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• A distributed system is set of processes, linked by a network
• No global information, no global time• Unpredictable communication delays• Concurrency and nondeterminism• Large probability of localized faults• Easy access by unauthorized users
What are the properties of global distributed systems?
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Additional Properties of the Internet
• A global network that is partitioned into several protection domains (Firewalls)
• Private sub networks with multiple reassignment of IP addresses across networks
• Dynamic reassignment of IP addresses -- ISP’s reuse a pool of IP addresses among customers
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Middleware for distributed global applications
• The middleware abstracts the complexity of the underlying system and provides the services of the OS to the application programmer.
• The middleware should support the distribution structure, scalability, openness, failure handling and security issues.
• Provides transparency (network and location) as much as possible or as desirable. The Network
The operating system
The middleware
The applications
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The middleware as a programming system for distributed applications
• Design a programming system from the start that is suitable for distributed applications (Oz language and its system Mozart)
• Extend an existing programming system with libraries to support distributed computing (JAVA)
• Provide an distribution layer that is language independent (CORBA), this layer might be needed anyway for communication with foreign software.
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Programming system for distributed applications
• The programming language by design provides abstractions necessary for distributed applications:– Concurrency and various communication abstraction
– Mobility of code (or more generally closures) and other entities
– Mechanisms for security at the language level -- the programming language by construction support all the concept needed for allowing arbitrary security levels (no holes)
– Notion of sited resources, how to plug and unplug resources
– Notion of a distributed/mobile component (for mobility of applications)
– Dynamic connectivity, transfer of entities and modification of various applications at runtime
– Abstraction of the network transport media
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Programming system for distributed applications
• The programming system (runtime system) by design provides mechanisms to support:
– Network transparency
– Well defined and extended distributed behavior for all language entities -- part of network awareness.
– Mechanisms for guaranteeing security on untrusted sites (fake implementations)
– Mechanism for limiting resource (memory and processor time) consumption by foreign computations at runtime
– Network layer that supports location transparency (mobile applications) (multiple) IP independent addressing
– Configurable and scalable network layer (multiple protocols, TCP, TTCP, Reliable UDP, …)
– Dynamic connectivity, fault/ connectivity detection
– Firewall enabled
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The programming system (runtime system)
Network layer
Memorymanagement layer
Protocol layer
Extended engine
}Shared computation spaceDistributed garbage collection
} Distribution protocols
} Local execution is here
Network TransportLevel
Messages between distributed nodes
Messages betweenthreads and nodes
StructuredMessages
Raw byte sequence
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The issues in distributed programming
Functionality
Fault tolerance
Distribution
Openness
Resource Control
Security
Scalability
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The issues in distributed programming
Functionality
Fault tolerance
Part of problem
Interaction
Distribution
Openness
Resource Control
Security
Scalability
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The issues in distributed programming
Functionality
Fault tolerance
Part of problem
Interaction
Distribution Security
Openness
Resource Control
Functionality
Distribution
Openness
Security
Resource Control
Fault tolerance
Scalability
Scalability
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Network transparency• It means:
– If you develop an application on a single machine, you can distribute the entities to different sites without changing the logical behavior (functionality/semantics) of the application
– If you connect to independent applications together they will logically behave as if they were running on a single machine.
• It does not means that you loose control over your applications
• However network transparency breaks when faults (network partitioning) occurs. Therefore awareness is also needed.
• The role of fault detection is to reflect abstracted failure to allow the programmer to develop fault tolerance mechanisms.
