Ode 2.0 User`s Manual - Department of Computer Science
Contents
1. eos formatrc file The server reads the file eos serverrc when starting up Clients read the initialization file eos clientrc before starting The following commands are provided for creating the initialization files e make_server_init e make_client_init e make_format_init These commands do not take any arguments The server and format initialization commands must be run on the host machine that will run the server The client initialization command should be run by each user who intends to run a client After these initialization files have been created they can be edited to change the information stored in them If the server is running on a machine different from the one specified as the value of the variable EOS_SERVER_HOST_NAME in file eos clientrc say as specified during initialization then the value of EOS_SERVER_HOST_NAME must be changed appropriately The log records generated by a transaction files with the prefix priv_ are stored in the directory specified by the variable EOS_PRIVATE_LOG_DIR in the file eos clientrce By default this directory is assumed to be t mp An alternative directory can be specifed changing this variable appropriately 6 2 Creating Database Areas Database areas are created formatted with the command odeareaformat UNIX is a registered trademark of USL odeareaformat 1l database_area o The area must be explicitly specif2. 3 The SunOS must be configured to support shared memory semaphores and message queues 4 Installing Ode The Ode system is organized as follows ode __ o 2 0 README 2 0 Makefil install 2 0 bin demo docs lib cmd front man include src lib tests 4 1 Installing Binaries Create a directory and unbundle the tar file tape in that directory tar xvf device or file If the file ends in Z then before unbundling with tar uncompress it as uncompress compressed file After unbundling the tar file read the file README 2 0 make the changes specified in the file install 2 0 and type install 2 0 4 2 Installing Source and Compiling Ode Create a directory and unbundle the tar file tape in that directory tar xvf device or file If the file ends in Z then before unbundling with t ar uncompress it as uncompress compressed file After unbundling the tar file read the file README 2 0 make the changes specified and recompile as specified 5 Environment Variables So as to invoke the client and server initialization commands server startup commands and the O compiler without explicitly specifying the full path add the following to your PATH variable ode directory path o 2 0 bin Also add ode directory path o 2 0 man to your MANPATH variable Ode databases reside in database areas which may
3. be UNIX files or raw disk partitions A database area may contain one or more databases Within programs applications databases may be specified by providing a full path name to the database area followed by the database name alternatively a database may be created or assumed to exist in an optional default area To use a default area the environment variable ODE_DEFAULT_AREA must be set to the path to the default area Suppose for example that databases are to reside in the database area DATABASES under your home directory To do this perform the following steps 1 Create area DATABASES under your home directory using the command odeareaformat 2 Next if you use csh or tcsh shells place the following instruction in your cshrc file setenv ODE_DEFAULT_AREA DATABASES If you use sh or ksh shells place the following instructions in your profile file ODE_DEFAULT_AREA DATABASES export ODE_DEFAULT_AREA 6 Database Area Server and Client Initialization 6 1 Initialization Configuration Files Ode uses three initialization files for storing parameters used for the server clients and for formatting areas Initialization files are located in the directory eos created automatically by the initialization commands located in the user s home directory Databases are located in database areas which are created formatted using the information stored in the file
4. facilities at the beginning and end of a transaction 9 2 Scanning a Type Extent A type extent refers to the collection of all objects of the same type in the database Users can examine the objects in a type extent class types only no primitive types or arrays by using the O for statement Some applications such as a database browser may want low level control for examining the objects in the database For example such an application may want to look at the first object in the database examine the next object or examine the previous object A cluster is a physical concept that refers to where groups of objects are stored By default all objects of a type are stored in the cluster named _cluster_t For example all employee are stored in the cluster_cluster_employee Class scan contains facilities for explicitly scanning the objects in a cluster these facilities are automatically made available to an application as a result of including the header file ode h RR RIK IK KR AR IR AR ARR IR AR ARR IRA IRA I AA I I A Ik Facilitates cluster and page oriented forward and backward scanning of objects in a given cluster The state of a scan is recorded by a cursor The cursor points to the current object in the cluster being scanned Scans are independent from each other even if they refer to the same cluster so that nested loops on the same cluster can be supported The inpage parameter of th
