Database Management for Multimedia Distributed Collaborative Writing
ABSTRACT Traditional computer applications have been designed to be run by one user at a time who does some work in a single medium, such as ASCII text, and very little regard has been given to the fact that people often work together. With the recent development of computer networks and the widespread deployment of networked workstations, automating the group writing process for geographically distributed users has become feasible. In this paper, a software package which supports distributed, real-time, multimedia collaborative work, known as the Distributed Collaborative Writing Aid (DCWA), is described. The DCWA has five major parts but the emphasis of this paper is on the DCWA's distributed database, which is crucial to making the entire system work together both logically and consistently. The database is a specialized, distributed system based on the client-server model implemented in C++ running under 4.3BSD Unix and provides dynamic management capabilities unique for a multimedia collab...
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Page 1
W riting
A?WesleyWear?YuGong? KaiH?Chang
Department ofComputerScience and Engineering
AuburnUniversity? Auburn?AL?????
fwwear?gong?k changg? en g?auburn?edu
Abstract?Traditionalc omputerapplic ationshave
be endesigned tobe runbyoneuseratatimewhodoes
someworkinasinglemedium? suchasASCIItext?
andverylittleregardhasbeengiventothefactthat
peopleoftenwork to gether?With therec entdevelop?
ment ofc omputernetworks and thewidespre ad deploy?
ment ofnetworkedworkstations? automating thegroup
writing proc essforgeographicallydistributedusers has
become feasible?Inthispaper?asoftwarepackage
whichsupportsdistributed?real?time?multimediacol?
laborativework? knownastheDistributedCollabora?
tiveWritingAid?DCWA?? isdescribed?TheDCWA
has?vemajorpartsbut theemphasisofthispaperis
onthe DCWA?sdistributeddatabase?whichiscrucial
to makingtheentiresystemworktogetherbothlogi?
callyandconsistently?Thedatabaseisaspecialize d?
distributedsystembasedontheclient?servermodelim?
plementedinC??runningunder???BSDUnixand
providesdynamicmanagementcapabilitiesuniquefor
amultimediacollaborativeworkingenvironment?
?Introduction
Computersarenowfamiliartoolsatworkand
athomeandareincreasinglyin?uencingtheway
inwhichpeopleinteract due torecentadv ances in
communicationnetworks?Electronicmail?bulletin
boards?newsgroups?distributed?lesystems?group
schedulers?video conferencing??letransferprotocol
?FTP??and?talk?servicesarebutafewexamples
ofcomputer?supportedinteractions?However?using
computerstoenhancepeople?scomm unicationcapa?
bilityhasmadesomeofthecurrentlimitations of the
technologyverynoticeable?Inparticular?traditional
computerapplicationshavebeendesignedtoberun
byoneuseratatimewhodoessomew ork inasingle
medium? suchasASCIItext?V erylittleregardhas
beengivento thefactthatpeopleoftenworktogether
andeventhescienti?cstudyofhuman?computerinter?
action?HCI? has emphasizedtheexplorationofissues
???????????????
whenonlyasingleuserinteractswithacomputer????
Sincemoderntechnology iscomplex?itisunusual
for anindividual to tacklethedesignofamajor
projectsingle?handedlyandusuallya team isassigned
toworkonthe project?Todayitis notuncommon for
do cuments toincludecon tributionsfrom man yp eople
thoughusuallyonepersonisresponsibleforcollecting
thev ariouspartsandmergingthemtogetherto form
the?hop efullycoherent? ?nalset of documents????
Inaddition?single?usercomputerapplicationscreate
barrierstogroupcollaborationandtheyarenormally
usedonlypassivelybythegroupto store?retrieve?
andpresentdata?Ifgroupmembersneedtoworkon
thesameitem? forexampleadocument? inmostcases
theymustworkinaninterleavedfashiontoprevent
inconsistency?Thismethod?however?meansthatall
theothermembersarepreventedfromworkingonthe
documentuntilthepersonis?nished?even iftheir
con tributionisina di?erentsectionofthedocumen t?
Thismethodisclearlyine?cientandastheneedfor
improving productivitycontinuestogrow?newtec h?
nologiesmustbedevelopedtoprovideactivesupport
fora teamandovercomeanygeographicaldistribution
oftheteam memb ers?? ??
