The advancement of artificial intelligence (AI) has led to an increased demand for both a greater volume and quality of data. In many companies, data is dispersed across multiple tables, yet AI models typically require data in a single table format. This necessitates the merging of these tables and the selection of optimal features for the model, a process known as Tabular Data Augmentation (TDA). With the rapid growth of TDA, automated tools have been developed to streamline this process. However, these state-of-the-art tools often make assumptions about user workflows that may not align with the actual needs of data specialists, potentially making them efficient yet not fully user-friendly. Additionally, without thorough evaluation through user studies, these tools may overlook critical steps in the TDA process.

This thesis is divided into two main parts. The first part is dedicated to uncovering the assumptions and oversights within current TDA research through an exhaustive review of recent literature. This is followed by conducting interviews with 19 data specialists. These discussions aim to verify the identified assumptions and reveal any missing elements in state-of-the-art research. The second part focuses on creating a new tool to meet the requirements identified from validated assumptions and the gaps discovered. This tool is then subjected to evaluation interviews to assess its effectiveness.

The findings indicate that data specialists prefer a TDA tool that offers enhanced control and deeper insights into the data augmentation process. To meet these preferences, Human in the Loop AutoTDA was developed, embodying the desired functionalities. Feedback from the evaluation phase confirmed that data specialists find Human in the Loop AutoTDA suitable for their TDA workflows, marking a significant advancement in the field.

This thesis embarks on the quest to efficiently compute similarities between data streams in real-time, a task burgeoning in importance with the advent of big data and real-time analytics. At the heart of this endeavor is the expansion of the Condor framework to accommodate new probabilistic data structures, tailored to meet the distinctive challenges posed by streaming data. A notable highlight is the adaptation of the DSTree data structure to a streaming environment, marking a significant stride towards achieving the stated goal. Through an implementation within the Condor framework, this research explores the core mechanisms for indexing and approximating similarities, paving the way for more refined analyses. Furthermore, a comparative study is conducted encompassing several probabilistic data structures, including HyperLogLog and Theta Sketches, examining their effectiveness in similarity search within a streaming environment, in comparison with the DSTree method. The evaluation of these methods will be done through a series of experiments, which are meticulously designed to measure the accuracy and efficiency of these structures, shedding light on their potential and limitations. he insights garnered from this study underscore the potential of probabilistic data structures in bolstering the speed and accuracy of similarity search in streaming data, while also hinting at promising avenues for further research.

Over the last two decades, the machine learning (ML) field has witnessed a dramatic expansion, propelled by burgeoning data volumes and the advancement of computational technologies. Deep learning (DL) in particular has demonstrated remarkable success across a wide range of domains, including healthcare, mobility, life sciences, and energy systems. This success has been further accelerated by the availability and efficiency of open-source ML frameworks like TensorFlow and PyTorch, making ML methodologies more accessible than ever.

However, this rapid growth has brought its own set of challenges. The proliferation of ML models and related artifacts, such as datasets, have brought abundant information during the ML lifecycle. The descriptive and property information of these artifacts is referred as metadata. Yet current practices, such as model cards used in public model zoos and tools to track metadata within scripts, cannot fully captured the metadata of these artifacts, let alone a standardized approach for their management, and access. In addition, the prevailing practice of managing ML/DL scripts via traditional software repositories, while adequate for software engineering, falls short in addressing the unique needs of ML workflows, such as model reuse and comparative analysis. These practices hinder the effective use of structured and comprehensive metadata representation. This disconnect points to a pressing need for improved methodologies and tools in the ML field.

In response to these challenges, this thesis delves into the development and exploitation of structured metadata representations within ML model zoos. In Chapter 2, we first propose a metamodel that represent different types of metadata, thus transforming the metadata from being merely descriptive to being queryable and machine-readable. The structured nature of our metamodel allows for more efficient querying and retrieval of information, which is a substantial improvement over the traditional, text-based descriptions.

Additionally, the thesis explores the use of metadata to optimize various ML processes, particularly in the selection of appropriate models for specific tasks, i.e., model inference and fine-tuning. In Chapter 3, we investigate the optimization of ML inference queries in heterogeneous model zoos using a Mixed-Integer-Programming-based optimizer. This optimizer, which considers multiple objectives such as accuracy and inference speed, provides a robust framework for model selection and execution planning. In Chapter 4, the research extends to model selection for fine-tuning. We investigate on predicting model performance, particularly accuracy, in scenarios where data domains shift, thus negating the need for constant model fine-tuning. By selectively choosing only the most promising candidates, this method substantially lowers the computational burden and associated costs of extensive model fine-tuning.

Overall, this thesis investigates the representation and application of metadata. The insights and methodologies presented not only improve the efficiency and effectiveness of ML workflows but also pave the way for further exploration in the integration of metadata within ML practices, highlighting the continual development and potential for advancements in ML.