5. A 1992 An Efficient Database Storage Structure for Large Dynamic Objects IEEE Data Engineering Conference Phoenix Arizona February 1992 Biliris A 1992 The performance of Three Database Storage Structures for Managing Large Objects ACM SIGMOD Conference San Diego California June 1992 Biliris A and E Panagos 1993 EOS User s Guide AT amp T Bell Laboratories Murray Hill NJ 07974 Dar S N Gehani and H V Jagadish 1992 CQL a SQL for a C based Object Oriented Database Proc of Int l Conf on Extending Database Technology Vienna Austria Biliris A S Dar and N Gehani Making C Objects Persistent Hidden Pointers To be published in Software Practice amp Experience Gehani N H V Jagadish and W D Roome 1993 OdeFS A File System Interface to an Object Oriented Database AT amp T Bell Laboratories Murray Hill NJ 07974 20 Ode 2 0 User s Manual A Biliris N Gehani D Lieuwen E Panagos T Roycraft AT amp T Bell Laboratories Murray Hill New Jersey 07974 ABSTRACT Ode is a database system and environment based on the object paradigm It offers one integrated data model for both database and general purpose manipulation The Ode 2 0 database is defined queried and manipulated using the database programming language O an extension of C O provides facilities for creating persistent objects defining sets and queries This document describes how to install Ode ho
6. Ode 2 0 User s Manual A Biliris N Gehani D Lieuwen E Panagos T Roycraft AT amp T Bell Laboratories Murray Hill New Jersey 07974 1 Introduction Ode is a database system and environment based on the object paradigm The primary interface for the Ode database is the database programming language O which is based on C A few facilities have been added to C to make it suitable for database applications O provides facilities for creating persistent objects which are stored in the database and for querying the database Future releases of Ode will support sets associating constraints and triggers with classes large objects versioned objects and concurrency control The Ode database is based on a client server architecture Each application runs as a client of the Ode database The storage manager used by Ode is EOS 2 Ode Release 2 0 Ode 2 0 is available to friendly users Ode 2 0 consists of the O compiler and the object manager manager library Database applications are written in O The O compiler produces C code which is then compiled with the C compiler Ode Object Manager Library Other Libraries O O C C object executable gt c __ _ Linker _ compiler compiler code code 3 Requirements for using Ode 1 A C compiler that conforms to C 2 1 or higher 2 Sun Sparcstation or a Sun with a Sparc processor running SunOS 4 1 x
7. R W position after this operation R W position n ERR putf FILE fp int b 0 get b n bytes of lo where n is all bytes on the right of b and write them in fp starting from the current R W position after this operation R W position n int size size of this int thresh int t set threshold to t returns the current threshold int level level of the tree pointing to segments int npages number of pages the object is stored ERR pack store the object in as few and large segms as possible ERR flat make the directory flat level 1 if it is not ERR cat const large amp lo int b int n int hint append to this the b n bytes of lo di For examples illustrating the use of class large see the example programs in files get c and store c in the directory ode_directory o 2 0 src demo 8 15 Versions Object versioning in Ode is orthogonal to type that is versioning is an object property and not a type property Versions of an object can be created without requiring any change in the corresponding object type definition all objects can be versioned and different objects of the same type can have a different number of versions O tracks both temporal and derived from relationships Suppose p v1 v2 and v3 are declared as persistent employ Ap saln ANZ AMS Assume that p refers to an employee object Then a new version of this object can be created and its
8. RK RIK I RR A KR IRA IR AR A AIA AR IA IR IRA IR I I A Ok A byte range is defined as a b n pair where b is the start byte and n the number of bytes of the byte range i e byte range b n includes the region of bytes from b up to and including byte btn 1 K E RR A E A A Re e A A e Re k e eee k A A k e A k k A k k k K k A k k k k k k k k k k k ee k k k k k k k k k k k class large private _handle public large large void ERR create int n char buf NULL int hint 0 const _cluster c NULL ERR open const persistent void p open this object for manipulation as large object close close the object R R read char p int b int n b can be eliminated if cursor R write char p int b int n R append char p int n int hint 0 R insert char p int b int n R remove int b int n R truncate int b R R read char buf int b int n read b n at this specific buf A Hf E A a a a a DA DB UUD H getf FILE fp int n get n bytes from fp starting from the current R W position and append them to possibly 0 length lo after this operation R W position n ERR getf FILE fp get all bytes from fp starting from the current R W position and append them to possibly 0 length lo after this operation R W position is at the end of fp ERR putf FILE fp int b int n get b n bytes from lo and write them in fp starting from the current
