A distributed real-time database (DRTDB) is a database distributed over a network on several nodes and where the transactions are associated with deadlines. The issues of concern in this kind of database are data consistency and the ability to meet deadlines. In addition, there is the possibility that the nodes, on which the database is distributed, are heterogeneous. This means that the nodes may be built on different platforms and written in different languages. This makes the integration of these nodes difficult, since data types may be represented differently on different nodes. The common object request broker architecture (CORBA), defined by the Object Management Group (OMG), is a distributed object computing (DOC) middleware created to overcome problems with heterogeneous sites. The project described in this paper aims to investigate the suitability of CORBA as a middleware in a DRTDB. Two extensions to CORBA, Fault-Tolerance CORBA (FT-CORBA) and Real-Time CORBA (RT-CORBA) is of particular interest since the combination of these extensions provides the features for object replication and end-to-end predictability, respectively. The project focuses on the ability of RT-CORBA meeting hard deadlines and FT-CORBA maintaining replica consistency by using replication with eventual consistency. The investigation of the combination of RT-CORBA and FT-CORBA results in two proposed architectures that meet real-time requirements and provides replica consistency with CORBA as the middleware in a DRTDB.

<p>Simulation is a good way to gain insight into a system, for example during development, without having to run or build the actual system. This is especially true for real-time systems, which often operate in hazardous environments or control critical entities in the 'real' world, making testing of these systems in their real environment unsafe during development.</p><p>When building simulations, one simulator is not likely to fit every type of simulation project. Therefore, different simulators, which focus on different aspects of simulation, are built. The High Level Architecture (HLA) from the Defense Modeling and Simulation Office (DMSO) is an architecture for distributed simulations providing a means to communicate between different simulations.</p><p>However, the HLA standard has limitations if viewed from a real-time perspective. For example, there is no built-in support for fault tolerance. In this thesis some of the limitations in HLA are identified and an extended architecture that uses a distributed active real-time database as a way to overcome these limitations is presented. One of the major advantages with this new extended HLA architecture is that it is still compliant with HLA, i.e., no modifications have been made to the HLA interfaces.</p>

<p>A distributed real-time database (DRTDB) is a database distributed over a network on several nodes and where the transactions are associated with deadlines. The issues of concern in this kind of database are data consistency and the ability to meet deadlines. In addition, there is the possibility that the nodes, on which the database is distributed, are heterogeneous. This means that the nodes may be built on different platforms and written in different languages. This makes the integration of these nodes difficult, since data types may be represented differently on different nodes. The common object request broker architecture (CORBA), defined by the Object Management Group (OMG), is a distributed object computing (DOC) middleware created to overcome problems with heterogeneous sites.</p><p>The project described in this paper aims to investigate the suitability of CORBA as a middleware in a DRTDB. Two extensions to CORBA, Fault-Tolerance CORBA (FT-CORBA) and Real-Time CORBA (RT-CORBA) is of particular interest since the combination of these extensions provides the features for object replication and end-to-end predictability, respectively. The project focuses on the ability of RT-CORBA meeting hard deadlines and FT-CORBA maintaining replica consistency by using replication with eventual consistency. The investigation of the combination of RT-CORBA and FT-CORBA results in two proposed architectures that meet real-time requirements and provides replica consistency with CORBA as the middleware in a DRTDB.</p>

<p>In a distributed real-time environment where it is imperative to make correct decisions it is important to have all facts available to make the most accurate decision in a certain situation. An example of such an environment is an Unmanned Aerial Vehicle (UAV) system where several UAVs cooperate to carry out a certain task and the data recorded is analyzed after the completion of the mission. This project aims to define and implement a time series architecture for use together with a distributed real-time database for the ability to store temporal data. The result from this project is a time series (TS) architecture that uses DeeDS, a distributed real-time database, for storage. The TS architecture is used by an application modelled from a UAV scenario for storing temporal data. The temporal data is produced by a simulator. The TS architecture solves the problem of storing temporal data for applications using DeeDS. The TS architecture is also useful as a foundation for integrating time series in DeeDS since it is designed for space efficiency and real-time requirements.</p>

Simulation is a good way to gain insight into a system, for example during development, without having to run or build the actual system. This is especially true for real-time systems, which often operate in hazardous environments or control critical entities in the 'real' world, making testing of these systems in their real environment unsafe during development. When building simulations, one simulator is not likely to fit every type of simulation project. Therefore, different simulators, which focus on different aspects of simulation, are built. The High Level Architecture (HLA) from the Defense Modeling and Simulation Office (DMSO) is an architecture for distributed simulations providing a means to communicate between different simulations. However, the HLA standard has limitations if viewed from a real-time perspective. For example, there is no built-in support for fault tolerance. In this thesis some of the limitations in HLA are identified and an extended architecture that uses a distributed active real-time database as a way to overcome these limitations is presented. One of the major advantages with this new extended HLA architecture is that it is still compliant with HLA, i.e., no modifications have been made to the HLA interfaces.

