您的位置：首页 > 其它

使用EF6和MVC5实现一个简单的选课系统--使用EF6处理并发操作（10/12）

2014-03-24 20:59 876 查看

先贴出来英文的，抽空翻译！

本讲源代码下载

TheContosoUniversitysamplewebapplicationdemonstrateshowtocreateASP.NETMVC5applicationsusingtheEntityFramework6CodeFirstandVisualStudio2013.Forinformationaboutthetutorialseries,seethe
firsttutorialintheseries.

Inearliertutorialsyoulearnedhowtoupdatedata.Thistutorialshowshowtohandleconflictswhenmultipleusersupdatethesameentityatthesametime.

You'llchangethewebpagesthatworkwiththe

Department

entity
sothattheyhandleconcurrencyerrors.ThefollowingillustrationsshowtheIndexandDeletepages,includingsomemessagesthataredisplayedifaconcurrencyconflictoccurs.

ConcurrencyConflicts

Aconcurrencyconflictoccurswhenoneuserdisplaysanentity'sdatainordertoeditit,andthenanotheruserupdatesthesameentity'sdatabeforethefirstuser'schangeiswrittentothedatabase.Ifyoudon'tenablethedetectionofsuchconflicts,whoever
updatesthedatabaselastoverwritestheotheruser'schanges.Inmanyapplications,thisriskisacceptable:iftherearefewusers,orfewupdates,orifisn'treallycriticalifsomechangesareoverwritten,thecostofprogrammingforconcurrencymight
outweighthebenefit.Inthatcase,youdon'thavetoconfiguretheapplicationtohandleconcurrencyconflicts.

PessimisticConcurrency(Locking)

Ifyourapplicationdoesneedtopreventaccidentaldatalossinconcurrencyscenarios,onewaytodothatistousedatabaselocks.Thisiscalledpessimistic
concurrency.Forexample,beforeyoureadarowfromadatabase,yourequestalockforread-onlyorforupdateaccess.Ifyoulockarowforupdateaccess,nootherusersareallowedtolocktheroweitherforread-onlyorupdateaccess,becausethey
wouldgetacopyofdatathat'sintheprocessofbeingchanged.Ifyoulockarowforread-onlyaccess,otherscanalsolockitforread-onlyaccessbutnotforupdate.

Managinglockshasdisadvantages.Itcanbecomplextoprogram.Itrequiressignificantdatabasemanagementresources,anditcancauseperformanceproblemsasthenumberofusersofanapplicationincreases.Forthesereasons,notalldatabasemanagementsystems
supportpessimisticconcurrency.TheEntityFrameworkprovidesnobuilt-insupportforit,andthistutorialdoesn'tshowyouhowtoimplementit.

OptimisticConcurrency

Thealternativetopessimisticconcurrencyisoptimisticconcurrency.Optimisticconcurrencymeansallowingconcurrency
conflictstohappen,andthenreactingappropriatelyiftheydo.Forexample,JohnrunstheDepartmentsEditpage,changestheBudgetamount
fortheEnglishdepartmentfrom$350,000.00to$0.00.

BeforeJohnclicksSave,JanerunsthesamepageandchangestheStart
Datefieldfrom9/1/2007to8/8/2013.

JohnclicksSavefirstandseeshischangewhenthebrowserreturnstotheIndexpage,thenJaneclicksSave.
Whathappensnextisdeterminedbyhowyouhandleconcurrencyconflicts.Someoftheoptionsincludethefollowing:

Youcankeeptrackofwhichpropertyauserhasmodifiedandupdateonlythecorrespondingcolumnsinthedatabase.Intheexamplescenario,nodatawouldbelost,becausedifferentpropertieswereupdatedbythetwousers.Thenexttimesomeonebrowsesthe
Englishdepartment,they'llseebothJohn'sandJane'schanges—astartdateof8/8/2013andabudgetofZerodollars.

