Implementing Collaborative Applications

Dewan CS 290-063 1

Implementing Collaborative ApplicationsPrasun Dewan

Department of Computer ScienceUniversity of North CarolinaCB 3175 Sitterson HallChapel Hill, NC 27599-3175

[email protected]:/www.cs.unc.edu/~dewan

Dewan CS 290-063 2

Infrastructures

ApplicationsA

ppli

cati

ons Infrastructures

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Evaluation Criteria

Flexibility

Automation

Efficiency

Reuse of Existing Code

Automation vs Flexibility Tradeoff

Programmer User

Favouring the Programmer

Programmer User

Favouring the User

Programmer User

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Shared Window System

Application

Window Window

User 1 User 2

Coupling

Audio/Video

WYSIWIS

+ Tightly-Coupled Meetings

+ Easy to Understand

+ Application-Independent

+ Automatable

Stefik ‘87

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User 1 User 2

X Server

X Client

X Server

Pseudo Server Pseudo Server

Centralized ArchitectureXTV (‘88) VConf (‘87) Rapport (‘88) NetMeeting

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User 1 User 2

X Server

X Client

X Server

X Client


Replicated ArchitectureRapport VConf

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User 1 User 2

Insert e,2Insert d,1Insert e,2 Insert d,1

dabc aebcdeabc daebc

Insert d,1 Insert e,2

X Server

X Client

X Server

X Client


Synchronization in Replicated Architecture

abc abc

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User 1 User 2

X Server

X Client

X Server


Migrate Central Client

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User 1 User 2

X Server X Server

X Client


Migrate Central ClientChung & Dewan ‘96

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Shared Window System

Application

Window Window

User 1 User 2

Coupling

Audio/Video

WYSIWIS

+ Tightly-Coupled Meetings

+ Easy to Understand

+ Application-Independent

+ Automatable

- Scroll Wars

- Window Wars

- No Flexibility

- Performance- Consistency

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File System

Process 1

File

Process 1

Read/Write

lrwx

User 1 User 2

Grove Outline IBIS Arguments GroupDraw Drawing Notes Document Prep Document

+ Sharing

+ Persistence

+ Access, Conc. Control

- Async

- Large-Grained

- Connected

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Relational DBMS

Process 1 Process 1

Insert/Delete/Select

IDS

User 1 User 2

Database

M

L+ Fine-Grained

+ Query Language

Author, Category, Status and Awareness Info

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DC -> Crep. schedule

Replicated Disconnected Databases

Process 1 Process 1

User 1 User 2

“Replica”

IDS M

“Replica”

IDS M

source determines

destination determines

read access delete replication old data replication replication schedule

write access type replicationrecord replication ACL rep.rep. param. rep. immediate rep.

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Coda: Connection Degree

Client 1 Client 2

User 1 User 2

Data

Cache Cache

Server

get uncached

send updatesend update & cache

SC

SCSC

DC -> SC

send updates

send updates & cache

DC -> SC

WC

trickle updates WCWC

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Interprocess Communication

Process 1 Process 1

User 1

Session Manager

User 2

register, get peers

+ Absolute Flexibility

- No Automation

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ISIS: Causal and Atomic Broadcast

P1 ISIS

P2

P3 ISIS

create_group, join_group, leave_group

ISISm1

m1 m2 m2

m1’ m1’

m1

m1 m2

t( m2) < t (m1) cbcast_group

t (m1) < t( m1’) t (m1’) < t (m1)

abcast_group

commuting operations

m1 = color redm1’ = magnify

non-commuting operations m1’ = Insert b, 1

m1 = Insert a, 2

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Multiple Views

Model

View View

Window Window

User

notify notifyrequest

Host 1

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Multiple Threads in Rendezvous

Model

View View

Window Window

User 1 User 2

Multiple threads

Prevent starvation

Host 1 Host 2

-Two-Phase Comm.

+Multi Views

-Central Arch.

