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REVIEW 5 minor 32 references

Symmetry all the way down

T0 review · 0 major / 5 minor · reviewed 2026-07-11 · grok-4.5

Pith's one-line read Asymmetric trust does not help solve hard distributed tasks beyond what ordinary symmetric quorums already allow.

desk verdict Tight depth-2 compiler plus matching impossibility: asymmetric trust adds no extra power for reliable broadcast or consensus. read the letter →

arxiv 2607.04887 v1 pith:TWYGVPQS submitted 2026-07-06 cs.DC

classification cs.DC
keywords asymmetrictrustByzantinequorumsystemsdepthhierarchycompilerreliablebroadcastconsensusconsistentfaulttolerance
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

Classical distributed systems force every participant to share the same failure assumptions. Asymmetric trust lets each participant keep its own private list of who it trusts, and this strictly enlarges the set of failure patterns the system can survive. The paper asks whether that extra survival power actually lets you solve harder problems such as reliable broadcast or consensus. Using a depth hierarchy that measures how much a process can still rely on others after faults occur, the authors prove the answer is no: every failure pattern in which a task that needs depth two or higher remains solvable can already be handled by an ordinary symmetric quorum system built from the asymmetric one. The only extra patterns that asymmetric trust alone tolerates leave every correct process with depth at most one, a regime in which only weak primitives such as consistent broadcast are possible. A simple compiler constructs the matching symmetric system, and the authors show that no such compiler can reach depth one. Thus the extra expressive power of asymmetric trust is real but is confined to the simplest tasks.

What carries the argument

The depth-parametrized compiler DC that expands every asymmetric quorum by replacing each of its members with one of that member’s own quorums; the resulting collection of unions is proved to satisfy the classical Q₃ consistency and availability properties precisely on the set of failure patterns that leave a process of depth ≥ 2.

What would settle it

Exhibit a concrete asymmetric quorum system and a failure pattern under which a process of depth two exists, yet the symmetric system produced by the compiler fails either consistency or availability; or exhibit any compiler that yields a valid symmetric system for all patterns that leave only depth-one processes.

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Extended reading notes

Core claim

For any Byzantine asymmetric quorum system, every failure scenario that still leaves at least one process with depth two or higher can also be tolerated by a suitably constructed Byzantine symmetric quorum system; consequently asymmetric trust never enlarges the set of scenarios in which tasks requiring depth two or more (reliable broadcast, consensus, …) are solvable.

Load-bearing premise

The depth hierarchy correctly separates task solvability so that depth at least two is necessary for reliable broadcast and consensus, and that this recursive depth is preserved when quorums are expanded by the compiler.

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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

0 major / 5 minor

Summary. The paper studies the precise power of asymmetric Byzantine quorum systems relative to classical symmetric ones, using the depth hierarchy of Amores-Sesar et al. (OPODIS ’25). It shows that every failure scenario in which at least one process has depth ≥ 2 (the regime needed for reliable broadcast and consensus) can also be tolerated by a symmetric quorum system obtained from a constructive compiler DC (Algorithm 1). The compiler expands each asymmetric quorum by the Cartesian product of the local quorums of its members; Lemmas 4.1–4.2 prove that the resulting system satisfies Q3 and the consistency/availability properties on F[2]. The extra failure patterns that only asymmetric systems tolerate leave all correct processes at depth ≤ 1, where only depth-1 tasks (e.g., consistent broadcast) remain solvable. Tightness is established by an impossibility (Lemma 4.4): no compiler can implement C[1]. The result holds for both known- and unknown-quorums settings, though only the former admits an efficient on-the-fly check.

Significance. If the lemmas hold, the paper cleanly delimits the advantage of asymmetric trust: it strictly enlarges the set of tolerable failure patterns, yet supplies no extra solvability for any task that requires depth two or higher. Practitioners can therefore replace complex asymmetric protocols for reliable broadcast or consensus by ordinary symmetric algorithms run on the compiled system, while still exploiting asymmetry for depth-1 primitives. The constructive compiler, the matching impossibility, and the explicit O(n) on-the-fly quorum check are concrete, reusable contributions that improve on the earlier C[∞] compiler. The work is self-contained once the depth hierarchy is taken as a black box, and the proofs rely only on standard intersection properties.

minor comments (5)
  1. Definition 3.3 states that “Any correct process pi has depth 0” and then adds higher depths; the wording is slightly ambiguous about whether depth 0 is the base case for every correct process or only for those that fail the inductive step. A one-sentence clarification would help.
  2. Table 1 lists “Asynchronous consensus [3]” at depth 10 and reliable broadcast at depth 3; a short remark that these numbers are the best currently known (not proven lower bounds) would prevent readers from treating them as tight.
  3. In the known-quorums cost analysis (Section 5.1) the claim of “at most 2n quorum checks” is correct for the two-step procedure, yet it would be useful to note that each individual check may itself be exponential in the worst case if the original asymmetric system is given explicitly as a power set.
  4. Several author self-citations appear as “Amores-Sesaret al.” (missing space or hyphen); a uniform spelling would improve polish.
  5. The arXiv identifier in the abstract header is 2607.04887; confirm that this matches the final submission identifier once the paper is published.

