Detailed Description

A ConnectionPool represents a database connection pool.

A connection pool can be used to get a connection to a database and execute statements. This class opens a number of database connections and allows callers to obtain and use a database connection in a reentrant manner. Applications can instantiate as many ConnectionPool objects as needed and against as many different database systems as needed. The following diagram gives an overview of the library's components and their method-associations:

The method ConnectionPool_getConnection() is used to obtain a new connection from the pool. If there are no connections available, a new connection is created and returned. If the pool has already handed out maxConnections Connections, the next call to ConnectionPool_getConnection() will return NULL. Use Connection_close() to return a connection to the pool so it can be reused.

A connection pool is created by default with 5 initial connections and with 20 maximum connections. These values can be changed by the property methods ConnectionPool_setInitialConnections() and ConnectionPool_setMaxConnections().

Supported database systems:

This library may be built with support for many different database systems. To test if a particular system is supported, use the method Connection_isSupported().

Life-cycle methods:

Clients should call ConnectionPool_start() to establish the connection pool against the database server before using the pool. To shutdown connections from the database server, use ConnectionPool_stop(). Set preferred properties before calling ConnectionPool_start(). Some properties can also be changed dynamically after the pool was started, such as changing the maximum number of connections or the number of initial connections. Changing and tuning these properties at runtime is most useful if the pool was started with a reaper-thread (see below) since the reaper dynamically changes the size of the pool.

Connection URL:

The URL given to a Connection Pool at creation time specifies a database connection in the standard URL format. The format of the connection URL is defined as:

database://[user:password@][host][:port]/database[?propertyName1][=propertyValue1][&propertyName2][=propertyValue2]...

The property names user and password are always recognized and specify how to log in to the database. Other properties depend on the database server in question. Username and password can alternatively be specified in the auth-part of the URL. If port number is omitted, the default port number for the database server is used.

MySQL:

Here is an example of how to connect to a MySQL database server:

mysql://localhost:3306/test?user=root&password=swordfish

In this case, the username, root and password, swordfish are specified as properties to the URL. An alternative is to use the auth-part of the URL to specify authentication information:

mysql://root:swordfish@localhost:3306/test

See mysql options for all properties that can be set for a mysql connection URL.

SQLite:

For a SQLite database, the connection URL should simply specify a database file, since a SQLite database is just a file in the filesystem. SQLite uses pragma commands for performance tuning and other special purpose database commands. Pragma syntax in the form name=value can be added as properties to the URL and will be set when the Connection is created. In addition to pragmas, the following properties are supported:

heap_limit=value - Make SQLite auto-release unused memory if memory usage goes above the specified value [KB].
serialized=true - Make SQLite switch to serialized mode if value is true, otherwise multi-thread mode is used (the default).

A URL for connecting to a SQLite database might look like this (with recommended pragmas):

sqlite:///var/sqlite/test.db?synchronous=normal&foreign_keys=on&journal_mode=wal&temp_store=memory

PostgreSQL:

The URL for connecting to a PostgreSQL database server might look like:

postgresql://localhost:5432/test?user=root&password=swordfish

As with the MySQL URL, the username and password are specified as properties to the URL. Likewise, the auth-part of the URL can be used instead to specify the username and the password:

postgresql://root:swordfish@localhost/test?use-ssl=true

In this example, we have also omitted the port number to the server, in which case the default port number, 5432, for PostgreSQL is used. In addition, we have added an extra parameter to the URL, so connection to the server is done over a secure SSL connection.

See postgresql options for all properties that can be set for a postgresql connection URL.

Oracle:

The URL for connecting to an Oracle database server might look like:

oracle://localhost:1521/servicename?user=scott&password=tiger

Instead of a database name, Oracle uses a service name which you typically specify in a tnsnames.ora configuration file. The auth-part of the URL can be used instead to specify the username and the password as in the example below. Here we also specify that we want to connect to Oracle with the SYSDBA role.

oracle://sys:password@localhost:1521/servicename?sysdba=true

See oracle options for all properties that can be set for an oracle connection URL.

