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Lecturer: Hadi SalimiLecturer: Hadi Salimi
Distributed Systems Lab,
School of Computer Engineering
I U i i f S i d T h lIran University of Science and Technology,
AgendaIn this chapter, we take a closer look at:
The role of processes in distributed systems
A brief introduction to virtualization technologies.
O h l d lik i i Other process‐related concepts like migraion.
ThreadsThe granularity of processes in a distributed system is not sufficient.
There is need for a more finer element inside processes.
Using threads, can bring different advantages and di ddisadvantages.
Traditional Thread UsagesThe spreadsheet example.
From an IPC point of view.p
Threads are more proper in some cases.
Are web browsers a good example?
Thread ImplementationThreads could be implemented as:
User mode threads
Easier for implementation
Low overhead
A thread blocks leads to process blockA thread blocks leads to process block
Kernel mode threads
Hard to implement
A blocked thread does not block the whole process
Hybrid threads (LWPs)
U h b id h d i dUses a hybrid approach as depicted.
Hybrid Thread ImplementationThreads are supported on both kernel and user modes.
Thread in Distributed SystemsThey could avoid a process from being blocked when a blocked system call has happened.
Threads could be used at:
Client Processes
Server Processes
Multithreaded ClientsA multi‐threaded browser:
Performs communication and page rendering simultaneously.
Could have multiple connections at the same time.
This could be more useful when the browser gets the page This could be more useful when the browser gets the page form a replicated server.
Multithreaded ServersAs the first example consider a file server:
In the case of a blocked thread, other thread could get , gclient requests.
VirtualizationThe concept of virtualization:
To present a concept in a new form
This technique has been applied for many decades.
Role of virtualization if distributed systems:
Legacy support
E i i tiEasier migration
More flexibilityMore flexibility
VirtualizationThis figure shows the role of interacting interfaces on a computer system.
Architecture of VMsInterfaces at different levels
An interface between the hardware and software consisting gof machine instructions
that can be invoked by any program.
An interface between the hardware and software, consisting of machine instructions
that can be invoked only by privileged programs, such as an operating system.
Architecture of VMsInterfaces at different levels
An interface consisting of system calls as offered by an g y yoperating system.
An interface consisting of library calls
generally forming what is known as an application programming interface (API)programming interface (API).
In many cases, the aforementioned system calls are hidden by an API.y
Architecture of VMsInterface stack on a computer system.
Architecture of VMsA VM that can provide a virtual environment for an application.
Architecture of VMsVirtualizing the hardware for an operating sytem using a Virtual Machine Monitor
Client‐Server ComputingClient
Networked User Interfaces
The X Window System
Compound Document
Client side software for distributed transparencyClient‐side software for distributed transparency
Server
General Design IssuesGeneral Design Issues
Server Clusters
Distributed Servers
Managing Server Clusters
PlanetLab
Networked User Interfaces
Two kind of clients:
Application‐specific protocols for clientspp p p
Example: X Window
General‐purpose protocols like TCP/IP
X Window Systems
The basic organization of the X Window System
Compound DocumentsToday modern documents are composed on many different parts, each of which belongs to an specialapplication.
A l d d ld As an example, a word processor document could contain:
Text, graphic, movie, charts, database tables, etc.
Like the X Window system most of the computations are Like the X Window system, most of the computations are done at server side.
Client‐Server ComputingClient
Networked User Interfaces
The X Window System
Compound Document
Client side software for distributed transparencyClient‐side software for distributed transparency
Server
General Design IssuesGeneral Design Issues
Server Clusters
Distributed Servers
Managing Server Clusters
PlanetLab
Clients and Distribution TransparencyThe client software could contain small processing elements as well.
The small elements could use to bring different types of d b h distribution transparency such as:
A tAccess transparency
Replication transparencyReplication transparency
Failure transparencyp y
Clients and Replication Transparency
A client can easily provide replication transparency through intercepting requests.
