# Publish/Subscribe-Middleware-Based Intelligent Transportation Systems: Applications and Challenges
## 2 Methodology  
This systematic literature review (SLR) aims to identify and analyze middleware research in the transportation domain, focusing on publish/subscribe middleware. To define the scope of this study, we conducted a keyword-based search in the Scopus database, retrieving publications where the terms middleware, publish/subscribe systems, and transportation appeared in the title, abstract, or keyword section. The distribution of retrieved publications is visually illustrated in Figure 2, highlighting the intersection of these three key domains. The shaded area in the Venn diagram represents the 87 publications that mention both \"publish/subscribe\" and \"transportation\" in their title, abstract, or keyword sections. Of these, 15 publications also explicitly mention \"middleware\", placing \begin{table}
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**RQ\#** & **Question** & **Section** & **Objective** \\ \hline RQ1 & What is Middleware? & Section 3 & Define middleware, explain its core functionalities, and highlight its importance in distributed systems. \\ \hline RQ2 & What is a publish/subscribe Middleware? & Section 3.1 & Define publish/subscribe middleware and explore the different publish/subscribe middleware types. \\ \hline RQ3 & What are the different applications of middleware in transportation? & Section 4 & Explore and understand middleware technology’s positive impacts and advantages on transportation systems and identify the specific benefits, such as improving efficiency, enhancing communication, and saving lives. \\ \hline RQ4 & What communication models and techniques are employed in transportation systems with the integration of middleware? & Section 4.3 & Identify and examine communication models used in middleware-based ITS in the literature. \\ \hline RQ5 & What are the challenges that middleware is facing in transportation? & Section 6 & Identify and analyze the obstacles, hindrances, and difficulties that middleware encounters when applied to real-world transportation scenarios. \\ \hline \hline \end{tabular}
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Table 1: Research questions. them at the intersection of all three terms. We consider all 87 publications relevant in this review, regardless of whether the term \"middleware\" appears explicitly, because publish/subscribe is a type of middleware. The inclusion of the \"middleware\" circle in the Venn diagram aims to highlight the relative scale of publish/subscribe research within the broader middleware domain: while over 29,000 papers mention middleware, only 3803 mention publish/subscribe, indicating that publish/subscribe represents a specialized subset of middleware research. This visualization underscores the focused and relatively underexplored nature of applying publish/subscribe middleware to ITS, reinforcing the motivation for this systematic review. A notable distinction in the Venn diagram is the separation of publish/subscribe from Middleware, as some research publications do not explicitly include the term \"Middleware\" in their metadata. For example, a study on Message Queuing Telemetry Transport (MQTT)-based vehicular communication may focus on the publish/subscribe model without mentioning middleware directly, even though it fundamentally relies on middleware for message exchange. Also, some publications use the word \"system\" instead of \"middleware\" [16]. The shaded region in the diagram represents the subset of research publications aligning with this review's focus--studies integrating publish/subscribe middleware into transportation applications. This approach ensures a comprehensive yet targeted selection of literature, capturing relevant peer-reviewed publications while avoiding studies that do not contribute meaningfully to the field. This focused review allows us to provide in-depth insights into the role, challenges, and future directions of publish/subscribe middleware in ITS while distinguishing it from other middleware architectures.