Example:

To obtain a connection pool for a MySQL database, the code below can be used. The exact same code can be used for PostgreSQL, SQLite and Oracle, the only change needed is to modify the Connection URL. Here we connect to the database test on localhost and start the pool with the default 5 initial connections.

URL_T url = URL_new("mysql://localhost/test?user=root&password=swordfish");
ConnectionPool_T pool = ConnectionPool_new(url);
ConnectionPool_start(pool);
//..
Connection_T con = ConnectionPool_getConnection(pool);
ResultSet_T result = Connection_executeQuery(con, "select id, name, photo from employee where salary>%d", anumber);
while (ResultSet_next(result))
{
     int id = ResultSet_getInt(result, 1);
     const char *name = ResultSet_getString(result, 2);
     int blobSize;
     const void *photo = ResultSet_getBlob(result, 3, &blobSize);
     // ...
}
Connection_close(con);
//..
ConnectionPool_free(&pool);
URL_free(&url);

Optimizing the pool size:

The pool is designed to dynamically manage the number of active connections based on usage patterns. A reaper thread is automatically started when the pool is initialized, performing two functions:

Sweep through the pool at regular intervals (default every 60 seconds) to close connections that have been inactive for a specified time (default 90 seconds).
Perform periodic validation (ping test) on idle connections to ensure they remain valid and responsive.

This dual functionality helps maintain the pool's health by removing stale connections and verifying the validity of idle ones.

Only inactive connections will be closed, and no more than the initial number of connections the pool was started with are closed. The property method, ConnectionPool_setReaper(), can be used to customize the reaper's sweep interval or disable it entirely if needed.

Clients can also call the method ConnectionPool_reapConnections() to prune the pool directly if manual control is desired.

The reaper thread is especially beneficial for pools maintaining TCP/IP Connections.

Realtime inspection:

Three methods can be used to inspect the pool at runtime. The method ConnectionPool_size() returns the number of connections in the pool, that is, both active and inactive connections. The method ConnectionPool_active() returns the number of active connections, i.e., those connections in current use by your application. The method ConnectionPool_isFull() can be used to check if the pool is full and unable to return a connection.

This ConnectionPool is thread-safe.

See also: Connection.h ResultSet.h URL.h PreparedStatement.h SQLException.h

Macros
#define	T ConnectionPool_T

Typedefs
typedef struct ConnectionPool_S *	T

Functions
T	ConnectionPool_new (URL_T url)
	Create a new ConnectionPool.

void	ConnectionPool_free (T *P)
	Disconnect and destroy the pool and release allocated resources.

Properties
URL_T	ConnectionPool_getURL (T P)
	Returns this Connection Pool's URL.

void	ConnectionPool_setInitialConnections (T P, int initialConnections)
	Sets the number of initial connections in the pool.

int	ConnectionPool_getInitialConnections (T P)
	Gets the number of initial connections in the pool.

void	ConnectionPool_setMaxConnections (T P, int maxConnections)
	Sets the maximum number of connections in the pool.

int	ConnectionPool_getMaxConnections (T P)
	Gets the maximum number of connections in the pool.

void	ConnectionPool_setConnectionTimeout (T P, int connectionTimeout)
	Set the Connection inactive timeout value in seconds.

int	ConnectionPool_getConnectionTimeout (T P)
	Gets the connection timeout value.

void	ConnectionPool_setAbortHandler (T P, void(abortHandler)(const char error))
	Sets the function to call if a fatal error occurs in the library.

void	ConnectionPool_setReaper (T P, int sweepInterval)
	Customize the reaper thread behavior or disable it.

Functions
void	ConnectionPool_start (T P)
	Prepares the pool for active use.

void	ConnectionPool_stop (T P)
	Gracefully terminates the pool.