Client‐Server ComputingClient
Networked User Interfaces
The X Window System
Compound Document
Client side software for distributed transparencyClient‐side software for distributed transparency
Server
General Design IssuesGeneral Design Issues
Server Clusters
Distributed Servers
Managing Server Clusters
PlanetLab
Server Design IssuesA Server is a process the implements a specific service on behalf of a group of clients.
Servers could process incoming requests or easily send it h ( f k)to another process. (e.g. using fork)
Cli d S d d i i Clients and Servers need an end point to communicate, so how to make it?
Publishing the endpoint globallyPublishing the endpoint globally
An agreement between client and server
Using a Naming Serviceg g
End Point Table
A case in which a client could find the end point by means of a special daemon server, equipped with an end point table.
Many Servers ‐ On End Point
Having a server for each end point may waste the resources.
This can be prevented by having a Super‐Server acts as a multiplexer multiplexer.
State ManipulationA stateless server does not keep information on the state of its clients and can change its own state without having to inform any client.
A simple time server
A fil d l dA file download server
When the request processes, the server completely forgetsabout the client.
Some unimportant information such as log files may be kept in stateless servers.
But loosing this information does not lead to any criticaldisasters.
These kind of states are called soft‐statesThese kind of states are called soft‐states.
Statefull ServersStatefull servers keep persistent information about clients.
As an example consider a document management server.
This server needs to keep track of the files or documents l k d b h li t f d t locked by each client for update.
Benefits:
Are usually fasterAre usually faster
Disadvantages:
A server crash could not be recovered easily A server crash could not be recovered easily.
State ManagementStatefull servers are usually persist their data on data layer.
Other info kept on processing layer, such as session info are usually soft state.
Cluster Computing
Three different layers of server cluster.
Code MigrationThere are cases in which passing programs in execution instead of their data simplifies the design of distributed systems.
h Moving a process to a remote machine is a time consuming activity.
This is usually done in support of load balancing.
The key idea behind code migration decisions is the ratio of communication on computation. p
Code Migration (Cont.)As an example, if a client application needs to process large amounts of data from a database, it would be more efficient if the application could be moved near the database.
In a similar case parts of the server could be used moved In a similar case, parts of the server could be used moved into client side.
Any examples?y p
Code Migration (Cont.)Code migration is also beneficial for decomposing an application into parts and distribute them on the system.
The principle of The principle of dynamicallyconfiguring a client to communicate to a server. The client first fetches the first fetches the necessary software, and then invokes the server.
Code Migration (Cont.)From a specific viewpoint, a process could be viewed as a triple:
Code segment: the part that contains executing code.
Resource segment the part that keeps references to Resource segment: the part that keeps references to resources.
Execution segment: the part that keeps execution state.
Code Migration (Cont.)Weak mobility:
Only the code segments migrates.
The migrated program should be started from beginning.
Example: Applets
Strong mobility:
Code and execution segments could be moved.
Th ld b d i t d d th t t dThe process could be paused, migrated and then restarted.
Models of Migration
Alternatives for code migration
Migration and Local ResourcesThe reason that makes migration difficult is that usually the resource segment cannot be moved.
As an example, suppose that a process that holds a TCP k f l h dl b d l socket or an open file handle cannot be migrated easily.
Th diff Three different resource types.
By identifier
By valueBy value
By type
Process‐Resource BindingsThe strongest binding is when a resource is refereed to by its identifier.
A URL f b A URL for a web resource
A remote machine’s end point
A weaker form of process to resource binding is by value. In this case any resource by that name could be used.
A shared object or dll
The weakest form is when a process refers to a resource by The weakest form is when a process refers to a resource by its type.
When a process needs a CD‐drive, a monitor, etc.
Resource TypesUnattached Resources: can be easily moved.
Data files that could be easily moved.
Fastened resource: can be moved, but brings a high cost
Local databases
Fixed resources: cannot be moved at allFixed resources: cannot be moved at all.
Local devices
Migration and Local Resources
Actions to be taken with respect to the references to local resources when migrating code to another machineresources when migrating code to another machine.
