WiredTiger supports read-only mode databases. When a database is opened in read-only mode, all modifications are disabled on the Connection handle, any sessions opened in that connection and any cursors opened in any of those sessions. For example, all cursor or session handle methods that modify the database will instead return errors.
When a database is opened in read-only mode, the database directory and content must already exist and have been shutdown cleanly.
readonly configuration affects other configuration settings. Where a default setting contradicts read-only operation, WiredTiger defaults are overridden to perform in a read-only mode. For example, LSM tree merges are turned off when LSM trees are configured, and log file archiving is disabled when logging is configured.
Where a user configured setting contradicts read-only operation, WiredTiger will return an error. For example, zero-filling log files is not allowed in read-only mode, and attempting to configure them will return an error.
Because recovery modifies the database, recovery cannot be done in read-only mode. A
wiredtiger.open call to open a database in read-only mode will fail if the database was not cleanly shutdown and recovery is required.
If logging is enabled on the database when opened in read-only mode, log file archiving and log file pre-allocation are disabled and the log files will not be modified any way.
If LSM trees are in use, read-only mode turns off all modification. Internal LSM operations such as merging, creating new chunks, creating bloom filters and dropping old chunks are disabled.
One unusual affect of read-only operations is the potential for multiple read-only database handles open on the same database at the same time. WiredTiger prevents multiple connection handles by writing a lock file, and this locking is done even in read-only mode. However, if the lock file cannot be written, that is, if the WiredTiger home directory does not have write permission, opening in read-only mode is still allowed to proceed. For that reason, multiple read-only connection handles could be open at the same time. Normal locking occurs if the lock file can be written in read-only mode, preventing multiple database connections.