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Pitt CS 2510 - The World Wide Web

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The World Wide WebOverall organization of the Web.Server Clusters (1)The principle of using a cluster of workstations to implement a Web service.Server Clusters (2)(a) The principle of TCP handoff.Server Clusters (3)(b) A scalable content-aware cluster of Web servers.Web (Proxy) CachingThe principle of cooperative caching; not only caching locally but also at neighborhoodServer ReplicationIn a CDN (content distribution network), parts of a document can come from different sources. Sources are chosen based on load, location, latency, cost, etc.Server Replication (2)Request handling in a cluster of Domino servers.Replication SchemesScheme DescriptionPull-pushA replicator task pulls updates in from a target server, and pushes its own updates to that target as wellPull-pullA replicator task pulls in updates from a target server, and responds to update fetch requests from that targetPush-onlyA replicator task only pushes its own updates to a target server, but does not pull in any updates from the targetPull-onlyA replicator only pulls in updates from a target server, but does not give any of its own updates to that targetPublish/subscribe systemRV = Rendezvous, or consistentcy/synchronization primitivesEvents (1)Processing listener events for subscriptions in TIB/Rendezvous.Events (2)Processing incoming messages in TIB/Rendezvous.Processesa) Priority scheduling of events through a queue group.b) A semantically equivalent queue for the queue group with the specific event objects from (a).Processes (2)Partial broadcasting of tuples and template tuplesAchieved through multicasting or unicastingReliable MulticastingThe principle of PGM (Pragmatic General Multicast)a) A message is sent along a multicast tree (n nodes, n-1 links: efficient)b) A router will pass only a single NACK for each messagec) A message is retransmitted only to receivers that have asked for it.Synchronization (1)interesting info: not in examThe organization of transactional messaging as a separate layer in TIB/Rendezvous.Synchronization (2)interesting info: not in examThe organization of a transaction in TIB/Rendezvous.Caching and ReplicationThe position of PAM with respect to security services.Client Caching (1)Client-side caching in NFS.Client Caching (2)Using the NFS version 4 callback mechanism to recall file delegation.Remote Procedure Call (PRC) FailuresThree situations for handling retransmissions.a) The request is still in progressb) The reply has just been returnedc) The reply was done earlier, but was lost.Client CachingThe use of local copies when opening a session in Coda.Cache invalidationa) Sending an invalidation message one at a time.b) Sending invalidation messages in parallel.Server ReplicationTwo clients with different copies of the same replicated file.Network partitions may cause problems and solutionsDisconnected Operation(network partitions on purpose)The state-transition diagram of a disconnected clientHoarding = gathering data; emulation = doing operationsCase study: NFS (network file system) (1)a) The remote access model.b) The upload/download modelNFS Architecture (2)The basic NFS architecture for UNIX systems.Communicationa) Reading data from a file in NFS version 3.b) Reading data using a compound procedure in version 4.Semantics of File Sharing (1)a) On a single processor, when a read follows a write, the value returned by the read is the value just written.b) In a distributed system with caching, obsolete values may be returned.Semantics of File Sharing (2)Four ways of dealing with the shared files in a distributed system.Method CommentUNIX semantics Every operation on a file is instantly visible to all processesSession semantics No changes are visible to other processes until the file is closedImmutable files No updates are possible; simplifies sharing and replicationTransaction All changes occur atomicallyFile Locking in NFS (1)interesting info : not in examNFS version 4 operations related to file locking.Operation DescriptionLock Creates a lock for a range of bytesLockt Test whether a conflicting lock has been grantedLocku Remove a lock from a range of bytesRenew Renew the lease on a specified lockFile Locking in NFS (2)interesting info: not in examThe result of an open operation with share reservations in NFS.a) When the client requests shared access given the current denial state.b) When the client requests a denial state given the current file access state.Current file denial stateNONE READ WRITE BOTHREAD Succeed Fail Succeed SucceedWRITE Succeed Succeed Fail SucceedBOTH Succeed Succeed Succeed Fail(a)Requested file denial stateNONE READ WRITE BOTHREAD Succeed Fail Succeed SucceedWRITE Succeed Succeed Fail SucceedBOTH Succeed Succeed Succeed


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