2 Hello World Example
In this chapter we will examine how to create a simple C++ application that relies on ODB for object persistence using the traditional "Hello World" example. In particular, we will discuss how to declare persistent classes, generate database support code, as well as compile and run our application. We will also learn how to make objects persistent as well as query, update and delete persistent objects.
The code presented in this chapter is based on the
hello
example which can be found in the
odb-examples
package of the ODB distribution.
2.1 Declaring a Persistent Class
In our "Hello World" example we will depart slighly from
the norm and say hello to people instead of the world. People
in our application will be represented as objects of C++ class
person
which is saved in person.hxx
:
// person.hxx // #include <string> class person { public: person (const std::string& first, const std::string& last, unsigned short age); const std::string& first () const; const std::string& last () const; unsigned short age () const; void age (unsigned short); private: std::string first_; std::string last_; unsigned short age_; };
In order not to miss anyone whom we need to greet, we would like
to save the person objects in a database. To achive this we declare
the person
class as persistent:
// person.hxx // #include <string> #include <odb/code.hxx> // (1) #pragma db object // (2) class person { ... private: person () {} // (3) friend class odb::access; // (4) #pragma db id auto // (5) unsigned long id_; // (5) std::string first_; std::string last_; unsigned short age_; };
To be able to save person objects in the database we had to make
five changes, marked with (1) to (5), to the orignal class
definition. The first change is the inclusion of the ODB
headers core.hxx
. This headers provides a number
of core ODB declarations, such as odb::access
, that
are used to define peristent classes.
The second change is the addition of db object
pragma just before the class definition. This pragma tells the
ODB compiler that the class that follows is persistent. Note
that making a class persistent does not mean that all objects
of this class will automatiacally be stored in the database.
You would still create ordinary or transient instances
of this class just as you would before. The difference is that
now you can make such transient instances persistent, as we will
see shortly.
The third change is the addition of the default constructor.
The ODB-generated database support code will use this constructor
when instantiating an object from the persistent state. As we have
done for the person
class, you can make the default
constructor private or protected if you don't want to make it
available to the ordinary users of your class.
With the fourth change we make the odb::access
class
friend of our person
class. This is necessary to make
the default constructor and the data members accessible to the
ODB support code. If your class has public default constructor and
public data members, then the friend
declaration is
unnecessary.
The final change adds a data member called id_
which
is preceded by another pragma. In ODB every persistent object must
have a unique, within its class, identifier. Or, in other words,
no two persistent instances of the same type have equal
identifiers. For our class we use an integer id. The
db id auto
pragma that preceeds the id_
member tells the ODB compiler that the following member is the
object's id. The auto
specifier indicates that it is
a database-assigned id. A unique id will be automatically generated
by the database and assigned to the object when it is made
persistent.
In this example we choose to add an identifier because none of
the existing members could serve the same purpose. However, if
a class already has a member with suitable properties, then it
is natural to use that member for an identifier. For example,
if our person
class contained some form of personal
identification (SSN in the United States or ID/passport number
in other countries), then we could use that as an id. Or, if
we stored an email associated with each person, then we could
have used that since each person is presumed to have a unqiue
email address:
class person { ... #pragma db id std::string email_; std::string first_; std::string last_; unsigned short age_; };
Now that we have the header file with the persistent class, let's see how to generate that database support code that we talked about.
2.2 Generating Database Support Code
The persistent class definition that we created in the previous section was particularly light on code that could actualy do the job and store the person't data to a database. There was no serialization or deserialization code, not even data member registration, that you would normally have to write by hand in other ORM libraries for C++. This is because in ODB code that translates between the database and C++ representations of an object is automatically generated by the ODB compiler.
To compile the person.hxx
header we created in the
previous section and generate the support code for the MySQL
database we invoke the ODB compiler from a terminal (UNIX) or
a command prompt (Windows):
odb -d mysql --generate-query person.hxx
We will use MySQL in the reminder of this chapter though other supported database systems can be used instead.
