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arxiv: 2605.20638 · v1 · pith:JJI3RDSRnew · submitted 2026-05-20 · 🧮 math.OC · cs.SY· eess.SY

Distributed and Decentralized Optimization Algorithms via Consensus ALADIN

Pith reviewed 2026-05-21 04:17 UTC · model grok-4.3

classification 🧮 math.OC cs.SYeess.SY
keywords distributed optimizationconsensus optimizationALADIN algorithmdecentralized optimizationquantized communicationconvergence guaranteesdirected graphsnon-convex optimization
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The pith

Consensus ALADIN extends ALADIN to directly solve distributed consensus optimization with global convergence for convex problems and local convergence for non-convex ones.

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

The paper develops Consensus ALADIN, or C-ALADIN, as an extension of the ALADIN framework tailored for consensus constraints in distributed optimization. It includes first-order and second-order variants that use Hessian approximations to limit information exchange. A decentralized implementation is introduced for operation over directed graphs using quantized communication and finite-time coordination protocols. The methods come with global convergence guarantees when the problem is convex and local guarantees otherwise, with the decentralized version approaching the solution within a quantization-dependent neighborhood. This matters for applications such as smart grids and machine learning where optimization must occur across networked agents with limited communication.

Core claim

C-ALADIN directly handles consensus constraints within the augmented Lagrangian-based alternating direction inexact Newton framework. It offers a first-order variant and a second-order variant that employs a Hessian approximation to avoid transmitting second-order information. The decentralized variant operates over directed graphs with quantized communication via a finite-time coordination protocol. Both versions guarantee global convergence for convex problems and local convergence for non-convex problems, while the decentralized iterates converge to a neighborhood of the optimum whose size is determined by the quantization level.

What carries the argument

Consensus ALADIN (C-ALADIN), which augments the ALADIN method to incorporate consensus constraints directly and uses finite-time coordination for decentralization over quantized directed graphs.

If this is right

  • The first- and second-order variants substantially reduce communication and computational costs relative to prior decentralized methods.
  • Global convergence holds for convex objective functions under the stated conditions.
  • Local convergence is guaranteed for non-convex problems near the solution.
  • The decentralized version converges to an optimum neighborhood bounded by the quantization level.
  • Fast local convergence is preserved despite the approximations and decentralization.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • If the finite-time coordination protocol succeeds, the approach could apply to other networked systems with directed topologies and bandwidth limits.
  • Quantization level acts as a tunable parameter balancing accuracy and communication overhead in practical deployments.
  • Hybrid methods combining C-ALADIN with event-triggered or asynchronous updates may further reduce resource use in dynamic environments.
  • Scalability to very large networks could be tested by applying the algorithm to distributed training tasks in machine learning.

Load-bearing premise

The finite-time coordination protocol must be realizable over the directed graph with the specified quantized communication to ensure the neighborhood convergence in the decentralized case.

What would settle it

A simulation on a directed graph with quantization levels too coarse for the coordination protocol to achieve consensus within the assumed bounds, showing whether the iterates remain within the predicted neighborhood or diverge.

Figures

Figures reproduced from arXiv: 2605.20638 by Apostolos I. Rikos, Jingzhe Wang, Karl H. Johansson, Xu Du.

Figure 1
Figure 1. Figure 1: Comparison of convergence performance among Pull￾FTERC [21], AsyAD-ADMM [55], QuAsyADMM [44], the proposed first￾order C-ALADIN (12), and its quantized decentralized version (27) under different quantization levels ∆ = 10−4, 10−5, 10−6 [PITH_FULL_IMAGE:figures/full_fig_p010_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Convergence Algorithm 4among Algorithm 1, the first-order C-ALADIN in (12), its quantized decentralized version (27), and the decentralized approximate second-order C-ALADIN (Algorithm 4) under different quantization levels ∆ = 10−4, 10−5, 10−6. 0 50 100 150 10-8 10-6 10-4 10-2 100 102 [PITH_FULL_IMAGE:figures/full_fig_p011_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Convergence comparison among Algorithm 1, Algorithm 3 with ∆ = 10−9, and GIANT [22]. of the inner consensus QP solver; therefore, only the case with ∆ = 10−9 is shown. In this setting, Algorithm 3 converges nearly as fast as Algorithm 1, reaching a neighborhood of the optimal solution. For comparison, the centralized GIANT method, which is based on primal decomposition, is included. B. Non-convex Case For … view at source ↗
read the original abstract

Distributed optimization has found widespread applications in smart grids, optimal control, and machine learning. This paper studies distributed consensus optimization. We extend the Augmented Lagrangian-based Alternating Direction Inexact Newton (ALADIN) framework to propose Consensus ALADIN (C-ALADIN) with a central coordinator, which directly handles consensus constraints. Our C-ALADIN algorithm admits both a first-order variant and a second-order variant that employs a Hessian approximation, avoiding direct transmission of second-order information while preserving fast local convergence. We then develop a decentralized version of C-ALADIN that operates over directed graphs with quantized communication, using a finite-time coordination protocol. For both versions, we establish global convergence guarantees for convex problems and local convergence guarantees for non-convex problems. For the decentralized case, the iterates converge to a neighborhood of the optimum determined by the quantization level. Numerical results demonstrate that our methods retain fast convergence while substantially reducing communication and computational costs compared to existing decentralized approaches.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 2 minor

