# Poco::Data

# class Session

Library: Data
Package: DataCore
Header: Poco/Data/Session.h

## Description

A Session holds a connection to a Database and creates Statement objects.

Sessions are always created via the SessionFactory:

    Session ses(SessionFactory::instance().create(connectorKey, connectionString));

where the first param presents the type of session one wants to create (e.g., for SQLite one would choose "SQLite", for ODBC the key is "ODBC") and the second param is the connection string that the session implementation requires to connect to the database. The format of the connection string is specific to the actual connector.

A simpler form to create the session is to pass the connector key and connection string directly to the Session constructor.

A concrete example to open an SQLite database stored in the file "dummy.db" would be

    Session ses("SQLite", "dummy.db");

Via a Session one can create two different types of statements. First, statements that should only be executed once and immediately, and second, statements that should be executed multiple times, using a separate execute() call. The simple one is immediate execution:

    ses << "CREATE TABLE Dummy (data INTEGER(10))", now;

The now at the end of the statement is required, otherwise the statement would not be executed.

If one wants to reuse a Statement (and avoid the overhead of repeatedly parsing an SQL statement) one uses an explicit Statement object and its execute() method:

    int i = 0;
    Statement stmt = (ses << "INSERT INTO Dummy VALUES(:data)", use(i));

    for (i = 0; i < 100; ++i)
    {
        stmt.execute();
    }

The above example assigns the variable i to the ":data" placeholder in the SQL query. The query is parsed and compiled exactly once, but executed 100 times. At the end the values 0 to 99 will be present in the Table "DUMMY".

A faster implementaton of the above code will simply create a vector of int and use the vector as parameter to the use clause (you could also use set or multiset instead):

    std::vector<int> data;
    for (int i = 0; i < 100; ++i)
    {
        data.push_back(i);
    }
    ses << "INSERT INTO Dummy VALUES(:data)", use(data);

NEVER try to bind to an empty collection. This will give a BindingException at run-time!

Retrieving data from a database works similar, you could use simple data types, vectors, sets or multiset as your targets:

    std::set<int> retData;
    ses << "SELECT * FROM Dummy", into(retData));

Due to the blocking nature of the above call it is possible to partition the data retrieval into chunks by setting a limit to the maximum number of rows retrieved from the database:

    std::set<int> retData;
    Statement stmt = (ses << "SELECT * FROM Dummy", into(retData), limit(50));
    while (!stmt.done())
    {
        stmt.execute();
    }

The "into" keyword is used to inform the statement where output results should be placed. The limit value ensures that during each run at most 50 rows are retrieved. Assuming Dummy contains 100 rows, retData will contain 50 elements after the first run and 100 after the second run, i.e. the collection is not cleared between consecutive runs. After the second execute stmt.done() will return true.

A prepared Statement will behave exactly the same but a further call to execute() will simply reset the Statement, execute it again and append more data to the result set.

Note that it is possible to append several "bind" or "into" clauses to the statement. Theoretically, one could also have several limit clauses but only the last one that was added will be effective. Also several preconditions must be met concerning binds and intos. Take the following example:

    ses << "CREATE TABLE Person (LastName VARCHAR(30), FirstName VARCHAR, Age INTEGER(3))";
    std::vector<std::string> nameVec; // [...] add some elements
    std::vector<int> ageVec; // [...] add some elements
    ses << "INSERT INTO Person (LastName, Age) VALUES(:ln, :age)", use(nameVec), use(ageVec);

The size of all use parameters MUST be the same, otherwise an exception is thrown. Furthermore, the amount of use clauses must match the number of wildcards in the query (to be more precise: each binding has a numberOfColumnsHandled() value which defaults to 1. The sum of all these values must match the wildcard count in the query. However, this is only important if you have written your own TypeHandler specializations. If you plan to map complex object types to tables see the TypeHandler documentation. For now, we simply assume we have written one TypeHandler for Person objects. Instead of having n different vectors, we have one collection:

    std::vector<Person> people; // [...] add some elements
    ses << "INSERT INTO Person (LastName, FirstName, Age) VALUES(:ln, :fn, :age)", use(people);

which will insert all Person objects from the people vector to the database (and again, you can use set, multiset too, even map and multimap if Person provides an operator() which returns the key for the map). The same works for a SELECT statement with "into" clauses:

    std::vector<Person> people;
    ses << "SELECT * FROM PERSON", into(people);