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2-Phase Model-View Protocol

Figure originally appears in [22]

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Clock: Optimized MVC

Figure originally appears in [22]

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Association

Xerox’s Colab. Broadcast Methods

Model

View

Window

User 1

Model

View

Window

User 2

bm

Broadcast method

Associates/Replicas

Associates/Replicas

lm

lm

lm

lmlm

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GroupKit: Environment Replication

Environment

Procedures

K1, V1

PR

K2, V2

I

K1, V1

QI

Multicast Session-based Calls

Active EnvironmentsK2, V2

I

ST Tall

U Vothers, usernum

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GroupKit: Open Session Management

K1, V1

K2, V2

PRI

K1, V1

K2, V2

QI

Registrar

User 1 User 2Session Manager

Session Manager

updateEntrant

userReqNewConf

newUserApprovednewinvite

newinvite

newUserArrived newUserArrived

+Flexible

-Collaboration Aware

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GroupKit Multicast Code & Events

Shared Idea List

proc insertIdea idea { insertColouredIdea blue $idea gk_toOthers ''insertColouredIdea red $idea''

Event Handlers

gk_bind newUserArrived { makeChatWindow %U } proc makeChatWindow usernum { ... }

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GroupKit Shared Environments

Group Hello World

gk_initConf $argv gk_defaultMenu .menubar pack .menubar -side top -fill x gk_newenv -bind -share message message greetings ''Hello World'' button .hello -text ''Hello World'' -command {message greetings ''[users local.username] says hello!''} pack .hello -side top message bind changeEnvInfo .hello configure -text [message greetings] after 2000 {.hello configure -text ''Hello World''} }

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Suite: Parameterized Generatorint[] v1

void updateV1 (int[] newVal)

C Application

Active Variables

void updateElemV1 (int index, newVal)Attributed Interaction Variables

int v1 (widget = w1, valueCpld = true, viewCpld = false, sync = incr, access = “rw”)

int v1 (widget = w2, valueCpld = true, viewCpld = true, sync = explicit, access = “r”)

transmit

bool validateElem V1(int newVal)

edit v1[2]commit

v

v1[2]

validate

Dialogue Managers

User 1 User 2

update(v2, newVal)

v1[2]

V1[2]

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Text Editor

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Text Editor

/*dmc Editable String */ String text = "hello world"; Load () { Dm_Submit (&text, "Text", "String"); Dm_Engage ("Text"); }

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Multiuser Outline

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Outline

/*dmc Editable Outline */typedef struct { unsigned num; struct section *sec_arr; } SubSection;typedef struct section { String Name; String Contents; SubSection Subsections;} Section;typedef struct { unsigned num; Section *sec_arr; } Outline;Outline outline;

Load () { Dm_Submit (&outline, "Outline", "Outline"); Dm_Engage ("Outline");}

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Talk

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Talk Program/*dmc Editable String */String UserA = "", UserB = "";int talkers = 0;Load () { if (talkers < 2) { talkers++; Dm_Submit (&UserA, "UserA", "String"); Dm_Submit (&UserB, "UserB", "String"); if (talkers == 1) Dm_SetAttr ("View: UserB", AttrReadOnly, 1); else Dm_SetAttr ("View: UserA", AttrReadOnly, 1); Dm_Engage_Specific ("UserA", "UserA", "Text"); Dm_Engage_Specific ("UserB", "UserB", "Text"); }}

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Command Interpreter

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Command Interpreter

/*dmc Editable CmdSeq */typedef struct { String Cmd, Result; } CmdRec;typedef struct { unsigned num; CmdRec *cmd_arr; } CmdSeq;CmdRec Init = {"", ""}; CmdSeq CmdList = {1, &Init };

void Execute (cmd_name, val) char *cmd_name; String *val;{ int index; char output_name[1024]; char *result;

sscanf (cmd_name, "(((CmdList)[%d]).Cmd)", &index); sprintf (output_name, "(((CmdList)[%d]).Result)", index); /* execute command using popen() and capture output */ result = ExecuteCommand (*val); Dm_Update (output_name, "String", &result); Dm_SequenceElementAppend ("CmdList", "CmdRec", &Init);}

Load () { Dm_Submit (&CmdList, "CmdList", "CmdSeq"); Dm_SetAttr ("Type: String", AttrAcceptOnReturn, 1); Dm_SetAttr ("Type: String", AttrUpdateProc, Execute); ... Dm_Engage ("CmdList");}

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WebForm 1

Cgi scripts

HTTP Server

Form 1 DisplayWeb Browsers

edit form1

User 1 User 2

Rendered HTML Document

Form 1 Display

update

+Wide-Area Naming and Sharing

get getnotify

+High-level Local UI

commit

-Async

-Jittery

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Implementation Taxonomy

User 1 User 2

X Server

X Client

X Server


User 1 User 2

X Server

X Client

X Server

X Client


Process 1

File

Process 1

lrwx

User 1 User 2

Model

View View

Window Window

User 1 User 2

Host 1 Host 2

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Single-User Architecture

Windows

User 1

Toolkit

PC

Model

View

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Layered Interaction with an Object{“John Smith”, 2234.57}