Thismethodofupdatingcanreducethenumberofconflictsthatcouldresultindataloss,butitcan'tavoiddatalossifcompetingchangesaremadetothesamepropertyofanentity.WhethertheEntityFrameworkworksthiswaydependsonhowyouimplement
yourupdatecode.It'softennotpracticalinawebapplication,becauseitcanrequirethatyoumaintainlargeamountsofstateinordertokeeptrackofalloriginalpropertyvaluesforanentityaswellasnewvalues.Maintaininglargeamountsofstate
canaffectapplicationperformancebecauseiteitherrequiresserverresourcesormustbeincludedinthewebpageitself(forexample,inhiddenfields)orinacookie.

YoucanletJane'schangeoverwriteJohn'schange.ThenexttimesomeonebrowsestheEnglishdepartment,they'llsee8/8/2013andtherestored$350,000.00value.ThisiscalledaClient
WinsorLastinWinsscenario.(Allvaluesfromtheclienttakeprecedenceoverwhat'sinthedatastore.)As
notedintheintroductiontothissection,ifyoudon'tdoanycodingforconcurrencyhandling,thiswillhappenautomatically.

YoucanpreventJane'schangefrombeingupdatedinthedatabase.Typically,youwoulddisplayanerrormessage,showherthecurrentstateofthedata,andallowhertoreapplyherchangesifshestillwantstomakethem.ThisiscalledaStore
Winsscenario.(Thedata-storevaluestakeprecedenceoverthevaluessubmittedbytheclient.)You'llimplementtheStoreWinsscenariointhistutorial.Thismethodensuresthatnochangesareoverwrittenwithoutauserbeingalertedtowhat'shappening.

DetectingConcurrencyConflicts

YoucanresolveconflictsbyhandlingOptimisticConcurrencyExceptionexceptions
thattheEntityFrameworkthrows.Inordertoknowwhentothrowtheseexceptions,theEntityFrameworkmustbeabletodetectconflicts.Therefore,youmustconfigurethedatabaseandthedatamodelappropriately.Someoptionsforenablingconflictdetection
includethefollowing:

Inthedatabasetable,includeatrackingcolumnthatcanbeusedtodeterminewhenarowhasbeenchanged.YoucanthenconfiguretheEntityFrameworktoincludethatcolumninthe

Where

clause
ofSQL

Update

Delete

commands.

Thedatatypeofthetrackingcolumnistypicallyrowversion

Therowversionvalue
isasequentialnumberthat'sincrementedeachtimetherowisupdated.Inan

Update

Delete

command,
the

Where

clauseincludestheoriginalvalueofthetrackingcolumn
(theoriginalrowversion).Iftherowbeingupdatedhasbeenchangedbyanotheruser,thevalueinthe

rowversion

column
isdifferentthantheoriginalvalue,sothe

Update

Delete

statement
can'tfindtherowtoupdatebecauseofthe

Where

clause.When
theEntityFrameworkfindsthatnorowshavebeenupdatedbythe

Update

Delete

command
(thatis,whenthenumberofaffectedrowsiszero),itinterpretsthatasaconcurrencyconflict.

ConfiguretheEntityFrameworktoincludetheoriginalvaluesofeverycolumninthetableinthe

Where

clause
of

Update

and

Delete

commands.

Asinthefirstoption,ifanythingintherowhaschangedsincetherowwasfirstread,the

Where

clause
won'treturnarowtoupdate,whichtheEntityFrameworkinterpretsasaconcurrencyconflict.Fordatabasetablesthathavemanycolumns,thisapproachcanresultinverylarge

Where

clauses,
andcanrequirethatyoumaintainlargeamountsofstate.Asnotedearlier,maintaininglargeamountsofstatecanaffectapplicationperformance.Thereforethisapproachisgenerallynotrecommended,anditisn'tthemethodusedinthistutorial.

Ifyoudowanttoimplementthisapproachtoconcurrency,youhavetomarkallnon-primary-keypropertiesintheentityyouwanttotrackconcurrencyforbyaddingtheConcurrencyCheckattribute
tothem.ThatchangeenablestheEntityFrameworktoincludeallcolumnsintheSQL

WHERE

clause
of

UPDATE

statements.

Intheremainderofthistutorialyou'lladdarowversiontracking
propertytothe

Department

entity,createacontrollerandviews,
andtesttoverifythateverythingworkscorrectly.

