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A Language and Architecture for Distributed Computing over the Internet

Carlos A. Varela Worldwide Computing Lab Department of Computer Science Rensselaer Polytechnic Institute http://www.cs.rpi.edu/wwc/ July 2002

Worldwide Computing

VISA

W W C

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Limitations of Java for Worldwide Computing

• passive objects
• shared memory
• synchronous communication
• non-universal naming
• nly primitive synchronization mechanisms

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Actor (Agent) Model

Actor

Thread (1) (3) (2) Mailbox

Internal variables Methods

State

Message

Actor

Thread (1) (3) (2) Mailbox Internal variables Methods State Message

Actor

Thread (1) (3) (2) Mailbox Internal variables Methods State Message

standardOutput<−print("World"); standardOutput<−print("Hello ") @ behavior HelloWorld { void act(){ } }

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Actor (Agent) Model (continued)

Actor mobility is simpler than object mobility:

• distributed memory
• universal naming

Coordination of actors in worldwide open systems remains difficult.

• non-blocking, asynchronous communication
• inherent concurrency – threads are encapsulated in objects

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Worldwide Computing Architecture

The World-Wide Computer consists of concurrent, distributed, and mobile Universal Actors.

• Universal naming strategy
• Run-time support: Theaters
• Remote communication protocol
• Migration support
• Preliminary performance results

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Naming in Worldwide Computing (Requirements)

The main goals of naming in worldwide computing are to provide:

• platform independence – names should appear coherent on all nodes

independently of underlying architecture

• scalability of name space management
• transparent actor migration
• penness by allowing unanticipated actor reference creation and protocols

that provide access through names

• both human and computer readability

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Proposed Solution: Universal Actor Names and Locators

WWC Theater UAN Server Actor Reference Internet Host B Internet Host A Actor UAN UAL

• Uniform Resource Identifiers (URI) syntax [Berners-Lee]
• UAN/UAL support transparent actor migration.

Sample UAN:

uan://wwc.osl.cs.uiuc.edu:3030/Agha/Calendar

Sample Universal Actor Locators for this WWC actor:

rmsp://agha.cs.uiuc.edu:4040/Agents/Calendar rmsp://howard.cs.uiuc.edu:4040/AghaCalendar rmsp://agha.pda.com:1234/Calendar

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Universal Actor Naming Protocol

The UANP defines how to interact with the WWC Naming Service. Similarly to HTTP , UANP is text-based, and includes methods for the following actions: Method Parameters Action

PUT

relative UAN, UAL Creates a new entry in the database

GET

relative UAN Returns the UAL entry in the database

DELETE

relative UAN Deletes the entry in the database

UPDATE

relative UAN, UAL Updates the UAL entry in the database An actor’s location can be cached for faster future accesses.

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World-Wide Computer Theaters

A WWC Theater provides runtime support to Universal

• Actors. A Theater contains:
• a remote communication module with a hashtable

mapping relative UALs to actual SALSA actor references, and

• a runtime system for universal and environment actors.

Environment Actors

RMSP Server Listener Hashtable

relative UAL Universal Actors Resources

World Wide Computing Theater

System

Universal Actor Run−Time System

SALSA Reference

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Remote Communication in Worldwide Computing (Requirements)

The main goals of a remote communication protocol in worldwide computing are to provide:

• asynchronous, non-blocking communication
• an interface to the naming service for target actor location
• data and code mobility

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Proposed Solution: Remote Message Sending Protocol

• RMSP defines how an actor sends a message to any other actor in the

World-Wide Computer

• RMSP is object-based, and includes support for message serialization, and

actor migration

• transparent programming language support for sending messages to local

and remote actors

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Migration in Worldwide Computing (Requirements)

The main goals of migration in worldwide computing are to provide:

• both fine-grained and coarse-grained mobility
• actor reference updating for more efficient local communication
• consistency protocols for shared memory.

Since actors do not have shared memory, actor migration is simpler and more efficient than object/thread migration.

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Actor Migration

Before migration of actor m from Theater 1 to Theater 2, its references to actors b and c are remote, while its reference to actor a is local.

• ✁

Actor m

Theater 3

Actor a Actor b Actor c

Theater 1

Local actor reference. Remote actor reference.

Theater 2

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Actor Migration (continued)

After migration of actor m, its reference to actor a becomes remote and its reference to actor b becomes local. Its reference to actor c remains unchanged.

• ✁

Theater 3

Actor a Actor b Actor c

Theater 1

Local actor reference. Remote actor reference. Actor m

Theater 2

Actor m forwarder

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World-Wide Computer Testbed

Machine Name Location OS-JVM Processor yangtze.cs.uiuc.edu Urbana, IL, USA Solaris 2.5.1-JDK 1.1.6 Ultra 2 vulcain.ecoledoc.lip6.fr Paris, France Linux 2.2.5-JDK 1.2pre2 PII, 350MHz solar.isr.co.jp Tokyo, Japan Solaris 2.6-JDK 1.1.6 Sparc 20

Time Ranges (with SALSA 0.3.2)

Local actor creation time 386

• s

Local message sending time 148

• s

LAN message sending time 30-60ms WAN message sending time 2-3 secs LAN actor migration time 150-160ms (minimal actor) LAN actor migration time 240-250ms (actor with 100Kb of data) WAN actor migration time 3-7secs (minimal actor) WAN actor migration time 25-30secs (actor with 100Kb of data)

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Simple Actor Language, System and Architecture SALSA is a dialect of Java, with support for:

• Concurrent programming with actors.
• Token-passing Continuations to control concurrency by

specifying customers for an actor message’s return value.

