Fog computing is an extension of cloud computing to overcome limitations of the latter. It brings data processing and storage closer to edge devices, while offering higher processing power and storage space compared to the edge. In edge or fog environments, data is often ephemeral, so storing data in memory provides considerable performance benefits. Edge or fog nodes may also fail or disconnect, so distributed database systems are used in order to provide durability and availability. Database performance is an important factor when designing a fog platform. We assess the performance of two distributed, in memory databases, Apache Ignite and Redis, measured as throughput, with respect to different parameters: number of nodes, number of replicas, persistency, network bandwidth, network latency and number of operations. In all cases, Redis outperforms Apache Ignite, however, Apache Ignite scales very well when the number of operations is increased.