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Publish-subscribe, Event brokers INF 123 Software architecture tdebeauv@uci.edu 1

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Outline Pubsub Event brokers 2

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PUBSUB 3

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Newspaper distribution Newspaper companies publish newspapers Readers (un)subscribe to a company for a newspaper Company sends newspaper to its subscribers 4

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Newspaper distribution 5

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Stock exchange Jobs wants to buy AAPL below $30 Gates wants to buy MSFT below $40 Buffett wants to sell MSFT and AAPL for $35 Traders are intermediaries Publication and subscription are decoupled Buffett does not know who he sold to Gates does not know who he bought from 6

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Stock exchange 2 NYSE provides feed of stock updates Humongous, fast Traders are only interested in parts of it Need filtering 7

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8 Eugster 2003: The many faces of publish/subscribe

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Constraints Subscribers register their interest Interest in a publisher, an event, or a content Publishers generate events Sometimes through intermediaries called message broker, or message bus, or even proxy Usually asynchronous Publishers dont wait for subscribers to publish Subscribers give a callback to be notified 9

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Gains Scalable one-way communication Usually over the network Well adapted to the loosely coupled nature of distributed large-scale applications [Eugster03] Can decouple subscribers from publishers Decoupling Privacy Content filtering 10

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Request-reply vs pubsub Request-reply Implies client-server Medium coupling Pull info Pubsub Less coupled Push info 11

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More about pubsub AKA producer-consumer Implies message queues Pubsub != client-server 12 CriteriaPubsubClient-server Message passingOne-wayTwo-way Request-replyImpossiblePossible Information flowPublishers push to subscribers Clients pull from server

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Pubsub + space partitioning = scale 13 http://www.cescg.org/CESCG-2000/JKrivanek/

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But What if everyone subscribes to one item? BarackObama: 43M followers on Twitter Bandwidth limits, slower updates Can/should cache What if everyone subscribes to everything? Then Pubsub is probably not the right style 14

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Apple Push Notifications 15 http://www.raywenderlich.com/32960/apple-push-notification-services-in-ios-6-tutorial-part-1

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Apple Push Notifications Subscription Device token = content of interest Delivered by APNS, for security why? Notification APNS intermediary between appServer and appClient For privacy why? 16

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Zoom-in/out Zoom on a region of the board Useful with a million small pieces Originally client-side The server sends all piece movement events Most events that the client receives are not rendered, and could be ignored Then server-side pubsub Only receive events near me Client subscription == client view Client model = subset of server model 18

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Near me Current view: x, y, w, h Subscribe to [x: x+w, y: y+h] Subscribe to [x-d: x+w+d, y-d: y+h+d] Partition the world statically in cells Subscribe to the cells that my view overlap with 19 User-oriented software development for real time online applications, Gorlatch 2008

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EVENT BROKERS 20

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Event broker vs pubsub Event broker = pubsub With an intermediary Where subscribers publish And publishers subscribe And usually with function callbacks rather than over the network 21

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Mini-map Receive player position message Change player position on main screen Change player position on mini-map 22

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Mini-map Main view and mini-map view Both subscribe to: My movements Quest markers The main view subscribes to: Other players movement The mini-map subscribes to: Zoom clicks 24

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Widgets GUI widgets N widgets interested in an event A widget may fire another event in response Enqueue new event? Or dispatch it right away? 25

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Publish events 26 http://ezide.com/games/writing-games.html

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Dispatch events 27 http://ezide.com/games/writing-games.html

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If youre interested http://msdn.microsoft.com/en- us/library/ff650653.aspx http://msdn.microsoft.com/en- us/library/ff650653.aspx http://ezide.com/games/writing-games.html The Many Faces of Publish/Subscribe, Eugster et al. 2003 http://code.google.com/p/pubsubhubbub/ http://www.capitalware.com/dl/docs/intro_p ub_sub.pdf http://www.capitalware.com/dl/docs/intro_p ub_sub.pdf 28