Data processing has heavily evolved in the last two decades, from single-node processing to distributed processing and from the MapReduce paradigm to the stream processing paradigm. At the same time, cloud computing has emerged as the primary means of deploying and operating a data processing system. In the cloud era, flexible resource allocation combined with flexible pricing schemes have brought forward new opportunities and have democratized access to computing resources. However, streaming dataflow or stream processing engines were originally designed for in-house clusters of fixed resources with limited needs for adaptivity. Therefore, they lack the mechanisms to adapt to unexpected changes in the needs of the processing workload. When solutions have been proposed in the literature, their experimental evaluation is limited hindering the progress of the field. The same applies to the native fault tolerance mechanisms that virtually every stream processing engine employs. In this thesis, we study the problem of adaptivity for streaming dataflow engines, and we focus on three major adaptivity subproblems: adaptivity to 𝑖) statistical changes, 𝑖𝑖) infrastructure failures, and 𝑖𝑖𝑖) input rate changes.
In Chapter 2, we study adaptivity to statistical changes through the important task of streaming similarity joins that is heavily affected by imbalanced loads, a by-product of statistical changes. We propose S3J ; the first adaptive distributed streaming similarity joins method in the general metric space that employs a two-layered adaptive partitioning scheme to reduce unnecessary similarity computations and distribute the load to the available workers. Our partitioning scheme is paired with an efficient load balancing scheme that leverages the existing partitioning in order to rebalance any imbalanced load. Our results show that S3J outperforms the employed baseline, inspired by a MapReduce method, in terms of partitioning efficiency. Additionally, our experiments show that the load balancing scheme can gradually defuse the imbalanced load and involve all the available workers in the processing.
The majority of the stream processing engines employ a checkpoint-based fault tolerance mechanism. In Chapter 3, we look at the adaptivity to infrastructure failures through the existing checkpointing protocols. We propose CheckMate, a principled experimental framework for evaluating checkpointing protocols for streaming dataflows. First, we summarize all the essential preliminaries required to study checkpoint-based fault tolerance. Then, we discuss in detail, implement, and evaluate in different scenarios the three main checkpointing protocols. Our evaluation shows that when the load is uniformly distributed, the implemented by most stream processing engines coordinated checkpointing protocol outperforms the alternatives. However, the uncoordinated prevails in the presence of skew, while it shows no domino effect when cyclic queries are employed.
Finally, in Chapter 4, we address the problem of adaptivity to input rate changes. Although multiple solutions have been proposed, their experimental evaluation is shallow and does not include detailed comparisons with other solutions. We propose a principled evaluation framework for stream processing autoscalers. We establish important metrics, queries, and workloads in order to provide guidelines for the evaluation of autoscaling solutions for stream processing. We discuss the state-of-the-art control-based autoscalers, and we evaluate them using the proposed framework. Our results show that, for complex queries, none of the evaluated autoscalers can adapt efficiently, while for simple stateless queries, a simple generic autoscaler outperforms the solutions tailored to stream processing.
We conclude this thesis by summarizing our main findings and discussing the limitations of our work. Based on the valuable insights we gained while designing and implementing the research work included in this thesis, we propose a series of interesting and important future research directions that are not limited to adaptivity problems but address stream processing in general.

Schema matching is a critical data integration process, which aims at capturing relevance between elements of different datasets; when datasets are tabular, it translates to the process of discovering related columns among them. Accurately discovering column matches is integral for several applications, such as entity resolution, data cleaning and data augmentation. While there exists a multitude of schema matching methods in the literature, we identify three major issues: i) there is no comprehensive study of comparing them in terms of effectiveness and efficiency, due to not available implementations and lack of evaluation datasets, ii) existing methods might be impractical and even inapplicable in certain modern settings, and iii) the heterogeneity and complexity of data can impede capturing relevance among columns for existing methods, as certain assumptions might not be holding for the entirety of underlying datasets. In this thesis, we tackle these issues by reviewing existing schema matching techniques and proposing novel methods capable to address challenges imposed by modern settings.
Starting with Chapter 2, we present an extensive comparison study on existing schema matching methods, by introducing Valentine. Specifically, Valentine constitutes an open-source experimental suite, which encompasses several state-of-the-art schema matching solutions. To guide the evaluation process towards modern applications, we extract four relatedness scenarios from the dataset discovery literature. To tackle the lack of existing datasets with ground truth, we devise a principled fabrication process. Our findings lead to insights that can help to improve future research on the field of schema matching, while they affect the design choices we make for novel methods we present in the following chapters.
Next, in Chapter 3, we turn our focus on applying schema matching among datasets stored in different data silos, which cannot be collocated and each contains information about column matches. Towards this direction, we introduce SiMa, a matching method that leverages existing matches in each silo, to build a column match prediction model, powered by the employment of a Graph Neural Network (GNN). To do so, SiMa transforms columns and matches among them in each silo to a graph, while it performs targeted negative edge sampling and incremental training to enhance the learning process. In our experimental evaluation, we show the benefits of using SiMa over state-of-the-art techniques, both in terms of effectiveness and efficiency.
Finally, Chapter 4 discusses the problem of discovering join relationships among datasets in a repository. To ameliorate the shortcomings of previous methods, we propose OmniMatch, a self-supervised method that can effectively capture both equi- and fuzzy-joins among tabular data. At the core of the method is the exploitation of a comprehensive set of similarity signals among columns, which are then transformed into a similarity graph. This graph, in conjunction with automatically generated positive and negative column match examples, enable the employment of a Relational Graph Convolution Network (RGCN) towards training a generalizable join prediction model. We compare the effectiveness of OmniMatch with several other state-of-the-art matching and column representation methods, while we verify the usefulness of utilizing a wide-spectrum of similarity signals to capture joins.
We conclude the thesis by reviewing our main findings, reflecting on our contributions and discussing potential limitations of the methods and approaches presented. Moreover, based on the insights we gain from surveying and developing novel matching methods, we discuss challenges and future directions in the field.

Similarity joins are operations which involve identifying similar pairs of records within one or multiple datasets. These operations are typically time-sensitive, as timely identification of relations can lead to increased profitability. Therefore, it is advantageous to analyze them using a stream processing system, which offers real-time capabilities. Due to the computational complexity of comparing numerous records, similarity joins can be resource-intensive.

To address this challenge, employing a distributed setting for executing the operations proves to be the most effective approach for resource management. In this research, we evaluate four distinct distributed systems designed for similarity joins in stream processing environments. The primary objective is to assess their individual strengths and weaknesses, as well as their overall efficiency. Our investigation reveals that certain solutions exhibit superior scalability and resource utilization, while highlighting the potential for further advancements in this domain.