WiredTiger databases are configured using the wiredtiger_open call, which takes a configuration string argument. In addition to this, the database configuration can be specified using the WiredTiger.config file or the WIREDTIGER_CONFIG environment variable.

When a WiredTiger database is created, the configuration string passed to wiredtiger_open is saved to a WiredTiger home directory file named WiredTiger.basecfg, and that configuration file is read whenever the database is subsequently opened.

The configuration string to the wiredtiger_open call allows applications to do per run settings (such as with or without statistics) or override original creation settings. The user config file and the environment variable allow system administrators to override application settings without recompilation.

Configuration ordering

When a database is created or opened, the order of configuration is:

Any WiredTiger.basecfg file,
which is overridden by the wiredtiger_open configuration string argument.
which is overridden by the WiredTiger.config file,
which is overridden by the WIREDTIGER_CONFIG environment variable,

WIREDTIGER_CONFIG environment variable

If the WIREDTIGER_CONFIG environment variable is set, it is read as a configuration string.

If the process is running with special privileges and WIREDTIGER_CONFIG is set, the wiredtiger_open function must be configured with the use_environment_priv flag, or the call to wiredtiger_open will fail. See Database Home Directory for more details.

WiredTiger.config file

If a file named WiredTiger.config appears in the WiredTiger home directory, it is read as a configuration string.

The file is minimally parsed in order to build configuration strings for the WiredTiger configuration parser:

A backslash (\) followed by any character other than a newline character leaves both characters untouched; otherwise, if a backslash is followed by a newline character, both the backslash and the newline character are discarded.
Any text between double-quote pairs (") is left untouched, including newline and white-space characters. Backslash characters escape double-quote characters: a backslash escaped double-quote character can neither start nor end a quoted string.
Comments are discarded. If the first non-white-space character following an unquoted and unescaped newline character is a hash mark (#), all characters up to the next newline character are discarded. The ending newline character cannot be escaped or quoted, once a comment line is identified, all characters to the next newline are simply discarded.
Otherwise, all lines are concatenated and the newline characters replaced with commas.

WiredTiger.basecfg file

When a WiredTiger database is created, non-default configuration information specified to wiredtiger_open is saved to a WiredTiger home directory file named WiredTiger.basecfg, and that configuration file is read whenever the database is subsequently opened.

For example, if a shared library extensions are required, those settings will be saved to the base configuration file so it's easier to write general-purpose applications that might not be aware of how a particular database was created and configured.

Users should not not edit this file, set the WIREDTIGER_CONFIG environment variable or create a WiredTiger.config file instead, as those settings override the settings in the base configuration file.

This file is a potential problem for applications that want to retain the ability to downgrade to previous releases. If a new database is created after an upgrade, the base configuration file might have configuration strings that would cause a previous version of the application to fail. For this reason, applications that may want to downgrade should configure the wiredtiger_open value config_base to false so the base configuration file is not written as part of database create.