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Lightweight RPCs

Lightweight RPCs. Many RPCs occur between client and server on same machine Need to optimize RPCs for this special case => use a lightweight RPC mechanism (LRPC) Server S exports interface to remote procedures Client C on same machine imports interface

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Lightweight RPCs

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  1. Lightweight RPCs • Many RPCs occur between client and server on same machine • Need to optimize RPCs for this special case => use a lightweight RPC mechanism (LRPC) • Server S exports interface to remote procedures • Client C on same machine imports interface • OS kernel creates data structures including an argument stack shared between S and C CS677: Distributed OS

  2. Lightweight RPCs • RPC execution • Push arguments onto stack • Trap to kernel • Kernel changes mem map of client to server address space • Client thread executes procedure (OS upcall) • Thread traps to kernel upon completion • Kernel changes the address space back and returns control to client • Called “doors” in Solaris CS677: Distributed OS

  3. Doors • Which RPC to use? - run-time bit allows stub to choose between LRPC and RPC CS677: Distributed OS

  4. Other RPC Models • Asynchronous RPC • Request-reply behavior often not needed • Server can reply as soon as request is received and execute procedure later • Deferred-synchronous RPC • Use two asynchronous RPCs • Client needs a reply but can’t wait for it; server sends reply via another asynchronous RPC • One-way RPC • Client does not even wait for an ACK from the server • Limitation: reliability not guaranteed (Client does not know if procedure was executed by the server). CS677: Distributed OS

  5. Asynchronous RPC • The interconnection between client and server in a traditional RPC • The interaction using asynchronous RPC 2-12 CS677: Distributed OS

  6. Deferred Synchronous RPC • A client and server interacting through two asynchronous RPCs 2-13 CS677: Distributed OS

  7. Remote Method Invocation (RMI) • RPCs applied to objects, i.e., instances of a class • Class: object-oriented abstraction; module with data and operations • Separation between interface and implementation • Interface resides on one machine, implementation on another • RMIs support system-wide object references • Parameters can be object references CS677: Distributed OS

  8. Distributed Objects • When a client binds to a distributed object, load the interface (“proxy”) into client address space • Proxy analogous to stubs • Server stub is referred to as a skeleton CS677: Distributed OS

  9. Proxies and Skeletons • Proxy: client stub • Maintains server ID, endpoint, object ID • Sets up and tears down connection with the server • [Java:] does serialization of local object parameters • In practice, can be downloaded/constructed on the fly (why can’t this be done for RPCs in general?) • Skeleton: server stub • Does deserialization and passes parameters to server and sends result to proxy CS677: Distributed OS

  10. Binding a Client to an Object Distr_object* obj_ref; //Declare a systemwide object referenceobj_ref = …; // Initialize the reference to a distributed objectobj_ref-> do_something(); // Implicitly bind and invoke a method (a) Distr_object objPref; //Declare a systemwide object referenceLocal_object* obj_ptr; //Declare a pointer to local objectsobj_ref = …; //Initialize the reference to a distributed objectobj_ptr = bind(obj_ref); //Explicitly bind and obtain a pointer to the local proxyobj_ptr -> do_something(); //Invoke a method on the local proxy (b) • (a) Example with implicit binding using only global references • (b) Example with explicit binding using global and local references CS677: Distributed OS

  11. Parameter Passing • Less restrictive than RPCs. • Supports system-wide object references • [Java] pass local objects by value, pass remote objects by reference CS677: Distributed OS

  12. DCE Distributed-Object Model • Distributed dynamic objects in DCE. • Distributed named objects CS677: Distributed OS

  13. Java RMI • Server • Defines interface and implements interface methods • Server program • Creates server object and registers object with “remote object” registry • Client • Looks up server in remote object registru • Uses normal method call syntax for remote methos • Java tools • Rmiregistry: server-side name server • Rmic: uses server interface to create client and server stubs CS677: Distributed OS

