Having been studied for few decades, research streams on SSOA have focused on the creation of optimiza-
tion algorithms and mathematical models for various problem types, including single, dual, multi-sourcing,
the incorporation of discounting and inflation, and how closed-loop supply chain and sustainable configura-
tions could be embedded (Aouadni et al. (2019); Pasquale et al. (2020); Naqvi and Amin (2021)).
However, according to a recent review, only 34% of the studies in the sample were conducted based on
real industrial cases or real collected data (Pasquale et al. (2020)). The literature consists mainly of supply
chains of limited sizes - our review shows that for real cased studies, the maximum scale of supply chain
studied under the SSOA literature consists of 15 suppliers and 90,000 variables (See Table 1).
In light of the above, the industrial trend shows an interesting conundrum. Companies with large scale
complex supply chains would be more likely to be in need of automated algorithms to configure their supply
chains, as the problem space becomes too complex to tackle manually. Recent studies show an increasing
trend in the complexity of supply chains (e.g. Christopher (2021)), add to the urgency of need for algorithms
that can handle high numbers of products and suppliers in SSOA optimization.
Large-scale SSOA across multiple-tiers brings about unique advantages as well as challenges. Current
little to no attention paid to the multi-tier and large-scale nature of the problem presents a gap between
theory and practice, as extant algorithms are not adopted to serve the needs of industries with complex
supply networks including aerospace engineering, industrial machinery and medical devices. Taking a multi-
tier perspective, where suppliers’ preferences to work with one another are incorporated is realistic and helps
improve cooperation. Large-scales of complex supply networks are another reality that have not yet been
tackled. Several industries have to select and assign hundreds, if not thousands of products/items during
procurement due to the complexity of engineered products being built. However, as our literature review
shows, scalability of existing approaches are hitherto unknown. On the other hand, the solution approach we
present that tackles these issues do increase the complexity of the problem due to the inherent dependency
of the tiers on each other. Moreover, the collection of data for multiple tiers is difficult and the OEM needs
to be in a position to be able to do so, for our approach to be applicable.
In this paper, we present a set of techniques for researchers to consider solving the SSOA at scale and
discuss how it can be extended to SSOA variants. We test our proposed approach using a real-life case
study involving 2 tiers, 7,200,020 decision variables and 70 suppliers, constituting the largest SSOA use case
to date. The case study includes a time constraint, in that the SSOA allocation needs to be done under
a reasonable time limit that would allow procurement officers to negotiate with suppliers proposing bids
during a multi-round bidding process taking place during a sourcing conference. Our work presents a novel
SSOA problem which considers allocations on two-tiers allowing suppliers to bid to work with each other and
penalize non-preferred allocations through the penalty constraints. The problem characteristics additionally
include NP-hardness and dual-sourcing constraints.
Our results show that there exists few methods in literature that can handle a real-time constraint.
Problem formulation, as well as modelling language and solver engine all play a role in the success of the
algorithm, whereas extant literature typically treats these solution components in isolation from one another.
We go on to argue that all three components need to be considered as they impact one another.
Thus, the research question of the study is given as: How to automate a dynamic/real-time real-life large-
scale double supplier allocations at two-tiers of a multi-item supply chain with dual-sourcing and penalty
constraints? The contributions of the study, while solving the research questions are summarized below.
•A novel multi-item multi-supplier order allocation problem with penalty constraints and dual-sourcing
is presented.
•The problem involves double allocations at two different tiers of the supply chain which result in
cooperation between tiers and facilitate supplier preferences to work with each other.
•Mixed-Integer Linear Programming models are developed for supplier order allocations at individual-
tiers as well as integrated allocation.
•A generic approach to solve large-scale problems using mathematical programming is presented.
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