9. a pointer to a dynamically allocated array i e of type char then it would not have a valid value across transactions 8 9 Queries Objects of class types only can be accessed by using the associative for loop as illustrated for pe in employee cout lt lt Name lt lt pe gt name lt lt Age lt pe gt age lt lt endl This for loop will iterate over all the employees in the database i e over the employee type extent After the completion of the loop pe will have the value NULL If we are only interested in subset of objects then we can use the suchthat clause to restrict the set of objects examined For example we can look at only employees older than 25 years as follows for pe in employee suchthat pe gt age gt 25 cout lt lt Name lt lt pe gt name lt lt Age lt pe gt age lt lt endl We can access a specific employee object as follows for pe in employee suchthat strcmp pe gt name O Shmueli 0 cout lt lt Name lt lt pe gt name lt lt Age lt pe gt age lt lt endl Joins can be performed using nested for loops or a loop with multiple loop variables For example for pe in employee pd in department 8 10 Named Persistent Objects Persistent objects can be named The names enable fast access of these objects for such objects it is not necessary to use the general for query statement to ac
10. allocate and access persistent arrays in a manner analogous to C A persistent array is allocated dynamically by specifying it s size As in C a default constructor must exist for the type and it will be used to initialize each object in the array persistent employ pe int size p eom w employ size Individual objects are accessed using the subscript operator pe 3 age 40 8 12 Pointer Arithmetic Pointer arithmetic can be performed with pointers to persistent objects much like with ordinary pointers 8 13 Casting Persistent Pointers to volatile Pointers Persistent pointers can be cast to volatile pointers The volatile pointer refers to the memory address of the persistent object Ode will bring the object from disk to the buffer pool if the object is not already in memory 8 14 Large Objects An object larger than a page is classified as large The current page size is 4K bytes 8 14 1 Header File large h Large objects are by default handled transparently However applications may find it more efficient to manipulate large objects by explicitly accessing portions of such objects Class Large provides functions for efficiently manipulating large objects Each O source file that uses special large object interface provided by the class large must include the file large h A truncated version of class Large which is defined in the O header file Large h is shown below
11. aphores and message queues Otherwise the server will not be able to start The system should be configured reboot required to support shared memory semaphores and message queues Currently a single database can be at most 2 1 gigabytes in size Persistent pointers cannot be assigned to volatile pointers without explicit casting Persistent objects cannot be passed by reference as illustrated below 18 persistent class employee class employee void f employee a int main employee e persistent employ pe trans f e allowed f pe not allowed If passing persistent objects was legal then parameter a would refer either to objects on disk or in memory depending upon what was passed Incidentally the current error message says that you cannot assign a pointer to a persistent object to an ordinary pointer reflecting the implementation of passing by reference in C 5 Persistent pointers cannot be members of union O translates a persistent pointer to a class with constructors object and C does not allow such members 15 7 O Keywords by constraint deactivate dual forall in is hypothetical pdelete perpetual persistent pthis pnew readonly suchthat tabort trans trigger vlatest newvers vdprev vtprev 16 More Documentation The Ode Object Database amp Environment is a collection of reports describing the Ode database its design and other related topics If you w
12. ause the generated C code invokes a constructor to perform the initialization constructor calls for default parameter values are not currently implemented in C pdelete will call the destructor if any of the object type as determined from its argument a persistent pointer If the pointer is of a base class and the object of a derived class then the destructor that will be invoked will be of the base class and not the derived class Thus virtual destructors are not of help when using pdelete If the use of virtual destructors is essential then pdelete should be packaged within a virtual functions to and these functions used to delete objects Not Implemented Sets forall loops Triggers Constraints Subclusters 6 15 4 1 15 5 1 15 6 7 Dual Pointers Syntax amp Semantic Modification A continue inside a for loop with multiple iterators gets the next iteration combination of values Static members the value of a static data member of a class is the same for all persistent instances created on the same invocation of the program The is operator is now gt The syntax is now persistent pointer gt object class Operator gt returns true if persistent pointer refers to an object of type object class Beware It is necessary to open a database to scan objects and create new objects However an object can be accessed or deleted assuming its oid is known without opening a database Users sho
