Editing Publish–subscribe pattern (section)

==Topologies==
In many publish–subscribe systems, publishers post messages to an intermediary [[message broker | message broker or event bus]], and subscribers register subscriptions with that broker, letting the broker perform the filtering. The broker normally performs a [[store and forward]] function to route messages from publishers to subscribers. In addition, the broker may prioritize messages in a [[Priority queue|queue]] before routing.{{Cn|date=October 2024}}

Subscribers may register for specific messages at build time, initialization time or runtime. In GUI systems, subscribers can be coded to handle user commands (e.g., click of a button), which corresponds to build time registration. Some frameworks and software products use [[XML]] configuration files to register subscribers. These configuration files are read at initialization time. The most sophisticated alternative is when subscribers can be added or removed at runtime. This latter approach is used, for example, in [[database trigger]]s, [[mailing list]]s, and [[RSS]].{{Cn|date=October 2024}}

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 [[Small-world routing#The Kleinberg model|Navigable Small-World topologies]]. Such Small-World topologies are usually implemented by decentralized or federated publish/subscribe systems.<ref name=":0">{{Cite book|last1=Chen|first1=Chen|last2=Tock|first2=Yoav|last3=Girdzijauskas|first3=Sarunas|title=Proceedings of the 12th ACM International Conference on Distributed and Event-based Systems |chapter=BeaConvey |date=2018|chapter-url=http://dl.acm.org/citation.cfm?doid=3210284.3210287|language=en|location=Hamilton, New Zealand|publisher=ACM Press|pages=64–75|doi=10.1145/3210284.3210287|isbn=9781450357821|s2cid=43929719|url=http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-228002 }}</ref> Locality-aware publish/subscribe systems<ref>{{Citation|last1=Rahimian|first1=Fatemeh|title=Locality-Awareness in a Peer-to-Peer Publish/Subscribe Network|date=2012|work=Distributed Applications and Interoperable Systems|volume=7272|pages=45–58|editor-last=Göschka|editor-first=Karl Michael|publisher=Springer Berlin Heidelberg|doi=10.1007/978-3-642-30823-9_4|isbn=9783642308222|last2=Le Nguyen Huu|first2=Thinh|last3=Girdzijauskas|first3=Sarunas|editor2-last=Haridi|editor2-first=Seif|doi-access=free}}</ref> construct Small-World topologies that route subscriptions through short-distance and low-cost links thereby reducing subscription delivery times.