ClickHouse/docs/en/operations/configuration_files.md

Configuration Files

The main server config file is config.xml. It resides in the /etc/clickhouse-server/ directory.

Individual settings can be overridden in the *.xml and *.conf files in the conf.d and config.d directories next to the config file.

The replace or remove attributes can be specified for the elements of these config files.

If neither is specified, it combines the contents of elements recursively, replacing values of duplicate children.

If replace is specified, it replaces the entire element with the specified one.

If remove is specified, it deletes the element.

The config can also define "substitutions". If an element has the incl attribute, the corresponding substitution from the file will be used as the value. By default, the path to the file with substitutions is /etc/metrika.xml. This can be changed in the include_from element in the server config. The substitution values are specified in /yandex/substitution_name elements in this file. If a substitution specified in incl does not exist, it is recorded in the log. To prevent ClickHouse from logging missing substitutions, specify the optional="true" attribute (for example, settings for macros).

Substitutions can also be performed from ZooKeeper. To do this, specify the attribute from_zk = "/path/to/node". The element value is replaced with the contents of the node at /path/to/node in ZooKeeper. You can also put an entire XML subtree on the ZooKeeper node and it will be fully inserted into the source element.

The config.xml file can specify a separate config with user settings, profiles, and quotas. The relative path to this config is set in the 'users_config' element. By default, it is users.xml. If users_config is omitted, the user settings, profiles, and quotas are specified directly in config.xml.

In addition, users_config may have overrides in files from the users_config.d directory (for example, users.d) and substitutions. For example, you can have separate config file for each user like this:

$ cat /etc/clickhouse-server/users.d/alice.xml
<yandex>
    <users>
      <alice>
          <profile>analytics</profile>
            <networks>
                  <ip>::/0</ip>
            </networks>
          <password_sha256_hex>...</password_sha256_hex>
          <quota>analytics</quota>
      </alice>
    </users>
</yandex>

For each config file, the server also generates file-preprocessed.xml files when starting. These files contain all the completed substitutions and overrides, and they are intended for informational use. If ZooKeeper substitutions were used in the config files but ZooKeeper is not available on the server start, the server loads the configuration from the preprocessed file.

The server tracks changes in config files, as well as files and ZooKeeper nodes that were used when performing substitutions and overrides, and reloads the settings for users and clusters on the fly. This means that you can modify the cluster, users, and their settings without restarting the server.

Original article