Mixing constants or variables with manipulators is allowed provided there are corresponding placeholders for the constants provided in the SQL string, such as in following example:

    std::vector<Person> people;
    ses << "SELECT * FROM %s", into(people), "PERSON";

Formatting only kicks in if there are values to be injected into the SQL string, otherwise it is skipped. If the formatting will occur and the percent sign is part of the query itself, it can be passed to the query by entering it twice (%%). However, if no formatting is used, one percent sign is sufficient as the string will be passed unaltered. For complete list of supported data types with their respective specifications, see the documentation for format in Foundation.

Transactions are supported via the begin(), commit() and rollback() methods. If the session is in autocommit mode, begin() will temporarily disable autocommit mode for the duration of the transaction. Calls to either commit() or rollback() will re-enable it. Poco::Data::Transaction, a convenient session wrapper class, automates this process and provides RAII-based transactional behavior. For more information, see Transation class documentation.

## Member Summary

Member Functions: begin, canTransact, close, commit, connector, createStatementImpl, dbmsName, getConnectionTimeout, getFeature, getLoginTimeout, getProperty, getTransactionIsolation, hasFeature, hasProperty, hasTransactionIsolation, impl, isAutocommit, isConnected, isGood, isTransaction, isTransactionIsolation, open, operator <<, operator =, reconnect, rollback, setConnectionTimeout, setFeature, setLoginTimeout, setProperty, setTransactionIsolation, swap, uri

## Constructors

### Session

Session(
Poco::AutoPtr < SessionImpl > ptrImpl
);

Creates the Session from SessionImpl.

### Session

Session(
const Session & param131
);

Creates a session by copying another one.

### Session

Session(
Session && param132
) noexcept;

Creates a session by moving another one.

### Session

Session(
const std::string & connection,
std::size_t loginTimeout = LOGIN_TIMEOUT_DEFAULT
);

Creates a new session, using the given connection (must be in "connection:///connectionString" format).

### Session

Session(
const std::string & connector,
const std::string & connectionString,
std::size_t loginTimeout = LOGIN_TIMEOUT_DEFAULT
);

Creates a new session, using the given connector (which must have been registered), and connectionString.

## Destructor

### ~Session

~Session();

Destroys the Session.

## Member Functions

### begin

void begin();

Starts a transaction. If `session` is in autocommit mode, it is switched to manual commit mode for the duration of the transaction and reverted back to the original mode after transaction completes (on rollback or commit() call).

### canTransact

bool canTransact();

Returns true if session has transaction capabilities.

### close

void close();

Closes the session.

### commit

void commit();

Commits and ends a transaction. If `session` was in autocommit mode when the transaction started (begin() call), it is switched back to autocommit mode.

### connector

std::string connector() const;

Returns the connector name for this session.

### createStatementImpl

SharedPtr < StatementImpl > createStatementImpl();

Creates a StatementImpl.

### dbmsName

const std::string & dbmsName() const;

Returns the DBMS name. The name must be set by the implementation. Defaults to "unknown".

### getConnectionTimeout

std::size_t getConnectionTimeout();

Returns the session connection timeout value.

### getFeature

bool getFeature(
const std::string & name
) const;

Look up the state of a feature.

Features are a generic extension mechanism for session implementations. and are defined by the underlying SessionImpl instance.

Throws a NotSupportedException if the requested feature is not supported by the underlying implementation.

### getLoginTimeout

std::size_t getLoginTimeout() const;

Returns the session login timeout value.

### getProperty

Poco::Any getProperty(
const std::string & name
) const;

Look up the value of a property.

Properties are a generic extension mechanism for session implementations. and are defined by the underlying SessionImpl instance.

Throws a NotSupportedException if the requested property is not supported by the underlying implementation.