13. cess them For example assume that db represents an open database The persistent employee referenced by pe whose name field has the value O Shmueli can be named oded by using the function set_name as follows db gt set_name oded pe db gt set_name returns zero if it is successful and a non zero value in case of an error In another transaction this object can be retrieved by using get_obj as follows pe persistent employ db gt get_obj oded get_obj returns zero the null persistent pointer in case of an error It is the user s responsibility to insure that the type of named object is the one to which it is cast The above object could also have been retrieved by using the for loop as for pe in employee suchthat strcmp pe gt name O Shmueli 0 but this may be slower The member function get__name of class database is used to retrieve the name of an object char name p if p db gt get_name pe name new char strlen p 1 strcpy name p If no name is associated with an object then get_name returns NULL 10 The relationship between names and objects is one to one i e an object may have at most one name and a name may correspond to at most one object The persistent object naming mechanism is intended for providing fast access to objects that have been stored in the database under a well known name 8 11 Persistent Arrays O allows the user to
14. d from outside a transaction body All other operations must be invoked from within the transaction body 8 3 1 open The static function open opens the database identified by name If the specified database is not found and the argument corresponding to the parameter create is specified to be non zero then the database is created with protection mode mode Specifying the argument corresponding to the parameter rdon1y to be ensures that the database is opened in read only mode Currently attempts to update the database will not be flagged as an error On successful completion it returns the pointer to the database object On failure open returns NULL 8 3 2 close Member function close is used to close a database previously opened by a calling database open close returns zero on success non zero on failure 8 3 3 remove Member function remove removes deletes a database previously opened by a calling database open remove returns zero on success non zero on failure 8 3 4 name Member function name returns the full path name of the specified database 8 3 5 valid Member function valid returns a true or false depending upon whether or not its argument refers to a persistent object In other words valid returns false if the persistent pointer is a dangling pointer and false otherwise 8 4 Transactions All code interacting with the database except database opening and closing or manipulating objects must be within a tra
15. d with the ki11 command 14 Protection Against Media Failure Ode does not currently provide automated facilities to protect against media failure User can protect themselves against media failure by explicitly making a backup copy of a database area which may contain one or more databases as follows 1 Make sure that no active transaction exists in the system 2 Checkpoint the database by giving the server the command checkpoint Shutdown the server with the command shutdown 3 Copy the database area to another disk or to any other place A database area is restored from a backup copy as follows 1 Remove the global log and checkpoint files their path names are specified in the initialization file serverrc the values of the variables EOS_CHECKPOINT_NAME and EOS_GLOBAL_ LOG_NAME wm 2 Remove all the private log records generated these are files beginning with the prefix priv_ the private log records are in the directory specified in the initialization file serverrc the value of the variable EOS_PRIVATE_LOG_DIR These records are not needed any more so they are removed to free space 3 Copy the backup copy of the database to the directory where the database was originally was before 4 Start the server 15 O Language Changes The original version of O is described in the document titled Ode Object Database and Environment The Language and the Data Model This section describes the chan
16. e scan functions control wether the scan is file oriented inpage is false or page oriented inpage is true In the latter case only objects in the current page of the cluster being scanned are returned To proceed to an other page within the cluster the inpage value must be set to false When scanning a cluster only first level ordinary objects are included in the scan FRR AR AR AR AR IR AR IR RAR AR IR AR IA IAA II k k k A class scan public scan const _cluster amp c make a scan for cluster scan const database amp db make a scan for db scan ERR first int inpage 0 ERR last int inpage 0 ERR next int inpage 0 ERR prev int inpage 0 r ERR at const persistent void amp oid seek at this object oid must be a member of the cluster being scanned persistent void current void if the cursor currently points to an object within the cluster it returns the oid of this object otherwise it returns _OID_NULL File escan c in the directory ode_directory o 2 0 src demo contains an example illustrating building a trivial browser by scanning a type extent 14 9 3 Catalog The catalog consists of elements of the predefined type metatype which is automatically made available as a result of including file ode h The types stored in the database can be examined by iterating over the metatype type extent Class metatype is defined a
