—The number of real-world applications that require QoS guarantees is constantly increasing and they often follow the publish/subscribe (pub/sub) messaging paradigm, which provides loosely coupled many-to-many communication. Many QoS-aware systems use overlay networks as they allow flexible routing. To provide QoS-aware pub/sub messaging in overlay networks, the messaging system should be adaptive to the changes in network conditions (such as delay and failures). However, many pub/sub systems depend on a fixed routing topology and it is costly to rebuild this topology in case of failures. This study seeks to address this challenge with Delay-Cognizant Reliable Delivery (DCRD), a novel and delay-aware dynamic routing algorithm to provide reliable message delivery for pub/sub overlay networks. For reliable message delivery, DCRD no longer uses a fixed routing topology. Instead, it dynamically switches among different links to bypass link failures and increase the chance to meet QoS ...