Circularity Check

0 steps flagged · score 1.0 of 10

No significant circularity: compiler C[2] and impossibility of C[1] are proved from B3/Q3 and the stated depth definition; self-citation of the authors' OPODIS '25 depth hierarchy frames interpretation but does not force the lemmas.

full rationale

The load-bearing chain is Definition 4.2 (C[d]), Algorithm 1 / Definition 4.3 (DC), then Lemmas 4.1–4.4 in Appendix A. Lemma 4.1 derives Q3(N) from B3(F) by contradiction on three expanded quorums and their originating asymmetric fail-prone sets. Lemma 4.2 shows consistency and availability of S = DC(Q) for every F in F[2] by taking a depth-≥2 process, expanding a quorum of depth-≥1 members, and invoking Corollary 4.1 / Lemma 4.3 (kernel containment from asymmetric consistency when F is foreseen by both parties). Lemma 4.4 exhibits a concrete 3-process B3 system whose F[1] violates Q3, so no symmetric system can cover all of F[1]. None of these steps is definitional of its conclusion, fitted to data, or reduced to an unverified uniqueness claim. The paper does rely on Amores-Sesar et al. (OPODIS '25) for the depth hierarchy and for the claim that reliable broadcast/consensus need depth ≥2 while consistent broadcast is depth-1; that citation supplies the interpretive link from the compiler theorems to 'task solvability,' but the mathematical statements 'DC implements C[2]' and 'no C[1]' are self-contained against the definitions restated in the manuscript. Score 1 reflects only that minor, non-load-bearing self-citation for framing—not circular derivation of the central results.

Assumptions & free parameters 0 free parameters · 4 assumptions · 1 invented entities

Pure theory paper. No fitted numbers. Load-bearing background is the standard Byzantine asynchronous model plus the B3 condition and the depth hierarchy taken from prior work of the same authors. The compiler itself is an invented algorithmic entity whose correctness is proved, not postulated.

assumptions (4)
  • domain assumption Asynchronous unauthenticated Byzantine model with authenticated channels (Section 2.1).
    Standard setting for the whole paper; all solvability claims are relative to it.
  • domain assumption B3-condition is necessary and sufficient for existence of an asymmetric Byzantine quorum system (Definition 3.2, citing Damgård et al. and Alpos et al.).
    Used to guarantee that the input to the compiler is well-formed.
  • domain assumption Depth hierarchy of Amores-Sesar et al. OPODIS ’25 correctly characterises which tasks are solvable at each depth (Table 1 and Section 3.2).
    The entire separation argument (extra tolerance only helps depth-1 tasks) rests on this characterisation.
  • ad hoc to paper All quorum systems may be taken to be canonical without loss of generality.
    Stated in Section 3; simplifies proofs but is asserted rather than proved for the non-canonical case.
invented entities (1)
  • Compiler DC (Algorithm 1) and the family of depth-parametrised compilers C[d] independent evidence
    purpose: Constructive reduction from asymmetric to symmetric quorum systems that preserves fault tolerance exactly on F[2].
    New algorithmic object introduced and proved correct in the paper; independent evidence is the explicit expansion procedure and the four lemmas.

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Cite this review

Pith. "Pith review of Symmetry all the way down." pith.science (2026). https://pith.science/paper/TWYGVPQS

@misc{pith2026260704887,
  author       = {Pith},
  title        = {Pith review of: Symmetry all the way down},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/TWYGVPQS}},
  note         = {Machine review of arXiv:2607.04887}
}
read the original abstract

Asymmetric trust generalizes classical symmetric quorum systems by allowing each process to specify its own failure assumptions. While this flexibility enables tolerance of strictly more failure scenarios, it is not known if, in these cases, it is actually possible to solve distributed tasks, and if so, which. We answer this question using the depth hierarchy for asymmetric trust (Amores-Sesar et al., OPODIS~'25), which characterizes how much a process must rely on others to solve a task. We prove that asymmetric trust does not increase the solvability of tasks requiring depth two or more, such as reliable broadcast or consensus. Specifically, for any Byzantine asymmetric quorum system, every failure scenario that permits solving a task requiring depth at least two can also be tolerated by a suitably constructed Byzantine symmetric quorum system. We show this via a compiler that transforms asymmetric quorum systems into symmetric ones. The additional failure patterns tolerated exclusively by asymmetric trust correspond to scenarios in which only simpler tasks requiring depth one or less (such as consistent broadcast) can be solved. We further prove that this result is tight in the depth hierarchy, meaning that there exist no compilers that produce symmetric quorum systems that are valid also in failure scenarios where correct processes have depths one or less. Our results clarify the precise power of asymmetric trust. While it strictly enlarges the set of tolerable failure patterns, it does not provide additional strength for solving tasks requiring depth two or higher.

Figures

Figures reproduced from arXiv: 2607.04887 by the authors.

Figure 1
Figure 1. The set of failure scenarios tolerated by any asymmetric quorum system Q (in striped red) is strictly larger than that which can be tolerated by the symmetric quorum system S = C(Q) derived by the compiler from Q (in green). However, in the failure patterns tolerated by Q but not S, only tasks requiring depth one can be solved, this excludes problems like reliable broadcast or consensus. Organization. The remainder … view at source ↗

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