Distributed Systems Remote Method Invocation

Distributed Systems Remote Method Invocation
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Published Date:01-08-2017
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Distributed Systems 5. Remote Method Invocation Werner Nutt 1ƒƒƒƒƒƒ Middleware Middleware offers an infrastructure that enables application processes to communicate with each other Processes issue requests to the transportation layer (i.e., the application takes the initiative, not the middleware) Applications access the middleware via APIs, e.g., – creation and manipulation of sockets Integration into programming languages – remote procedure call (RPC) – remote method invocation (RMI) For higher level APIs, data has to be transformed before it can be shipped (“data marshalling”) Protocols for Client/Server Interaction (“Request/Reply”) 3ƒƒƒƒƒ Why Middleware? Distributed computing environments are heterogeneous: Networks – ATM, Ethernet, etc. have different protocols Computer hardware – data types (integers) can be represented differently Operating systems – e.g., TCP module can be part of OS (Unix/Linux) or not Programming languages – e.g., different paradigms (functional, OO, etc.) – e.g., data structures (arrays, records) can be represented differently Applications implemented by different developers 4Middleware Hides Heterogeneity Applications RMI, RPC and events Middleware Request reply protocol layers External data representation Operating System 5ƒƒ Middleware Characteristics Location transparency – client/server need not know their location Sits on top of OS, independent of – Communication protocols: use abstract request-reply protocols over UDP, TCP – Computer hardware: use external data representation e.g. CORBA CDR – Operating system: use e.g. socket abstraction available in most systems – Programming language: e.g. CORBA supports Java, C++ 6ƒƒƒƒƒƒ Middleware Programming Models Commonly used models: Distributed objects and remote method invocation (Java RMI, Corba) Remote Procedure Call (Web services) Remote SQL access (JDBC, ODBC) Distributed transaction processing CORBA: provides remote object invocation between – a client program written in one language and – a server program written in another language commonly used with C++ 7ƒƒ Objects object object Data Data interface m1 m1 implementation implementation m2 m2 m3 m3 of methods of methods Object = data + methods – logical and physical encapsulation – accessed by means of references – first class citizens, can be passed as arguments Interaction via interfaces – define types of arguments and exceptions of methods 8Î ÎÎ ƒƒƒƒ The Object Model Programs are (logically and physically) partitioned into objects distributing objects natural and easy Interfaces – the only means to access data make them remote Actions –via method invocation – interaction, chains of invocations – may lead to exceptions part of interface Garbage collection – reduces programming effort, error-free (Java, not C++) 9ƒƒ The Distributed Object Model: Ideas local C remote E local invocation invocation remote invocation invocation F B A local invocation D Objects are distributed – client-server relationship at the object level Extended with – Remote interfaces – Remote Method Invocation (RMI) – Remote object references 10Î ƒƒƒƒ The Distributed Object Model: Principles local C remote E local invocation invocation remote invocation invocation F B A local invocation D Each process contains objects, some of which can receive remote invocations, others only local invocations Objects that can receive remote invocations are called remote objects The remote interface specifies which methods can be invoked remotely Objects need to know the remote object reference of an object in another process in order to invoke its methods How do they get it? 11ƒƒ Remote Object References Object references – used to access objects, which live in processes – can be passed as arguments and results – can be stored in variables Remote object references – object identifiers in a distributed system –must be unique in space and time – error returned if accessing a deleted object – can allow relocation (see CORBA) 12Î ƒƒƒ Remote Object Reference Construct unique remote object reference – IP address, port, interface name – time of creation, local object number (new for each object) Use in the same way as local object references If used as address cannot support relocation 32 bits 32 bits 32 bits 32 bits interface of port number Internet address time object number remote object 13Î ƒƒƒƒ Remote Interfaces Specify externally accessible methods –no direct references to variables (no global memory) –local interface is separate Parameters – input, output or both (no output parameters in Java why? ) – call by value and call by reference No pointers – but references No constructors –but factory methods 14ƒƒ A Remote Object and its Interface remoteobject Data remote interface m4 m1 implementation m5 m2 m6 of methods m3 CORBA: Interface Definition Language (IDL) Java RMI: like other interfaces, extends class Remote 15ƒƒ Handling Remote Objects Exceptions (Java: RemoteException) –raised in remote invocation – clients need to handle exceptions – timeouts in case server crashed or too busy Garbage collection –distributed garbage collection may be necessary – combined local and distributed collector – cf. Java reference counting (remote object knows in which processes live proxies, extra communication to inform server about creation and delection of proxies) 16ÎÎÎ ƒƒ RMI Issues Local invocations – executed exactly once Remote invocations – via Request-Reply – may suffer from communication failures retransmission of request/reply message duplication, duplication filtering no unique semantics… 17Invocation Semantics Invocation Fault tolerance measures semantics Retransmit request Duplicate Re-execute procedure message filtering or retransmit reply No Not applicable Not applicable Maybe Yes No Re-execute procedure At-least-once Yes Yes Retransmit reply At-most-once 18ƒƒƒƒ Maybe Invocation Remote method –may execute or not at all, invoker cannot tell – useful only if failures are rare Invocation message lost... – method not executed Result not received... – was method executed or not? Server crash... –before or after method executed? – if timeout, result could be received after timeout … 19ƒƒƒ At-least-once Invocation Remote method – invoker receives result (executed at least once) or exception (no result, executed one or more times) – retransmission of request messages Invocation message retransmitted … – method may be executed more than once – arbitrary failure (wrong result possible) – method must be idempotent (repeated execution has the same effect as a single execution) Server crash... – dealt with by timeouts, exceptions 20ƒƒƒ At-most-once Invocation Remote method – invoker receives result (executed once) or exception (no result) – retransmission of reply and request messages – receiver keeps history with results (how long?) – duplicate filtering Best fault-tolerance ... – arbitrary failures prevented if method called at most once Used by CORBA and Java RMI 21

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