add proposal/discussion on capacity planning

Author: florolf

archive/2026-05-10-fcfs-capacity-allocation.md

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+# Proposal: Dynamic witness capacity allocation
+
+This is a proposal I made to increase witness utilization, originally posted on
+Matrix. I also had some in-person discussions about this with rgdd, notes on
+those below.
+
+## Original proposal
+
+Before the witness network, the process for a log operator for finding witness
+was:
+
+ (1) Find out that a witness/witness operator (conflated for now) exists
+ (2) Vet the witness
+ (3) Ask the witness to configure the log and wait for that to happen
+
+(or more likely: Ask somebody to set up a witness for them)
+
+The witness operator perspective mostly comes in at step 3 where they need to
+decide if they can (and want, but let's suppose this is always the case for
+now) support the log. This is not a trivial "yes" because there always will be
+capacity limits to consider.
+
+The witness network helps with 1 and 3 by providing a list of witnesses and
+managing their configuration. This means that for the log operator, they only
+need to talk to the witness network maintainers and once their log gets
+accepted, they can have their pick from any witness that configures the
+respective log lists (after a short while once they have updated them).
+
+This currently works by the witness network maintainers packing logs into
+specific "performance tiers" defined by the maximum number of logs and
+witnessing requests per second (qps) a given witness can support. Witnesses are
+supposed to configure log lists starting from the lowest performance profile up
+to the highest one that they can still support in aggregate (i.e. including the
+lower tiers).
+
+In the following, we'll mostly consider the qps dimension of resource usage,
+since this is the much tighter bottleneck in practice.
+
+The way logs get assigned to performance profiles is currently not documented,
+but seems to follow a strategy that tries to minimize qps utilization per
+performance profile (i.e. even though the 10qps list would have capacity to
+accommodate a 1qps log, 1qps logs get allocated to the 100qps list first since
+adding a 1qps log to the former would use up 10% of its capacity).
+
+There are a number of problems with the current approach:
+
+ - Logs will only make use of a subset of witnesses available to them, but the
+   log list capacity planning can't know which, so it must happen based on
+   worst case (all logs in a list fully utilize all witnesses available to
+   them), which leads to underutilization within a given performance profile.
+
+ - Similarly, a witness must assume that all logs from the lists it configures
+   will make use of it, so it configures less log lists than it could
+   actually handle in practice, also leading to underutilization and less
+   witnesses being available to logs in higher profiles.
+
+ - On top, this scheme requires picking somewhat arbitrary bucket sizes for the
+   performance profiles.
+
+The core issue here is that the witness network itself tries to do capacity
+planning for the witness operators and tries to do so for all of them
+simultaneously. Furthermore, it does this in advance, without knowing how log
+operators will make use of witnesses.
+
+An alternative approach could be to remove this capacity planning component
+from the witness network and only have it be a place where witnesses and logs
+can advertise their existence. To that end, consider the following architecture:
+
+ - There is only one list/pool of logs (containing the same data as
+   today, including estimated qps). When a log is retired it is marked as
+   inactive. (Could also be removed from the list, but the following description
+   is clearer that way)
+
+ - As long as they are below capacity, witnesses keep importing *all* logs in
+   this list. They also provide an interface where log operators can query if
+   their log is configured (i.e. the witness would accept add-checkpoints
+   requests for it). A log will be advertised as supported as long as
+   activating that individual log would not exceed that witness's capacity.
+
+ - Witnesses keep track of utilized qps. They do this by aggregating the
+   advertised qps from the log list over all the logs that have sent them at
+   least one checkpoint and are not marked as inactive in the log list.
+
+ - Once a witness has reached its locally-configured qps limit, it stops
+   advertising support for/accepting checkpoints from logs from which it
+   hasn't received any checkpoints yet.
+
+ - It still keeps updating the log list to see if logs have been marked as
+   inactive, which might free up capacity if one such log has previously been
+   active on this witness. In that case, it starts advertising logs again.
+
+This resolves the problems described above:
+
+ - If a log does not decide to make use of a witness for one reason or another,
+   the capacity for that log is not needlessly reserved on that witness.
+
+ - All witnesses with spare capacity are available to all logs.
+
