Installing and Updating an Extension

ClickHouse supports custom extensions via [User Defined Functions (UDFs)], external dictionaries, and shared libraries that extend its core capabilities with custom logic, formats, or integrations. These behave similarly to modules or plugins in other systems and must be configured at server startup. Common examples include integration with geospatial libraries, custom UDFs, or external dictionary sources like MySQL or HTTP. 

 In Elestio-hosted ClickHouse instances or any Docker Compose-based setup, extensions can be added by mounting external libraries or configuration files and referencing them in config.xml or users.xml . This guide walks through how to install, load, and manage ClickHouse extensions using Docker Compose along with best practices and common troubleshooting steps. 

 Installing and Enabling ClickHouse Extensions 

 ClickHouse extensions are typically compiled as shared objects ( .so ) files or defined as configuration files for dictionaries or formats. These files must be mounted into the container and referenced explicitly in the server’s configuration files. 

 Example: Load Custom Shared Library UDF 

 Suppose you have a compiled UDF called libexample_udf.so . To include it in a Docker Compose setup: 

 Update  docker-compose.yml 

 Mount the shared library into the container: 

 services:

 clickhouse:

 image: clickhouse/clickhouse-server:latest

 volumes:

 - ./modules/libexample_udf.so:/usr/lib/clickhouse/user_defined/libexample_udf.so

 - ./configs/config.xml:/etc/clickhouse-server/config.xml

 ports:

 - "8123:8123"

 - "9000:9000" 

 

 

 ./modules/libexample_udf.so : local path to the shared library on the host. 

 

 

 /usr/lib/clickhouse/user_defined/ : default directory for user libraries inside the container. 

 

 

 Make sure the file exists before running Docker Compose. 

 Configure  config.xml to Load the UDF 

 In your custom config.xml : 

 <user_defined>

 <function>

 <name>example_udf</name>

 <type>udf</type>

 <library>libexample_udf.so</library>

 </function>

</user_defined> 

 The library path must match the volume mount location. 

 Restart the ClickHouse Service 

 After updating the Compose and configuration files, restart the service: 

 docker-compose down

docker-compose up -d 

 This will reload ClickHouse with the specified UDF. 

 Verify the Extension is Loaded 

 Connect using the ClickHouse CLI or HTTP interface and run: 

 SELECT example_udf('test input'); 

 If successful, the function will return expected results from the loaded library. You can also confirm the server loaded your shared library by inspecting logs: 

 docker-compose logs clickhouse 

 Look for lines that indicate the library was found and loaded. 

 Managing External Dictionaries 

 ClickHouse supports loading external data sources (like MySQL, HTTP APIs, or files) as dictionaries 

 Mount Dictionary Configuration 

 In docker-compose.yml : 

 volumes:

 - ./configs/dictionaries/:/etc/clickhouse-server/dictionaries/ 

 Reference in  config.xml 

 <dictionaries_config>/etc/clickhouse-server/dictionaries/*.xml</dictionaries_config> 

 Example dictionary file ( mysql_dictionary.xml ): 

 <dictionary>

 <name>mysql_dict</name>

 <source>

 <mysql>

 <host>mysql-host</host>

 <user>root</user>

 <password>password</password>

 <db>test</db>

 <table>cities</table>

 </mysql>

 </source>

 <layout><flat /></layout>

 <structure>

 <id>id</id>

 <attribute>

 <name>name</name>

 <type>String</type>

 </attribute>

 </structure>

</dictionary> 

 Use the dictionary in queries: 

 SELECT dictGetString('mysql_dict', 'name', toUInt64(42)); 

 Updating or Removing Extensions 

 ClickHouse doesn’t support unloading UDFs or dictionaries at runtime. To modify or remove an extension: 

 1.  Stop the container : 

 docker-compose down 

 2. Edit config files : 

 

 

 Replace or remove the <function> entry in config.xml or dictionary config. 

 

 

 Replace or remove the .so file if applicable. 

 

 

 3. Restart the container : 

 docker-compose up -d 

 Always test changes in staging before deploying to production. 

 Troubleshooting Common Extension Issues 

 

 

 

 

 Issue 

 

 

 Cause 

 

 

 Resolution 

 

 

 

 

 

 

 ClickHouse fails to start 

 

 

 Invalid config or missing .so file 

 

 

 Run docker-compose logs clickhouse and fix missing files or XML syntax 

 

 

 

 

 UDF not recognized 

 

 

 Wrong library path or missing permissions 

 

 

 Ensure volume mount is correct and file is executable inside container 

 

 

 

 

 Dictionary not available 

 

 

 Config file not found or misconfigured XML 

 

 

 Double-check dictionaries_config and validate with SHOW DICTIONARIES 

 

 

 

 

 Segmentation fault 

 

 

 Invalid shared library or ABI mismatch 

 

 

 Recompile UDF for correct platform, verify against installed ClickHouse version 

 

 

 

 

 Query fails silently 

 

 

 Dictionary or UDF not fully loaded 

 

 

 Recheck server logs for errors during startup 

 

 

 

 

 Security Considerations 

 ClickHouse extensions especially shared libraries run with the same privileges as the ClickHouse server. Be cautious: 

 

 

 Only load trusted .so files from verified sources. 

 

 

 Ensure clickhouse user has restricted permissions inside the container. 

 

 

 Never expose dictionary or UDF paths to writable directories from external systems. 

 

 

 Avoid using custom UDFs or dictionaries from unknown sources in production environments without a thorough code review.