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REVIEW 4 major objections 5 minor 83 references

Automata Learning -- Expect Delays!

T0 review · 4 major / 5 minor · reviewed 2026-08-05 · deepseek-v4-flash

Pith's one-line read Active learning of automata with stochastic delays works best split into two stages — learn the input/output logic first, then route a postman tour that samples every transition's delay enough times — beating sampling inside classic L* by a

desk verdict A genuinely new two-phase method for learning stochastic transition delays, with the central caveat that its state-separation case rests on an uncertified d-step confluence parameter the paper itself admits can fail. read the letter →

arxiv 2508.16384 v1 pith:64KYD4XJ submitted 2025-08-22 cs.FL cs.SE

classification cs.FLcs.SE MSC 68Q4568Q32
keywords activeautomatalearningMealydelaymachinesstochastictransitiondelaysRuralPostmanProblemL*algorithmd-stepconfluencedistributionsamplingblack-boxtiminganalysis
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

This paper is the first, by its own account, to build a formal framework for actively learning automata whose transitions carry stochastic delays. Its central claim is that delay sampling should be separated from logical behavior learning: first learn the minimal Mealy machine with classic L* querying, then use the learned structure to compute a minimum-cost postman route that visits every transition at least k times, so each delay distribution can be estimated from k samples. When identical input/output behaviors can stem from different delay characteristics, the paper expands the learned machine into a history-indexed machine Md and merges back only states whose sampled delays are statistically indistinguishable. The payoff the paper demonstrates is practical: across a wide benchmark range, the two-stage method outperforms the naive sampling-based L* by a large margin, and a relational-database join-order case study shows the method can discover that the order of operations changes completion time from tens to hundreds of milliseconds.

What carries the argument

Mealy delay machine (MDM): a Mealy machine with a per-transition CDF of delay, the object to be learned. The d-step confluence criterion — two I/O-equivalent states must reconverge within d steps — justifies the expanded machine Md, whose states are (minimal-state, last-d-steps history) pairs, injectively bisimilar to the true machine (Theorem 1) and of size O(|S'|·|I|^d) (Theorem 2). The Rural Postman Problem over the learned graph, solved by min-cost flow, yields the cheapest tour whose edge multiplicities give k samples per transition, with resets modeled as edges to the root. A final merge collapses states with statistically equal sampled delays.

What would settle it

Take a system whose delays depend on history but whose logical states never reconverge (the paper's non-confluent example), run the learner with d=1, and inspect the delay samples on one expanded state: if the empirical distribution is bimodal, or its deviation from the true branch CDFs does not shrink as k grows, the d-step confluence premise is what keeps the reported delays trustworthy. Complementary check: on a d-step confluent system, estimated CDFs should converge to the true ones as k grows at the predicted rate.

Watch

Extended reading notes

Core claim

Mealy delay machines (MDMs) are Mealy machines whose transitions carry continuous probability distributions of delay, observable only through samples. The paper's central proposal: learn the minimal underlying Mealy machine with classic L* query learning, then solve a directed Rural Postman Problem over that machine to build the cheapest input sequence visiting every transition k times. Since identical input/output histories can hide different delays, the learner expands the learned machine into Md, whose states carry the last d steps of history, samples Md, then merges states with statistically indistinguishable delays. The paper claims this is the first formal framework for active learning

Load-bearing premise

The load-bearing premise is that the true system is d-step confluent for the learner's chosen d — any two input/output-identical states must reconverge within d steps — because if d is too small, delay samples on a state of the expanded machine blend several genuinely different distributions and the reported delay curves are a mixture, not a faithful estimate.