If you haven't installed the common ODB runtime library
(libodb
) or installed it into a directory where
the C++ compiler doesn't search for headers by default,
then you may get the following error:
person.hxx:10:24: fatal error: odb/core.hxx: No such file or directory
To resolve this you will need to specify libodb
headers
location with the -I
preprocessor option, for example:
odb -I.../libodb -d mysql --generate-query person.hxx
Here .../libodb
represents the path to the
libodb
directory.
The above invocation of the ODB compiler produces three C++ files:
person-odb.hxx
, person-odb.ixx
,
person-odb.cxx
. You normally don't use types
or functions contained in these files directly. Rather, all
you have to do is include person-odb.hxx
in
C++ files where you are performing database operations
with classes from person.hxx
as well as compile
person-odb.cxx
and link the resulting object
file to your application.
You may be wondering what is the --generate-query
option for. It instructs the ODB compiler to generate
optional query support code that we will use later in our
"Hello World" example. Another option that we will find
useful is --generate-schema
. This option
makes the ODB compiler generate a fourth file,
person.sql
, which contains the database
schema for the classes defined in person.hxx
:
odb -d mysql --generate-query --generate-schema person.hxx
If you would like to see the list of all the available options, refer to the ODB Compiler Command Line Manual.
Now that we have the persistent class and the database support code, the only part that is left is the application code that does something useful with all this. But before we move on to the fun part, let first learn how to build and run an application that uses ODB. This way when we have some application code to try, there are no more delays before we can run it.
2.3 Compiling and Running
Assuming that the main()
function with some application
code is saved in driver.cxx
and the database support
code and schema are generated as described in the previous section,
to build our application we will first need to compile all the C++
source files and then link them with two ODB runtime libraries.
On UNIX, the compilation part can be done with the following commands
(for Microsoft Visual Studio setup, see the odb-examples
package):
c++ -c driver.cxx c++ -c person-odb.cxx
Similar to the ODB compilation, if you get an error stating that
a headers in odb/
or odb/mysql
directory
in not found. In this case you will need to use the -I
preprocessor option to specify the location of the common ODB runtime
library (libodb
) and MySQL ODB runtime library
(libodb-mysql
).
Once the compilation is done, we can link the application with the following command:
c++ -o driver driver.o person-odb.o -lodb-mysql -lodb
Notice that we link our application with two ODB libraries:
libodb
which is a common runtime library and
libodb-mysql
which is a MySQL runtime library
(if you use another database, then the name of this library
will change accordingly). If you get an error saying that
one of these libraries could not be found, then you will need
to use the -L
linker option to specify their locations.
Before we can run our application we need to create a database
schema using the generated person.sql
file. For MySQL
we can use the mysql
client program, for example:
mysql --user=odb_test --database=odb_test < person.sql
The above command will login to a local MySQL server as user
odb_test
without a password and use database
named odb_test
. Note that after executing this
command all data stored in the odb_test
database
will be deleted.
Once the database schema is ready, we run our application using the same login and database name:
./driver --user odb_test --database odb_test
2.4 Making Objects Persistent
Now that we have the infrastructure work out of the way, it
is time to see our first code fragment that interracts with the
database. In this section we will learn how to make person
objects persistent:
// driver.cxx // #include <memory> // std::auto_ptr #include <iostream> #include <odb/database.hxx> #include <odb/transaction.hxx> #include <odb/mysql/database.hxx> #include "person.hxx" #include "person-odb.hxx" using namespace std; using namespace odb; int main (int argc, char* argv[]) { try { auto_ptr<database> db (new mysql::database (argc, argv)); unsigned long john_id, jane_id, joe_id; // Create a few persistent person objects. // { person john ("John", "Doe", 33); person jane ("Jane", "Doe", 32); person joe ("Joe", "Dirt", 30); transaction t (db->begin_transaction ()); db->persist (john); db->persist (jane); db->persist (joe); t.commit (); // Save object ids for later use. // john_id = john.id (); jane_id = jane.id (); joe_id = joe.id (); } } catch (const odb::exception& e) { cerr << e.what () << endl; return 1; } }
Let's examine this code piece by piece. At the beginnig we include
a bunch of headers. Those include odb/database.hxx
and
odb/transaction.hxx
which define database
system-independant odb::database
and
odb::transaction
interfaces. Then we include
odb/mysql/database.hxx
which defines the
MySQL implementation of the database
interface. Finaly,
we include person.hxx
and person-odb.hxx
which define our persistent person
class.
