threads on a single machine might share memory.
They use a contrived example of an independent
FIREmessage appearing before an unrelated
FIRE OUTmessage, and thus the system might appear to not be in a
FIREstate, because it misapplied the unrelated
Can’t say “together”
As stated above, applications often require transactional semantics. CATOCS does not help with the serialization or atomicity between groups of messages. A system with this property obviates the need for CATOCS.
Can’t say “whole story”
Happens-before might not be enough. Applications might require linearizability or sequential consistency.
Can’t say “efficiently”
They claim CATOCS protocols don’t show any efficiency gains over state-level techniques, and in fact are very inefficient. Unfortunately the paper does not provide actual measurements. False causality could also be an issue; happens-before enforces ordering that the application might not care about.
Cheriton and Skeen would prefer to see state-level and application-specific ordering techniques instead.
Birman sees this paper as a critique of Isis, and claims that Cheriton and Skeen misrepresented the true debate. CATOCS should not be considered in isolation, but when transactional semantics are required, techniques like virtual synchrony should be used in conjunction with CATOCS.
Birman makes the point that application developers should not even need to consider their semantic ordering needs, instead the network should provide guarantees for them, reducing user-visible design complexity.
He also claims that their assumptions about overhead are completely off.
This seems founded in a more general debate — should systems developers aim for efficiency and performance first, giving application developers total control, but leaving them to layer safety accordingly, or should they apply an unknown cost to all users, making strong semantics an indelible part of the system?
In the space of datastores, the former argument seems to have “won”. Most application developers do not run their databases with serializability or even other forms of slightly weaker consistency. There is a move towards general key/value stores which do not provide transactions or any ordering guarantees and might not necessarily pay the penalty of writing to disk for durability. It seems as though application developers have chosen performance over safety, and developed techniques to accommodate inconsistencies on their own (one of which might be simply ignoring them).
We found it extremely difficult to reason about these two papers without looking at a real system with a concrete design.