REDS: A Reconfigurable Event Dispatching Infrastructure Gianpaolo Cugola Dipartimento di Elettronica e Informazione Politecnico di Milano, Italy joint work with G. P. Picco, P.Costa, D.Frey, C. Ghezzi, M.Migliavacca, A.L.Murphy

Politecnico di Milano IS-MANET Meeting, July 21st Publish-Subscribe Application components can publish asynchronous messages (event notifications), and/or declare their interest in message classes by issuing a subscription Subscriptions are collected by a message dispatcher component, responsible for routing messages to matching subscribers High degree of decoupling among components Message Dispatcher C1C1 C2C2 C3C3 C1C1 Temperature > 20 o C Temperature = 25 o C C4C4 Temperature < 40 o C C4C4

Politecnico di Milano IS-MANET Meeting, July 21st Design Space Existing publish-subscribe middleware differ along several dimensions: –The format of messages –The expressivity of the subscription language –The architecture of the dispatcher –The routing strategy (in presence of a distributed dispatcher) –The forwarding strategy

Politecnico di Milano IS-MANET Meeting, July 21st Format of Messages Untyped sequences of values –Tuples Untyped, record-like, messages –Sequences of fields, each with a name and a value Typed, record-like, messages –Similar to the above case with typed fields Typed objects –Full-fledged objects with their own attributes and methods

Politecnico di Milano IS-MANET Meeting, July 21st Subscription Language The expressivity of the subscription language allows one to distinguish between: –Subject-based The set of subjects is determined a priori Analogous to multicast –Content-based Subscriptions contain expressions (message filters) that allow clients to filter messages based on their content The set of filters is determined by client subscriptions A single message may match multiple subscriptions

Politecnico di Milano IS-MANET Meeting, July 21st Architecture of the Dispatcher Centralized –A single component is in charge of collecting subscriptions and forward messages to subscribers Distributed –A set of message brokers organized in an overlay network cooperate to collect subscriptions and route messages –The topology of the overlay network and the routing strategy adopted may vary Message Dispatcher

Politecnico di Milano IS-MANET Meeting, July 21st Unrooted Tree Topology Message forwarding –Every broker stores only subscriptions coming from directly connected clients –Messages are forwarded from broker to broker... –...and delivered to clients only if they are subscribed

Politecnico di Milano IS-MANET Meeting, July 21st Unrooted Tree Topology Subscription forwarding –Every broker forwards subscriptions to the others –Subscriptions are never sent twice over the same link –Messages follow the routes laid by subscriptions

Politecnico di Milano IS-MANET Meeting, July 21st Tree Topology Hierarchical –Both messages and subscriptions are forwarded by brokers toward the root of the tree –Messages flow down only if a matching subscription had been received along that route

Politecnico di Milano IS-MANET Meeting, July 21st Forwarding Strategy Each time a broker or the unique, centralized dispatcher receives a message it has to match it against the list of received filters to determine the list of recipients The efficiency of this process may vary, depending on the complexity of the subscription language, but also on the forwarding algorithm chosen –It greatly influences the overall performance of the system

Politecnico di Milano IS-MANET Meeting, July 21st Reconfiguration In presence of a distributed dispatcher, topological reconfiguration of the dispatching infrastructure should be considered They can be caused by: –Physical mobility of the message brokers –Changes in the overlay network induced by application requirements (e.g., in P2P networks) or by system optimizations (e.g., for load balancing) No publish-subscribe system to date deals with such kind of topological reconfiguration –Some deal only with the mobility of clients

Politecnico di Milano IS-MANET Meeting, July 21st Dealing with Reconfiguration Involves solving three problems: –Reconstruct the overlay network Currently we have: –MAODV-like approach for mobility –Custom, optimized solution for large- scale P2P environments In some environments, may be under control of some user (e.g., sysadm) –Rearrange the subscription tables Content-based, subscription forwarding routing defines a new problem, for which the existing subject-based or multicast solutions are no longer applicable –Minimize message loss We have a solution based on epidemic algorithms [ICDCS04] HW + Network Overlay Maintainance Reconfigurable Subscription Forwarding Message Recovery physical link break logical link break

Politecnico di Milano IS-MANET Meeting, July 21st A Strawman Approach Problem: a potentially very large number of subscriptions may end up removed and immediately re-inserted The endpoints of a vanishing link behave as if they received unsubscriptions from the other end (and similarly subscriptions for those on the new link) A B C D

Politecnico di Milano IS-MANET Meeting, July 21st Understanding Propagation A subscription is propagated following the unique route up to the closest splitter, if it exists; to the whole tree, otherwise splitter –A splitter is either a subscriber, or a dispatcher whose routing table has two or more entries for the given subscription A B C D Adding a subscription is usually cheap The more splitters, the shorter the path a subscription must follow