Once we are in main()
, the first thing we do is create
the MySQL database object. Notice that this is the last line in
driver.cxx
that mentions MySQL explicitly; the rest
of the code works though the common interfaces and is database
system-independant. We use the argc
/argv
mysql::database
constructor which automatically
extract the database parameters, such as login name, passowrd,
database name, etc., from the command line. In your own applications
you may prefer to use other versions of the mysql::database
constructors which allow you to pass this information directly
(@@ ref MySQL database).
Next we create three person
objects. Right now they are
transient objects, which means that if we terminate the application
at this point, they will be gone without any evidence of them ever
existed. The next line starts a database transaction. We discuss
transactions in detail later in this manual. For now all we need
to know is that all ODB database operations must be performed within
a transaction and that a transaction is an atomic unit of work; all
database operations performed within a transaction either succeed
(commited) together or are automatically undone (rolled back).
Once we are in a transaction, we call the persist()
database function on each of our person
objects.
At this point the state of each object is saved in the database.
However, note that this state is not permanent until and unless
the transaction is commited. If, for example, our application
crashes at this point, there will still be no evidence of our
objects ever existed.
In our case one more thing happens when we call persist()
on a person
object. Remember that we decided to use
database-assigned identifiers for our objects. The call to
persist()
is where this assignment happens. Once
this function returns, the id_
member contains this
object's unique identifier.
After we have persisted our objects, it is time to commit the
transaction and make the changes permanent. Only after the
commit()
function returns succefully are we
guaranteed that the objects are made persistent. Following
the crashing example, if our application terminates after
the commit for whatever reason, the objects' state in the
database will remain intact. In fact, as we will discover
shortly, our application can be restarted and load the
orignal objects from the database. Note also that a
transaction must be commited explicitly with the
commit()
call. If the transaction
object leaves scope without the transaction beeing
explicitly commited or rolled back, it will be automatically
rolled back. This behavior allows you not to worry about
exceptions being thrown within a transaction; if they
cross the transaction boundaries, the transaction will
be automatically rolled back and all the changes made
to the database undone.
After the transaction has been commited, we save the persistent
objects' ids in local variables. We will use them later in this
chapter to perform other database operations on our persistent
objects. You might have noticed that our person
class doesn't have the id()
function that we use
here. To make our code work we need to add a simple accessor
with this name that returns the value of the id_
data member.
The final bit of code in our example is the catch
block that handles the ODB exceptions. We do this by catching
the base ODB exception and printing the diagnostics. (@@ Ref
exceptions)
Let's now compile (see @@ Ref "Compiling and Running") and then run our first ODB application:
mysql --user=odb_test --database=odb_test < person.sql ./driver --user odb_test --database odb_test
Our first application doesn't print anything except for error
messages so we can't really tell whether it actually stored the
objects' state in the database. While we will extend our application
to be more enternaining, for now we can use the mysql
client to examine the database content. It will also give us a feel
for how the object are stored:
mysql --user=odb_test --database=odb_test Welcome to the MySQL monitor. mysql> select * from person; +----+-------+------+-----+ | id | first | last | age | +----+-------+------+-----+ | 1 | John | Doe | 33 | | 2 | Jane | Doe | 32 | | 3 | Joe | Dirt | 30 | +----+-------+------+-----+ 3 rows in set (0.00 sec) mysql> quit
In the next section we will examine how to query persistent objects from our application.