AddanOptimisticConcurrencyPropertytotheDepartmentEntity

InModels\Department.cs,addatrackingpropertynamed

RowVersion

publicclassDepartment
{
publicintDepartmentID{get;set;}

[StringLength(50,MinimumLength=3)]
publicstringName{get;set;}

[DataType(DataType.Currency)]
[Column(TypeName="money")]
publicdecimalBudget{get;set;}

[DataType(DataType.Date)]
[DisplayFormat(DataFormatString="{0:yyyy-MM-dd}",ApplyFormatInEditMode=true)]
[Display(Name="StartDate")]
publicDateTimeStartDate{get;set;}

[Display(Name="Administrator")]
publicint?InstructorID{get;set;}

[Timestamp]
publicbyte[]RowVersion{get;set;}

publicvirtualInstructorAdministrator{get;set;}
publicvirtualICollection<Course>Courses{get;set;}
}

TheTimestampattribute
specifiesthatthiscolumnwillbeincludedinthe

Where

clause
of

Update

and

Delete

commands
senttothedatabase.TheattributeiscalledTimestampbecause
previousversionsofSQLServerusedaSQLtimestampdata
typebeforetheSQLrowversionreplaced
it.The.Nettypefor

rowversionis
abytearray.

IfyouprefertousethefluentAPI,youcanusetheIsConcurrencyTokenmethod
tospecifythetrackingproperty,asshowninthefollowingexample:

modelBuilder.Entity<Department>()
.Property(p=>p.RowVersion).IsConcurrencyToken();

Byaddingapropertyyouchangedthedatabasemodel,soyouneedtodoanothermigration.InthePackageManagerConsole(PMC),enterthefollowingcommands:

Add-MigrationRowVersion

Update-Database

ModifytheDepartmentController

InControllers\DepartmentController.cs,adda

using

statement:

usingSystem.Data.Entity.Infrastructure;

IntheDepartmentController.csfile,changeallfouroccurrencesof"LastName"to"FullName"sothatthedepartment
administratordrop-downlistswillcontainthefullnameoftheinstructorratherthanjustthelastname.

ViewBag.InstructorID=newSelectList(db.Instructors,"InstructorID","FullName");

Replacetheexistingcodeforthe

HttpPost

Edit

method
withthefollowingcode:

[HttpPost]
[ValidateAntiForgeryToken]
publicasyncTask<ActionResult>Edit(
[Bind(Include="DepartmentID,Name,Budget,StartDate,RowVersion,InstructorID")]
Departmentdepartment)
{
try
{
if(ModelState.IsValid)
{
db.Entry(department).State=EntityState.Modified;
awaitdb.SaveChangesAsync();
returnRedirectToAction("Index");
}
}
catch(DbUpdateConcurrencyExceptionex)
{
varentry=ex.Entries.Single();
varclientValues=(Department)entry.Entity;
vardatabaseEntry=entry.GetDatabaseValues();
if(databaseEntry==null)
{
ModelState.AddModelError(string.Empty,
"Unabletosavechanges.Thedepartmentwasdeletedbyanotheruser.");
}
else
{
vardatabaseValues=(Department)databaseEntry.ToObject();

if(databaseValues.Name!=clientValues.Name)
ModelState.AddModelError("Name","Currentvalue:"
+databaseValues.Name);
if(databaseValues.Budget!=clientValues.Budget)
ModelState.AddModelError("Budget","Currentvalue:"
+String.Format("{0:c}",databaseValues.Budget));
if(databaseValues.StartDate!=clientValues.StartDate)
ModelState.AddModelError("StartDate","Currentvalue:"
+String.Format("{0:d}",databaseValues.StartDate));
if(databaseValues.InstructorID!=clientValues.InstructorID)
ModelState.AddModelError("InstructorID","Currentvalue:"
+db.Instructors.Find(databaseValues.InstructorID).FullName);
ModelState.AddModelError(string.Empty,"Therecordyouattemptedtoedit"
+"wasmodifiedbyanotheruserafteryougottheoriginalvalue.The"
+"editoperationwascanceledandthecurrentvaluesinthedatabase"
+"havebeendisplayed.Ifyoustillwanttoeditthisrecord,click"
+"theSavebuttonagain.OtherwiseclicktheBacktoListhyperlink.");
department.RowVersion=databaseValues.RowVersion;
}
}
catch(RetryLimitExceededException/*dex*/)
{
//Logtheerror(uncommentdexvariablenameandaddalineheretowritealog.
ModelState.AddModelError(string.Empty,"Unabletosavechanges.Tryagain,andiftheproblempersistscontactyoursystemadministrator.");
}