Thereisthusagrowinginterestinanewgenera?
tionofmulti?usercomputerapplicationswhichassist
groupsofpeopleworkingtogether?supportingcollab?
orationeveniftheteammembersare geographically
distributed?Thesenewtoolshavebeen group edto?
gether underthenameofComputer?SupportedCo?
operativeWork?CSCW??????????? Thecollection
ofhardw areandsoftwarewhichsupportsCSCW is
alsoknownas groupware?? ??Inthepast?central?
izedgroupwareonbothmainframesandPCswasex?
ploredasameansforachievingo?ceautomation???
butnowthefocushasbeenshiftedto thedevelopment
ofdistributed?netw orkedgroup wareforCSCW????
?Groupware?isalso meanttobeamoretechnically
orientedlabeltodi?eren tiate itfrom?group?oriented?
products? whicharesimplyadd?onstosingle?user
productssothatgroupsofpeoplemaywork together
usingthesemore traditionalapplications????
Anotherexisting limitationtousershasbeenthe
relianceonapplicationswhichsupportonly onetype
ofinformationmedium? Thusa new?eld hasemerged
ov erthe past fewyearsknown asmultimedia? Mul?
timedia applicationsallow theuser tomanipulatein?
Page 2
therapiddevelopmentofnetw orking technologyand
with thecompletion ofaBroadband IntegratedSer?
vices DigitalNetw ork?B?ISDN?basedon?beroptics?
largeramountsofallkindsofdata?suchastext? au?
dioandvideo?maybetransmittedthroughasingle
channele?cientlyandcheaply????ascomparedto
thenetworks ofjustafewyearsago?
ThisreportintroducesatoolknownastheDis?
tributedCol laborativeWritingAid?DCWA?which
wasdevelopedto providemultimediaCSCWonwork?
stationsconnected tothe Internetandrunningthe
UNIXoperatingsystem?TheDCWAcanhelpusers
co op erateonanywriting task?suchasprogramming?
reportwriting?notetaking?etc??logically?conve?
niently?ande?ciently?Thecoding oftheDCWAhas
beendividedinto?vemajorparts andthisreport?s
emphasiswillbeontheDCWA?sdistributeddatabase?
whichiscrucialtomakingtheentiresystemworkto?
getherbothlogicallyandconsistently ?The nextsec?
tionexploresthefeaturesofsomerelatedsystemsand
thencomparesandcontraststheDCWA withthem?
Thepaperisorganizedasfollows?Inthenextsec?
tion?anoverviewofresearchonCSCWingeneraland
itsdatabasesubsystemsinparticularisprovided?then
wedescribethedesignofthenewdatabasesystemfor
theDCWAin Section??Theimplementationissues
arebrie?ydiscussedinSection??InSection??wepro?
videexamplestodemonstratethecapabilityofthe
system?Section?concludesthepaperanddiscusses
futurework?
?LiteratureReview
Groupwareforgeographicallydistributedusersis
usuallybaseduponanarchitecturethatallo wsmul?
tiplepro cesses to communicatewith eachother?????
Therefore?afundamentalrequirementforCSCWis
theprovisionformessageexchangeamongthepartic?
ipantsandithasbeensuggested that messagesshould
beorganized ina structuredway toachievee?ciency
?????Thatis?protocolsmustbedesigned todiscrim?
inateb etween co operation information andmessages
oflittle relevance? e?g?? thecognitive?ltering inIn?
formationLens????andthesemi?automaticagentsin
ObjectLens?????Asecondfundamentalrequirement
isthepro visionofmechanismsfor informationsharing
andcooperation?????Ithasbeenarguedthatinfor?
mationtobeusedincooperativeworkneedstobe
takenoutofthelimitofa?personalwall??thus all
usersshouldhavecommonaccesstoallinformation
?????
???TheGroupCollaborationProcess
Thegroupprocessmaybedividedintothosebehav?
iors?calledtaskbehaviors?whichareaimedatachiev?
ingthegroup?s taskandthosebehaviors?calledmain?
tenancebehaviors?whichare aimedatmain tainingthe
groupasacohesiveunit?Thesetwotypesofbehav?
iorareantagonisticandthegroupprocessusuallyin?
volvesbothofthemasprogressismadetowardsthe
interactiononthegroup?Othersarguethatsocial
protocolsshouldbedeterminedbythe groupmembers
independentofthe CSCW system? andstillothersar?
guethatamiddlegroundmustbetaken?????