2.4 Querying Persistent Objects
So far our application doesn't resemble a typical "Hello World" example. It doesn't print anything except for error messages. Let's change that and teach our application to say hello to people from our database. To make it a bit more interesting, let's say hello only to people over 30:
// driver.cxx // ... int main (int argc, char* argv[]) { try { ... // Create a few persistent person objects. // { ... } typedef odb::query<person> query; typedef odb::result<person> result; // Say hello to those over 30. // { transaction t (db->begin_transaction ()); result r (db->query<person> (query::age > 30)); for (result::iterator i (r.begin ()); i != r.end (); ++i) { cout << "Hello, " << i->first () << "!" << endl; } t.commit (); } } catch (const odb::exception& e) { cerr << e.what () << endl; return 1; } }
The first half of our application is the same as before and is replaced with "..." in the above listing for brievety. Again, let's examine the rest of it piece by piece.
The two typedef
s create convenient aliases for two
template instantiations that will be used a lot in our application.
The first is the query type for the person
objects
and the second is the result type of that query.
Then we begin a new transaction and call the query()
database function. We pass a query expression
(query::age > 30
) which limits the returned objects
only to those with age greater than 30. We also save the result
of the query in a local variable.
The next few lines perform a pretty standard for-loop iteration over the result sequence printing hello for every returned person. Then we commit the transaction and we are node. Let's see what this application will print:
mysql --user=odb_test --database=odb_test < person.sql ./driver --user odb_test --database odb_test Hello, John! Hello, Jane!
That looks about right but how do we know that the query actually
used the database instead of just using some in-memory artifacts of
the earlier persist()
calls. One way to test this
would be to comment out the first transaction in our application
and re-run it without re-creating the database schema so that the
objects that were persisted during the previous run will be returned.
Alternatively, we can just re-run the same application without
re-creating the schema and notice that we now how duplicate
objects:
./driver --user odb_test --database odb_test Hello, John! Hello, Jane! Hello, John! Hello, Jane!
What happens here is that the previous run of our application
persisted a set of person
objects and when we re-run
the application, we persist another set with the same names but
with different id. When we later run the query, matches from
both sets are returned. We can change the line where we print
the "Hello" string as follows to illustrate this point:
cout << "Hello, " << i->first () << " (" << i->id () << ")!" << endl;
If we now re-run this modified program, we will get the following output:
./driver --user odb_test --database odb_test Hello, John (1)! Hello, Jane (2)! Hello, John (4)! Hello, Jane (5)! Hello, John (7)! Hello, Jane (8)!
The identifiers 3, 6, and 9 that miss from the above list belong to the "Joe Dirt" objects which are not selected by this query.
2.5 Updating Persistent Objects
While making objects persistent and then querying them are useful oprations, most applications will also need to change the object's state and then make these changes persistent. Let's illustrate this by updating Joe's age who just had a birthday:
// driver.cxx // ... int main (int argc, char* argv[]) { try { ... unsigned long john_id, jane_id, joe_id; // Create a few persistent person objects. // { ... // Save object ids for later use. // john_id = john.id (); jane_id = jane.id (); joe_id = joe.id (); } // Joe Dirt just had a birthday, so update his age. // { transaction t (db->begin_transaction ()); auto_ptr<person> joe (db->load<person> (joe_id)); joe->age (joe->age () + 1); db->store (*joe); t.commit (); } // Say hello to those over 30. // { ... } } catch (const odb::exception& e) { cerr << e.what () << endl; return 1; } }
The beginning and the end of this transaction are the same as
the previous two. Once within a transaction, we call the
load()
database function to instantiate a
person
object with Joe's persistent state. We
pass Joe's object identifer that we stored earlier when we
made this object persistent.
