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, load, update and delete persistent objects, as well as query the database for persistent objects that match a certain criteria.
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 variants of the mysql::database
constructor 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 the database for persistent objects matching a certain criteria.
2.4 Querying Database for 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 selecting some of them using queries ara two useful operations, 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 the query facility 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 queries 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
@@
In this chapter we will continue to use and exapand the
person
persistent class that we have developed in the
previous chapter.
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 and will not discuss them further in
this revision of the manual.
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 Database
Before an application can make use of a persistence services
offered by ODB, it has to create a database instance. A
database instance is the representation of the place where
the application stores its persistent objects. You create
a database instance by instantiating one of the database
system-specific classes. For example odb::mysql::database
would be such a class for the MySQL database system. You will
also normally pass a database name as an argument to the
database
class' constructor. The following
code fragment shows how we can create a database instance
for the MySQL database system:
#include <odb/database.hxx> #include <odb/mysql/database.hxx> auto_ptr<odb::database> db ( new odb::mysql::database ( "test_user" // database login name "test_password" // database password "test_database" // database name ));
The odb::database
class is an abstract base class
that defines a common interface for all database system-specific
classes provided by ODB. You would normally work with the database
instance via this interface unless there is a specific
functionality that your application depends on and which is
only exposed by a particular system's database
class.
The odb::database
interface defines functions for
starting transactions and manipulating persistent objects.
These are discussed in detail in the reminder of this chapter
as well as the next chapter which is dedicated to the topic of
querying the database for persistent objects. For details on the
system-specific database
classes, refer for
(@@ ref Database Systems).
3.3 Transactions
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.
A transaction is started by calling the
database::begin_transaction()
function. The returned transaction handle is stored in
an instance of the odb::transaction
class which is
defined in the odb/transaction.hxx
header file.
A source code fragment that uses ODB transactions should include
this header file. The odb::transaction
class 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; } }
Note that in the above discussion of atomicity, consistency, isolation, and durability, all of these 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; } }
3.4 Making Objects Persistent
A newly created instance of a persistent class is transient.
We use the database::persist()
function template
to make a transient instance persistent. This function has two
overloaded variants with the following signatures:
template <typename T> typename object_traits<T>::id_type persist (const T& object); template <typename T> typename object_traits<T>::id_type persist (T& object);
The first persist()
variant expects a constant reference
to an instance being persisted and is used on objects with
application-assigned object ids (@@ref pragma id/auto). The second
variant expects an unrestricted reference and, if the object id is
assigned by the database, it updates the passed instance's id member
with the assigned value. Both variants return the object id of the
newly persistent object.
If the database already contains an object of this type with this
id, the persist()
functions throw the
odb::object_already_persistent
exception. This should
never happen for database-assigned object ids as long as the
number of objects persisted does not exceed the value space of
the id type.
When calling the persist()
function we don't need to
explicitly specify the template type since it will be automatically
deduced from the argument being passed. The odb::object_traits
template used in the signature above is part of the database support
code generated by the ODB compiler.
The following example shows how we can call this function:
person john ("John", "Doe", 33); person jane ("Jane", "Doe", 32); transaction t (db->begin_transaction ()); db->persis (john); unsigned long jane_id (db->persist (jane)); t.commit (); cerr << "Jane's id: " << jane_id << endl;
Notice that in the above code fragment we have created instances that we were planning to make persistent before starting the transaction. Likewise, we printed Jane's id after we have commited the transaction. As a general rule, you should avoid performing operations within a transaction's scope that can be performed before the transaction starts or after it terminates. An active transaction consumes both your application's resources, such as a database connection, as well as the database server's resources, such as object locks. By following the above rule you make sure these resources are made available to other threads in your application and to other applications for as long as possible.
3.5 Loading Persistent Objects
Once an object is made persistent, and you know its object id, it
can loaded by the application using the database::load()
function template. This function has two overloaded variants with
the following signatures:
template <typename T> typename object_traits<T>::pointer_type load (const typename object_traits<T>::id_type& id); template <typename T> void load (const typename object_traits<T>::id_type& id, T& object);
Given an object id, the first variant allocates a new instance
of the object class in the dynamic memory, loads its state from
the database, and returns the pointer to the new instance. The
second variant loads the object's state into an existing instance.