??? MultimediaandItsSolution
Asalientcharacteristic ofinteractive?multimedia
documentsisthenotionofsupporting?nonsequential
viewing??aswellas?nonsequentialauthoring??As
theconsequence?thereisnotaprede?nedorderin
whichthedi?erentsegmentsofthedocumentshould
beaccessed?Onesolutionofnonsequentialdo cument
structureis tomanageacollection ofno desandlinks
????? In essence?the heterogeneousmedia areaccessed
via anhomogeneous linking mechanism?? ??
???DatabaseSystemsforCSCW
Themainchallengesassociatedwithdesigninga
database forCSCWare?
?Implementingthepartitioning methodwhichwill
breakthedo cumentsintomanageablesections?
?Main tainingthispartitioned structureasc hanges
aremadebyeachuser?
?Updatingthemastercopyasneeded?
?Determiningiftwousersaretryingtomodifythe
samesection?
Intraditionaldatabasesystems?theinformationre?
triev
riding
alis
user
primarily
need
based
?and
onthenotion
load?
of
in
k
the
eys?How?
ever?thesekeysarestaticandmaybeextremelystan?
dardized orsopersonalthattheyareuselesstoall
membersofthegroupexceptfor theircreator????
Also?concurrencycontrol?neededtomediateaccess
toanysharableobjects?isusuallyachiev edthrough
simpleloc king?though othersystems usetransaction
mechanisms ??? ??Flo orcontrolora turn?takingmec h?
anismisanotherwayofpro viding accesscontrol tothe
sharedob jects?Ithas been recommendedthatsys?
temssupportman ysuc h?oorcon trolpoliciessoas to
suitthevarious users?needs ??? ??
Until recen tly?mostmultimediaob jectshaveb een
storedin?lesby using?do cumentmanagementsys?
tems?tostore thedocumen ts?index andattribute in?
formation?where oneof theattributesindicatesthe
physicallocationofthe?le?However? thesesystems
usuallyk eepjournal ?les topro videforrecovery after
system failures?Unfortunately?thispresentsa disad?
vantage tousingcommercial? relationaldatabase sys?
temssince thedatabasejournal ?les tendtobebottle?
neckssince everytransaction isrecordedinthejournal
????? Itis also important torealize that?do cumen ts
rather than simplyn umerical datawillbetheov er?
database ????s??
????
Page 3
imen talsystem providingasync hronous collab orative
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CSCWcanalsobeaccomplishedthroughm ultime?
diacomm unicationchannels?TheMERMAID ????
and SPIN???? systems areprototyp es thatprovide
real?time conferencingenvironmen tsforgeographi?
callydistributedparticipantsbyusingsync hronous
textual?audio?andvideocommunications?Oneof
thedrawbacksassociatedwiththesesystemsisthat
thecost
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Inaddition
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to providingaconferencingservice?theCOGNOTER
?collectsideas
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videsou
IsWhatISee?capability?
ah that are
ope????ishypertext
videsservices?usersareiden?
ti?edaseithercoauthorsorcommenters?Itallowsall
commenoncumentonly
ileged users?i?e??thecoauthors?can modifythedocu?
orComment?pro videsgroup
editingenvironmentandallo wsupto?? review ers to
on documen How ever? onlytheorigi?
nalauthorcanactually modifythe documen t?The
Aquanetsystemdev elopedat XeroxPARCisahy?
pertexttoolwhichfacilitates collab orativeknowledge
structuring? It providesa?WhatY ouSee IsWhatI
Did??WYSIWID?viewon thesharedstructurebut
doesnotprovidesynchronousshared viewsorcom?
municationfacilities?????
rIBIS????
h ypertextsystemsprovidingvariouscollab oration
Inindeptmousersyorkon
theirowntaskswithout interferingwitheachother?
Intheloosely?coupledmode?usersmaysharecertain
publicinformationwhileworkingontheirowntasks
andinthetightly?coupledmode?users sharethesame
view andresources? e?g??mouseand?le?whichare
strictly controlled toavoidcon?icts? Major improv e?
men tsof SEPIAover rIBISincludeautomaticmo de
switchingandthe useofcomp ositenodes? SEPIA
alsopro videsanaudiocommunicationchannelb e?
tw eenparticipants?
designtopic from memb ers ofahardwareplatform
project team?andthensuggests goalsandsubgoals to
them?TheO?ceWorks system????arranges meetings
foragroup ofparticipan tsbychecking eachpartici?
pant?sschedule?
