chapter 5.4 distributed process implementaion prepared by: karthik v puttaparthi...
TRANSCRIPT
OUTLINE Logical Model of Local and Remote
Processes Application Scenarios Remote Service Remote Execution Process Migration Mobile Agents References
Logical Model [1] Front end Stub Processes facilitate the
creation of and communication between processes.
Stub Processes serve as a logical link making the physical boundary between the local and remote processes transparent.
Local Process Remote Process
Stub Process Stub Process
CLIENT SERVER
Application Scenarios [1] There are three significant application
scenarios depending on how request messages are interpreted.
Remote Service – Interpreted as a request for known service at the remote site.
Remote Execution – Contain a program to be executed at the remote site.
Process Migration – Represent a process being migrated to the remote site for continuing execution.
REMOTE SERVICE
Its primary application : resource sharing in distributed systems.
Request message for remote service can be generated at three different software levels.
• As remote procedure calls at the language level• As remote commands at the operating system level• As interpretive messages at the application level
Remote Procedure Calls At the Language Level [3]
At language level for remote service requests
Most suitable model : RPC It is service oriented and provides both access and location transparency
At the System Level,
There are some frequently used commands that need to invoke remote objects. These commands can be built in as part of the command shell and recognized by the local operating system.
Example: rsh host –l user ls in Unix can list the home directory files of the user, user on a remote host machine, host.
Remote Commands at the Operating System Level [3]
User Want to send message Remote host
Some user-defined operations at remote hostinvoke
Remote commands are limited only to shell commands. The concept can be generalized to process messages.
Stub process at Server site has capability of
-- Interpreting messages sent from client stub -- invoking respective operations
Based on the content of message
Example is ftp. The stubs are the ftp daemon processes. They interpret messages such as get and put for downloading and uploading files.
Interpretive Messages at the Application Level
Remote operations are initiated by
Remote procedure calls, remote commands and interpretive messages
Constrained to Services that are supported at the remote host
Primary Implementation Issues
I/O Redirection Security
Client stub
Server stub
copies Client process’ standard input to the remote command
Standard output and error of the remote command to client process
copies
Access to remote resources required authorization
REMOTE EXECUTION [1]• Remote operation initiated by client is created by the client
• Message sent from client to server is client program to be executed at the server
•Remote Execution or Processor Pool model – Selected host may be a system with specific resources for load sharing.
Selected host
System with specific resources for resource sharing
Remote Service
System with purpose of load sharing
Remote Execution
Implementation issueLoad sharing algorithm location independence system heterogeneity protection and security
Load Sharing Algorithm [4] Practical Implementation Each Process server maintains list of
registered hosts. First step is to select remote host Client process server indicate Resource
requirement to process server at remote site
If authenticated and its resource request can be met, Server grants permission for remote execution
LOCATION INDEPENDENCE
Process created by remote execution require
Coordination to accomplish common task
So, it is necessary to support logical view for the processes
Each remote process is represented by an agent process at the originating host
SYSTEM HETEROGENEITY
If remote execution is invoked on heterogeneous host
It is necessary to recompile the program
Overhead may be too high
Solution
Use canonical machine-independent intermediate language for program execution.
PROTECTION AND SECURITY
Remote execution powerful
Foreign code image can be Trojan horse
To Achieve Protection and Security,
• Accept only remote execution in source or intermediate codes
• Language used to describe a remote execution could be restricted to exclude potential problems
• Run-time checking of parameters and stack overflow is also necessary to protect the integrity of the remote hosts
PROCESS MIGRATION [2]
Process P1::::
Executionsuspended
Source Site Destination Site
ExecutionResumed
::::
Process P1
Transfer of control
Time
Freezingtime
LINK REDIRECTION [2]
Transfer ofaddress space
Sourcenode
Destinationnode
SuspendedMigrationdecision
resumed
Freezingtime
Transfer ofaddress space
Sourcenode
Destinationnode
Suspended
Migrationdecision
resumed
Freezingtime
Total Freezing Pretransferring
Merits: easy implementationDemerits: long delay time
Merits: freezing time reduceDemerits: total time extended
MESSAGE FORWARDING
Three types of messages:
1. Received when the process execution is stopped on the sourcenode and has not restarted on the destination node
2. Received on the source node after the execution started ondestination node
3. Sent to the migrant process after it started execution ondestination node
MESSAGE FORWARDING [2]
Sender
Origin
Dest 1
Dest 2
Receiver
Migrate
Migrate again
Resend
Resend again
Sender
Origin
Dest 1
Dest 2
Receiver
Migrate
Migrate again
Resending messagesAsk origin site
SendSend
Forward
Messages of type 1 and 2 are either dropped or negatively ack-ed, the sender is notified and it needs to locate the migrant process
Origin node keeps the info on the current location of the process created there, all messages are sent to origin whichforwards them to migrant process
MESSAGE FORWARDING [2]
Sender
Origin
Dest 1
Dest 2
Receiver
Migrate
Migrate again
Link traversal
Link
Link
Send
Forward
Forward
Send
Messages are queued and sent to destination node as part of migration procedure
Link is left on source node to redirectmessages, link contains the system-wide unique id of a process and its last known location
MESSAGE FORWARDING
Sender
Origin
Dest 1
Dest 2
Receiver
Migrate
Migrate again
Link Update
Send
New location
Send
New location
Send
Current location
During the transfer the source node sends the notification (link update) of the transfer to all the nodes to which the process communicates:
Messages are forwarded by the source node
Messages are sent directly to the destination node
MOBILE AGENTS [5]
• It is composition of computer software and data which is able to move from one computer to another autonomously and continue its execution on the destination computer.
• It refers to a process that can transport its state from one environment to another, with its data intact
• Mobile agents can decide when and where to move next
• When a mobile agent decides to move, it saves its own state and transports this saved state to next host and resume execution from the saved state.
Common applications include:• Resource availability, discovery, monitoring • Information retrieval • Network management • Dynamic software deployment
Mobile Agents
A mobile agent visits a collection of ORBs.
In each ORB along its path, it can install new components on the disk, dynamically link new components, inspect the state and configuration of the
ORB and the applications on top of it, reconfigure ORBs and applications.
A Flexible Framework Different NetworkBrokers support
different agent flavors. For example: simple, lightweight, script-based agents
(carrying data and DCP commands only). powerful, heavyweight, Java-based
agents (carrying data, bytecode, and dynamic state, taking autonomous decisions).
Simple agents are suitable for PDAs, embedded systems.
Reconfiguration with Mobile Agents
SysAdmins use a GUI to build agents for reconfiguration inspection
GUI is used to1. Build distribution
graph2. Select
reconfiguration and inspection commands
3. Visualize results.
Process Migration vs. Mobile Agents
Process Migration Mobile Agents
Navigational Autonomy
Migration decision is made by system.
Agents decide where and where to go
Code Execution Programs are fully compiled and executed in native mode.
Most agents are coded in Java and are interpreted by their execution engine.
Strong/Weak Migration
Execution is resumed where it has been suspended.
Java-based agents resume their execution from the top of a given method.
I/O State Long-term I/Os are forwarded to processes migrated to the destination.
Agents relinquish I/O connections every time they depart for their next destination.
References1. Distributed operating system and
algorithms Randy chow, Theodore Johnson
2. A Framework for Process Migration in Software DSM Environments ieeexplore.ieee.org/iel5/6662/17807/00823407.pdf …. ( 2000 )
3. http://birrell.org/andrew/papers/ImplementingRPC.pdf
4. http://www.informs-cs.org/wsc02papers/061.pdf
5. http://en.wikipedia.org/wiki/Mobile_agent
Thank You!!