Editorial extensions

If this is right

  • Sampling effort decouples from the L* query tree: machines with large alphabets that push sampling-based L* past 10^8 actions (OpenSSH, TCP servers) stay feasible when the expanded machine Md is used for sample collection.
  • Raising d never loses distinctions already made, since Md is injectively bisimilar to Md+1 (Lemma 2); a larger d can only improve the faithfulness of the learned delay structure, at exponential state cost.
  • The merge step, together with the stutter-free and unique-root assumptions, recovers a minimal MDM that matches the true delay structure when the assumptions hold, so the final model is both minimal and delay-accurate.
  • The database case study shows the method works against real measured delays: it learns that join-completion time depends on the last one or two joins in the order, information invisible to logic-only learning.
  • The k-sample budget can be set before learning from accuracy targets: the deviation bound for empirical CDFs (or, for exponential delays, a closed-form relative-error formula) gives the k needed for a chosen error and confidence.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • A natural extension the paper leaves implicit is an adaptive sampling budget: instead of a fixed global k, allocate extra samples to transitions whose estimated delay distributions are still far apart or high-variance, reusing the same postman-route machinery with per-edge weights.
  • The paper's admitted mixture problem is itself diagnosable: when the true machine is not d-step confluent at the chosen d, samples on a state of the expanded machine should show excess dispersion or multi-modality, so a variance or modality check on collected samples could offer a data-driven way to raise d.
  • The database join-order study is the seed of a general black-box performance profiler: the same pipeline could attach measured timing distributions to logical states of compilers, network stacks, or query planners, and flag histories whose delay differs despite identical logical results.
  • The exponential dependence of Md on d and alphabet size sets the practical ceiling; the empirical tables show the unique-root assumption is the lever that keeps real benchmarks tractable, and a statistical stopping criterion for d would make the method's main tuning parameter self-tuning.
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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

4 major / 5 minor

Summary. The paper proposes an active automata learning framework for Mealy machines with stochastic transition delays (Mealy delay machines, MDMs). It separates the learning of I/O behavior from the collection of delay samples: first an ordinary L*-style algorithm learns the underlying minimal Mealy machine, then the learner plans a low-cost circuit of inputs—via a reduction to the directed Rural Postman Problem—that visits every transition at least k times to estimate delay CDFs. In the more general case where I/O-equivalent states may have distinct delay behavior, the paper constructs an expanded machine Md under assumptions of stutter-freeness and d-step confluence, samples delays on Md, and then merges statistically equivalent delay states. Theoretical results include injective bisimilarity of the true machine to Md (Theorem 1), a bound on |Md| (Theorem 2), monotonicity in d (Lemma 2), and a general product construction (Theorem 3). The empirical evaluation compares the sampling-aware L* baseline with the Md-based method on a public benchmark suite and a PostgreSQL join-order case study, reporting substantially fewer actions for the new method.

Significance. If the results hold, this is a useful first step toward active learning of automata with stochastic delays, and the separation of I/O learning from delay sampling is a natural and potentially impactful idea. The paper ships an artifact, contains a formal semantics for MDMs in Appendix A, and provides a reproducible benchmark setup, which are strengths. However, the central correctness guarantee for the state-separation part depends on a parameter d that is neither known in advance nor certified, and the empirical evaluation uses a mean-based equality criterion and single runs. These issues do not invalidate the algorithmic idea but limit the strength of the claims as currently stated.