?
a
Design
user
of
able
the
to
Database
select
System
no
Toreducecomplexity?wedecided todesign the
databasecomponent usingtheclien t?server mo del?All
thedatabase processesrunningonthegroup mem?
bers?hostswillbeconsidered?clients??exceptfor
one? whichwillbedesignated asthe?server??The
databaseserveris onlya?serv er?in thesense that it
serv esotherdatabases?Users?andmore importantly?
thedatabase?ssibling processes?CR?UI?andNA?will
notbe able todistinguishwhethertheir DB process is
aclient or theserver? That is?the server hasall the
functionalitythataclientdoes? asw ell as additional
capabilities?Itis th usacentrally controlleddatabase?
How ever? data aredistributed asm uch aspossible?As
suc h?actuallyonlya fewthingsneed tobe requested
from this cen tral site?
Mostofthesiblingprocesses?requests canbeful?
?lledlocallyand themain use oftheserver is toman?
agethe edit loc ksofthenodesinthelogical structure
ofthedocument?explained laterinSection?????It
wasdecided that maintaininginformationconsistency
bymanagingthegroup?s edits in thisfashionwould
be easier thandistributing this informationandthen
runninga ?con?ict?correction?algorithm? Asbefore?
willbe anyleafdetow orkon
which isnot curren tlylock edby anothergroup mem?
ber?
Ingeneral? the databaseprocessshouldpro videser?
vicesforbackupandrecovery? de?nitionofnodeat?
tributes?searc hingthe logical view to de?neapar?
tialview?sharing and con?ict resolution?and?nally?
run?timemo di?cationof thelogicalstructure?Some
detailsof thesecapabilities arediscussedb elow?
???BackupandRecov ery
To implement backupandrecov ery? theDCWA
database distinguishesbetw een ?sav es?and ?com?
mits??Sav es ofa nodewillbe localand maybeun?
done?and ev enredone? until theuser hasthe conten ts
of theno de like hew antsit? Commits?onthe other
hand?are sent to the DB serverandp ermanen tlyre?
placetheoldversion of theno de?Only the con tent
ofthat no dewillbereplaced? thecontent ofother
nodes in thesame physical ?lewill notbe a?ectedby
a commit?though theDBma yneed toupdate infor?
mation about those nodes?such as theirstartandend
points?
Ho wever?the ability tostoremultiple committed
versionsof anode tocreatea?history list?hasbeen
deferred tosomefutureversion ofthisproject since
thisasp ect needstobediscussed inmore detail among
the DCWAworking groupas howb est toservea group
of userswith suchafeature?F orexample?when should
the to olallowanoldversion tobe deleted forever?
Page 4
groupiftheyantneedit? Finally?
databasewillgivethe groupabilityde?ne
oaluepairs?Incase? thegroup not
ited?
only
The
assigns
DCW
v
Adatabase
alues
allo
sp
ws
eci?es
thegroup
the
toassign
of
sev
new
eral
attributes?
attributes to each node?F orexample? in an
academic writingapplication?a no de may correspond
toa segment oftextwhich mentionsa certainc hemical
compound? Theuser mayw ant to recordthisfact in
thecorresponding logicalnodeasaquick reminderfor
the future?Moreimportantly?the attributescan be
usedbythesearchfacility?seebelow?togainimme?
diateinsight to thesemantic alstructure?as opposed
totheorganizationalstructureof thelogicalview?
Therefore? inadditiontotheactualnodecontent?
eachnodeofthe logicalstructurewillalsocontainat?
tributesthatdescribethenatureofthenode?Some
attributeswillhavetheirassociatedvaluesassigned
bythedatabase?suchasthenode?sidnumberand
the timethe contents of thenodewerelastmodi?