17. ely to deadlock because they request only read locks and faster because no log records written Note that a read only transaction can be run only if the data being read has been previously created Otherwise the transaction will attempt to update the catalog and thats not consistent with the read only semantics Hypothetical transactions have the form hypothetical trans Hypothetical transaction allow users to pose what if scenarios as often done with spread sheets Users can change data and see the impact of these changes without changing the database Only the transaction making the changes sees the changes because they are not made visible to other users No log records are written 8 5 Note on Locks Write locks have more overhead than read locks Unfortunately the O compiler cannot always determine whether or not a persistent object is being accessed for reading only or for update In such cases the code generated assumes that the object will be updated which means that a write lock will be obtained For example the implicit coercion of apersistent char persistent string pointer toa char transient string pointer by passing it as an argument to a function expecting a char argument will cause a write lock to be obtained on the persistent string The printf function is an example of where this takes place A read lock is obtained if the coercion is toa const char as is the case when the C streams output opera
18. ges to O and implementation restrictions 15 1 Additions 1 Large objects can be manipulated in a file like fashion using the class large 2 Users can access information in the catalog which consists of persistent objects of type met at ype 3 Objects in a type extent can be scanned using the facilities provided by class scan Unlike the for loop these facilities can be used to examine objects in an application specific order scanning can be 15 3 dd 16 suspended and resumed arbitrarily Databases must be explicitly opened and closed Users can access multiple databases from within a single program but only one database can be accessed at a time Three flavors of transactions have been added 1 Update transactions are written as trans 2 Hypothetical transactions are written as hypothetical trans 3 Read only transactions are written as readonly trans Named persistent objects Objects of primitive types can now be made persistent Pointer arithmetic allowed on persistent objects Restrictions Persistent pointer types cannot be used to differentiate between signatures of overloaded functions For example int insert persistent employ pe int insert persistent item pi the code produced by O will be flagged by the C compiler as an error because each persistent pointer is translated to an object of type __pref Persistent pointer parameters cannot be given default values bec
19. id stored in v1 as follows vl newvers p 26 Ode records the fact that the object referenced by v1 was derived from the object referenced by p Function vdprev version derived from previous can be used to extract this information The call vdprev v1 will yield p and the call vdprev p will yield NULL assuming that p is not a version of another employee object Ode also records that the fact that object referenced by v1 was created after that referenced by p assuming that no other versions of this employee object were created in the meantime Function vtprev version temporally previous can be used to extract this information The call vtprev v1 will yield p and the call vtprev p will yield NULL To further illustrate the information recorded by Ode suppose that the following statements are now executed v2 newvers vl v3 newvers p The versions created above are related as follows p temporal relationship r derived from relationship Calling vdprev v2 will yield v1 vdprev v3 will yield p vtprev v2 will yield v1 and vtprev v3 will yield v2 Function vlatest version latest is used for finding the most recently derived version of a particular object For instance vlatest p will yield v3 vlatest v1 will yield v2 vlatest v2 will yield v2 and vlatest v3 will yield v3 Function vroot version root is used to find the initial object that an object is di
20. ied even though the environment variable ODE_DEFAULT_AREA may have been set The server should not be not running when odeareaformat is executed This command will overwrite an existing area with the same name if one exists See the man page for details about odeareaformat For example a database area test 1 may be created in the directory usr nhg odedb as follows odeareaformat l usr nhg odedb test1l o 6 3 Starting the Server Before client programs can be run the Ode server must be started by executing the command odeserver Only one server can be running on a machine The server is typically run in a separate window in the foreground because the server accepts commands from st din and because it prints messages The two server commands that are used most often are checkpoint and shutdown The checkpoint command causes the server to flush its buffers and write out all committed transactions to disk The shutdown command causes the server to terminate See the man page for details about odeserver and its commands 6 4 Client Programs Client programs applications are executables produced by the O compiler 7 Changes in Ode 2 0 with respect to Ode 1 1 1 Only objects of classes for which a persistent forward declaration has been given before the class definition is encountered can be made persistent For example persistent class employee class employee Previously only objects of classes that followed the incl