Both functions throw odb::object_not_persistent
if
there is no object of this type with this id in the database.
When we call the first variant of load()
we need to
explicitly specify the object type. We don't need to do this for
the second variant because the object type will be automatically
deduced from the second argument, for example:
transaction t (db->begin_transaction ()); person* jane (db->load<person> (jane_id)); db->load (jane_id, *jane); t.commit ();
If we don't know for sure whether an object with a gived id
is persistent, we can use the find()
function
instead of load()
:
template <typename T> typename object_traits<T>::pointer_type find (const typename object_traits<T>::id_type& id); template <typename T> bool find (const typename object_traits<T>::id_type& id, T& object);
If an object with this id is not found in the database, the first
variant of find()
returns a NULL
pointer
while the second variant leaves the passed instance unmodified and
returns false
.
If we don't know an object's identifier, then we can use queries to find the object (or objects) matching some other criteria (@@ ref query). Note, however, that loading an object's state using its identifier can be significantly faster that doing a query.
3.5 Updating Persistent Objects
If a persistent object has been modified, we can store the updated
state in the database using the database::update()
function template:
template <typename T> void update (T& object);
If the object passed to this function does not exist in the
database, update()
throws the
odb::object_not_persistent
exception.
Below is an example of the funds transfer that we talked about
in the earlier section on transactions. It uses the hypothetical
bank_account
persistent class:
void transfer (database& db, unsigned long from_acc, unsigned long to_acc, unsigned int amount) { bank_account from, to; transaction t (db.begin_transaction ()); db.load (from_acc, from); if (from.balance () < amount) throw insufficient_funds (); db.load (to_acc, to); to.balance (to.balance () + amount); from.balance (from.balance () - amount); db.update (to); db.update (from); t.commit (); }
3.6 Deleting Persistent Objects
To delete a persistent object's state from the database we use the
database::erase()
function template. If the application
still has an instance of the erased object, this instance becomes
transient. The erase()
function has the following
overloaded variants:
template <typename T> void erase (const T& object); template <typename T> void erase (const typename object_traits<T>::id_type& id);
The first variant uses an object itself to delete its state from
the database. The second variant uses the object id to identify
the object to be deleted. If the object to be deleted does not
exist in the database, both variants throw the
odb::object_not_persistent
exception.
We have to specify the object type when calling the second variant
of erase()
. The same is unnecessary for the first
variant because the object type will be automomatically deduced
from its argument. The following example shows how can call
this function:
const person& john = ... transaction t (db->begin_transaction ()); db->erase (john); db->erase<person> (jane_id); t.commit ();
4 Querying the Database
If you don't know the identifiers of the objects that you are looking for, you can use queries to search the database for objects matching a certain criteria. ODB provides flexible and powerful query support that offers two distinct levels of abstraction from the database system query language such as SQL.
At the high level you are presented with an easy to use yet powerful object oriented query language, called ODB query language. This query language is modeled after and is integrated into C++ allowing you to write expressive and safe queries that look and feel like plain C++. We have already seen examples of these queries in the introductory chapters. Below is another, more interesting, example:
typedef odb::query<person> query; typedef odb::result<person> result; unsigned short age; query q (query::first == "John" && query::age < query::_ref (age)); for (age = 10; age < 100; age += 10) { result r (db->query<person> (q)); ... }
At the low level, queries can be written as predicates using
the database system-native query language such as the
WHERE
predicate from the SQL SELECT
statement. This language will be refered to as native query
language. At this level ODB still takes care of converting
query parameters from C++ to the database system format. Below
is the re-implementation of the above example using SQL as
the native query language:
query q ("first = 'John' AND age = " + query::_ref (age));
Note that at this level you also loose the static typing of query expressions. For example, if we wrote something like this:
query q (query::first == 123 && query::agee < query::_ref (age));
We would get two errors during the C++ compilation. The first would
indicate that we cannot compare query::first
to an
integer and the second would pick the misspelling in
query::agee
. On the other hand, if we wrote something
like this:
query q ("first = 123 AND agee = " + query::_ref (age));
It would compile without any errors and would trigger an error only when executed by the the database system.
You are also allowed to combine the two query languages in a single query, for example:
query q ("first = 'John'" + (query::age < query::_ref (age)));
4.1 ODB Query Language
An ODB query is an expression that tell the database system whether
any given object matches our criteria. As such a query expression
always evaluates to true
or false
. At
the higher lever, an expression consist of other expressions
combined with logical operators such as &&
(AND),
||
(OR), and !