• John Smith•

• John Smith•

• John Smith•

X

Abstraction

Interactor =

Absrtraction Representation

+

Syntactic Sugar

Interactor/Abstraction

Interactor/Abstraction

Interactor

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Single-User Architecture

Interactor

User 1

Interactor

Interactor

Abstraction

Interactor

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Generic ArchitectureSemantics

Interactor Base

Interactor Interactor

User 1 User 2

Hardware

Ste

m

Bra

nch

InteractorVersion VersionInteractor

Interactor InteractorHardware

Bra

nch

Abstraction

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Example Layers Model

View

Windows Windows

User 1 User 2

Toolkit Toolkit

PC PC

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Varying Replication Degree

Windows

User 1 User 2

Toolkit

PC PCDegree 0

Pure Centralized

Team Workstation

Model

View

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Windows

User 1 User 2

Toolkit

PC PC

Degree 1

“Centralized”

Shared Window Systems

Model

View

Windows Windows

XTV

Rapport

NetMeeting

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User 1 User 2

Model

View

Windows Windows

Toolkit Toolkit

PC PC

Degree 2

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User 1 User 2

Model

View

Windows Windows

Toolkit Toolkit

PC PC

Degree 3 View Hybrid

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User 1 User 2

Model

View

Windows Windows

Toolkit Toolkit

PC PC

Replicated

View

Model

+ Divergence + Concurrency + Distribution

- Duplication - Non- Idempotent Operations

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Varying Concurrency Degree

User 1 User 2

Model

View

Windows Windows

Toolkit Toolkit

PC PC

View

Degree 0

Serial

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User 1 User 2

Model

View

Windows Windows

Toolkit Toolkit

PC PC

View

Semi-concurrent

Degree 1 < R

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User 1 User 2

Model

View

Windows Windows

Toolkit Toolkit

PC PC

ViewDegree = Replication Degree

Concurrent

Suite Rendezvous Weasel Clock

+ Fairness + Concurrency + Distribution

- Scheduling Overhead - Programming Overhead

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Varying Distribution Degree

User 1 User 2

Model

View

Windows Windows

Toolkit Toolkit

PC PC

View

Degree 0

Single-Site

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User 1 User 2

Model

View

Windows Windows

Toolkit Toolkit

PC PC

ViewRendezvous

Degree 1 < Concurrency Degree

Semi-Distributed

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User 1 User 2

Model

View

Windows Windows

Toolkit Toolkit

PC PC

ViewSuite Weasel Clock

Degree = Concurrency Degree

Distributed

+ Local Response + Compact Communication

- Programming Overhead- Pointers

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Varying Collaboration Awareness

User 1 User 2

Model

View

Windows Windows

Toolkit Toolkit

PC PC

View

Degree 0

Hardware-support

Team Workstation

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User 1 User 2

Model

View

Windows Windows

Toolkit Toolkit

PC PC

View

Degree 1

Window-support

Shared Window Systems

XTV

Rapport

NetMeeting

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User 1 User 2

Model

View

Windows Windows

Toolkit Toolkit

PC PC

View

+ Sharing Flexibility

- Interaction Flexibility

Suite Weasel Clock

Lower layer cannot see upper layers

shared interactor => shared abstraction

shared abstraction => shared interactor

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Partitioned Support

User 1 User 2

Model

View

Windows Windows

Toolkit Toolkit

PC PC

View

+ Separation of Concerns

- Duplication - Coordination

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Adding Collaboration Awareness

Colab. Transp.

Colab. Aware

Extend Single-User Class

JCE

Colab. Transp. Colab. Aware

Ad-HocSuite

Colab. Aware

Colab. Transp.

Extend Colab. Aware Class

Sync

Colab. Aware Colab. Transp.

Colab. Aware DelegateColab. Beans

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Colab. Transparent Delegate

Colab. Aware Proxy Colab. Transp.