With the instantiated object in hand we increment the age
and call the store()
database function to update
the object's state in the database. Once the transaction is
commited, the changes are made permanent in the database.
If we now run this application, we will see Joe in the output since he is now over 30:
mysql --user=odb_test --database=odb_test < person.sql ./driver --user odb_test --database odb_test Hello, John! Hello, Jane! Hello, Joe!
What if we didn't have an identifier for Joe? Maybe this object was made persisted in another run of our application or by another application altogether. Provided that we have only one Joe Dirt in the database, we can use query to come up with an alternative implementation of the above transaction:
// Joe Dirt just had a birthday, so update his age. An // alternative implementation without using the object id. // { transaction t (db->begin_transaction ()); result r (db->query<person> (query::first == "Joe" && query::last == "Dirt")); result::iterator i (r.begin ()); if (i != r.end ()) { auto_ptr<person> joe (*i); joe->age (joe->age () + 1); db->store (*joe); } t.commit (); }
2.5 Deleting Persistent Objects
The last operation that we will discuss in this chapter is deleting the persistent object from the database. The following code fragment shows how we can delete an object given its identifier:
// John Doe is no longer in our database. // { transaction t (db->begin_transaction ()); db->erase<person> (john_id); t.commit (); }
To delete John from the database we start a transaction, call
the erase()
database function with John's object
id, and commit the transaction. After the transaction is commited
the erased object is no longer persistent.
If we don't have an object id handy, we can use query to find and delete the object:
// John Doe is no longer in our database. An alternative // implementation without using the object id. // { transaction t (db->begin_transaction ()); result r (db->query<person> (query::first == "John" && query::last == "Doe")); result::iterator i (r.begin ()); if (i != r.end ()) { auto_ptr<person> john (*i); db->erase (*john); } t.commit (); }
2.5 Summary
This chapter presented a very simple application which, nevertheless,
excercised all core database functions: persist()
,
query()
, load()
, store()
,
and erase()
. We also saw that writing an application
that uses ODB involves the following steps:
- Declare persistent classes in header files.
- Compile these headers to generate database support code.
- Link the application with the support code and two ODB runtime libraries.
Do not be concerned if, at this point, much appears unclear. The intent of this chapter is to give you only a general idea of how to persist C++ objects with ODB. We will cover all the details throughout the remainder of this manual.
3 Working Title
3.1 Base Concepts
The term database can refer to three distinct things: a general notion of a place where an application stores its data, a software implementation for managing this data (for example MySQL), and, finally, some database software implementations may manage several data stores which are usually distinguished by name. This name is also commonly referred to as database.
In this manual, when we use just the word database, we
refer to the first meaning above, for example,
"The store()
function saves the object's state to
the database." The term Database Management System (DBMS) is
often used to refer to the second meaning of the words database.
In this manual we will use the term database system
for short, for example, "Database system-independant
application code." Finally, to distinguish the third meaning
from the other two we will use the term database name,
for example, "The second option specfies the database name
that the application should use to store its data."
In C++ there is only one notion of a type and an instance
of a type. For example, a fundamental type, such as int
,
is, for the most part, treated the same as a user defined class
type. However, when it comes to persistence, we have to place
certain restrictions and requirements on certain C++ types that
can be stored in the database. As a result, we devide persistent
C++ types into two groups: object types and value
types. An stances of an object type is called an object
and an instance of a value type — a value.
An object is an independant entity. It can be stored, updated, and deleted in the database independant of other objects or values. An object has an identifier, called object id, that is unique among all instances of an object type within a database. An object consits of data members which are either values or references to other objects. In contrast, a value can only be stored in the database as part of an object and doesn't have its own unique identifier.
An object type is a C++ class. Because of this one to one
relationship, we will use terms object type
and object class interchangably. In contrast,
a value type can be a fundamental C++ type, such as
int
or a class type, such as std::string
.