(NOT). For example:
typedef odb::query<person> query; query q (query::first == "John" || query::age == 31);
At the core of every query expression lie simple expressions which
involve one or more object data members, values, or parameters. To
refer to an object member you use an expression such as
query::first
above. The names of members in the
query
class are derived from the names of data members
in the object class by removing the common member name decorations,
such as leading and trailing underscores, the m_
prefix,
etc.
In a simple expressions an object member can be compared to a value, parameter, or another member using a number of predefined operators and functions. The following table gives an overview of the available expressions:
Operator | Description | Example |
---|---|---|
== |
equal | query::age == 31 |
!= |
unequal | query::age != 31 |
< |
less than | query::age < 31 |
> |
greater than | query::age > 31 |
<= |
less than or equal | query::age <= 31 |
>= |
greater than or equal | query::age >= 31 |
in() |
one of the values | query::age.in (30, 32, 34) |
in_range() |
one of the values in range | query::age.in_range (begin, end) |
is_null() |
value is NULL | query::age.is_null () |
is_not_null() |
value is not NULL | query::age.is_not_null () |
The operator precedence in the query expressions are the same as for equivalent C++ operators. You can use parentheses to make sure the expression is evaluated in the desired order. For example:
query q ((query::first == "John" || query::first == "Jane") && query::age < 31);
4.2 Parameter Binding
An instance of the odb::query
class encapsulates two
parts of information about the query: the query expression and
the query parameters. Parameters can be bound to C++ variables
either by value or by reference.
If a parameter is bound by value, then the value for this parameter is copied from the C++ variable to the query instance at the query construction time. On the other hand, if a parameter is bound by reference, then the query instance only stores a reference to the bound variable. The actual value for the parameter is only extracted at the query execution time. Consider, for example the following two queries:
string name ("John"); query q1 (query::first == query::_val (name)); query q2 (query::first == query::_ref (name)); name = "Jane"; db->query<person> (q1); // Find John. db->query<person> (q2); // Find Jane.
The odb::query
class provides two special functions,
_val()
and _ref()
, that allow you to
bind the parameter either by value or by reference, respectively.
In the embedded query language, if the binding is not specified
explicitly, the value semantics is used by default. In the
native query language, binding must always be specified
explicitly. For example:
query q1 (query::age < age); // By value. query q2 (query::age < query::_val (age)); // By value. query q3 (query::age < query::_ref (age)); // By reference. query q4 ("age < " + age); // Error. query q5 ("age < " + query::_val (age)); // By value. query q6 ("age < " + query::_ref (age)); // By reference.
A query that only has by-value parameters does not depend on any other variables and is self-sufficient once constructed. A query that has one or more by-reference parameter depends on the bound variables until the query is executed. If one such variable goes out of scope and you execute the query, the behavior is undefined.
4.3 Executing a Query
Once we have the query instance ready and by-reference parameters
initialized, we can execute the query using the
database::query()
function template. It has two
overloaded variants:
template <typename T> result<T> query (bool cache = true); template <typename T> result<T> query (const odb::query<T>&, bool cache = true);
The first variant is used to return all persistent objects of a
given type stored in the database. The second variant uses the
passed query instance to only return objects matching the
query criteria. The cache
argument determines
whether the object states should be cached in the application's
memory or if it should be returned by the database system
one by one as the iteration over the result progresses. The
result caching is discussed in detail in the next section.
When calling the query()
function we have to
explicitly specify the object type we are querying. For example:
typedef odb::query<person> query; typedef odb::result<person> result; result all (db->query<person> ()); result johnes (db->query<person> (query::first == "John"));
Note that it is not required to explicitly create a named query variable before executing it. For example, the following two queries are equivalent:
query q (query::first == "John"); result r1 (db->query<person> (q)); result r1 (db->query<person> (query::first == "John"));
Normally you would create a named query instance if you are planning to run the same query multiple times and would use the in-line version for those that are executed only once.