21. ined within a database area file or partition A default database area may be designated by an enviroment variable ODE_DEFAULT_AREA and a database db within that area as SODE_DEFAULT_AREA db If this variable is undefined followed by a followed by a as in host lt path to area database gt Only one database can be open at a given time FRR AR e E AR AA IR AR ARR I AR ARR IR AR k k k k IR k k k k k k II k k k I class database public static database open open or create a database const char name the name of the db int rdonly 0 if true read only else r w int create 1 if true and no such db exists create db int trunc 0 if true clear the db 3 it returns NULL on failure ERR close void close this database ERR remove void remove this database char name void the full path name of this database static database amp of const persistent void p db of ob pointed by p int valid const persistent void p valid pointer functions for the named object directory char get_name persistent void id name of obj or NULL persistent void amp get_obj char name null if no such name ERR set_name char name persistent void id int overwrite 0 give name to this object ERR remove_name persistent void id remove name of this obj Only the following database operations e open e close and e remove can be invoke
22. nsaction block Ode uses 2 version 2 phase 2V2P locking by 2 phase locking will be available as a compile time option which simultaneously allows multiple transactions to read an object and one transaction to write an object Currently there are three flavors of transactions update read only and hypothetical Update transactions have the form trans Here is a code fragment illustrating the opening and closing of a database named alex include lt ode h gt main database db if db database open alex NULL error cannot open database alex trans db gt close The database must be closed and opened outside the transaction body Read only transactions have the form readonly trans Persistent objects should not be updated in read only transactions Programs containing read only transactions should be compiled with the ODE_READONLY constant defined using the D option when invoking the O compiler e g OO D ODE_READONLY All source files involving read only transactions should be compiled with the above option even though some or all of these files may contain update transactions The update transactions will incur an extra check for each persistent object dereference If a persistent object is updated within a read only transaction then the semantics are undefined in any case the database will not be updated Read only transactions are less lik
23. nstructors for the type whose objects are or will be made persistent and 2 persistent forward declarations also be given for all classes involved in the definition of a persistent class One exception to this is a class used to declare volatile pointers within a persistent class 8 8 Creation and Deletion of Persistent Objects Persistent objects are created and deleted using the operators pnew and pdelete Except for the fact that these operators allocate objects in the database their semantics are similar to those of the operators new and delete Operators pnew and pdelete cannot be overloaded Operator pnew returns a pointer to persistent object Such pointers are referred to as persistent pointers object ids Persistent pointers are declared like ordinary pointers except that the type qualifier persistent is used Note that the value O and the constant NULL are automatically coerced to null persistent pointers Persistent objects are referenced using persistent pointers much like heap objects are referenced using volatile pointers Here is some example code illustrating the creation and manipulation of persistent objects persistent class employee class employee public char name MAX int age persistent employ pe pe pnew employee pe gt age 40 Volatile pointers embedded within persistent objects will not have legitimate values across transactions For example had name been implemented as
