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.

The effect of using the eventual consistency model is evaluated, compared to the effect of immediate consistency, by increasing the stabilization time in both models and using the immediate consistent system as a baseline for evaluations. The immediate consistent system performs better if the information and the decisions are replicated adequately fast throughout the system. When the stabilization time increases the effectiveness of eventual consistency emerges, which is most obvious when time constraints make it difficult to propagate information and decisions. By using a simulation to extract data for evaluations, it is verified in this research that as the stabilization time between different parts of a system increases, the eventual consistency will always outperform the immediate consistent system. This statement is valid in all situations where consistency models are useful. Of secondary importance in the research, by adding more decision layers to the eventual consistency model, the performance output is increased significantly, as swift and less well calculated decisions can be thwarted and corrected using the second decision layer.

A distributed, real-time database could be used to implement a shared whiteboard architecture used for communication between mobile nodes, in an ad-hoc network. This kind of application implies specific requirements on how the database handles replication and consistency between replicas (global consistency). Since mobile nodes are likely to disconnect from the network and connect again at unpredictable times, and since a node may be disconnected an arbitrary amount of time, this needs to be treated as normal operation, and not as failures. The replication scheme used in the DeeDS architecture, and the PRiDe replication protocol are both suitable for a shared whiteboard architecture as described above. Since the mobile nodes are likely to be some kind of hand-held device (e.g., used by rescue personnel to exchange information), the database system should be suitable for use in embedded systems. The Polyhedra Real-Time Relational Database (RTRDB) and the TimesTen database are two such systems. A problem is that neither of these two database systems have a replication scheme suitable for use in the previously described type of architecture. This dissertation presents two design proposals for how to extend the Polyhedra RTRDB with support for multi-master replication of data using full replication and eventual consistency. One design proposal is based on the DeeDS architecture and the other is based on the PRiDe replication protocol. The proposal based on DeeDS puts a number of requirements on the underlying database and is not easy to port to another DBMS since it makes use of Polyhedra specific API’s. The proposal based on PRiDe on the other hand requires no instrumentation of the underlying database and is thus easier to port to other database systems.

Replication primarily provides data availability by having multiple copies over different systems and is exploited to make distributed systems scalable in num- bers and geographical areas. Placing a replica closer to the source of request can also significantly reduce the time required to service the request, improv- ing applications’ performance. However, modifications done at a single copy need to be propagated to all the standing copies to maintain the data’s consis- tency. Over the years, numerous strategies have been proposed for handling the tradeoff between consistency and availability, of which the majority pro- vides either strong consistency or eventual consistency. These models do not provide sufficient compatibility for developing modern applications for geo- distributed (edge) environments.Conflict-Free Replicated Data Types (CRDT) provides a new model of consistency referred to as strong eventual consistency. In principle, CRDTs guarantee conflict-free merge even when the updates arrive out of order using simple mathematical properties. Lasp is a coordination free distributed pro- gramming model for building modern distributed applications using CRDTs. Lasp uses a gossip protocol for disseminating state changes to all replicas in the system. The current implementation of gossip in Lasp is agnostic to the application’s behavior in propagating the updates efficiently to critical repli- cas in the system. In the thesis, we introduce an application-specific feature to optimize the dissemination of updates in Lasp. The proposed algorithm propagates the updates by catering to the different consistency requirements of the replicas in the system. The experimental results on a topology of 100 replicas found that the update latency at critical replicas with high consistency requirements is reduced by 40–50%, and the total bandwidth consumption in the system is reduced by 4–8% without significant repercussion on other repli- cas in the system. / Datareplikering erbjuder primärt tillgänglighet genom att tillhandahålla mul- tipla kopior fördelat över olika system, och utnyttjas för att göra distribuerade system skalbara i antal och över geografiska områden. Att placera en replika nära källan till en förfrågan kan dessutom signifikant reducera tiden det krävs att besvara förfrågan vilket förbättrar applikationens prestanda. Modifikatio- ner gjorda på en av kopiorna måste dock propageras till alla stående kopior för att upprätthålla datans konsistens . Över tid har många strategier föreslagits för att hantera avvägningen mellan konsistens och tillgänglighet, där majorite- ten erbjuder antingen stark eller eventuell konsistens. Dessa modeller erbjuder inte tillräcklig kompatibilitet för utveckling av moderna applikationer för geo- distribuerade (edge) miljöer.Konfliktfria replikerade datatyper (CRDT) erbjuder en ny modell av konsi- stens som kallas stark eventuell konsistens. I princip garanterar CRDTer kon- fliktfria sammanslagningar trots att uppdateringar sker i oordning, genom att använda dess matematiska egenskaper. Lasp är en koordineringsfri distribue- rad programmeringsmodell för att bygga moderna distribuerade applikationer med hjälp av CRDTer. Lasp använder skvallerprotokoll för att sprida tillstånds- förändringar till alla replikor i systemet. Den nuvarande implementationen av skvaller i Lasp är agnostiskt för applikationens beteende relaterat till effektiv propagering av uppdateringar till kritiska replikor i systemet. I det här exa- mensarbetet introducerar vi applikationsspecifik funktionalitet för att optime- ra spridandet av uppdateringar i Lasp. Den föreslagna algoritmen sprider upp- dateringarna genom att tillgodose de olika konsistenskraven för replikorna i systemet. Experimentella resultat i en topologi av 100 replikor visade att upp- dateringslatensen vid kritiska replikor med höga konsistenskrav minskas med 40–50% och att den totala bandbreddskonsumtionen i systemet minskas med 4–8% utan signifikanta negativa följder för andra replikor i systemet.