major comments (4)
  1. [Sec. 5.2, 'Selection of d' and Def. 6] The correctness of the Md construction and the subsequent merging step requires the SUL to be stutter-free and d-step confluent for the chosen d. The paper admits that d is generally unknown and that some MDMs have no suitable d (Fig. 2b). If d is too small, a state of Md corresponds to multiple true states and sampling yields 'an uncontrolled mixture of multiple CDFs,' as the authors state. The later merging step compares those already-mixed samples and cannot split them, so D' may not be a faithful delay model. Lemma 2 only gives monotonicity, not a stopping rule or certificate. The benchmarks choose d with knowledge of the true system (M1 requires d≥2; the database SUL is 3-step confluent), so the failure mode is not exercised. Please either provide a data-driven method to choose/certify d, or explicitly restrict the formal claims to 'given a d that is known to satisfy the confluence
  2. [Sec. 6, 'Experimental setup' and Appendix C] The distribution-equality test used for state merging is a heuristic based on the sample mean: two distributions are considered different only if the relative deviation of the means exceeds 20% (and the absolute deviation is at least 0.01). This does not test equality of CDFs as required by Def. 4 and the problem statement. Two distributions with the same mean but different shapes (e.g., exponential vs. Erlang with the same mean) will be merged, yielding an incorrect D'. Please either use a proper CDF-level test (e.g., Kolmogorov–Smirnov) or explicitly narrow the target class to distributions whose differences are captured by the mean.
  3. [Sec. 4.2 and Appendix D] The text says the sampling sequence will be constructed 'with minimum total cost,' but then notes the general RPP is NP-complete and, when mandatory edges are not weakly connected, the implementation resorts to a minimum spanning arborescence heuristic. Appendix D further states that 'we used a heuristic approach to find a solution to the RPP.' Therefore the evaluated procedure is not the optimal one in general. Please state which experiments use the exact min-cost flow formulation and which use the heuristic, and report the heuristic's suboptimality gap where possible.
  4. [Fig. 4 and Appendix D] The empirical evaluation reports single runs without repetitions, confidence intervals, or statistical tests. More importantly, it reports only the number of steps and resets, not the quality of the learned delay model (e.g., whether the merged D' correctly recovers the true delay states and whether the estimated CDFs are close to the true ones). Since the equality tests are stochastic, a single run may silently produce an incorrect D'. Please report repeated runs and accuracy metrics for the learned delays.
minor comments (5)
  1. [Sec. 2, Def. 5 vs. Def. 8] The definition of a sink in Sec. 2 uses δ(s,i)=s for all i, while Def. 8 uses δ(s,i)∼IO s for all i. Align the terminology.
  2. [Sec. 4.2, last paragraph] 'minimum spanning arborescence of minimum weight' is redundant; use 'minimum weight spanning arborescence.'
  3. [Fig. 4] The y-axis is labeled 'Total number of steps and resets,' but the plot shows a single total. Clarify whether resets are included in the total or counted separately.
  4. [Appendix D tables] The repeated column header 'Using expansion Md' is confusing; the left block appears to be the adapted L* baseline. Reformat the tables to make the columns unambiguous.
  5. [Sec. 7, d=0 result] The text says for d=0 the algorithm 'reports that ... the commit will be delayed with a mean duration of 191 ms,' then immediately says this is not a meaningful average. Rephrase to avoid appearing contradictory.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the derivation chain is self-contained, and the selection-of-d limitation is a correctness risk, not a circular reduction.

full rationale

The derivation chain is self-contained. The MDM model (Def. 3) and the learning problem are defined independently of the proposed algorithm; the algorithm separates I/O learning from delay sampling and uses an RPP tour to cover transitions (Sec. 4.2), which is an optimization over the learned MM, not a prediction fitted to the delay data. The expansion construction Md (Sec. 5.2) is defined purely from the learned minimal MM and the parameters d and the loop-free/stutter-free/confluence assumptions (Defs. 5-7); Theorem 1 is proved from these definitions and does not import the conclusion. The state-merging step compares fresh delay samples and is explicitly optional. The self-citations (e.g., [9,27,31,45]) provide background semantics and model-family context; none carries a load-bearing uniqueness theorem or ansatz. The limitation the paper itself states in Sec. 5.2 -- that a too-small d yields 'an uncontrolled mixture of multiple CDFs' -- is a correctness risk for the parameter choice, not a circular step: the output D' is not defined in terms of the samples in a way that forces the claimed performance. The empirical threshold of 20% is derived from the exponential sample-size calculation in Appendix C and the synthetic benchmarks use exponential delays; this is a transparent calibration match, but it does not make the method's performance claim reduce by construction, and the same threshold is applied to both compared methods. Overall, no prediction or first-principles result in the paper is equivalent to its inputs by construction.

Assumptions & free parameters 4 free parameters · 6 assumptions · 0 invented entities

The method is parameterized by k, d, and statistical thresholds, and rests on structural assumptions (stutter-free, d-step confluence, sink delay-ignorant) about the system under learning. These assumptions are clearly stated, but the most fragile one, d-step confluence, cannot be checked a priori and its failure can invalidate the sampling scheme.