In a distributed real-time environment where it is imperative to make correct decisions it is important to have all facts available to make the most accurate decision in a certain situation. An example of such an environment is an Unmanned Aerial Vehicle (UAV) system where several UAVs cooperate to carry out a certain task and the data recorded is analyzed after the completion of the mission. This project aims to define and implement a time series architecture for use together with a distributed real-time database for the ability to store temporal data. The result from this project is a time series (TS) architecture that uses DeeDS, a distributed real-time database, for storage. The TS architecture is used by an application modelled from a UAV scenario for storing temporal data. The temporal data is produced by a simulator. The TS architecture solves the problem of storing temporal data for applications using DeeDS. The TS architecture is also useful as a foundation for integrating time series in DeeDS since it is designed for space efficiency and real-time requirements.

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A fully replicated distributed real-time database provides high availability and predictable access times, independent of user location, since all the data is available at each node. However, full replication requires that all updates are replicated to every node, resulting in exponential growth of bandwidth and processing demands with the number of nodes and objects added. To eliminate this scalability problem, while retaining the advantages of full replication, this thesis explores Virtual Full Replication (ViFuR); a technique that gives database users a perception of using a fully replicated database while only replicating a subset of the data. We use ViFuR in a distributed main memory real-time database where timely transaction execution is required. ViFuR enables scalability by replicating only data used at the local nodes. Also, ViFuR enables flexibility by adaptively replicating the currently used data, effectively providing logical availability of all data objects. Hence, ViFuR substantially reduces the problem of non-scalable resource usage of full replication, while allowing timely execution and access to arbitrary data objects. In the thesis we pursue ViFuR by exploring the use of database segmentation. We give a scheme (ViFuR-S) for static segmentation of the database prior to execution, where access patterns are known a priori. We also give an adaptive scheme (ViFuR-A) that changes segmentation during execution to meet the evolving needs of database users. Further, we apply an extended approach of adaptive segmentation (ViFuR-ASN) in a wireless sensor network - a typical dynamic large-scale and resource-constrained environment. We use up to several hundreds of nodes and thousands of objects per node, and apply a typical periodic transaction workload with operation modes where the used data set changes dynamically. We show that when replacing full replication with ViFuR, resource usage scales linearly with the required number of concurrent replicas, rather than exponentially with the system size.

<p>Recovery is a fundamental service in database systems. In this work, we present a new mechanism for diskless real-time recovery in fully replicated distributed real-time database systems. Traditionally, recovery has relied on disk-resident redundant data. Unfortunately, disks cannot always be used in real-time systems since these systems are sometimes used in environments which do not allow the use of disks. Also, minimizing the amount of hardware can save money, especially in mass-produced products. Instead of loading the database from disk, our recovery mechanism enables a restarted node to retrieve a copy of the database from an arbitrary remote node. The recovery mechanism does not violate timeliness during normal processing and, during recovery, all nodes except for the recovering node can guarantee the timeliness of critical transactions. The mechanism uses fuzzy checkpointing to copy the database to the recovering node. Fuzzy checkpointing has been chosen since it copies the database without regard to concurrency control and, thus, does not increase data contention in the database. We conclude that the suggested recovery mechanism is a feasible option for fully replicated distributed real-time database systems.</p>

Recovery is a fundamental service in database systems. In this work, we present a new mechanism for diskless real-time recovery in fully replicated distributed real-time database systems. Traditionally, recovery has relied on disk-resident redundant data. Unfortunately, disks cannot always be used in real-time systems since these systems are sometimes used in environments which do not allow the use of disks. Also, minimizing the amount of hardware can save money, especially in mass-produced products. Instead of loading the database from disk, our recovery mechanism enables a restarted node to retrieve a copy of the database from an arbitrary remote node. The recovery mechanism does not violate timeliness during normal processing and, during recovery, all nodes except for the recovering node can guarantee the timeliness of critical transactions. The mechanism uses fuzzy checkpointing to copy the database to the recovering node. Fuzzy checkpointing has been chosen since it copies the database without regard to concurrency control and, thus, does not increase data contention in the database. We conclude that the suggested recovery mechanism is a feasible option for fully replicated distributed real-time database systems.