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Version 11.0.0
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Version 11.0.0
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Caution: the Architecture Guide is not updated in lockstep with the code base and is not necessarily correct or complete for any specific release.
A connection is a handle to a WiredTiger database instance. The connection has exclusive access to the database through a file lock, hence only one connection can be opened at a time. Internally, a connection is represented by WT_CONNECTION.
A connection is initialized when WT_CONNECTION::wiredtiger_open is called by the user application. WT_CONNECTION::wiredtiger_open accepts a list of configuration items (see Database Configuration) that can be used to enable different WiredTiger features and tune their behavior. Those features are for example related to Eviction, Logging, Checkpoints, Caching, statistics, etc. All the different available configuration settings are described in the documentation for WT_CONNECTION::wiredtiger_open.
WT_CONNECTION::wiredtiger_open also performs different sanity checks depending on the configuration item "create". When "create" is specified and a database does not already exist, a new database is created along with specific WiredTiger files such as the turtle file and other metadata files. If a database already exists, whether "create" is specified or not, WiredTiger will try to open it and check for the existence of the different required WiredTiger files. If "create" is not specified, WiredTiger expects a previously created database where it is executed. If the existing database is corrupted and cannot be opened, either WT_RUN_RECOVERY or WT_TRY_SALVAGE error (see Error handling) is returned to the user application and the connection is not created. In this case, a recovery operation will be required to bring the database to a consistent state (see WiredTiger command line utility for more details) before a connection can be successfully established with the database.
Once the database has been successfully opened, internal worker threads are started to provide global services used at runtime. Those services consist of different threads to handle statistics, logging, eviction, checkpoint and cache management. The sweeping server that manages the active and inactive dhandles is started too, see Data Handles for more information.
Finally, before the connection is completely initialized, the database is set to a consistent state by running rollback to stable.
At runtime, database-wide operations can be executed using the connection interface. For instance, it is possible to reconfigure WiredTiger features and behavior using WT_CONNECTION::reconfigure instead of closing the connection and calling WT_CONNECTION::open again. However, almost all CRUD operations on the database are executed in the context of a Session which can be created using WT_CONNECTION::open_session. See the WT_CONNECTION:: documentation to discover other available APIs related to WiredTiger connections.
A connection also keeps tracks of global information, see WT_CONNECTION_IMPL defined in connection.h. Finally, a WT_CONNECTION handle may be shared between threads, see Multithreading for more information.
When a connection is no longer required, it can be closed using WT_CONNECTION::close. As a result, any resource held by the connection (i.e sessions) is freed unless configured differently and the database is restored to a consistent state if necessary. It is worth noting that this final step might take some time as it may involve running the rollback to stable operation.