Colab. Aware Proxy

XTV

X Server

X Client

Pseudo Server COLA

Called Object

Calling Object

Adapter Object

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Replication Granularity

Coarse Grained

Fine-Grained

XTV

Suite, DistView

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A1 A2

Autonomous Name Assignment

Translator

Replica Naming

XTV

Global Name Assignment

SuiteName Manager

Efficient

Reusable

A A

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Peer-Peer Coupling

II’

Remote Input

Shared X

I

I’

Peer Input

XTV

I

Windows

I’

I

I I I

Peer Multi-layer InputSuite

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Peer-Peer Coupling

MM’

Remote Method

GroupKit

M

M’

Peer Method

Colab.

Copy

Getters Setters

Object Object

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Peer-to-Peer Coupling

Windows

Update

Update’Data

Event

Data

Remote Active Update

GroupKit

Event

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Display Consistency

C1 C2

C1 C2

ABCAST

ISIS

C1 C2

C2 C1

Optimistic

Rep. SW

C1 U (C2)

Check &Abort

Xerox’s Colab

C2 U (C1)

C1 C2

Undo/redo

C2 C1 U (C2) C2

move rotate

Commute check

GroupDesign

rotate move

insert a,1 insert b,3

Operation Transform

Grove

insert b,2 insert a,1

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Algorithm

t (Ii Ij) { if Pi > Pj { t = Ii(Ci, Pi+1) } elsif Pi < Pj { t = Ii(Ci, Pi) } elsif Ci = Cj { t = null } else { ???? }

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Algorithm

t (Ii, pi, Ij, pj) { if Pi > Pj {t = Ii(Ci, Pi+1) } elsif Pi < Pj {t = Ii(Ci, Pi) } elsif Ci = Cj {t = null } elseif pi > pj {t = Ii(Ci, Pi)) else {t = Ii(Ci, Pi+1)}

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Inserting at Same Position

Site 1

O1 = Insert (A, 1) O2 = Insert (B, 1) O3 = Insert (C, 1)

Site 2 Site 3

A B C

O2 = Insert (B, 1)

O3 = Insert (C, 1)

O3 = Insert (C, 1)

O1 = Insert (A, 1) O2 = Insert (B, 1)

BC C

O1 = Insert (A, 2)O1 = Insert (A, 3)

A

O1 = Insert (A, 1)O1 = Insert (A, 2)

A

O2 = Insert (B, 2)

CBA

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Tricky Case

Site 1

O1 = Insert (A, 1)

O2 = Insert (B, 1) O3 = Insert (C, 1)

Site 2 Site 3

O3 = Insert (C, 1)

O1 = Insert (A, 1)

C C

O1 = Insert (A, 2)

AABC

O3 = Insert (C, 1) O1 = Insert (A, 1)

ABC

O2 = Insert (B, 1)

CAB

O2 = Insert (B, 2)

O2 = Insert (B, 1)O2 = Insert (B, 2)

ABC

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Implementing Undo

C1

Full Check Point

Full Check Point

C2

Full Check Point

C1

Check Point C1’s Effects

Check Point C2’s Effects

C2

Partial Check Point

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Implementing Undo

C1

Full Check Point

Partial Check Point

C2

C3

Partial Check Point

C1

Inverse(C1)

Inverse(C2)

C2

Inverse Commands

Hybrid Approach

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Ins 1, B

Ins 1, A

Del 1, C Ins 1, C

Selective Undo

Del 1, C

Ins 1, A

Ins 1, B

Del 1, C

U

U

U

R

R

Ins 2, C

Ins 3, C

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Multi-Layered Undo

C CO U UO C CO U UO

C CO U UO

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History & Command Object

C CO R ROC CO U UO

C CO

History

RU

C

Add

C

Add

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Distributed Undo

56

5.0 5.0

Inc

Inc

Inc

Inc

6.0 6.0

Undo

Undo

Undo

Undo

5.0 5.0

5

Inc Undo

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Collaborative Undo

56

5.0 5.0

Inc

Inc

Inc

Inc

6.0 6.0

Undo

Requesting Object

Originating Object

Undo (c)

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Composable Support

User 1 User 2

Model

View

Windows Windows

Toolkit Toolkit

PC PC

View

+ Separation of Concerns - Coordination

Com

posa

ble

Log

ger

Collaboration Bus

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Coupling Awareness

IPC

Manual

Object ObjectCopy

Semi-Automatic

Getters Setters

Object Object

Copy

Automatic Copy

Data Data Replay

Automatic Log/Replay

Object Object

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Collaboration BusCollaboration Bus