?ed? Otherattributes?suc hastheno de?sname ?label?
and anykeyw ords?willhavetheirassociatedvalues
assignedbythegroupmembers?However?with two
exceptionsdetailedlater?thereisnorequirementfor
theset yp es ofattributes to haveavalueassignedby
thedon?tworthe
thetotheir
wnattribute?vthis
butalso namesthe
??? SearchFacility
Thedo cumen ts?and theirvarioussections?which
the group iswritingcanb ecomenumerous and? th us?
thelogicalstructurewouldprobablybeavery compli?
catedtree hierarchy? Inordertohelpausertolo cate
a desiredportionofthedocument quickly? searchca?
pabilitiesmust be pro vided?
In groupwriting?
the
esp
A
ecially
from
inthe
CSCW
case of
systems?
?divi?
sion oflab or?writing? each writeronly concen trates
onhis?hero wnpart and repeated men tioning of the
samep ointbecomespossible? Ithasalsob een ob?
serv edthat collab oratorsneedtoknow the detailsof
ano dewrittenby another inorder to use theno de
appropriately? Iftheno decontainsCprogram co de?
forexample?thetediousness oftheco deoftengives
rise totheneedfora readerto consultwiththewriter
directlyfor clari?cation?Inboth thesecases?anau?
tomatic searchfacilit y ofthe nodes?attributeswould
helpacollaboratorgainknowledge aboutwhatthe
others havewrittenwithout having toactually read
theirw ork orask themdirectly? Itwould alsohelpa
user quicklylocate the node?s?he?shewantstoedit?
Thesearchfacilityallowsausertospecify queries
aboutthe logical view?Sofar?fewpeoplehavereal?
izedthede?niteneedforasearchfacilitywithCSCW
writingsinceinsingle userwriting?thewritertends
toknow?andalmostalwaysrememb er?ev erydetail
ofthedocument?Infact? searchfacilitiesareacom?
monfeatureinmostcommercialdatabasesandthis
featureshouldbeavailableinaCSCWenvironment
too? Theuse of no deattributes andsearch techniques
distinguishes DCWother
andinformationaboutthesenodeswillbesenttothe
UIfordisplay?replacingtheentirelogicalviewwith
onlythosenodes?Ifaremoteusercreatesanew node
ormodi?esan oldone?such that italso matches the
criteria? ittoo will thenbesent tothe UI fordisplay
asso onasthelocalDBisinformedaboutit?
???SharingandCon?ict Resolution
The abilitytomaintainthe consistencyof the?leas
itisbeing modi?edbytheusersishandledbylocking
andunlockingthe nodesofthelogical structure?To
maintainconsistency?aDBserver? asdiscussedabove?
will beresp onsibleforcontrollingthelockingmecha?
nism?
Therehavebeenanumberofreal?time conferencing
toolsthatincludecon?ict resolutions?suchasMER?
MAID ?????SPIN?????andthe textualbulletinboard
in?????However? theirmethodsarenotquitease?ec?
tiveforresolving con?ictarisingindocumentcompo?
sition?
???Run?TimeModi?cationof theLogical
Structure
It is
commit?
p ossiblethatthe groupcreatesaprimitive logi?
cal structureatthev erybeginning of theirprojectand
will
?
need
Implemen
to re?neit asthe writing
of
pro cess
Database
pro ceeds?
Inan yeven t?a groupmemb er shouldbepro videdwith
thecapability tode?nenew nodes ordeleteunneeded
nodes duringthe writing?without disturbingthe other
users?
TheDB serverwillalsoberesponsible forthis con?
?ict resolutionandtreatsitjustlikeedits?Afterthe
serverlocksasubtreeof thelogicalview formodi?ca?
tion?the usermay makethenecessarychanges?save?
undo? andredojustasif shew ereediting theconten t
ofa node?Whentheuser commitsthe modi?cation?
theDB serv erwill mergethe existinglogicalviewwith
the modi?ed subtree? writethe entirelogical view into
the?le on disk?and then sendthe new subtree to
allthe DB clients? The term subtreew asused since
all descendants oftheselected node arelock edasw ell?
Should theuser needto modify the entirelogical view?
thero otno de shouldbe selected?
Finally? this pro ceduremayalsobe usedtoc hange
thev alues of thosenodeattributes which areassigned
by thegroup?Theuser shouldlocktheno defor logical
view modi?cation?c hange theattribute?sv alueand
then
tationthe
System
Thedatabase isa specialized?distributedprogram
based onthe client?serv er modelimplemented inC??