Connection_T	ConnectionPool_getConnection (T P)
	Get a connection from the pool.

Connection_T	ConnectionPool_getConnectionOrException (T P)
	Get a connection from the pool.

void	ConnectionPool_returnConnection (T P, Connection_T connection)
	Returns a connection to the pool.

int	ConnectionPool_reapConnections (T P)
	Reaps inactive connections in the pool.

int	ConnectionPool_size (T P)
	Gets the current number of connections in the pool.

int	ConnectionPool_active (T P)
	Gets the number of active connections in the pool.

bool	ConnectionPool_isFull (T P)
	Checks if the pool is full.

Class functions
const char *	ConnectionPool_version (void)
	Gets the library version information.

Variables
int	ZBDEBUG
	Library Debug flag.

Macro Definition Documentation

◆ T

#define T ConnectionPool_T

Typedef Documentation

◆ T

typedef struct ConnectionPool_S* T

Function Documentation

◆ ConnectionPool_new()

T ConnectionPool_new ( URL_T url )

Create a new ConnectionPool.

The pool is created with 5 initial connections. Maximum connections is set to 20. Property methods in this interface can be used to change the default values.

Parameters

url	The database connection URL. It is a checked runtime error for the url parameter to be NULL. The pool does not take ownership of the `url` object but expects the url to exist as long as the pool does.

Returns: A new ConnectionPool object

See also: URL.h

◆ ConnectionPool_free()

void ConnectionPool_free ( T * P )

Disconnect and destroy the pool and release allocated resources.

Parameters

P	A ConnectionPool object reference

◆ ConnectionPool_getURL()

URL_T ConnectionPool_getURL ( T P )

Returns this Connection Pool's URL.

Parameters

P	A ConnectionPool object

Returns: This Connection Pool's URL

See also: URL.h

◆ ConnectionPool_setInitialConnections()

void ConnectionPool_setInitialConnections	(	T	P,
		int	initialConnections )

Sets the number of initial connections in the pool.

Parameters

P	A ConnectionPool object
initialConnections	The number of initial pool connections. It is a checked runtime error for initialConnections to be < 0

See also: Connection.h

◆ ConnectionPool_getInitialConnections()

int ConnectionPool_getInitialConnections ( T P )

Gets the number of initial connections in the pool.

Parameters

P	A ConnectionPool object

Returns: The number of initial pool connections

See also: Connection.h

◆ ConnectionPool_setMaxConnections()

void ConnectionPool_setMaxConnections	(	T	P,
		int	maxConnections )

Sets the maximum number of connections in the pool.

If max connections has been reached, ConnectionPool_getConnection() will return NULL on the next call.

Parameters

P	A ConnectionPool object
maxConnections	The maximum number of connections this connection pool will create. It is a checked runtime error for maxConnections to be less than initialConnections.

See also: Connection.h

◆ ConnectionPool_getMaxConnections()

int ConnectionPool_getMaxConnections ( T P )

Gets the maximum number of connections in the pool.

Parameters

P	A ConnectionPool object

Returns: The maximum number of connections this pool will create.

See also: Connection.h

◆ ConnectionPool_setConnectionTimeout()

void ConnectionPool_setConnectionTimeout	(	T	P,
		int	connectionTimeout )

Set the Connection inactive timeout value in seconds.

The method ConnectionPool_reapConnections(), if called, will close inactive Connections in the pool which have not been in use for connectionTimeout seconds. The default connectionTimeout is 90 seconds.

The reaper thread, see ConnectionPool_setReaper(), will use this value when closing inactive Connections.

Parameters

P	A ConnectionPool object
connectionTimeout	The number of `seconds` a Connection can be inactive (i.e., not in use) before the reaper closes the Connection. (value > 0)

◆ ConnectionPool_getConnectionTimeout()

int ConnectionPool_getConnectionTimeout ( T P )

Gets the connection timeout value.