Service 2, Instance 1: Meeting Environment

No Computer-Supported Control

Service 1:Single User

Ship Walk-through

Colab. Bus Com

posi

tions

Translations

Reference Model

Service 2, Instance 2:Meeting Environment

Computer-Supported Floor ControlIntra-Service Interoperation(Translation):

CombinedMeeting

Environment

Inter-Service Interoperation (Composition):

Multi-Person Ship Walk-through

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9. References

Given below are references to papers from which some of this material is drawn. These include threesurvey papers I have written on desiging [12] and implementing [11,14] collaborative applications, papers onthe following collaborative applications: Grove [17], PREP [34,35], GroupSketch and GroupDraw [23] MUD [8],Coordinator/ActionWorkflow [33], Quilt [19], and FLECSE [10]; papers on the following issues: SessionManagement [16], Coupling [13], Undo [3,6,37], and Awareness [15,20,28] and papers on the followingcollaboration systems: Team Workstation [25], Xerox's Colab programming environment [38], VConf [31], Rapport[18], XTV [1], Shared X [21], MMConf [7], Rendezvous [24], Suite [9], ISIS [4], Coda [29], Rover [27],GroupDesk [20] and PLAYBACK [32].

[1] Hussein Abdel-Wahab and Kevin Jeffay, "Issues, Problems and Solutions in Sharing X Clients on Multiple Displays," Internetworking: Research and Experience 5 (94), pp. 1-15.

[2] Steve Benford, Chris Brown, Gail Reynard, and Chris Greenhalgh, "Shared Spaces: Transportation, Artificiality, and Spatiality," Proceedings of the ACM Conference on Computer Supported Cooperative Work, November 1996, pp. 77-86.

[3] Thomas Berlage, "A Selective Undo Mechanism for Graphical User Interfaces," ACM Transactions on Computer-Human Interaction 1:3 (September 1994), pp. 269-294.

[4] K. Birman and T. Joseph, "Exploiting Virtual Synchrony in Distributed Systems," Proceedings of the Eleventh ACM Symposium on Operating Systems Principles, 8-11 November 1987, pp. 123-138. In ACM Operating Systems Review 21:5.

[5] W. Buxton and A. Sellen, "Interfaces for Multiparty Video Conferences," Technical Report , University of Toronto, 191.

[6] Rajiv Choudhary and Prasun Dewan, "A General Multi-User Undo/Redo Model," Proceedings of European Conference on Computer Supported Work, October 1995, pp. 231-246.

[7] Terrence Crowley, Paul Milazzo, Ellie Baker, Harry Forsdick, and Raymond Tomlinson, "MMConf: An Infrastructure for Building Shared Multimedia Applications," Proc. of ACM Conference on Computer Supported Cooperative Work, October 1990, pp. 329-342.

Dewan CS 290-063 83

[8] Pavel Curtis, "Muddings: Social Phenomena in Text-Based Virtual Reality," CSL-92-4, Xerox Palo Alto Research Center, 1992.

[9] Prasun Dewan and Rajiv Choudhary, "A High-Level and Flexible Framework for Implementing Multiuser User Interfaces," ACM Transactions on Information Systems 10:4 (October 1992), pp. 345-380.

[10] Prasun Dewan and John Riedl, "Toward Computer-Supported Concurrent Software Engineering," IEEE Computer 26:1 (January 1993), pp. 17-27.

[11] Prasun Dewan, "Tools for Implementing Multiuser User Interfaces ," Trends in Software: Issue on User Interface Software 1, Wiley (1993), pp. 149-172.

[12] Prasun Dewan, Rajiv Choudhary, and HongHai Shen, "An Editing-based Characterization of the Design Space of Collaborative Applications," Journal of Organizational Computing 4:3 (1994), pp. 219-240.

[13] Prasun Dewan and Rajiv Choudhary, "Coupling the User Interfaces of a Multiuser Program," ACM Transactions on Computer Human Interaction 2:1 (March 1995), pp. 1-39.

[14] Prasun Dewan, "Multiuser Architectures," Proc. of IFIP WG2.7 Working Conference on Engineering for Human-Computer Communication, August 1995, Chapman & Hall, 1996, pp. 247-270.

