Using Caché Objects - InterSystems Documentation

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Object Concurrency Setting 1 2 3 4 Meaning Atomic Shared Shared / Retained Exclusive Description When an object is loaded, the %LoadData method creates a shared lock for the object, provided that the object occupies more than one storage node in the database. No locks are held for objects stored in a single node. %LoadData releases the lock after it finishes loading the object. When an object is initially saved to the database (inserted), %SaveData holds an exclusive lock during the save, provided that the object occupies more than one storage node in the database. %SaveData holds no locks for objects stored in a single node. When a previously saved object is saved to the database (updated), %SaveData holds an exclusive lock during the save. When an object is loaded, the %LoadData method creates a shared lock for the object. %LoadData releases the lock after it finishes loading the object.When an object is initially saved to the database (inserted), %SaveData holds an exclusive lock during the save, provided that the object occupies more than one storage node in the database. %SaveData holds no locks for objects stored in a single node.When a previously saved object is saved to the database (updated), %SaveData holds an exclusive lock during the course of the save. A shared lock is acquired either when an existing object is opened or when a new object is initially saved to the database (inserted); the lock is released when the object is destructed. Additionally, when an object is initially saved to the database, %SaveData holds an exclusive lock during the save, provided the object occupies more than one storage node in the database. An exclusive lock is acquired either when an existing object is opened or when a new object is initially saved to the database (inserted); the lock is released when the object is destructed. It is important to set concurrency options at an appropriate level for your application. Consider the following scenario: 1. Process A opens an object. 2. Process B deletes that object from disk. 202 Using Caché Objects
Version Checking 3. Process A saves the object using %Save and receives a success status. Examining the data on disk shows that the object was not written to disk. This is an example of concurrent operations that occurred without adequate concurrency control. In this example, Process B should not have been allowed to delete the object if Process A really intended to write the object back to disk. Since Process A did not modify the object, no physical write occurred and unexpected results were observed. Had Process A modified the object, then %Save would have succeeded as before but the object would have been written to disk. These inconsistent results are just one example of allowing concurrent access without adequate controls in place at the application level. (The inconsistent behavior exhibited by a successful call to %Save and the object not appearing on disk is not a bug in %Save. It is a concurrency failure which, by definition, will leave the database in an inconsistent state.) The solution to this problem is to have Process A open the object with concurrency 3 or 4; in this case, Process B would then either have been denied access (failed with a concurrency violation) or would have had to wait until Process A released the object. A.2 Version Checking Caché implements version checking using a class parameter called VERSIONPROPERTY. All persistent classes have this parameter. When defining a persistent class, the procedure for enabling version checking is: 1. Create a property of type %Integer that holds the updatable version number for each instance of the class. 2. Set the value of the property's InitialExpression keyword to 0. 3. Set the value of VERSIONPROPERTY to the name of that property. The value of VERSIONPROPERTY cannot be changed to a different property by a subclass. This incorporates version checking into updates to instances of the class. When version checking is implemented, the property specified by VERSIONPROPERTY is automatically incremented each time an instance of the class is updated (either by objects or SQL). Prior to incrementing the property, Caché compares its in-memory value to its stored value. If they are different, then a concurrency conflict is indicated and an error is returned; if they are the same, then the property is incremented and saved. Using Caché Objects 203
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Using Caché Objects Version 5.2 01
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Table of Contents 1 Introduction to
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7 Properties ......................
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13.1 Inheritance and SQL ..........
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20.8 %OnNew .......................
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List of Figures Embeddable Objects
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1 Introduction to Caché Objects Th
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1.2 Class Definitions and the Class
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Development Tools Caché Class Libr
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• The Caché EJB Binding—a vers
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2 Object-Oriented Database Developm
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Encapsulation Caché exploits inher
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3 The Caché Object Model The Cach
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Referring to an Object — OREF, OI
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Types of Classes 3.2.1 Transient Ob
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Types of Classes Embeddable Objects
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A class inherits all of the specifi
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2. It resolves inheritance—it det
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Caché Classes • Class Parameters
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Caché Classes ClientDataType Descr
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Caché Classes Internally, this wor
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Packages Class Packages Any of thes
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Packages Letting the compiler resol
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Packages • “%Library” —any
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Methods where count and state are d
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Methods 6.4 Method Language You hav
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Methods 6.6 Instance and Class Meth
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Methods Write dog.Speak() Which cou
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Properties 7.1 Property Keywords Yo
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Properties Property Behavior The pr
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Properties 7.5 Attribute Properties
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Properties You can manipulate a col
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8 Class Queries Class queries provi
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Query Basics Final Specifies that s
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Caché ObjectScript. These queries
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The data in Row is the result of ex
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9 Indices Indexes provide a mechani
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Index Collation 9.2 Index Collation
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10 Using Objects with Caché Object
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Executing Methods 10.1.3 Executing
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Opening Objects 10.3 Opening Object
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where oref is the OREF of the speci
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For example, to populate the Home p
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change the second vaccination recor
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There are two types of array proper
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Saving Objects 10.5 Saving Objects
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mation. Again, this method deletes
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Set rset = ##class(%ResultSet).%New
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Executing Queries // Create a Resul
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Data Types 11.1 Available Types Cac
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Data Types • INDEXSUBSCRIPTS —
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Data Types CLIENTDATATYPE Values Va
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Data Types SQLCATEGORY Values Value
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Data Types The property is restrict
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Object Persistence 1. Its primary (
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Object Persistence Note: In a multi
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Object Persistence // Open person "
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Object Persistence For example: //
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Object Persistence If a persistent
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Object Persistence The storage clas
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13 Objects and SQL A key aspect of
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The Object-SQL Projection 13.2 The
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The Object-SQL Projection 13.2.2.1
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The Object-SQL Projection • The p
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If the list for a particular instan
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Triggers are, however, fired by the
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14 Relationships A relationship is
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Relationship Basics the relationshi
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An Invoice class class MyApp.Invoic
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14.4 Persistent Behavior of Relatio
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Streams Stream data may be stored i
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Streams 15.2.2 Copying Data between
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Streams Set stream=##class(%Stream.
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16 Class Projections Class projecti
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RemoveProjection The RemoveProjecti
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17 Object Synchronization Object sy
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About Updates • If two changes re
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About Updates Two Synchronized Data
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Page 167 and 168: 18 Method Generators A powerful fea
Page 169 and 170: Method Generator Context 6. It copi
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Page 174 and 175: The Caché Data Population Utility
Page 182 and 183: Using Callback Methods Since differ
Page 184 and 185: Using Callback Methods ReturnValue
Page 186 and 187: Using Callback Methods 20.6 %OnDele
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Page 191 and 192: 21 Object-Specific ObjectScript Fea
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Page 200 and 201: Dynamic Dispatch 22.2 Content of Me
Page 202 and 203: Dynamic Dispatch %DispatchMethod ca
Page 205 and 206: 23 Class Definition Classes The Cac
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Page 212 and 213: Internet Classes For more informati
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