Add a 'principle' about optimisation.

There are some key principles which we used when
putting together the Haskell node - it feels like
these should be captured somewhere! This one in
particular seems very relevant to people designing
alternate node implementations.

Author: Nicholas Clarke

src/principles/optimise-worst.md

@@ -0,0 +1,79 @@
+# Principles
+
+## Optimise only for the worst case
+
+Algorithms often possess different performance characteristics in the best,
+average and worst cases. Quick sort has, for example, `O(n*log(n))` time
+complexity in the average case, but `O(n^2)` in the worst case.
+
+Often in software we are interested in optimising for the average case, since
+over time it tends to result in the highest overall performance. In Cardano,
+however, we take a different approach: we want to only optimise the worst case
+performance, and indeed, pick algorithms where _worst case is the same as best
+(or average) case_.
+
+### Motivation
+
+There are two motivating ideas behind this principle:
+
+1. _Performance will come to be relied upon_. This is an instance of Hyrum's
+   law in action:
+
+   > With a sufficient number of users of an API, it does not matter what you
+   > promise in the contract: all observable behaviors of your system will be
+   > depended on by somebody.
+
+   If we manage to increase average performance, tools will be developed that
+   come to expect this behaviour. If blocks can be processed faster, for
+   example, there may be pressure to use that extra time to allow larger script
+   execution budgets.
+
+2. Forcing honest nodes to do more work provides an attack opportunity for the
+   adversary. In the worst case, an adversary could potentially force nodes to
+   fail to adopt or forge blocks, and hence potentially gain control of the
+   chain.
+
+### Example: UTxO locality
+
+As an example, there is certain evidence that the UTxO set exhibits a degree of
+temporal locality. That is, there are a number of long-lived UTxO entries that
+are unlikely to be spent, while recently created entries are quite likely to
+come up again.
+
+We could choose to take advantage of this to organise the UTxO along the form
+of a MRU (most recently used) cache. This would likely speed up UTxO lookup
+in the average case, particularly were the UTxO stored on disk.
+
+However, an attacker could then choose to deliberately craft a number of
+transactions containing UTxO that were created a long time ago. A block filled
+with such transactions, while being perfectly legitimate, might take
+significantly longer to process than a regular block. The attacker could use
+this delay to gain an advantage in block forging and thus magnify their
+effective stake.
+
+### Considerations
+
+Whilst we have written the above as a general principle, there are of course
+various considerations that effect how much we want to follow it:
+
+1. The situation we most want to avoid is where an attacker could control an
+   input such that they can force the worst-case behaviour. The UTxO locality
+   example above is one such.
+2. Still problematic are cases where an attacker cannot control the performance
+   but can predict it, or less serious still, observe it. Either way a
+   prepared attacker could take advantage of the degraded performance to launch
+   an attack on the chain.
+3. Either of these cases are exascerbated if the same behaviour is coordinated
+   across all nodes, since this allows an attacker to exploit a performance
+   drop across the entire network.
+
+In situations where the inputs to a function are random or not observable by
+an adversary, or where the effects are purely local, it may therefore still be
+sensible to optimise for the average case. An example of such might be in
+local state query computations, since these are triggered only by a (trusted)
+local connection and do not lie on a critical path for block forging or
+adoption.
+
+The key point is that, in the Cardano setting, optimisations should be carefully
+considered and it is certainly not the case that better average-case performance
+is always desirable!