What is a distributed system

• A system where the software components that make up the system execute on two (or typically more) computers.

Key Challenges

• Heterogeneity
  • Varieties and differences in networks, computer hardware, OS, programming languages, implementations.
  • Address heterogeneity through the use of middleware.
• Scalability
  • A scalable system operates effectively when there is a significant increase in the number of resources and the number of users. Scalability is a mean for controlling performance loss and avoiding performance bottlenecks.
• Transparency
  • Concealment from the user and application programmer of the separation of components in a distributed system.
  • Access transparency: local and remote resources can be accessed using identical operations.
  • Location transparency: resources can be accessed without knowledge of their physical or network location.
• Openness
  • Can the system be extended and re-implemented
  • Degree to which new resources-sharing services can be added and be made available for use by a variety of client programs.
  • Specification and documentation of key interfaces must be published.
  • Open Systems: An open system is one where the components are built to common public standards.
• Security
  • A variety of points that need to be protected: data flow, network, coordination and allocation service, data management / consistency service.

RPC

• Definition: RPC is a protocol that lets a program call a function on a remote machine as if it were a local function call.
• Components:
  • Client Stub: Acts like a local function but sends a request over the network
  • Server Stub (Skeleton): Receives the request and invokes the real function
  • Transport: Often uses HTTP, TCP, or HTTP/2

Java RMI

• A distributed Java program is one which the objects making up the program reside on two or more separate computers (virtual machines).
• Java Remote Method Invocation (for object oriented programming) provides support for clients invoking methods on remote servers.
  • Write code in C -> RPC (Remote Procedure Call)
• Connection
  • Server publishes its name and location to the RMI Registry
    • through the RMI URLs
  • Client asks the RMI Registry about the server.
  • Both the RMIRegistry and Server listen on sockets for remote requests.
  • Clients need to identify both the host where the server resides and the socket number (port) that it is listening on.
• Invocation
  • Server gives its Stub (local proxy) to the RMIRegistry, which gives it to the client.
    • Stub implements the same method signatures as the Server.
  • The Client invokes methods on the Stab as if it were invoking the methods directly on the Server.
  • The Stub is then response for sending the request to the Server VM.
    • The Stab serialises the method invocation across a socket connection connecting the Client and Server.
      • It sends the method name, and the parameters of the method invocation.
      • It waits for a return object or exception objects to be sent back.
      • Synchronous method invocation
• Server side
  • The server has a local proxy (Skeleton) for the Client.
  • Skeleton creates a socket on which to listen for Client requests.
  • Skeleton receives requests, de-serialises them, and invokes the method on the Server on behalf of the Client.
  • Skeleton sends return result or exception objects back to Stub.
• An Open Java RMI System
  • The communications protocol used when sending data between the Stub and Skeleton
  • The application specific definition of the methods available for remote method invocation