ViewBag.InstructorID=newSelectList(db.Instructors,"ID","FullName",department.InstructorID);
returnView(department);
}

Theviewwillstoretheoriginal

RowVersion

valueinahiddenfield.
Whenthemodelbindercreatesthe

department

instance,thatobject
willhavetheoriginal

RowVersion

propertyvalueandthenewvalues
fortheotherproperties,asenteredbytheuserontheEditpage.ThenwhentheEntityFrameworkcreatesaSQL

UPDATE

command,
thatcommandwillincludea

WHERE

clausethatlooksforarow
thathastheoriginal

RowVersion

value.

Ifnorowsareaffectedbythe

UPDATE

command(norowshavethe
original

RowVersion

value),theEntityFrameworkthrowsa

DbUpdateConcurrencyException

exception,
andthecodeinthe

catch

blockgetstheaffected

Department

entity
fromtheexceptionobject.

varentry=ex.Entries.Single();

Thisobjecthasthenewvaluesenteredbytheuserinits

Entity

property,
andyoucangetthevaluesreadfromthedatabasebycallingthe

GetDatabaseValues

method.

varclientValues=(Department)entry.Entity;
vardatabaseEntry=entry.GetDatabaseValues();

The

GetDatabaseValues

methodreturnsnullifsomeonehasdeleted
therowfromthedatabase;otherwise,youhavetocasttheobjectreturnedtothe

Department

class
inordertoaccessthe

Department

properties.

if(databaseEntry==null)
{
ModelState.AddModelError(string.Empty,
"Unabletosavechanges.Thedepartmentwasdeletedbyanotheruser.");
}
else
{
vardatabaseValues=(Department)databaseEntry.ToObject();

Next,thecodeaddsacustomerrormessageforeachcolumnthathasdatabasevaluesdifferentfromwhattheuserenteredontheEditpage:

if(databaseValues.Name!=currentValues.Name)
ModelState.AddModelError("Name","Currentvalue:"+databaseValues.Name);
//...

Alongererrormessageexplainswhathappenedandwhattodoaboutit:

ModelState.AddModelError(string.Empty,"Therecordyouattemptedtoedit"
+"wasmodifiedbyanotheruserafteryougottheoriginalvalue.The"
+"editoperationwascanceledandthecurrentvaluesinthedatabase"
+"havebeendisplayed.Ifyoustillwanttoeditthisrecord,click"
+"theSavebuttonagain.OtherwiseclicktheBacktoListhyperlink.");

Finally,thecodesetsthe

RowVersion

valueofthe

Department

object
tothenewvalueretrievedfromthedatabase.Thisnew

RowVersion

value
willbestoredinthehiddenfieldwhentheEditpageisredisplayed,andthenexttimetheuserclicksSave,only
concurrencyerrorsthathappensincetheredisplayoftheEditpagewillbecaught.

InViews\Department\Edit.cshtml,addahiddenfieldtosavethe

RowVersion

property
value,immediatelyfollowingthehiddenfieldforthe

DepartmentID

property:

@modelContosoUniversity.Models.Department

@{
ViewBag.Title="Edit";
}

<h2>Edit</h2>

@using(Html.BeginForm())
{
@Html.AntiForgeryToken()

<divclass="form-horizontal">
<h4>Department</h4>
<hr/>
@Html.ValidationSummary(true)
@Html.HiddenFor(model=>model.DepartmentID)
@Html.HiddenFor(model=>model.RowVersion)

TestingOptimisticConcurrencyHandling

RunthesiteandclickDepartments:

RightclicktheEdithyperlinkfortheEnglishdepartmentandselectOpen
innewtab,thenclicktheEdithyperlinkfortheEnglishdepartment.Thetwotabsdisplaythesameinformation.

