removed at runtime. This latter approach is used, for example, in database triggers, mailing lists, and RSS. The Data Distribution Service (DDS) middleware does not use a broker in the middle. Instead, each publisher and subscriber in the pub/sub system shares meta-data about each other via IP multicast. The publisher and the subscribers cache this information locally and route messages based on the discovery of each other in the shared cognizance. In effect, brokerless architectures require publish/subscribe system to construct an overlay network which allows efficient decentralized routing from publishers to subscribers. It was shown by Jon Kleinberg that efficient decentralised routing requires Navigable Small-World topologies. Such Small-World topologies are usually implemented by decentralized or federated publish/subscribe systems. Locality-aware publish/subscribe systems construct Small-World topologies that route subscriptions through short-distance and low-cost links thereby reducing subscription delivery times. History One of the earliest publicly described pub/sub systems was the "news" subsystem of the Isis Toolkit, described at the 1987 Association for Computing Machinery (ACM) Symposium on Operating Systems Principles conference (SOSP '87), in a paper "Exploiting Virtual Synchrony in Distributed Systems. 123–138". Advantages Loose coupling Publishers are loosely coupled to subscribers, and need not even know of their existence. With the topic being the focus, publishers and subscribers are allowed to remain ignorant of system topology. Each can continue to operate as per normal independently of the other. In the traditional tightly coupled client–server paradigm, the client cannot post messages to the server while the server process is not running, nor can the server receive messages unless the client is running. Many pub/sub systems decouple not only the locations of the publishers and subscribers but also decouple them temporally. A common strategy used by middleware analysts with such pub/sub systems is to take down a publisher to allow the subscriber to work through the backlog (a form of bandwidth throttling). Scalability Pub/sub provides the opportunity for better scalability than traditional client-server, through parallel operation, message caching, tree-based or network-based routing, etc. However, in certain types of tightly coupled, high-volume enterprise environments, as systems scale up to become data centers with thousands of servers sharing the pub/sub infrastructure, current vendor systems often lose this benefit; scalability for pub/sub products under high load in these contexts is a research challenge. Outside of the enterprise environment, on the other hand, the pub/sub paradigm has proven its scalability to volumes far beyond those of a