### getTransactionIsolation

Poco::UInt32 getTransactionIsolation();

Returns the transaction isolation level.

### hasFeature

bool hasFeature(
const std::string & name
) const;

Returns true if session has the named feature.

### hasProperty

bool hasProperty(
const std::string & name
) const;

Returns true if session has the named property.

### hasTransactionIsolation

bool hasTransactionIsolation(
Poco::UInt32 ti
);

Returns true if and only if the transaction isolation level corresponding to the supplied bitmask is supported.

### impl

SessionImpl * impl();

Returns a pointer to the underlying SessionImpl.

### isAutocommit

bool isAutocommit() const;

Returns true if and only if the session is in autocommit mode, false otherwise. If the session does not support autocommit, it is assumed not to be in auto commit mode. This function looks for the "autoCommit" feature and returns its value. It is recommended for all back-end implementations to support this feature.

### isConnected

bool isConnected() const;

Returns true if and only if session is connected, false otherwise.

### isGood

bool isGood() const;

Returns true if and only if the session is good and can be used, false otherwise.

### isTransaction

bool isTransaction();

Returns true if and only if a transaction is in progress, false otherwise.

### isTransactionIsolation

bool isTransactionIsolation(
Poco::UInt32 ti
);

Returns true if and only if the transaction isolation level corresponds to the supplied bitmask.

### open

void open(
const std::string & connect = ""
);

Opens the session using the supplied string. Can also be used with default empty string to reconnect a disconnected session. If the connection is not established, a ConnectionFailedException is thrown. Zero timout means indefinite

### operator <<

template < typename T > Statement operator << (
const T & t
);

Creates a Statement with the given string as SQLContent.

### operator =

Session & operator = (
const Session & param133
);

Assignment operator.

### operator =

Session & operator = (
Session && param134
) noexcept;

Move assignment.

### reconnect

void reconnect();

Closes the session and opens it.

### rollback

void rollback();

Rolls back and ends a transaction. If `session` was in autocommit mode when the transaction started (begin() call), it is switched back to autocommit mode.

### setConnectionTimeout

void setConnectionTimeout(
std::size_t timeout
);

Sets the session connection timeout value.

### setFeature

void setFeature(
const std::string & name,
bool state
);

Set the state of a feature.

Features are a generic extension mechanism for session implementations. and are defined by the underlying SessionImpl instance.

Throws a NotSupportedException if the requested feature is not supported by the underlying implementation.

### setLoginTimeout

void setLoginTimeout(
std::size_t timeout
);

Sets the session login timeout value.

### setProperty

void setProperty(
const std::string & name,
const Poco::Any & value
);

Set the value of a property.

Properties are a generic extension mechanism for session implementations. and are defined by the underlying SessionImpl instance.

Throws a NotSupportedException if the requested property is not supported by the underlying implementation.

### setTransactionIsolation

void setTransactionIsolation(
Poco::UInt32
);

Sets the transaction isolation level.

### swap

void swap(
Session & other
);

Swaps the session with another one.

### uri

std::string uri() const;

Returns the URI for this session.

### uri

static std::string uri(
const std::string & connector,
const std::string & connectionString
);

Utility function that teturns the URI formatted from supplied arguments as "connector:///connectionString".

## Variables

### LOGIN_TIMEOUT_DEFAULT

static const std::size_t LOGIN_TIMEOUT_DEFAULT = SessionImpl::LOGIN_TIMEOUT_DEFAULT;

### TRANSACTION_READ_COMMITTED

static const Poco::UInt32 TRANSACTION_READ_COMMITTED = 0x00000002L;

### TRANSACTION_READ_UNCOMMITTED

static const Poco::UInt32 TRANSACTION_READ_UNCOMMITTED = 0x00000001L;

### TRANSACTION_REPEATABLE_READ

static const Poco::UInt32 TRANSACTION_REPEATABLE_READ = 0x00000004L;

### TRANSACTION_SERIALIZABLE

static const Poco::UInt32 TRANSACTION_SERIALIZABLE = 0x00000008L;