Summary. The manuscript extends the ALADIN framework to Consensus ALADIN (C-ALADIN) for distributed consensus optimization problems. It introduces a centralized coordinator version with both first-order and second-order variants (the latter using a Hessian approximation to avoid transmitting second-order information), followed by a decentralized variant that operates over directed graphs using quantized communication and a finite-time coordination protocol. Global convergence guarantees are established for convex problems and local convergence guarantees for non-convex problems; in the decentralized case the iterates are shown to converge to a neighborhood of the optimum whose size is determined by the quantization level. Numerical results are presented to demonstrate retained fast convergence with reduced communication and computational costs relative to existing decentralized methods.

Significance. If the stated convergence guarantees hold under the paper's assumptions, the work provides a useful extension of ALADIN-type methods to consensus-constrained problems with practical communication constraints. The combination of a Hessian-approximation second-order variant, quantization, and a finite-time protocol on directed graphs addresses real constraints in applications such as smart grids and networked control, while the numerical comparisons illustrate concrete efficiency gains. The explicit neighborhood result for the quantized decentralized case is a concrete, falsifiable prediction that strengthens the contribution.

major comments (1)
  1. [§5.3] §5.3 (Decentralized convergence analysis) and the finite-time coordination protocol description: the neighborhood convergence claim for the decentralized iterates depends on the protocol achieving the required approximate consensus in finite time on a directed graph under uniform quantization. Standard finite-time consensus results on directed graphs require strong connectivity together with weight-balancing or push-sum mechanisms; persistent bounded quantization errors can prevent exact finite-time termination or inflate the effective neighborhood. The manuscript does not explicitly verify that the protocol construction satisfies these conditions while preserving the ALADIN update structure, which is load-bearing for the decentralized guarantee.
minor comments (2)
  1. [Abstract] The abstract states that the methods 'substantially reduc[e] communication and computational costs' but does not name the specific baseline algorithms used in the numerical comparison; adding this would improve clarity.
  2. [Section 2] Notation for the consensus constraint and the quantization operator is introduced without a dedicated table or summary; a short notation table would aid readability.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their thorough review and constructive comments on our manuscript extending the ALADIN framework to consensus optimization problems. We address the major comment point by point below and have made revisions to improve the clarity of the decentralized analysis.

read point-by-point responses
  1. Referee: [§5.3] §5.3 (Decentralized convergence analysis) and the finite-time coordination protocol description: the neighborhood convergence claim for the decentralized iterates depends on the protocol achieving the required approximate consensus in finite time on a directed graph under uniform quantization. Standard finite-time consensus results on directed graphs require strong connectivity together with weight-balancing or push-sum mechanisms; persistent bounded quantization errors can prevent exact finite-time termination or inflate the effective neighborhood. The manuscript does not explicitly verify that the protocol construction satisfies these conditions while preserving the ALADIN update structure, which is load-bearing for the decentralized guarantee.

    Authors: We thank the referee for highlighting this important aspect of the analysis. Upon review, we acknowledge that the original manuscript could benefit from a more explicit verification of the protocol's properties. In the revised manuscript, we have added a new paragraph in §5.3 that explicitly states the assumptions on the directed graph (strong connectivity and the use of a weight-balancing or push-sum mechanism as per standard finite-time consensus protocols). We verify that under uniform quantization, the protocol achieves approximate consensus in finite time, with the approximation error bounded by the quantization level. This bounded error is then incorporated into the convergence analysis as a perturbation, preserving the ALADIN update structure and leading to convergence to a neighborhood of the optimum. We believe this addition addresses the concern while maintaining the integrity of the decentralized version. revision: yes

Circularity Check

0 steps flagged

No significant circularity detected in derivation chain

full rationale

The paper extends the existing ALADIN framework to C-ALADIN for direct handling of consensus constraints, introduces first- and second-order variants, and develops a decentralized version via a finite-time coordination protocol on directed graphs with quantization. Global convergence for convex problems and local convergence for non-convex problems, with neighborhood size determined by quantization level, are established through standard analysis of the algorithm iterates and protocol properties rather than by redefinition, fitting to the target result, or load-bearing self-citation chains. No equations or steps reduce the claimed guarantees to inputs by construction; the work remains self-contained against external benchmarks from optimization theory.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The central claims rest on standard assumptions from distributed optimization literature such as convexity or smoothness of local objective functions and the existence of a finite-time coordination protocol under quantization.

axioms (2)
  • domain assumption Local objective functions are convex (for global convergence) or twice continuously differentiable (for local convergence)
    Invoked to establish the stated convergence guarantees for the C-ALADIN variants.
  • domain assumption A finite-time coordination protocol exists over the directed graph with quantized communication
    Required for the neighborhood convergence result in the decentralized case.

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