Distributed OS

Commn. Primitives High level construct with which program use underlying network 2 commn. Models : Msg Passing & RPC Msg Passing – 2 primitives: SEND & RECEIVE Design issues : buffered option – msg copied 3 times unbuffered option : program issuing SEND should avoid resuing user buffer till msg. is transmitted

Commn. Primitives Non-blocking primitives:SEND returns control to user process as soon as msg is copied to kernel buffer Flexibile to perform computation & commn. Complex : programs may become time dependent, difficult to debug Blocking primitives: SEND will not return control until msg. is sent (unreliable blocking), or until ack. Received (reliable blocking) Corres. RECEIVE will not return control till msg. is copied to user buffer (unreliable) or till auto. Sends ack (reliable blocking) For large systems, unbuffered & non blocking primitives

Commn. Primitives Sync. primitives – SEND primitive is blocked till corres. RECEIVE is executed at Rx. System – Rendezvous Unblocking sync: copy msg to buffer at Tx.system. Async. Primitives No block, msg. are buffered Comples: creating, managing & destroying buffers, msg. meant for processes that have already died.

RPC Issues with Msg.passing: Pairing request msg with responses. Data repn. (diff. arch, languages etc.) Knowing address of remotes m/c or server Care for commn. & system failures Programs – time dependent Hides all these from programmer, PC extended for control and data access across nework Interaction between Client and Server, Client that needs a service invokes procedure at Server

RPC Basic operation: On RPI, Calling process (Client) suspended, parameters passed to remote server , where procedure will be executed. On completion, results passed to Client and it resumes execution. Design Issues: Structure: ref. fig.4.5 Stub procedures can be generated at compile time or linked at run time.

RPC – Design Issues Binding: process that determines remote procedure , m/c on which to be executed on RPI Also checks for compatibility of parameters & procedure types 2 approaches : use of binding server – Servers register services they provide with BS, Client stub obtains address by querying BS Client specifies m/c & service required, Server returns port number for commn.

RPC – Design Issues Parameter & result passing: Stub procedure has to convert paramters, results to approp. Repn. Std. format Passing parameters by value – stub copies paramters in to msg. By reference- complex , semantics of parameters in local & remote procedures can be different

RPC – Design Issues Error handling in DS - difficult RPC can fail due to system or commn. Failure Unwanted executions – orphans Semantics (interpretations) : Atleast once semantics Exactly once semantics Atmost once semantics Correctness:

RPC – Design Issues Other Issues: Low latency RPC, min. delay, datagram protocol, Ex: Future High throughput RPC, bulk data transfer- TCP, Ex. Stream RPI blocks calling process, limits concurrency, MultiRPC, PARPC Incremental results cannot be returned to calling process while in progress Protocol flexibility Channel model- pipe for transmitting data and incremental results, channel groups for sequencing of calls