• Join Continuations to provide a synchronization barrier

for multiple actor computations into a single continuation.

• Universal Naming to bind and locate actors in the WWC

using UANs and UALs.

• RMSP and Migration to send messages to remote

actors and to migrate actors across WWC Theaters. Language Implementation

• SALSA Actor Library
• Join Continuation Code Generation

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SALSA Architecture

Program.salsa

SALSA Source Code Java Source Code

Program.java

SALSA Actor Library

>> javac Program.java >> salsac Program.salsa Java Bytecode

Program.class

>> java Program Java Virtual Machine

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SALSA Hello World Example

module helloworld; behavior HelloWorld { void act(String arguments[]){ standardOutput <- print("Hello ") @ standardOutput <- println("World!"); } }

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Actor Model Support

• All SALSA behaviors inherit from the

UniversalActor class. To create a new actor

instance:

HelloWorld helloWorld = new HelloWorld();

• To send messages to acquaintance actors:

acquaintance <- message(arg1, arg2,...);

For example:

standardOutput <- print("Hello "); hello();

• Sending a message returns immediately (it is

asynchronous) with a void return value.

• Only an actor itself can change its internal state through

assignments to its instance variables. No shared memory or static variables are allowed in SALSA.

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Token-Passing Continuations

• SALSA messages are potential Java method
• invocations. Message passing is asynchronous.
• Continuations allow to specify a customer for a

message’s return value (called token):

acquaintance<-m1(args) @ customer<-m2(arg0, ... ,token, ..., argn);

For example:

fractal<-computePixel() @ screen<-draw(token);

• The return type of computePixel() needs to

match the formal argument type of

draw(argument). SALSA allows method

• verloading and will choose the most specific method

according to the token run-time type.

• a<-m with no arguments, is syntactic sugar for

a<-m(token). For example:

fractal<-computePixel() @ screen<-draw;

• It is possible to ”chain” continuations. For example:

a1<-m1() @ a2<-m2 @ a3<-m3(token,10);

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Join Continuations

• A join statement allows to provide a synchronization barrier for multiple actor

computations into a single continuation:

join(a1<-m1(args), a2<-m2(args), ... ) @ customer <- n; join(actorArray<-m()) @ customer <- n;

For example:

join(author1<-writeChapter(1), author2<-writeChapter(2)) @ editor<-review @ publisher<-print;

will only send the message review(token) to the editor when both authors have finished writing their chapters. The token passed as an argument is an array containing the return values of the messages inside the

join statement.

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Universal Actor Naming in SALSA

behavior Agent { void printItinerary(){...} void act(String[] args){ Agent a = new Agent(); try { a<-bind("uan://yangtze.cs.uiuc.edu:3030/agent", "rmsp://yangtze.cs.uiuc.edu:4040/agent"); } catch (Exception e){ standardOutput<-println(e); } } }

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RMSP and Actor Migration in SALSA

Getting a remote actor reference by name and sending a message:

Agent a = new Agent(); a<-getReferenceByName("uan://yangtze.cs.uiuc.edu/agent") @ a<-printItinerary();

Getting the reference by location:

Agent a = new Agent(); a<-getReferenceByLocation("rmsp://yangtze.cs.uiuc.edu/agent") @ a<-printItinerary();

Migrating an agent to a remote WWC Theater:

Agent a = new Agent(); a<-getReferenceByName("uan://yangtze.cs.uiuc.edu/agent") @ a<-migrate("rmsp://vulcain.ecoledoc.lip6.fr/agent");

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Example 1: Multicast Protocols

Multicast a2 a1 a3

m1 m2 m3

d

a2<-m2(), a3<-m3() ); join ( a1<-m1(),

Acknowledged multicast

• join ( a1<-m1(),

m1 m2 m3 done(); ack ack ack

d a2 a1 a3

a2<-m2(), a3<-m3() ) @ done();

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Group knowledge multicast (Fagin, Halpern, Moses and Vardi, 1995) a2 a1 a3

join ( a1<-m1(), a2<-m2(), a3<-m3() ) @ a3<-ok() ) @ done(); join ( a1<-ok(), a2<-ok(),

• m1

m2 m3 done();

• k
• k
• k

d

ack ack ack ack ack ack

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Lines of Code Comparison for Multicast Protocols

Foundry SALSA Java Shared Code 40 10 34 Basic Multicast 146 27 115 Acknowledged Multicast 60 21 134 Group-knowledge Multicast 73 24 183 TOTAL 319 82 466

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SALSA Actor Library

Object

source target method arguments continuation tokenPosition withMessage mailbox send(Message) process(Message) run() actor messages put(Message) get() isEmpty() bind(UAN, UAL) getReferenceByName(UAN) getReferenceByLocation(UAL) migrate(UAL) uan ual

Thread Actor Message salsa.language java.lang Mailbox JoinDirector UniversalActor

messages tokens continuation tokensSet process() ack(i, token)

wwc.naming UAL UAN URI

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Join Continuation Code Generation

m1 m2 mn n(tokens); ack ack ack

jd a a

process();

a

join ( a1<-m1(), a2<-m2(),

1 2 n

a3<-m3(), ... ) @ cust<-n;

cust src

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