Parameters

P	A ConnectionPool object

Returns: The time an inactive Connection may live before it is closed

◆ ConnectionPool_setAbortHandler()

void ConnectionPool_setAbortHandler	(	T	P,
		void(*	abortHandler )(const char *error) )

Sets the function to call if a fatal error occurs in the library.

In practice, this means Out-Of-Memory errors or uncaught exceptions. Clients may optionally provide this function. If not provided, the library will call abort(3) upon encountering a fatal error if ZBDEBUG is set; otherwise, exit(1) is called. This method provides clients with a means to close down execution gracefully. It is an unchecked runtime error to continue using the library after the abortHandler was called.

Parameters

P	A ConnectionPool object
abortHandler	The handler function to call should a fatal error occur during processing. An explanatory error message is passed to the handler function in the string `error`

See also: Exception.h

◆ ConnectionPool_setReaper()

void ConnectionPool_setReaper	(	T	P,
		int	sweepInterval )

Customize the reaper thread behavior or disable it.

By default, a reaper thread is automatically started when the pool is initialized, with a default sweep interval of 60 seconds. This method allows you to change the sweep interval or disable the reaper entirely.

The reaper thread closes inactive Connections in the pool, down to the initial connection count. An inactive Connection is closed if its connectionTimeout has expired or if it fails the ping test. Active Connections (those in current use) are never closed by this thread.

This method can be called before or after ConnectionPool_start(). If called after start, the changes will take effect on the next sweep cycle.

Parameters

P	A ConnectionPool object
sweepInterval	Number of seconds between sweeps of the reaper thread. Set to 0 or a negative value to disable the reaper thread, before calling ConnectionPool_start().

◆ ConnectionPool_start()

void ConnectionPool_start ( T P )

Prepares the pool for active use.

This method must be called before the pool is used. It will connect to the database server, create the initial connections for the pool, and start the reaper thread with default settings, unless previously disabled via ConnectionPool_setReaper().

Parameters

P	A ConnectionPool object

Exceptions

SQLException If a database error occurs.

See also: SQLException.h

◆ ConnectionPool_stop()

void ConnectionPool_stop ( T P )

Gracefully terminates the pool.

This method should be the last one called on a given instance of this component. Calling this method closes down all connections in the pool, disconnects the pool from the database server, and stops the reaper thread if it was started.

Parameters

P	A ConnectionPool object

◆ ConnectionPool_getConnection()

Connection_T ConnectionPool_getConnection ( T P )

Get a connection from the pool.

The returned Connection (if any) is guaranteed to be alive and connected to the database. NULL is returned if a database error occurred or if the pool is full and cannot return a new connection.

This example demonstrates how to check if the pool is full before attempting to get a connection, and how to handle potential errors:

if (ConnectionPool_isFull(p)) {
    // Consider increasing pool size before trying to get a connection
    // ConnectionPool_setMaxConnections(p, ...)
}
 
Connection_T con = ConnectionPool_getConnection(p);
if (!con) {
    if (ConnectionPool_isFull(p)) {
        // Pool is full
        fprintf(stderr, "Connection pool is full. Cannot acquire a new connection.\n");
    } else {
        // A database error occurred. This could be due
        // to network issues or database unavailability
        fprintf(stderr, "Database error: Unable to acquire a connection.\n");
    }
} else {
    // Use the connection...
}

Parameters

P	A ConnectionPool object

Returns: A connection from the pool or NULL if a database error occurred.

See also: Connection.h; ConnectionPool_setMaxRetries(T P, int maxRetries)

◆ ConnectionPool_getConnectionOrException()

Connection_T ConnectionPool_getConnectionOrException ( T P )

Get a connection from the pool.

The returned Connection is guaranteed to be alive and connected to the database. The method ConnectionPool_getConnection() above is identical except it will return NULL if the pool is full or if a database error occured. This method will instead throw an SQLException in both cases with an appropriate error message.

