template<typename T, typename T_Group>
QgsConnectionPool class
Template class responsible for keeping a pool of open connections.
Contents
- Reference
This is desired to avoid the overhead of creation of new connection every time.
The methods are thread safe.
The connection pool has a limit on maximum number of concurrent connections (per server), once the limit is reached, the acquireConnection() function will block. All connections that have been acquired must be then released with releaseConnection() function.
When the connections are not used for some time, they will get closed automatically to save resources.
Public functions
- auto acquireConnection(const QString& connInfo, int timeout = -1, bool requestMayBeNested = false) -> T
- Try to acquire a connection for a maximum of timeout milliseconds.
- void invalidateConnections(const QString& connInfo)
- Invalidates all connections to the specified resource.
- void releaseConnection(T conn)
- Release an existing connection so it will get back into the pool and can be reused.
Function documentation
template<typename T, typename T_Group>
T QgsConnectionPool<T, T_Group>::
Try to acquire a connection for a maximum of timeout milliseconds.
| Returns | initialized connection or nullptr if unsuccessful |
|---|
If timeout is a negative value the calling thread will be blocked until a connection becomes available. This is the default behavior.
template<typename T, typename T_Group>
void QgsConnectionPool<T, T_Group>::
Invalidates all connections to the specified resource.
The internal state of certain handles (for instance OGR) are altered when a dataset is modified. Consquently, all open handles need to be invalidated when such datasets are changed to ensure the handles are refreshed. See the OGR provider for an example where this is needed.