  14. Java RMI and Synchronization • Java supports Monitors: synchronized objects • Serializes accesses to objects • How does this work for remote objects? • Options: block at the client or the server • Block at server • Can synchronize across multiple proxies • Problem: what if the client crashes while blocked? • Block at proxy • Need to synchronize clients at different machines • Explicit distributed locking necessary • Java uses proxies for blocking • No protection for simultaneous access from different clients • Applications need to implement distributed locking CS677: Distributed OS

  15. Persistence and Synchronicity in Communication • General organization of a communication system in which hosts are connected through a network 2-20 CS677: Distributed OS

  16. Persistence • Persistent communication • Messages are stored until (next) receiver is ready • Examples: email, pony express CS677: Distributed OS

  17. Persistence • Transient communication • Message is stored only so long as sending/receiving application are executing • Discard message if it can’t be delivered to next server/receiver • Example: transport-level communication services offer transient communication • Example: Typical network router – discard message if it can’t be delivered next router or destination CS677: Distributed OS

  18. Synchronicity • Asynchronous communication • Sender continues immediately after it has submitted the message • Need a local buffer at the sending host • Synchronous communication • Sender blocks until message is stored in a local buffer at the receiving host or actually delivered to sending • Variant: block until receiver processes the message • Six combinations of persistence and synchronicity CS677: Distributed OS

  19. Persistence and Synchronicity Combinations • Persistent asynchronous communication (e.g., email) • Persistent synchronous communication 2-22.1 CS677: Distributed OS

  20. Persistence and Synchronicity Combinations • Transient asynchronous communication (e.g., UDP) • Receipt-based transient synchronous communication 2-22.2 CS677: Distributed OS

  21. Persistence and Synchronicity Combinations • Delivery-based transient synchronous communication at message delivery (e.g., asynchronous RCP) • Response-based transient synchronous communication (RPC) CS677: Distributed OS

  22. Message-oriented Transient Communication • Many distributed systems built on top of simple message-oriented model • Example: Berkeley sockets CS677: Distributed OS

  23. Berkeley Socket Primitives CS677: Distributed OS

  24. Message-Passing Interface (MPI) • Sockets designed for network communication (e.g., TCP/IP) • Support simple send/receive primitives • Abstraction not suitable for other protocols in clusters of workstations or massively parallel systems • Need an interface with more advanced primitives • Large number of incompatible proprietary libraries and protocols • Need for a standard interface • Message-passing interface (MPI) • Hardware independent • Designed for parallel applications (uses transient communication) • Key idea: communication between groups of processes • Each endpoint is a (groupID, processID) pair CS677: Distributed OS

  25. MPI Primitives CS677: Distributed OS

  26. Message-oriented Persistent Communication • Message queuing systems • Support asynchronous persistent communication • Intermediate storage for message while sender/receiver are inactive • Example application: email • Communicate by inserting messages in queues • Sender is only guaranteed that message will be eventually inserted in recipient’s queue • No guarantees on when or if the message will be read • “Loosely coupled communication” CS677: Distributed OS

  27. Message-Queuing Model CS677: Distributed OS

  28. Stream Oriented Communication • Message-oriented communication: request-response • When communication occurs and speed do not affect correctness • Timing is crucial in certain forms of communication • Examples: audio and video (“continuous media”) • 30 frames/s video => receive and display a frame every 33ms • Characteristics • Isochronous communication • Data transfers have a maximum bound on end-end delay and jitter • Push mode: no explicit requests for individual data units beyond the first “play” request CS677: Distributed OS

  29. Examples Single sender and receiver One sender Multiple receivers CS677: Distributed OS

  30. Quality of Service (QoS) • Time-dependent and other requirements are specified as quality of service (QoS) • Requirements/desired guarantees from the underlying systems • Application specifies workload and requests a certain service quality • Contract between the application and the system CS677: Distributed OS

  31. Specifying QoS: Token bucket • The principle of a token bucket algorithm • Parameters (rate r, burst b) • Rate is the average rate, burst is the maximum number of packets that can arrive simultaneously CS677: Distributed OS

  32. Setting Up a Stream: RSVP CS677: Distributed OS

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