It is also possible to create queries from other queries by combinding them using logical operators. For example:
result find_minors (database& db, const query& name_query) { return db.query<person> (name_query && query::age < 18); } result r (find_underage (db, query::first == "John"));
4.3 Query Result
The result of executing a query is zero, one, or more objects
matching the query criteria. The result is represented as the
odb::result
class template:
typedef odb::query<person> query; typedef odb::result<person> result; result johnes (db->query<person> (query::first == "John"));
It is best to view an instance of odb::result
as a handle to a stream, such as a file stream. While you can
make a copy of a result or assign one result to another, the
two instances will refer to the same result stream. Advancing
the current position in one instance will also advance it in
another. The result instance is only usable within a transaction
it was created in. Trying to manipulate the result after the
transaction has terminates leads to undefined behavior.
The odb::result
class template conforms to the
standard C++ sequence requirements and has the following
interface:
namespace odb { template <typename T> class result { public: typedef odb::result_iterator<T> iterator; public: result (); result (const result&); result& operator= (const result&); void cache (); iterator begin (); iterator end (); }; }
The default constructor creates an empty result set. The
cache()
function caches the returned objects'
state in the application's memory. We have already mentioned
result caching when we talked about query execution. As you
may remember the database::query()
function
cahces the result unless instructed not to by the caller.
The result::cache()
function allows you to
cache the result at a later stage if it wasn't already
cached during query execution.
If result is cached, the entire state of the returned objects is stored in the application's memory. Note that the actual objects are still only instantiated on demand during result iteration. It is the raw database state that is cached in memory. In contrast, for uncached results the object state is sent by the database system one object at a time as the iteration progresses.
Uncached results can improve performance of both the application
and the database system in situations where you have a large
number of objects in the result or if you will only examine
a small portion of the returned objects. However, uncached
results have a number of limitations. There can only be one
uncached result in a transaction. Creating another result
(cached or uncached) by calling database::query()
will invalidate the first uncached result. Furthermore,
executing any other database function, such as update()
or erase()
will also invalidate the uncached result.
To iterate over the objects in a result we use the
begin()
and end()
functions
together with the odb::result<T>::iterator
type, for example:
result r (db->query<person> (query::first == "John")); for (result::iterator i (r.begin ()); i != r.end (); ++i) { ... }
The result iterator is an input iterator which means that the only two position operations that are support are to move to the next object and determine whether we have reached the end of the result stream. In fact, the result iteraror can only be in two states: the current position and the end position. If you have two iterators pointing to the current position and then you advance one of them, the other will advance as well. This, for example, means that it doesn't make sense to store an iterator that points to some object of interest in the result stream with the intent of dereferncing it after the iteration is over. Instead, you would need to store the object itself.
The result iterator has the following dereference functions that can be used to access the pointed-to object:
namespace odb { template <typename T> class result_iterator { public: typename object_traits<T>::pointer_type operator-> () const; typename object_traits<T>::pointer_type operator* () const; void load (T& x); }; }
When you call the ->
operator, the iterator
will allocate a new instance of the object class in the
dynamic memory, load its state from the returned database
state, and return the pointer to the new instance. In
case of this operator, the iterator still maintains the
ownership of the returned object and will return the
same pointer for subsequent calls until advanced to
the next object. For example:
result r (db->query<person> (query::first == "John")); for (result::iterator i (r.begin ()); i != r.end ();) { cout << i->last () << endl; // Create an object. cout << i->age () << endl; // Use the same object. ++i; // Free the object. }
The *
operator is similar to ->
(notice that it also returns a pointer) except that it
relinquishes the ownership of the allocated object. In
fact, it is very similar to the database::load()
variant which returns a dynamically allocated instance.
Note also that subsequent calls to this operator or to
->
return the same object. The following
example shows how we can use this operator:
result r (db->query<person> (query::first == "John")); for (result::iterator i (r.begin ()); i != r.end (); ++i) { auto_ptr p (*i); // Create an object. cout << p->last () << endl; // Use the same object. cout << i->age () << endl; // Use the same object. p.reset (); // Free the object. cout << i->first () << endl; // Error, object was deleted. }
The result_iterator::load()
function allows
you to load the current object's state into an existing
instance. It is similar to the second overloaded variant
of the database::load()
function. For example:
result r (db->query<person> (query::first == "John")); person p; for (result::iterator i (r.begin ()); i != r.end (); ++i) { i.load (p); cout << p.last () << endl; cout << i.age () << endl; }