24. ould like a copy please send mail to ode allegra att com along with your mailing address 17 Help and Bug Reports To get help with installation or using O and to report potential bugs send mail to ode allegra att com When reporting potential bugs or problems please send a complete source file causing the potential bug or problem It is important that this file be complete and as small as possible with extraneous stuff deleted 18 Additional Interfaces to be Provided in Future Releases Besides O several other Ode interfaces are under development These interfaces which will be available in later releases of Ode are 1 OdeView A graphical X window based interface to Ode 2 CQL An SQL like interface to Ode 19 10 19 OdeFS A UNIX system file like interface to Ode Ode Publications Partial List Agrawal R and N Gehani 1989 Ode Object Database amp Environment SJGMOD Portland Oregon Agrawal R and N Gehani 1989 Rationale for the Design of Persistence and Query Processing Facilities in the Database Programming Language O Expanded Version with R Agrawal 2nd International Workshop on Database Programming Languages Portland Oregon Agrawal R S Buroff N Gehani and D Shasha Object Versioning in Ode 199 IEEE Data Engineering Conference Kobe Japan Agrawal R N Gehani and J Srinivasan 1990 OdeView The Graphical Interface to Ode SIGMOD Atlantic City 1990 Biliris
25. rectly or indirectly derived from the farthest back one can go by applying vdprev to a version repeatedly before hitting NULL For our example vroot p vroot v1 vroot v2 and vroot v3 all evaluate to p All the objects derived from some initial object are in some sense related The most recently created version in this relation can be computed given any version v using vlatest vroot v Given this latest version all related objects can be found by following the vt prev links back to the root For examples illustrating the use of class large see the example programs in files vers c versEmp c and listvers c in the directory ode_directory o 2 0 src demo 9 Specialized Facilities for Advanced Users The specialized facilities are automatically made available to users by including the header file ode h 9 1 Flexible Transaction Begin and End Instead of using transaction blocks users can begin a transaction by calling _transaction begin commit a transaction by calling 13 _transaction commit and abort a transaction by calling _transaction abort Using these member functions means that O will not be able to detect nested transaction calls which are not supported jumping in and out of transaction code and other potential errors Also some facilities such as those for versioning will not be automatically available In case of trans blocks the O compiler generates code to initialize and close versioning
26. s metatype describes all types in a database class metatype public char name MAX_TYPE_NAME type name unsigned int size size of instance of this int basecnt how many base classes _PERS_PTR base_spec base_list list of base classes class base_spec persistent class descriptor public Access access one of private public protected boolean is_virtual is it a virtual function _PERS_PTR metatype baseclass persistent descriptor of base class base_spec base_spec Access a boolean iv _PERS_PTR metatype p access a is_virtual iv baseclass p Note that the definition of class metatype is likely to change File catalog c in the directory ode_directory o 2 0 src demo contains an example illustrating how information in the catalog can be accessed 10 Miscellaneous Facilities The O compiler OO defines the constant __oplusp1lus to allow users to determine whether or not the program is being compiled with the O compiler 11 Examples For examples of O programs see the directory ode directory path o 2 0 bin src demo 12 Deadlocks Deadlocks can result when multiple transactions access the same objects concurrently In such cases the Ode server will abort one or more of transactions involved in the deadlock to break the deadlock Currently abortion of a transaction by the Ode server results in the termination of the client process containing the tran
27. saction 13 Recovery The Ode server EOS creates log files in the file specified in the initialization file ecos serverrc 13 1 System Failure After the occurrence of a system failure restarting the Ode server will automatically return the database to the last consistent state prior to failure as recorded in the log The log is scanned and updates made by the committed transactions are redone in exactly the same order as they were originally performed If a system failure occurs during the restart the subsequent restart performs the same work again in an idempotent fashion 15 13 2 Restarting the Server If the server crashes then the following things might happen Shared memory segments remain in the system Semaphores remain in the system Message queues remain in the system The disk daemon process eos_diskd was not notified and is therefore still running 5 The log process eosserver same name as the server was not killed Prone Presence of shared memory segments semaphores and message queues which are inter process communication IPC facilities can be detected by using the UNIX command ipcs If any IPC facilities used by the crashed server process have not been freed then these should be removed freed by using the UNIX command ipcrm before restarting the server If the disk daemon process eos_diskd and the log process eosserver are around check by executing the command ps aux then they should be kille
