COMP2310 Week 7 - Message-based Synchronisation

Synchronisation

• Synchronisation model

  • Asynchronous
  • Synchronous
  • Remove invocation

• Addressing (Name space)

  • Direct communication
  • Mail-box communication

• Message structure

  • Arbitrary
  • Restricted to 'basic' types
  • Restricted to un-typed communications

Message Protocols

Synchronous

• Delay either the sender or receiver until both have reached communication stage

Sender process

• Sender delayed until
  • Receiver becomes available
  • Reciever acknowledges reception

Receiver process

• Receiver delayed until
  • Sender becomes available
  • Sender acknowledges transmission

Asynchronous

• Neither sender nor receiver is blocked

• Message not transferred directly, rather a buffer is required to store messages.

  • A policy is required for buffer sizes and buffer overflow situations

• Sender is allowed to continue with any tasks and doesn't need to be waiting or be interrupted immediately to recieve the message(s)

Emulated

Asynchronous

• Asynchronous messaging able to be emulated with underlying synchronous messages
• Intermediate process needs to be accepting messages at all times
  • Recieves sent messages asynchronously and hold onto them until receiver is ready for them
• Also needs to send out messages on request
• While messages are blocked, they are not actually delayed

Synchronous

• Both processes voluntarily suspend themselves until transaction is complete
• Processes aren't actually synchronised -> No immediate communication ever takes place
• Sender, but not receiver, knows when transaction is complete

Remote Invocation

• Delay sender or receiver until the first rendezvous point
• Sending data and performing side-effecting computation, returning it at the end
• Pass parameters
• Keeps sender blocked while receiver executes local procedure before releasing both out of rendezvous

Emulated

• Two synchronous messages (possibility of two combined middle async)

No result (Active message)

• Don't care about results
  • Both processes are actually synchronised at time of invocation
• i.e. perform some side effect

Synchronous vs Asynchronous Communications

• Want synchronous when we want synchronisation

• Want most recent data -> Asynchronous

  • Needs a buffer and policy for overflow cases

• Synchronous message passing in distributed systems requires hardware support

• Asynchronous message passing requires buffers and overflow policies

Addressing (Namespace)

• Direct

send <message> to <process-name>
wait for <message> from <process-name>

• Indirect

send <message> to <mailbox>
wait for <message> from mailbox>

• Asymmetrical addressing
  • Client-server paradigm

send <message> to ...
wait for <message>

Message Structure

• Communication systems only handle streams (packets) of basic element types
• Complexity varies
• Conversion routines for data-structures other than basic element types exist
  • Manually (POSIX, C)
  • Semi-automatically (CORBA)
  • Automatic (Compiler generated)
  • Typed-persistent (Ada, CHILL, Occam2)