Politecnico di Milano IS-MANET Meeting, July 21st Our First (Simple) Idea [ICDCS03] It is fundamental to perform subscriptions before unsubscriptions –This way, the network is kept dense of subscriptions (and splitters), and the impact of reconfiguration is reduced Unsubscriptions are delayed using a timer, set by both the end-points of a broken link This simple idea is: –grounded on a thorough understanding of how the subscription forwarding strategy works –significantly effective in reducing overhead –easy to implement An additional optimization is provided for when the broken and new link share a dispatcher

Politecnico di Milano IS-MANET Meeting, July 21st Reconfiguration Path [SAC04] It is the sequence of dispatchers connecting the old link with the new link Only the dispatchers on this path are affected Our second algorithm propagates a control message along the path, carrying the set of subscriptions to be added and/or deleted at each node A B C D

Politecnico di Milano IS-MANET Meeting, July 21st REDS Goal: designing and implementing a reconfigurable publish- subscribe middleware In which sense reconfigurable? –Logical/Software (Static) : a well organized component-based design allows system integrators to build their REDS system by choosing among a set of predesigned components (or to write their own versions) to determine several aspects The format of messages and subscriptions The wire-protocol used to transport messages and subscriptions The architecture of the dispatcher The routing and the forwarding strategies –Physical (Dynamic) : REDS brokers are designed from the ground to deal with reconfiguration How? –Through an open framework of Java classes, which realizes the design above and allow system integrators to choose among a set of predefined components or build their own

Politecnico di Milano IS-MANET Meeting, July 21st The Client API

Politecnico di Milano IS-MANET Meeting, July 21st The Architecture of the Broker ** transport layer routing and forwarding layer

Politecnico di Milano IS-MANET Meeting, July 21st The Architecture of the Broker The Core –Holds the shared data structures (the subscription table and the list of neighbors) –Mediates the communication among the other components The simplest implementation delegates all its operations to the other modules, without adding any further processing –Manages internal messages, delivering them to registered processors Internal messages are used to exchange information among a set of distributed brokers (e.g., to communicate current load or current location) The Router –Implements the specific routing strategy adopted The ConnectionManager –Is in charge of managing the dispatching network in presence of reconfigurations New clients/brokers appearing Links/brokers vanishing The SubscriptionTable –Implements the specific forwarding strategy adopted The Transport and Neighbor –Implements the specific transport protocol adopted (e.g., TCP based, HTTP based,...) –Include methods to open and close links toward other brokers/clients

Politecnico di Milano IS-MANET Meeting, July 21st Currently Available Modules A transport layer using TCP links to move messages and filters along the network A router that implements the subscription forwarding strategy A simple, but general subscription table that can store any kind of messages and filters A configuration manager that implements the delaying unsubscriptions approach previously described –It can be coupled with an overlay manager that implements an MAODV-like strategy to keep the tree of brokers connected

Politecnico di Milano IS-MANET Meeting, July 21st Modules We Are Building A core that implements our gossip-based algorithm to recover messages lost due to reconfigurations or link errors Two subject-based routers –One using a single unrooted tree spanning all brokers –One using different trees, one for each subject, spanning only the routers that subscribed to the corresponding subject An overlay manager similar in its interface to the MAODV- like one, but tailored to large-scale p2p environments A new subscription table implementing a more efficient forwarding strategy designed for record-like messages

Politecnico di Milano IS-MANET Meeting, July 21st Modules We Planned A message-forwarding router A hierarchical router A configuration manager that implements the reconfiguration path strategy An UDP transport layer

Politecnico di Milano IS-MANET Meeting, July 21st Future Work Adding new modules to REDS Porting to REDS the Jedi mechanism to allow clients to move from broker to broker Introducing into REDS mechanisms to automatically rearrange the overlay network of dispatchers (also changing the routing strategy, if necessary) to optimize routing –Can leverage-off our previous work on reconfiguration –Needs some mechanism to trigger reconfiguration, measure traffic, and determine best topology/strategy based on the measured traffic profile –We already have some promising results coming from simulations we are running Use REDS in production

Politecnico di Milano IS-MANET Meeting, July 21st Other Research Areas LIME: Linda in a Mobile Environment –A tuple-based middleware for mobile hosts –Based on transliently-shared tuple spaces (host-level tuple space and federated tuple space) Sensor networks (motes + tiny os) –Porting LIME on this platform –Goal: provide the ability to access the data context provided by the sensors in range Different from TinyDB, which instead provides access to all the sensors in the network (with high latency) Emphasis on one-hop rather than multi-hop access Base stations are mobile, and travel across a space disseminated of sensors