ChangeafieldinthefirstbrowsertabandclickSave.

ThebrowsershowstheIndexpagewiththechangedvalue.

ChangeafieldinthesecondbrowsertabandclickSave.

ClickSaveinthesecondbrowsertab.Youseeanerrormessage:

ClickSaveagain.Thevalueyouenteredinthesecondbrowsertabissavedalongwiththeoriginalvalueofthedata
youchangedinthefirstbrowser.YouseethesavedvalueswhentheIndexpageappears.

UpdatingtheDeletePage

FortheDeletepage,theEntityFrameworkdetectsconcurrencyconflictscausedbysomeoneelseeditingthedepartmentinasimilarmanner.Whenthe

HttpGet

Delete

method
displaystheconfirmationview,theviewincludestheoriginal

RowVersion

value
inahiddenfield.Thatvalueisthenavailabletothe

HttpPost

Delete

method
that'scalledwhentheuserconfirmsthedeletion.WhentheEntityFrameworkcreatestheSQL

DELETE

command,
itincludesa

WHERE

clausewiththeoriginal

RowVersion

value.
Ifthecommandresultsinzerorowsaffected(meaningtherowwaschangedaftertheDeleteconfirmationpagewasdisplayed),aconcurrencyexceptionisthrown,andthe

HttpGet
Delete

methodiscalledwithanerrorflagsetto

true

in
ordertoredisplaytheconfirmationpagewithanerrormessage.It'salsopossiblethatzerorowswereaffectedbecausetherowwasdeletedbyanotheruser,sointhatcaseadifferenterrormessageisdisplayed.

InDepartmentController.cs,replacethe

HttpGet

Delete

method
withthefollowingcode:

publicActionResultDelete(int?id,bool?concurrencyError)
{
if(id==null)
{
returnnewHttpStatusCodeResult(HttpStatusCode.BadRequest);
}
Departmentdepartment=db.Departments.Find(id);
if(department==null)
{
returnHttpNotFound();
}

if(concurrencyError.GetValueOrDefault())
{
if(department==null)
{
ViewBag.ConcurrencyErrorMessage="Therecordyouattemptedtodelete"
+"wasdeletedbyanotheruserafteryougottheoriginalvalues."
+"ClicktheBacktoListhyperlink.";
}
else
{
ViewBag.ConcurrencyErrorMessage="Therecordyouattemptedtodelete"
+"wasmodifiedbyanotheruserafteryougottheoriginalvalues."
+"Thedeleteoperationwascanceledandthecurrentvaluesinthe"
+"databasehavebeendisplayed.Ifyoustillwanttodeletethis"
+"record,clicktheDeletebuttonagain.Otherwise"
+"clicktheBacktoListhyperlink.";
}
}

returnView(department);
}

Themethodacceptsanoptionalparameterthatindicateswhetherthepageisbeingredisplayedafteraconcurrencyerror.Ifthisflagis

true

,
anerrormessageissenttotheviewusinga

ViewBag

property.

Replacethecodeinthe

HttpPost

Delete

method
(named

DeleteConfirmed

)withthefollowingcode:

[HttpPost]
[ValidateAntiForgeryToken]
publicActionResultDelete(Departmentdepartment)
{
try
{
db.Entry(department).State=EntityState.Deleted;
db.SaveChanges();
returnRedirectToAction("Index");
}
catch(DbUpdateConcurrencyException)
{
returnRedirectToAction("Delete",new{concurrencyError=true});
}
catch(DataException/*dex*/)
{
//Logtheerror(uncommentdexvariablenameafterDataExceptionandaddalineheretowritealog.
ModelState.AddModelError(string.Empty,"Unabletodelete.Tryagain,andiftheproblempersistscontactyoursystemadministrator.");
returnView(department);
}
}

Inthescaffoldedcodethatyoujustreplaced,thismethodacceptedonlyarecordID:

publicActionResultDeleteConfirmed(intid)

You'vechangedthisparametertoa

Department

entityinstancecreated
bythemodelbinder.Thisgivesyouaccesstothe

RowVersion

property
valueinadditiontotherecordkey.

publicActionResultDelete(Departmentdepartment)

Youhavealsochangedtheactionmethodnamefrom

DeleteConfirmed

Delete

.
Thescaffoldedcodenamedthe

HttpPost

Delete

method

DeleteConfirmed

to
givethe

HttpPost

methodauniquesignature.(TheCLRrequires
overloadedmethodstohavedifferentmethodparameters.)Nowthatthesignaturesareunique,youcanstickwiththeMVCconventionandusethesamenameforthe

HttpPost

and

HttpGet

delete
methods.