If a value consists of other values then is is called a
composite value and its type — a
composite value type. Otherwise the the value is
called simple value and its type — a
simple value type. Note that the distinction between
simple and composite values is conceptual rather than
representational. For example, std::string
is a simple value type because conceptually string is a
single value even though the representation of the string
class may contain several data member each of which would be
considered a value. In fact, the same value type can be
viewed (and mapped) as both simple and composite by different
applications.
Seeing how all these concepts map to the relational model will hopefully make these distinctions more clear. In a relational database an object type is mapped to a table and a value type is mapped to one or more columns. A simple value type is mapped to a single column while a composite value type is mapped to several columns. Conversly, an object is stored as a row in this table and a value is stored as one or more cells in this row. A simple value is stored in a single cell while a composite value occupies several cells.
Going back to the distinction beetween simple and composite values, consider a date type which has three integer data members: year, month, and day. In one application it can be conidered a composite value and each member will get its own column in the relational database. In another application it can considered as a simple value and stored a single column as a number of day from some predefined date.
Until now, we have been using the term persistent class to refer to object classes. We will continue to do so even though a value type can also be a class. The reason for this assimetry is the subordinate nature of value types when it comes to database operations. Remember that values are never stored directly but rather as part of an object that contains them. As a result, when we say that we want to make a C++ class persistent or persist an instance of a class in the database, we invariably refer to an object class rather than a value class.
To make a C++ class a persistent object class we need to declare
it as such using the db object
pragma:
#pragma db object class person { ... };
The other pargma that we need to use is the db id
which designates one of the data members as an object id:
#pragma db object class person { private: #pragma db id unsigned long id_; };
These two pragmas are the minimum required to declare a persistent class. Other pragmas can be used to fine-tune the persistence-related properties of a class and its members.
You may be wondering whether we aslo have to do declare value types
as persistent. We don't need to do anything special for simple value
types such as int
or std::string
since the
ODB compiler knows how to map them to the database system types and
how to convert between the two. On the other hand, if a simple value
is unknown to the ODB compiler then you will need to provide the
mapping to the database system type and, possibly, the code to
convert between the two. For more information on this see @@ Ref
Custom value types/pragma value type. Composite value types are
not yet supported by ODB.
Normally, you would use object types to model real-world entities,
things that have their own identity. For example, in the
previous chapter we created a person
class to model
a person which is a real-world enitity. Name and age, which we
used as data members in our person
class are clearly
values. It is hard to think of age 31 or name "Joe" as having their
own identity.
A good test to determine whether something is an object or a value is to consider if other objects might reference it. A person is clearly an object because it can be refered to by other object's such as a spouce, an employer, or a bank. On the other hand, a person's age or name is not something that other objects would normally refer to.
Also, when an object represents a real entity, it is easy to choose a suitable object identifier. For example, for a person there is an established notion of an identifier (SSN, student id, passport number, etc). Another alternative is to use person't email address as an identifier.
Note, however, that these are only guidelines. There could
be goot reasons to make something that would normally be
a value an object. Consider, for example, a database that
stores a vast number of people. Many of the person objects
in this database have the same names and surnames and the
overhead of repeating them in every object may negatively
affect the performance. In this case we could make first name
and last name each an object and only store references to
these objects in the person
class.
An instance of a persistent class can be in one of two states: transient and persistent. A transient instance only has a representation in the applciation's memory and will ceas to exist when the application terminates unless it is explicitly made persistent. A persistent instance has a representation in both the application's memory and the database. A persistent instance will remain even after the application terminates unless and until it is explicitly deleted from the database. In other words, a transient instance of a persistent class behaves just like an instance of any ordinary C++ class.
3.2 Transactions and Concurrency
A transaction is an atomic, consistent, isolated and durable (ACID) unit of work. All database operations can only be performed within a transaction and each thread of execution in an application can have only one active transaction at a time.