28. tor lt lt is used to print a string since it expects a const char 8 6 Transaction Abortion Transactions can be explicitly aborted by using the t abort statement which has the form tabort In such a case control simply flows to the statement if any following the transaction block The Ode system may also abort transactions to break deadlocks In this case the client application is aborted 8 7 Persistent Objects Objects of any class type and of the primitive types char short int long float and double can be made persistent Definitions of classes whose objects are to be made persistent must be preceded by persistent forward declarations which have the form persistent class class name For example persistent class employee class employee If a class is to be used both in a C program and in an O program then the preferred way of declaring a class as persistent is to give a persistent forward declaration But if a class is to be only used in an O program then instead of giving a forward declaration first and then giving its complete declaration the class can alternatively be declared to be persistent directly persistent class employee Specifying a class C as persistent is not necessary in a file that contains code which does not manipulate or otherwise reference persistent objects of type C It is important that 1 a persistent forward declaration be included in all files that contain co
29. trying to create access or otherwise manipulate database objects must first open or create a database After manipulating objects in the database it must be closed Another one can then be opened When a simple name with no embedded is used O creates the databases in the database area specified by the environment variable ODE_DEFAULT_AREA Thus ODE_DEFAULT_AREA must be set to the name of a database area in which the databases to be created CI A database name can also be given as a full path name i e starting with full pathname of database area database 8 1 O Compiler The O compiler is called OO It is used in a manner similar to the C compiler which is called CC See the man page for details about OO 8 2 Header File ode h Each O source file that uses O database facilities must include the file ode h If the O source file is just a C file i e no O facilities are used then this header file need not be included 8 3 Class database Class database which is automatically made available by including the header file ode h provides functions for manipulating closing opening etc the database and naming persistent objects A truncated version of class database which is automatically made available to an application as a result of including the header file ode h is shown below RK RIK KA KR KK IR IR AR ARIA ARIA IRA II I I OK A database is conta
30. ude directive include lt ode h gt could be made persistent The above use of the include directive to specify classes whose objects could be made persistent was abandoned because this approach was error prone and inappropriate from software engineering considerations Note that the header file must still be included prior to using any of the O facilities 2 The database must be created opened closed or removed from outside the transaction body 3 The large object class interface header file 10 h has been renamed to large h 4 Object versioning is now supported 5 Ode now supports full fledged transactions Three flavors of transactions are supported update read only and hypothetical transactions 6 Client server architecture is supported 7 2 version 2 phase locking is used 8 O Database Facilities We now give a brief description of O and how O programs are written We will only describe the facilities that O adds to C The original version of O is described in the document titled Ode Object Database and Environment The Language and the Data Model Changes to O and current O restrictions are described later We assume that the reader is well versed in C Stroustrup 1991 An O database is identified by the name of the file in which the database resides Multiple databases can be accessed from an O program but only one database can be accessed at any given time An O program
31. uld be careful when writing applications that manipulate multiple databases A database can be opened in read only mode currently attempts to update the database will not be flagged as an error As in the case of volatile objects the special variable this is a volatile pointer for persistent objects It is not a pointer to the persistent object instead it points to the in memory copy of the persistent object However within a member function the special variable pt his can be used much like the special variable this Provided the member function has been called in conjunction with a persistent object pthis will yield a persistent pointer of the right type Otherwise it will yield the null value pthis may not work correctly if this points to a base class object in the presence of multiple inheritance static members of a class are shared between class objects Such components are not made persistent because C does not implement them as belonging to the object Members of persistent objects that are pointers to volatile objects will not in general have meaningful values across transactions Applications accessing the catalog are likely to be affected by future releases of Ode since the definition of class met at ype is likely to change Modifying the schema class definitions corresponding to existing objects will lead to unspecified behavior such as core dumps Notes The SunOS must be configured to support shared memory sem
32. w to start the storage manager a brief summary of O using O changes to O 4 and 6 other items of interest eRe D It also contains a list of all the Ode publications
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