free parameters (4)
  • k = 1000
    Number of delay samples per transition. Derived in Appendix C from a 0.081 relative error bound at 99% confidence for exponential distributions, but chosen globally as a constant for the method.
  • d = varied 0..5 in experiments
    Confluence depth used for state separation. It is a user-selected parameter that must be large enough to distinguish delay-relevant histories; too small a d can mix CDFs (Sec. 5.2).
  • distribution equality thresholds = 20% relative deviation, 0.01 absolute deviation
    Thresholds used to decide whether two sets of delay samples come from the same distribution. Calibrated in Appendix C for exponential samples; not validated for other delay families.
  • RPP edge cost = 1 (constant)
    Each input action and each reset is assumed to cost 1 in the RPP formulation. Context-specific costs are stated as supported but only the constant choice is evaluated.
assumptions (6)
  • domain assumption The teacher provides a perfect equivalence oracle for the underlying time-abstract Mealy machine.
    Stated in Sec. 1 and used throughout: 'the teacher has full knowledge of the time-abstract behavior' and the problem statement assumes 'an EO for the underlying MM'. This is standard in AAL but strong for real systems.
  • domain assumption Stutter-free condition (Def. 5): self-loops in the minimal MM indicate no state change in the MDM.
    Required for the expansion construction in Sec. 5.2; if violated, transitions between distinct states with the same I/O behavior would be missed.
  • domain assumption d-step confluence (Def. 6): I/O-equivalent states have identical delay behavior after at most d steps.
    Load-bearing for the expanded machine Md and for state separation. The authors note that non-confluent cases (Fig. 2b) lead to uncontrolled CDF mixtures.
  • domain assumption Sink delay-ignorant (Def. 8): sink nodes do not split by delay behavior.
    Assumed across all empirical studies (Sec. 6) to reduce the expansion size; not theoretically justified beyond convenience.
  • domain assumption The learner obtains independent samples from each transition's delay CDF on every traversal.
    Formalized in the MDM semantics (Appendix A); independence underlies the DKW/Chebyshev sample-size arguments and the statistical tests.
  • domain assumption Visiting each transition at least k times yields statistically sufficient estimates of the delay distributions.
    The problem statement in Sec. 3 fixes k as the decisive accuracy parameter; the paper does not address how to choose k when delays are not exponential.

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Cite this review

Pith. "Pith review of Automata Learning -- Expect Delays!." pith.science (2026). https://pith.science/paper/64KYD4XJ

@misc{pith2026250816384,
  author       = {Pith},
  title        = {Pith review of: Automata Learning -- Expect Delays!},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/64KYD4XJ}},
  note         = {Machine review of arXiv:2508.16384}
}
abstract

This paper studies active automata learning (AAL) in the presence of stochastic delays. We consider Mealy machines that have stochastic delays associated with each transition and explore how the learner can efficiently arrive at faithful estimates of those machines, the precision of which crucially relies on repetitive sampling of transition delays. While it is possible to na\"ively integrate the delay sampling into AAL algorithms such as $L^*$, this leads to considerable oversampling near the root of the state space. We address this problem by separating conceptually the learning of behavior and delays such that the learner uses the information gained while learning the logical behavior to arrive at efficient input sequences for collecting the needed delay samples. We put emphasis on treating cases in which identical input/output behaviors might stem from distinct delay characteristics. Finally, we provide empirical evidence that our method outperforms the na\"ive baseline across a wide range of benchmarks and investigate its applicability in a realistic setting by studying the join order in a relational database.

Figures

Figures reproduced from arXiv: 2508.16384 by the authors.

Figure 1
Figure 1. Minimal MM M′ The useful case. To make the analysis approachable, we pro￾pose restrictions on what the delay behavior induces compared to the I/O behavior alone. The first one states that self-loops in the underlying minimal MM indicate that no changes to the system are made. This prevents transitions between distinct states of D that have the same I/O behavior, which is why we use the term stutter-free: Definition … view at source ↗
Figure 2
Figure 2. Confluent and non-confluent expansions of [PITH_FULL_IMAGE:figures/full_fig_p010_2.png] view at source ↗
Figure 3
Figure 3. Expanded Md for minimal MM M′ from [PITH_FULL_IMAGE:figures/full_fig_p012_3.png] view at source ↗
Figures from the paper (2 more)
Figure 4
Figure 4. Figure 4: Number of total actions for different models and selections o [PITH_FULL_IMAGE:figures/full_fig_p014_4.png]
Figure 5
Figure 5. Figure 5: Impact of k (d = 0) Evaluation. RQ1: Across all cases, the sampling￾based L ∗ is outperformed by the approach using the expanded MM Md by a large margin [PITH_FULL_IMAGE:figures/full_fig_p015_5.png]

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Pith tools

Reviewed August 5, 2026 · model on record in the stance chip above.