By atomicity we mean that when it comes to making changes to the database state within a transaction, either all the changes succeed or none at all. Consider, for example, a transaction that transfers funds between two objects representing bank accounts. If the debit function on the first object succeeds but the credit function on the second fails, the transaction is rolled back and the database state of the first object remains unchanged.
By consistency we mean that a transaction must take all the objects stored in the database from one consistent state to another. For example, if a bank account object must reference a person object as its owner and we forget to set this reference before making the object persistent, the transaction will be rolled back and the database will remain unchanged.
By isolation we mean that the changes made to the database state during a transaction are only visible inside this transaction until and unless it is commited. Using the above example with bank transfer, the results of the debit operation performed on the first object is not visible to other transactions until the credit operation is successfully completed and the transaction is commited.
By durability we mean that once the transaction is committed, the changes that it made to the database state are permanent and will survive failures such as an application crash. From now the only way to alter this state is to execute and commit another transaction.
Note that all of the above guarantees only apply to the object's state in the database as opposed to the object's state in the application's memory. It is possible to roll a transaction back but still have changes from this transaction in the application's memory. An easy way to avoid this potentiall inconsistency is to instantiate persistent objects withing the transaction's scope. Consider, for example, this two implementations of the same transaction:
void update_age (database& db, person& p) { transaction t (db.begin_transaction ()); p.age (p.age () + 1); db.store (p); t.commit () }
In the above implementation, if the store()
call fails
and the transaction is rolled back, the state of the person
object in the database and the state of the same object in the
application's memory will differ. Now consider an
alternative implementation which only instantiates the
person object for the duration of the transaction:
void update_age (database& db, unsigned long id) { transaction t (db.begin_transaction ()); auto_ptr<person> p (db.load<person> (id)); p.age (p.age () + 1); db.store (p); t.commit () }
Of course, it may be not always be possible to write the application in this style. Oftentimes we need to access and modify application's state of persistent objects out of transactions. In this case it may make sense to try to roll back the changes made to the application state if the transaction was rolled back and the database state remains unchanged. One way to do this is to re-load the object's state from the database:
void update_age (database& db, person& p) { try { transaction t (db.begin_transaction ()); p.age (p.age () + 1); db.store (p); t.commit () } catch (...) { transaction t (db.begin_transaction ()); db.load (p.id (), p); t.commit (); throw; } }
A transaction is started by calling the begin_transaction()
database function. The returned transaction handle is stored in
an instance of the odb::transaction
class which has
the following interface:
namespace odb { class transaction { public: typedef odb::database database_type; void commit (); void rollback (); database_type& database (); static transaction& current (); static bool has_current (); }; }
The commit()
function commits a transaction and
rollback()
rolls it back. Unless the transaction
has been finalized, (explicitly commited or rolled back),
the destructor of the odb::transaction
class will
automatically roll it back when the transaction instance goes
out of scope.
The database()
function returns the database this
transaction is working on. The current()
static
function returns the currently active transaction for this
thread. If there is no active transaction, this function
throws the odb::not_in_transaction
exception.
You can check whether there is a transaction in effect using
the has_current()
static function.
If two or more transaction access or modify more than one object
and are executed concurrently by different applications or by
different threads within the same application, then it is possible
that these transactions will try to access objects in an incompatible
order and deadlock. The canonical example of a deadlock are
two transactions in which the first has modified object1
and is waiting for the second transaction to commit its changes to
object2
so that it can update object2
. At
the same time the second transaction has modified object2
and is waiting for the first transaction to commit its changes to
object1
because it also needs to modify object1
.
As a result none of the two transactions can complete.
The database system detects such situations and automatically
aborts the waiting operation in one of the deadlocked transactions.
In ODB this translates to the odb::deadlock
exception
being thrown from one of the database functions. You would normally
handle a deadlock by restarting the transaction, for example:
for (;;) { try { transaction t (db.begin_transaction ()); ... t.commit () break; } catch (const odb::deadlock&) { continue; } }