[15] Paul Dourish and Sarah Bly, "Portholes: Supporting Awareness in a Distributed Work Group," Proceedings of CHI '92., 1992, pp. 541-547.

[16] Keith Edwards, "Session Management for Collaborative Applications," Proceedings of the ACM Conference on Computer Supported Cooperative Work, October 1994, pp. 323-330.

[17] Clarence A. Ellis, Simon J. Gibbs, and Gail L. Rein, "Groupware: Some Issues and Experiences," CACM 34:1 (January 1991), pp. 38-58.

[18] J.R. Ensor, S.R. Ahuja, D.N. Horn, and S.E. Lucco, "The Rapport Multimedia Conferencing System: A Software Overview," Proceedings of the 2nd IEEE Conference on Computer Workstations, March 1988, pp. 52-58.

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[19] R. Fish, R. Kraut, M. Leland, and M. Cohen, "Quilt: a Collaborative Tool for Cooperative Writing," Proceedings of ACM SIGOIS Conference, 1988, pp. 30-37.

[20] Ludwig Fuchs, Uta Pankoke-Babatz, and Wolfgang Prinz, "Supporting Cooperative Awareness with Local Event Mechanisms," Proceedings of European Conference on Computer Supported Work, October 1995, pp. 247-262.

[21] Daniel Garfinkel, Bruce Welti, and Thomas Yip, "HP Shared X: A Tool for Real-Time Collaboration," Hewlett-Packard Journal, April 1994, pp. 23-24.

[22] T.C. Nicholas Graham, Tore Urnes, and Roy Nejabi, "Efficient Distributed Implementation of Semi- Replicated Synchronous Groupware ," Proceedings of the Ninth Conference on User Interface Software and Technology, October 1996, pp. 1-10.

[23] Saul Greenberg, Mark Roseman, Dave Webster, and Ralph Bohnet, "Issues and Experiences Designing and Implementing Two Group Drawing Tools," Proceedings of 25th Annual Hawaii International Conference on System Sciences, January 1992, pp. Vol. 4: 139-150.

[24] Ralph Hill, Tom Brinck, Steven Rohall, John Patterson, and Wayne Wilner, "The Rendezvous Architecture and Language for Constructing Multiuser Applications," ACM Transactions on Computer Human Interaction 1:2 (June 1994).

[25] Hirsoshi Ishii and Masaaki Ohkubo, "Design of a Team Workstation," Multi-User Interfaces and Applications, North Holland, , pp. 131-142.

[26] Hiroshii Ishii and Minoru Kobayashi, "Integration of Inter-Personal and Shared Workspace: Clearboard Design and Experiments," Proceedings of the ACM Conference on Computer Supported Cooperative Work, November 1992, pp. 33-42.

[27] Anthony D. Joseph, Alan F. deLespinasse, Joshua A. Tauber, David K. Gifford, and M. Frans Kaashoek, "Rover: A Toolkit for Mobile Information Access," Proceedings of the 15th Symposium on Operating System Principles, 1995.

[28] Alain Karsenty, Christophe Tronche, and Michel Beaudouin-Lafon, "GroupDesign: Shared Editing in a Heterogeneous Environment," Usenix Computing Systems 6:2 (Spring 1993), pp. 167-195.

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[29] J. J. Kistler and M. Satyanaraynan, "Disconnected Operation in the Coda File System," ACM Transactions on Computer Systems 10:1 (February 1992), pp. 3-25.

[30] Hideaki Kuzuoka, Toshio Kosuge, and Masatomo Tanaka, "GestureCam: A Video Communication System for Sympathetic Remote Collaboration," Proceedings of the ACM Conference on Computer Supported Cooperative Work, October 1994, pp. 35-43.

[31] J.C. Lauwers and K.A. Lantz, "Collaboration Awareness in Support of Collaboration Transparency: Requirements for the Next Generation of Shared Window Systems," Proceedings of ACM CHI'90, April 1990, pp. 303-312.

[32] Nelson R. Manohar and Atul Prakash , "The Session Capture and Replay Paradigm for Asynchronous Collaboration," Proceedings of European Conference on Computer Supported Work, October 1995, pp. 149-164.

[33] Raul Medina-Mora, Terry Winograd, Rodrigo Flores, and Fernando Flores, "The Action workflow Approach to Workflow Technology," ACM CSCW'92 Proceedings, October 1992, pp. 281-288.

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Implementing Collaborative Applications

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