????running under???BSD Unix?
Ausershouldbeginhis?herDCWAeditingsession
asfollows?
Step ?? Opena?lecon tainingalogicalview orcreate
anewlogicalview?
Page 5
Step ??Getthecontentofthedocumentforthese?
lectednodeandstart towork?
Todeterminewhichno de toselect?theusermayrely
onpastexperience?i?e??theuserknowswhichnode
he?shewants?ormaymakeaqueriedsearchofthe
availablenodesto limithis?her view ofthetree?
Whenauser getsanodeforread?only?hemaybe
informed?when itisreceivedorperhapslater?that
another user is currentlyeditingtheno de?scontents?
The usershould decide whetherornottowatchthe
changesastheyareb eing made? Thiscapabilitymay
betoggledon ando?astheuserchooses untilthe
remoteeditor?nishes?Whenausergetsanodefor
editing?the nodebecomeslo ck edto otherusers who
w ant toeditthatnode?thoughthey may stillreadit?
as described above? Thesame is truefor usersthat
locksubtrees of the logicalview for mo di?cation?
Intheremainderof thissection?webrie?yexplain
theimplementationofimportantcomponentsin the
DCWAdatabase?
???Logical View
Usersareable to de?nealogical structurewithout
any textor graphicobjectinthenodes?Ifthe group
do esn?twant togettoocomplicatedthe logicalview
canbe as simpleasone ro ot no defollow edbyasman y
childnodesas necessary?for example?
?
Paper??
?? Abstract
?? Intro
?? Point?
?? Point?
?? Point?
???
??Conclusion
?? Notes
??Bibliography
??AppendixI
??AppendixII
Thetermsthatthegroupusestonamenodeswhen
de?ningthelogicalstructureisunimp ortanttothe
database?A node?snameisjustoneofmanyat?
tributesthatitmain tainsforeachnode??Seethe
subsectionb elowon node attributes??The database
iden ti?es and di?erentiates no desbya no de idwhich
is maintained internallyby thedatabase itself andnot
assigned ?p erhapserroneously?by the group?
Anotherexampleofalogical view isseen in Figure
?? whichrepresen tsap ossiblestructureforaC pro?
gram?le?Notethat thelogicalstructureis notthe
?ledependencydiagramusedforwritingaMakefile?
It ismerely anorganizationalstructureshowingthe
comp ositionoftheprogram?The?ledependencyin
aMakefiledealswithsemanticdependencies? and is
the reasonforov erla yingtheseman ticnetwork?ex?
plained later?on thelogicalstructure?
?
This istheway logicaltrees aredisplay edin theDCW A?
....................
...........
..........
Source File
Header
Function 1
Description
Preprocessor
directives
Global
variables
Main
Figure ??AnotherGraphical Example ofaLogical
Structure
???NodeAttributes
The basisfor modeling seman ticintheDCWA
database is thecapabilityofassigningattributeto
each?organizational? node? Thusmeaningsareas?
signed totheno deand it may thenberelated to
othernodesthroughmanyrelationaloperationssuch
asclassi?cation? categorization?inheritance? etc?The
follo wing isthelist ofattributesthat are maintained
foreach no de? aswell asanindicationof thet ype of
informationeachwillhold?
?NodeId?Aninteger?
?Name?Acharacterstringlabelforthenode?
?NodeLevel?OneofRoot?Intermediate?orLeaf?
?Creationtime?Anumberoftypetime?t?
?Creator?Acharacterstring holdingausername?
?LastChangetime?Anumb eroftypetime?t?
? LastEditor?Acharacterstring holdingauser
name?
?Topic?Ac haracterstring?
?Description?Acharacterstring?
? Keywor ds?Ac haracterstringwitheachk eyword
separatedbya space?
?UserDe?nedAttributes?Ac haracterstringwith
eachattribute separatedbyaspace?
?User De?nedA ttributeV alues?Ac haracterstring
with eachvalue separatedbyaspace?
?ReadAccessList?Acharacterstringwith each
usernameseparatedbya space? seetheno de
accessp olicybelo w?
? EditAccess List?Acharacterstringwith each
usernameseparatedbyaspace?
?Lo gical ViewMo di?c ationAccessList?Ac har?
acterstringwitheach user nameseparatedbya
space?
?LocalUserReading?Aboolean?