Ifaconcurrencyerroriscaught,thecoderedisplaystheDeleteconfirmationpageandprovidesaflagthatindicatesitshoulddisplayaconcurrencyerrormessage.

InViews\Department\Delete.cshtml,replacethescaffoldedcodewiththefollowingcodethataddsanerrormessagefield
andhiddenfieldsfortheDepartmentIDandRowVersionproperties.Thechangesarehighlighted.

@modelContosoUniversity.Models.Department

@{
ViewBag.Title="Delete";
}

<h2>Delete</h2>

<pclass="error">@ViewBag.ConcurrencyErrorMessage</p>

<h3>Areyousureyouwanttodeletethis?</h3>
<div>
<h4>Department</h4>
<hr/>
<dlclass="dl-horizontal">
<dt>
Administrator
</dt>

<dd>
@Html.DisplayFor(model=>model.Administrator.FullName)
</dd>

<dt>
@Html.DisplayNameFor(model=>model.Name)
</dt>

<dd>
@Html.DisplayFor(model=>model.Name)
</dd>

<dt>
@Html.DisplayNameFor(model=>model.Budget)
</dt>

<dd>
@Html.DisplayFor(model=>model.Budget)
</dd>

<dt>
@Html.DisplayNameFor(model=>model.StartDate)
</dt>

<dd>
@Html.DisplayFor(model=>model.StartDate)
</dd>

</dl>

@using(Html.BeginForm()){
@Html.AntiForgeryToken()
@Html.HiddenFor(model=>model.DepartmentID)
@Html.HiddenFor(model=>model.RowVersion)

<divclass="form-actionsno-color">
<inputtype="submit"value="Delete"class="btnbtn-default"/>|
@Html.ActionLink("BacktoList","Index")
</div>
}
</div>

Thiscodeaddsanerrormessagebetweenthe

h2

and

h3

headings:

<pclass="error">@ViewBag.ConcurrencyErrorMessage</p>

Itreplaces

LastName

with

FullName

in
the

Administrator

field:

<dt>
Administrator
</dt>
<dd>
@Html.DisplayFor(model=>model.Administrator.FullName)
</dd>

Finally,itaddshiddenfieldsforthe

DepartmentID

and

RowVersion

properties
afterthe

Html.BeginForm

statement:

@Html.HiddenFor(model=>model.DepartmentID)
@Html.HiddenFor(model=>model.RowVersion)

RuntheDepartmentsIndexpage.RightclicktheDeletehyperlinkfortheEnglishdepartmentandselectOpen
innewtab,theninthefirsttabclicktheEdithyperlinkfortheEnglishdepartment.

Inthefirstwindow,changeoneofthevalues,andclickSave:

TheIndexpageconfirmsthechange.

Inthesecondtab,clickDelete.

Youseetheconcurrencyerrormessage,andtheDepartmentvaluesarerefreshedwithwhat'scurrentlyinthedatabase.

IfyouclickDeleteagain,you'reredirectedtotheIndexpage,whichshowsthatthedepartmenthasbeendeleted.

Summary

Thiscompletestheintroductiontohandlingconcurrencyconflicts.Forinformationaboutotherwaystohandlevariousconcurrencyscenarios,seeOptimistic
ConcurrencyPatternsandWorking
withPropertyValuesonMSDN.Thenexttutorialshowshowtoimplementtable-per-hierarchyinheritanceforthe

Instructor

and

Student

entities.

LinkstootherEntityFrameworkresourcescanbefoundintheASP.NET
DataAccess-RecommendedResources.

内容来自用户分享和网络整理，不保证内容的准确性，如有侵权内容，可联系管理员处理

标签：

相关文章推荐

新的分享

章节导航