Communication in Distributed Systems
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The document discusses communication methodologies in distributed systems, focusing on network programming, remote procedure calls (RPC), remote method invocation (RMI), and message-oriented communication. It outlines the historical evolution of these concepts, detailing the steps and concerns involved in RPC and the transition to RMI, which enhances distribution transparency. It also briefly covers web services and their interoperability, emphasizing their reliance on XML for communication.
Communication in Distributed Systems
- 1. Communication in Distributed Systems CS4262 Distributed Systems Dilum Bandara [email protected] Some slides extracted from Dr. Srinath Perera & Dr. Rajkumar Buyya’s Presentation Deck
- 2. Outline Network Programming Remote Procedure Calls Remote Method Invocation Message Oriented Communication 2
- 3. Network Programming Communication is typically over IP HPC Clusters may use Infiniband or Myrinet Unicast, multicast, anycast, broadcast Only unicast is globally routable 2 main approaches TCP – connection oriented UDP – connection less Lot more popular than you think! 3
- 4. End-to-End Communication 4Source: Communication Networks by Alberto Leon-Garcia (Author), Indra Widjaja, 2000
- 5. TCP Socket Calls 5 Source: Communication Networks by Alberto Leon- Garcia (Author), Indra Widjaja, 2000
- 6. UDP Socket Calls 6 • No “handshake” • No simultaneous close • No fork() for concurrent servers Source: Communication Networks by Alberto Leon- Garcia (Author), Indra Widjaja, 2000
- 7. Messages Used to convey data among nodes Has a limited size Typically has a header Multiple messages may be send through same connection Identify message boundaries based on size, unique symbol patterns, etc. Messages can be clear text, binary, XML, JSON, etc. 7
- 8. Communication in Distributed Systems – History Started with RPC Went to distributed objects Come back to RPC + Message Oriented Communication 8
- 9. Remote Procedure Calls (RPCs) Extend local procedure calls to work across computers Call a procedure on a remote machine “just” as you would on local machine Send the call as a message & get response/fault as a message 9 Source: http://pubs.opengroup.org/onlinepubs/9629399/chap6.htm
- 10. RPC in Action… 10Source: Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007
- 11. RPC – Steps 1. Client procedure calls client stub in normal way 2. Client stub builds message, calls Middleware 3. Middleware sends message to remote Middleware 4. Remote Middleware gives message to server Skeleton 5. Server stub unpacks parameters, calls local implementation 6. Local implementation does work, returns result to Skeleton 7. Skeleton packs it in message, calls local Middleware 8. Server's Middleware sends message to client's Middleware 9. Client's Middleware gives message to client stub 10. Stub unpacks result, returns to client 11
- 12. RPCs Introduced by Birrell & Nelson in 1984 Pre-RPC – Most applications were built directly over Internet primitives Initial idea – Mask distributed computing system using a “transparent” abstraction Looks like normal procedure call Hides all aspects of distributed interaction Supports an easy programming model Today, RPC is the core of many distributed systems, e.g., Web Services But now acknowledge other aspects like failures & asynchronous nature, communication latency more 12
- 13. RPCs (Cont.) 13 Source - wikipedia.org
- 14. Message Exchange Patterns With local calls, you call, & wait for response But with RPC, there are other possibilities Send/Receive Send Only (Fire & Forget) Send with Ack (Send Robust) There are 2 models for client API Polling Callback 14
- 15. RPC Concerns How to make RPC similar to a local procedure call? Communications Location transparency Parameter passing Heterogeneity OS, arch., language Failures Failure transparency Stubs 15 Source: http://technet.microsoft.com/en- us/library/cc738291%28v=ws.10%29.aspx
- 16. Data in Messages Data is “marshalled (encoded)” into a message & “demarshalled (decoded)” from it Data structures Flattened on transmission Rebuilt upon reception Data types Base types – Ints, floats Flat types – structures, arrays Complex types – pointers With Web services we convert them to XML Issues Big-endian vs. little-endian, strings may require padding, alignment (16/32/64-bits), Floating point representations 16
- 17. Asynchronous & Deferred Synchronous RPC 17 Source: Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007
