# Publish/Subscribe-Middleware-Based Intelligent Transportation Systems: Applications and Challenges
## 3 Background of Middleware  
Researchers and vendors have defined middleware in various ways. Microsoft describes it as a software layer that facilitates communication and data management between an operating system (OS) and applications, acting as a hidden translation layer that enables seamless interaction across distributed components [19]. [PERSON] [20] defined middleware as a bridge between applications and system-level services governed by processor architecture. [PERSON] et al. [21] emphasized its role in managing communication and data exchange across distributed systems, enabling shared access to databases and services. [PERSON] and [PERSON] [22] offered a broader perspective, describing middleware as software that abstracts the complexities of distributed systems while facilitating interaction among applications, networks, hardware, and operating systems. \begin{table}
\begin{tabular}{p{42.7 pt} p{341.4 pt}} \hline
**Attribute** & **Description** \\ \hline Focus & The papers should focus on publish/subscribe-based middleware implementation in transportation, addressing specific application areas, associated challenges, or potential security and privacy concerns. \\ \hline Type & We consider all scientific publications, including peer-reviewed articles, conference papers, and review papers. \\ \hline Relevance & The papers must utilize publish/subscribe-based middleware as a communication channel in transportation applications. \\ \hline \multirow{2}{*}{Recency} & We included all published papers from 2009 to 2024 to capture historical developments and emerging trends. \\ \hline \end{tabular}
\end{table} 
Table 2: Table of attributes and descriptions. \begin{table}
\begin{tabular}{p{42.7 pt} p{341.4 pt}} \hline
**Attribute** & **Description** \\ \hline Duplicates & We excluded articles with substantial content overlap to maintain diversity and originality in the review. \\ \hline Sources & We excluded all non-peer-reviewed materials, such as websites, blogs, and opinion pieces, to ensure the academic integrity of the review paper. However, we use middleware or transportation companies’ and governmental websites for the latest examples in the field. \\ \hline Completeness & We eliminated publications that were either retracted, incomplete, not accessible, or invalid. \\ \hline Language & We excluded all papers published in languages other than English. \\ \hline \end{tabular}
\end{table} 
Table 3: Table of exclusion criteria. Figure 4 illustrates how middleware operates as an intermediary layer between distributed applications and the underlying system software and hardware [23]. Each application interacts with the middleware through its Application Programming Interface (API), abstracting away the complexity of direct communication with system components such as OSs, drivers, and utilities. This decoupling allows applications to operate independently of specific hardware or system configurations, improving portability and scalability across heterogeneous platforms. For instance, in intelligent transportation systems (ITSs), middleware enables different stakeholders and infrastructure components to use independent platforms while interacting seamlessly. It achieves this by managing data exchange, service invocation, and resource access across multiple distributed nodes. Researchers have demonstrated several applications of middleware in the transportation sector. [PERSON] and [PERSON] [24] used middleware to manage diverse communication protocols and data formats, enhancing device compatibility and interoperability within IoT-based ITS in Smart Cities. [PERSON] et al. [25] reported how middleware facilitates the integration of machine learning (ML) and autonomous vehicles in ITS. [PERSON] et al. [26] studied DDS applications in Smart Traffic Systems (STSs), including real-time data processing, urban traffic management, vehicle-to-everything (V2X) communication, and Cooperative Intelligent Transport Systems (C-ITSs). However, this paper focuses on the publish/subscribe-middleware-based ITS.