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.

Configuration ordering

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

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

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, 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.

Users should not edit this file, set the WIREDTIGER_CONFIG environment variable or create a WiredTiger.config file instead.