ClickHouse DB::Exception: No elements in configuration

The NO_ELEMENTS_IN_CONFIG error is raised when a required configuration section exists but is empty -- it has no child elements where at least one is expected. For example, defining a <remote_servers> block with no clusters inside it, or a <storage_configuration> with no disks defined, will trigger this error. The message typically reads DB::Exception: No elements in config for 'section_name'.

Impact

This error usually prevents ClickHouse from starting, because the empty section is one that the server depends on during initialization. Even if the server does manage to start, the feature associated with the empty section will not function. For instance, an empty <remote_servers> section means no distributed queries will work, and an empty <storage_configuration> means tiered storage will not be available.

Common Causes

Placeholder section left empty -- A section was added to the config in preparation for future use but never populated with the required child elements.
Override file blanked a section -- A config.d/ fragment used replace="replace" on a section but provided no children, effectively emptying it.
Commented-out content -- All child elements within a section were commented out, leaving the section present but empty.
Template or automation error -- A configuration management tool (Ansible, Puppet, Chef) rendered a template with an empty loop, producing an empty section.
Accidental deletion -- Content was inadvertently removed during manual editing.

Troubleshooting and Resolution Steps

Identify the empty section: The error message names the section. Locate it in the configuration:
```
grep -rn 'section_name' /etc/clickhouse-server/config.xml /etc/clickhouse-server/config.d/
```
Check the preprocessed config: Examine the merged configuration to see what ClickHouse actually sees:
```
cat /var/lib/clickhouse/preprocessed_configs/config.xml
```
Search for the empty section -- it may appear as <section_name></section_name> or <section_name/>.

Populate the section with required elements: Add the necessary child elements. For example, if <remote_servers> is empty:

<remote_servers>
    <my_cluster>
        <shard>
            <replica>
                <host>clickhouse-node-1</host>
                <port>9000</port>
            </replica>
        </shard>
    </my_cluster>
</remote_servers>

Remove the section entirely if not needed: If you do not need the feature, remove the empty section rather than leaving it in place. ClickHouse will use its defaults or simply not enable the feature:
```
# Remove or comment out the entire section
```

Check override files for accidental blanking: If a config.d/ file uses replace="replace" on a section, ensure it provides the necessary children:

<!-- This will cause NO_ELEMENTS_IN_CONFIG -->
<storage_configuration replace="replace">
</storage_configuration>

<!-- This is correct -->
<storage_configuration replace="replace">
    <disks>
        <default/>
    </disks>
    <policies>
        <default>
            <volumes>
                <main>
                    <disk>default</disk>
                </main>
            </volumes>
        </default>
    </policies>
</storage_configuration>

Verify template rendering: If you use automation tools, inspect the rendered template output on the target server to ensure loops and conditionals produced valid content.

Best Practices

Do not add configuration sections until you are ready to populate them. An absent section is fine; an empty one is an error.
When using replace="replace" in override files, always verify that the replacement includes all required child elements.
Test configuration templates with realistic data before deploying to production.
Review the preprocessed config file after changes to verify the final merged result looks correct.
Add validation steps to your deployment pipeline that check for empty required sections.

Frequently Asked Questions

Q: Which configuration sections require child elements?
A: Sections like <remote_servers>, <storage_configuration>, <dictionaries> (when present), and <graphite_rollup> require child elements if the section tag is present. If you do not need these features, simply remove the section entirely.

Q: Can an empty <profiles> section cause this error?
A: Typically not, because ClickHouse has a built-in default profile. However, an empty <users> section would prevent any user from connecting. The error depends on whether ClickHouse strictly requires children for that specific section.

Q: I removed the section but still get the error. What is going on?
A: Check config.d/ for other fragments that might re-introduce the section. Also verify you edited the correct file -- the preprocessed config in /var/lib/clickhouse/preprocessed_configs/ shows the final merged state.

Q: Does this error occur with YAML configuration too?
A: Yes. An empty mapping in YAML (e.g., storage_configuration: with nothing under it) will produce the same error. Either populate the section or remove the key entirely.