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Application Logic
Graphic Entities
BS
DB
CMWM CMWM
Board Server
Display Broadcaster
CM - Client ManagerWM- Window Manager
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Transparent distribution
Graphic Entities
BS
DB
CMWM CMWM
Board Server
Display Broadcaster
CM - Client ManagerWM- Window Manager
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Programming systems for distributed applications
• The programming language by design provides abstractions necessary for distributed applications:– Concurrency and various communication abstractions
– Mobility of code (or more generally closures) and other entities
– Mechanisms for security at the language level -- the programming language by construction support all the concept needed for allowing arbitrary security levels (no holes)
– Notion of sited resources, how to plug and unplug resources
– Notion of a distributed/mobile component (for mobility of applications)
– Dynamic connectivity, transfer of entities and modification of various applications at runtime
– Abstraction of the network transport media
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Language Entities
records, procedures, classes
objects
single-assignmentvariable
cells
ports, threads
functors, components
Concurrency and asynchronous communication abstractions are importantto hide latency in distributed applications
Cheap light weight threads are necessarycombined with dataflow synchronizationto overlap computation and communication
time
T1 : m(X)
T0: …X…
T0
X=Value
Object
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Language Entities
records, procedures, classes
objects
single-assignmentvariable
cells
ports, threads
functors, components
Concurrency and asynchronous communication abstractions are importantto hide latency in distributed applications
Just use Ports for asynchronous communication, no thread creation
time
T0 : send m(X) to P
T0: …X…
T0
X=Value
receive minvoke Object
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JAVA’s remote method invocation• JAVA’s basic mechanism for communication is
extending the sequential method invocation into invocation on remote objects
• It is possible to create a thread, but it is both expensive and synchronization with the main thread is awkard
• A future abstraction has to programmed explicitly combined with the call-back in a separate thread
time
T0 : invoke m(X) to object
T0: …X…
T0
X=Value
Object
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Language Entities
records, procedures, classes
objects
single-assignmentvariable
cells
ports, threads
functors, components
Mobility of code (or more generally closures) and other entitiesEvery entity in the language is a runtime value (access by reference not by a string)
In particular procedures and classes are values that can be sent in a message, or given as a parameter to remote invocationNo name collision at remote site
C variable send m(.) to O
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Distributed Lexical Scoping/Mobility
C variable send m(.) to site 2
E2
E1
E1
E2
C variable
E2
E1
E1
E2
Site 1Site 2
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Advantages of distributed lexical scoping
• Distribution transparency under procedure/class/object mobility– programs can be tested and understood on one site, and behave the
same when distributed on different sites
• Security, given correct implementation:– a procedure transferred to a site will not accidentally acquire
unauthorized access to entities residing on the site.
– Can easily implement capability based security, allowing subset of possible operations on an entity
• Example a procedure referring to a file object, only allowing reads
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Programming system for distributed applications
• The programming system (runtime system) by design provides mechanisms to support:
– Network transparency
– Well defined and extended distributed behavior for all language entities -- part of network awareness.
– Mechanisms for guaranteeing security on untrusted sites (fake implementations)
– Mechanism for limiting resource (memory and processor time) consumption by foreign computations at runtime
– Network layer that supports location transparency (mobile applications) (multiple) IP independent addressing
– Configurable and scalable network layer (multiple protocols, TCP, TTCP, Reliable UDP, …)
– Dynamic connectivity, fault/ connectivity detection
– Firewall enabled
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Distributed behavior
Stateless Replication Eager
Lazy
record, procedure, class
object-record
Single assignment
Eagerelimination
single assign.variable
Stateful Localization Mobile
Stationary
cell, object-state
port, thread
resourcesPrivate
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System support• Object could not be copied by value as in JAVA
because this will loose transperancy• Object could be either stationary, or mobile with a
consistency protocol that is fault tolerant. Mozart support both.
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Resource Security with distributed components
requires Open.file
Open
New Module Manager
I do not trust youYou can only read/writein TEMP
Importedto site
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Conclusions• I have shown that making a programming language
distributed (Internet ready!) is not just a taking a centralized language and extending it with a distribution layer.
• The design should take distribution into account from scratch.
• I have just touched upon the subject, the topic I did not cover are just as important.
• Look at the mozart demo, where you would also see the distributed component architecture where new functionality can be added on the fly and make available to all participants in a Mozart instant messenger (ICQ like) Internet service.