+ - Witness operators have fine-grained control over the capacity of their
+   witness and witnesses can reach closer to 100% utilization.
+
+When a witness is at capacity, a witness operator can easily deploy another
+witness which will start picking up new/different logs (since a single log is
+unlikely to use multiple witnesses from the same operator unless for
+redundancy).
+
+But other than increased complexity, there are also some further downsides:
+
+ - Unlike today, a log can't be certain it will get picked up by the witnesses
+   it likes (or possibly any witness) if it has been accepted into the network.
+   Thus, a log operator needs to query individual witnesses to see if they have
+   picked up the log. But to some extent this already the case today since log
+   list downloads might only happen weekly for example.
+
+ - Since logs decide which witnesses they claim, ecosystem diversity can be
+   affected by log choices. I.e. a 1qps log takes a hypothetical 1qps witness
+   fully out of the ecosystem, but it would likely be better for resilience
+   to partition that same witness among 10 0.1qps logs.
+
+ - There is also the potential for race conditions. I.e. a log operator looks
+   at all the witnesses with spare capacity, carefully vets them and a few
+   seconds before they start making use of it, somebody else claims all spare
+   capacity of that witness.
+
+## Discussion notes
+
+Discussion based on this with rgdd during the 2025-05 Sigsum community meeting.
+I'm writing this from memory a few days later so it's probably a bit
+inaccurate.
+
+ - The main issue with managing the witness network is that we're dealing with
+   a scarce resource (witness capacity), if every witness could do 500 qps and
+   had unlimited storage, all witnesses could just be free-for-all and we
+   wouldn't have to have these discussions. However, this is not the case.
+
+ - The goal of the witness network is not only to help coordinate between logs
+   and witnesses, but also manage this scarce resource in a thoughtful way.
+   This is the part I wasn't aware of as being a deliberate decision, which
+   invalidates the above proposal to an extent.
+
+   At the same time, the witness network aims to serve the long tail of logs.
+   The assumption is that heavy hitters such as MTC will curate their own set
+   of witnesses for policy reasons anyway.
+
+   So this fits together well - for example an average Sigsum log does 0.1qps.
+   Others might do even less or only produce a checkpoint sporadically. Thus
+   even 10qps of capacity could serve a lot of logs (possibly *all* of the
+   long-tail ones).
+
+ - A part of the awkwardness is that the witness network maintainers do not
+   want to be in the position to be able to DoS logs. Thus, they deliberately
+   aren't able to cause deconfiguration of logs. However, logs (especially
+   things like CT logs) retire frequently. Ideally, logs would be able to
+   signal this to the world (and witnesses in particular) cryptographically,
+   but the proposed mechanism (tombstones) has not been fleshed out yet.
+
+   Putting this off was "fine" since there was enough spare capacity as not to
+   have to worry about this now.
+
+ - However, CT logs getting started to be admitted to the witness network
+   compounded this issue and prompted the creation of the 100k log list (which
+   in turn prompted the above proposal). Maybe creating such a big list was a
+   mistake.
+
+ - Maybe eventually having multiple 10qps lists (maybe grouped somehow so that
+   witness operators can choose which parts of the ecosystem to support) would
+   be better. This would also help with better bin-packing on the witness side.
+
+   Probably leaving the 100k CT log list as is though?
+
+Further notes added by rgdd:
+
+ - Seems like 100qps might have been an unnecessarily big jump, which, e.g.,
+   have made it difficult for some (potential) witness operators to configure
+   it.
+
+ - When doing some napkin math, the current 10qps list would likely be able to
+   accomodate a lot of the "long tail" / lower-frequency logs; and perhaps one
+   or two high-profile ones with higher qps like Go's checksum database.
+
+ - From CT, we're probably expecting something like 10 qps.
+
+ - From MTC, we're probably talking about a qps in the same ballpark (?)
+
+ - We don't have that many other high-qps logs right now, and having something
+   like 10qps reserved for that will probably serve us well for some time.
+
+ - So if it increases the number of participating witnesses, then it might be a
+   better trade-off to have several 10qps lists (.2, .3) where we basically
+   have one which is the "longer tail one" and another which is the "higher-qps
+   one". And the "higher qps-one" we expect to fill up a bit quicker, and when
+   it's full we will create another one. Or maybe we should even create multipe
+   ones right away, and witnesses configure as many as they can even though,
+   e.g., .3 is not being populated quite yet?
+
+ - Working on defining tombstone for proper deallocation = worth while to do
+   soon since CT is interested in taking part (and sharding is frequent there).