Highly-Available Applications on Unreliable Infrastructure: Microservice Architectures in Practice

In contrast to applications relying on specialized and expensive highly-available infrastructure, the basic approach of microservice architectures to achieve fault tolerance - and finally high availability - is to modularize the software system into small, self-contained services that are connected via implementation-independent interfaces. Microservices and all dependencies are deployed into self-contained environments called containers that are executed as multiple redundant instances. If a service fails, other instances will often still work and take over. Due to the possibility of failing infrastructure, these services have to be deployed on several physical systems. This horizontal scaling of redundant service instances can also be used for load-balancing. Decoupling the service communication using asynchronous message queues can increase fault tolerance, too. The Deutsche Bahn AG (German railway company) uses as system called EPA for seat reservations for inter-urban rail services. Despite its high availability, the EPA system in its current state has several disadvantages such as high operational cost, need for special hardware, technological dependencies, and expensive and time-consuming updates. With the help of a prototype, we evaluate the general properties of a microservice architecture and its dependability with reference to the legacy system. We focus on requirements for an equivalent microservice-based system and the migration process; services and data, containerization, communication via message queues; and achieving similar fault tolerance and high availability with the help of replication inside the resulting architecture.