Context: Continuous practices, i.e., continuous integration, delivery, and deployment, are the software development industry practices that enable organizations to frequently and reliably release new features and products. With the increasing interest in and literature on continuous practices, it is important to systematically review and synthesize the approaches, tools, challenges, and practices reported for adopting and implementing continuous practices.Objective: This research aimed at systematically reviewing the state of the art of continuous practices to classify approaches and tools, identify challenges and practices in this regard, and identify the gaps for future research. Method:We used systematic literature review (SLR) method for reviewing the peer-reviewed papers on continuous practices published between 2004 and 1st June 2016. We applied thematic analysis method for analysing the data extracted from reviewing 69 papers selected using predefined criteria. Results:We have identified thirty approaches and associated tools, which facilitate the implementation of continuous practices in the following ways: (1) -reducing build and test time in continuous integration (CI)‖; (2) -increasing visibility and awareness on build and test results in CI‖; (3) -supporting (semi-) automated continuous testing‖; (4) -detecting violations, flaws and faults in CI‖; (5) -addressing security and scalability issues in deployment pipeline‖, and (6) -improving dependability and reliability of deployment process‖. We have also determined a list of critical factors such as -testing (effort and time)‖, -team awareness and transparency‖, -good design principles‖, -customer‖, -highly skilled and motivated team‖, -application domain‖, and -appropriate infrastructure‖ that should be carefully considered when introducing continuous practices in a given organization. The majority of the reviewed papers were validation (34.7%) and evaluation (36.2%) research types. This review also reveals that continuous practices have been successfully applied to both greenfield and maintenance projects. Conclusion:Continuous practices have become an important area of software engineering research and practice. Whilst the reported approaches, tools, and practices are addressing a wide range of challenges, there are several challenges and gaps which require future research work for: improving the capturing and reporting of contextual information in the studies reporting different aspects of continuous practices; gaining a deep understanding of how software-intensive systems should be (re-) architected to support continuous practices; addressing the lack of knowledge and tools for engineering processes of designing and running secure deployment pipelines.

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Recently, many software organizations have been adopting Continuous Delivery and Continuous Deployment (CD) practices to develop and deliver quality software more frequently and reliably. Whilst an increasing amount of the literature covers different aspects of CD, little is known about the role of software architecture in CD and how an application should be (re-) architected to enable and support CD. We have conducted a mixed-methods empirical study that collected data through indepth, semi-structured interviews with 21 industrial practitioners from 19 organizations, and a survey of 91 professional software practitioners. Based on a systematic and rigorous analysis of the gathered qualitative and quantitative data, we present a conceptual framework to support the process of (re-) architecting for CD. We provide evidence-based insights about practicing CD within monolithic systems and characterize the principle of "small and independent deployment units" as an alternative to the monoliths. Our framework supplements the architecting process in a CD context through introducing the quality attributes (e.g., resilience) that require more attention and demonstrating the strategies (e.g., prioritizing operations concerns) to design operations-friendly architectures. We discuss the key insights (e.g., monoliths and CD are not intrinsically oxymoronic) gained from our study and draw implications for research and practice.

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