This example demonstrates how to get a connection, and how to handle potential errors:

Connection_T con = NULL;
TRY
{
     con = ConnectionPool_getConnectionOrException(p);
     // Use the connection...
}
ELSE
{
    // The error message in Exception_frame.message will specify
    // if the pool was full or the database error that occured
    fprintf(stderr, "Error: %s\n", Exception_frame.message);
}
FINALLY
{
    if (con) Connection_close(con);
}
END_TRY;

Parameters

P	A ConnectionPool object

Returns: A connection from the pool

Exceptions

SQLException If a database connection cannot be obtained. The error message is available in Exception_frame.message

See also: Connection.h

◆ ConnectionPool_returnConnection()

void ConnectionPool_returnConnection	(	T	P,
		Connection_T	connection )

Returns a connection to the pool.

The same as calling Connection_close() on a connection. If the connection is in an uncommitted transaction, rollback is called. It is an unchecked error to attempt to use the Connection after this method is called.

Parameters

P	A ConnectionPool object
connection	A Connection object

See also: Connection.h

◆ ConnectionPool_reapConnections()

int ConnectionPool_reapConnections ( T P )

Reaps inactive connections in the pool.

An inactive Connection is closed if and only if its connectionTimeout has expired or if the Connection failed the ping test against the database. Active Connections are not closed by this method.

Parameters

P	A ConnectionPool object

Returns: The number of Connections that were closed

See also: ConnectionPool_setConnectionTimeout; ConnectionPool_setInitialConnections; Connection_ping

◆ ConnectionPool_size()

int ConnectionPool_size ( T P )

Gets the current number of connections in the pool.

Parameters

P	A ConnectionPool object

Returns: The total number of connections in the pool.

◆ ConnectionPool_active()

int ConnectionPool_active ( T P )

Gets the number of active connections in the pool.

I.e., connections in current use by your application.

Parameters

P	A ConnectionPool object

Returns: The number of active connections in the pool

◆ ConnectionPool_isFull()

bool ConnectionPool_isFull ( T P )

Checks if the pool is full.

The pool is full if the number of active connections equals max connections and the pool is unable to return a connection.

Parameters

P	A ConnectionPool object

Returns: true if pool is full, false otherwise

Note: A full pool is unlikely to occur in practice if you ensure that connections are returned to the pool after use.

◆ ConnectionPool_version()

const char * ConnectionPool_version ( void )

Gets the library version information.

Returns: The library version information

Variable Documentation

◆ ZBDEBUG

int ZBDEBUG

extern

Library Debug flag.

If set to true, emit debug output

Detailed Description

Supported database systems:

Life-cycle methods:

Connection URL:

MySQL:

SQLite:

PostgreSQL:

Oracle:

Example:

Optimizing the pool size:

Realtime inspection:

Macros

Typedefs

Functions

Variables

Macro Definition Documentation

◆ T

Typedef Documentation

◆ T

Function Documentation

◆ ConnectionPool_new()

◆ ConnectionPool_free()

◆ ConnectionPool_getURL()

◆ ConnectionPool_setInitialConnections()

◆ ConnectionPool_getInitialConnections()

◆ ConnectionPool_setMaxConnections()

◆ ConnectionPool_getMaxConnections()

◆ ConnectionPool_setConnectionTimeout()

◆ ConnectionPool_getConnectionTimeout()

◆ ConnectionPool_setAbortHandler()

◆ ConnectionPool_setReaper()

◆ ConnectionPool_start()

◆ ConnectionPool_stop()

◆ ConnectionPool_getConnection()

◆ ConnectionPool_getConnectionOrException()

◆ ConnectionPool_returnConnection()

◆ ConnectionPool_reapConnections()

◆ ConnectionPool_size()

◆ ConnectionPool_active()

◆ ConnectionPool_isFull()

◆ ConnectionPool_version()

Variable Documentation

◆ ZBDEBUG