- 18. Directory Service Directory service or a UUDI registry UDDI - Universal Description, Discovery & Integration 18 Source: Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007
- 19. RPC – What Can Go Wrong? Failures Network failure Client failure Server failure Assuming only network idiosyncrasies for now… Ignoring possibility of process & machine crashes …Need to overcome Limited levels of message loss Message disorder/reorder Message duplication 19
- 20. Overcoming Only Communication Failures Each message carry Unique sequence number Timestamp from a global clock (in most RPC protocols) Global clock assumed to return roughly same value throughout network, up to clock synchronization limits Enables server to detect & discard Very old messages Duplicate copies RPCs use acknowledgements If timeout with no ack resend packet Leads to the issue of replayed requests 20
- 21. Overcoming Lost Packets 21 Client Server Time out Sends request Retransmit Ack for request Reply Ack for reply
- 22. Overcoming Lost Packets (Cont.) Acks are expensive Retransmissions are costly For big messages, send packets in bursts & Ack a burst at a time 22
- 23. Machines Could Fail Too… What does a failed request mean? Network failure, machine failure or both! If a process fails, but machine remains operational, OS will provide sufficient status information to accurately determine “process failure” Client that issued request would not know, if server processed the request or not 23
- 24. RPC RMI Evolution By expanding RPCs to invocation on remote objects Object-oriented equivalent of RPC Distribution transparency is enhanced Separation between interfaces & objects implementing these interfaces is crucial for distributed systems State of remote objects is not distributed Only interfaces implemented by the object are distributed (available on other machines) 24
- 25. RMI in Action 25
- 26. RMI in Action (Cont.) 26 Source: Introduction to Java Programming, Comprehensive Version (7th Edition) by Y. Daniel Liang
- 27. RMI in Action (Cont.) Client binds to a distributed object residing at a server machine An implementation of object’s interface, called a proxy, is loaded to the client’s address space Proxy – Analogous to a client stub in RPC Marshals message invocations into messages – object serialization Unmarshals reply messages to return method invocation result to client On server side Incoming invocation requests are passed to a server stub, called a skeleton Unmarshals invocation request into proper method invocations Marshals replies & forwards reply messages to client side proxy27
- 28. Remote Objects & Remote Interfaces RMI is interface-oriented Remote interface implemented by remote object Only methods specified by remote interfaces can be accessed via RMI A remote object may implement several sets of remote interfaces simultaneously 28Source: Introduction to Java Programming, Comprehensive Version (7th Edition) by Y. Daniel Liang
- 29. Remote Exceptions If a remote method throws an exception Remote side Catch all uncaught exceptions thrown from remote methods Find the caller Map to network request Client side Keep listening Any time an exception could be sent from a remote side Unmarshall network requests to exception objects Re-throw remote exception objects locally, as if exception was thrown from current method 29
- 30. Remote Exceptions (Cont.) RMI software itself can also throw exceptions Network timeout Other system related They have accepted those failures & have decided to expose it 30
- 31. Web Services RPC like Distributed Communication medium that supports interoperability Not that much different from RMI, RPC, etc., except interoperability & lack of distributed objects Use XML to communicate Request messages, description, & discovery all use standard XML based formats 31
- 32. Web Services (Cont.) Publish, find, & use 32 UDDI – Universal Description, Discovery & Integration
- 33. Web Services (Cont.) Are described using WSDL Web Service Description Language standard Published & found through a UUDI registry Invoked through SOAP standard XML based protocol for accessing Web Services Can transmit through any transport HTTP most common SMTP, JMS, UDP, TCP, VFS are known 33
- 35. Summary 35
Editor's Notes
- #17: Big-endian vs. little-endian - http://h71000.www7.hp.com/doc/82final/6443/zk-6654a.gif
- #19: DCE RPC is a facility for calling a procedure on a remote machine as if it were a local procedure call
- #33: UDDI a platform-independent, XML-based registry by which businesses worldwide can list themselves on the Internet, and a mechanism to register and locate web service applications.
- #34: JMS – Java Message Service VFS – Virtual File System