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HTTP Interface

The HTTP interface lets you use ClickHouse on any platform from any programming language in a form of REST API. The HTTP interface is more limited than the native interface, but it has better language support.

By default, clickhouse-server listens for HTTP on port 8123 (this can be changed in the config). HTTPS can be enabled as well with port 8443 by default.

If you make a GET / request without parameters, it returns 200 response code and the string which defined in http_server_default_response default value "Ok." (with a line feed at the end)

Also see: HTTP response codes caveats.

Sometimes, curl command is not available on user operating systems. On Ubuntu or Debian, run sudo apt install curl. Please refer this documentation to install it before running the examples.

Web UI can be accessed here: http://localhost:8123/play.

The Web UI supports displaying progress during query runtime, query cancellation, and streaming results. It has a secret feature for displaying charts and graphs for query pipelines.

Web UI is designed for professionals like you.

In health-check scripts use GET /ping request. This handler always returns "Ok." (with a line feed at the end). Available from version 18.12.13. See also /replicas_status to check replica's delay.

Send the request as a URL 'query' parameter, or as a POST. Or send the beginning of the query in the 'query' parameter, and the rest in the POST (we'll explain later why this is necessary). The size of the URL is limited to 1 MiB by default, this can be changed with the http_max_uri_size setting.

If successful, you receive the 200 response code and the result in the response body. If an error occurs, you receive the 500 response code and an error description text in the response body.

When using the GET method, 'readonly' is set. In other words, for queries that modify data, you can only use the POST method. You can send the query itself either in the POST body or in the URL parameter.

Examples:

As you can see, curl is somewhat inconvenient in that spaces must be URL escaped. Although wget escapes everything itself, we do not recommend using it because it does not work well over HTTP 1.1 when using keep-alive and Transfer-Encoding: chunked.

If part of the query is sent in the parameter, and part in the POST, a line feed is inserted between these two data parts. Example (this won't work):

By default, data is returned in TabSeparated format.

You use the FORMAT clause of the query to request any other format.

Also, you can use the 'default_format' URL parameter or the 'X-ClickHouse-Format' header to specify a default format other than TabSeparated.

The POST method of transmitting data is necessary for INSERT queries. In this case, you can write the beginning of the query in the URL parameter, and use POST to pass the data to insert. The data to insert could be, for example, a tab-separated dump from MySQL. In this way, the INSERT query replaces LOAD DATA LOCAL INFILE from MySQL.

Examples

Creating a table:

Using the familiar INSERT query for data insertion:

Data can be sent separately from the query:

You can specify any data format. The 'Values' format is the same as what is used when writing INSERT INTO t VALUES:

To insert data from a tab-separated dump, specify the corresponding format:

Reading the table contents. Data is output in random order due to parallel query processing:

Deleting the table.

For successful requests that do not return a data table, an empty response body is returned.

Compression​

You can use compression to reduce network traffic when transmitting a large amount of data or for creating dumps that are immediately compressed.

You can use the internal ClickHouse compression format when transmitting data. The compressed data has a non-standard format, and you need clickhouse-compressor program to work with it. It is installed with the clickhouse-client package. To increase the efficiency of data insertion, you can disable server-side checksum verification by using the http_native_compression_disable_checksumming_on_decompress setting.

If you specify compress=1 in the URL, the server will compress the data it sends to you. If you specify decompress=1 in the URL, the server will decompress the data which you pass in the POST method.

You can also choose to use HTTP compression. ClickHouse supports the following compression methods:

  • gzip
  • br
  • deflate
  • xz
  • zstd
  • lz4
  • bz2
  • snappy

To send a compressed POST request, append the request header Content-Encoding: compression_method. In order for ClickHouse to compress the response, enable compression with enable_http_compression setting and append Accept-Encoding: compression_method header to the request. You can configure the data compression level in the http_zlib_compression_level setting for all compression methods.

info

Some HTTP clients might decompress data from the server by default (with gzip and deflate) and you might get decompressed data even if you use the compression settings correctly.

Examples

Default Database​

You can use the 'database' URL parameter or the 'X-ClickHouse-Database' header to specify the default database.

By default, the database that is registered in the server settings is used as the default database. By default, this is the database called 'default'. Alternatively, you can always specify the database using a dot before the table name.

The username and password can be indicated in one of three ways:

  1. Using HTTP Basic Authentication. Example:
  1. In the 'user' and 'password' URL parameters (We do not recommend using this method as the parameter might be logged by web proxy and cached in the browser). Example:
  1. Using 'X-ClickHouse-User' and 'X-ClickHouse-Key' headers. Example:

If the user name is not specified, the default name is used. If the password is not specified, the empty password is used. You can also use the URL parameters to specify any settings for processing a single query or entire profiles of settings. Example:http://localhost:8123/?profile=web&max_rows_to_read=1000000000&query=SELECT+1

For more information, see the Settings section.

For information about other parameters, see the section "SET".

Using ClickHouse sessions in the HTTP protocol​

You can also use ClickHouse sessions in the HTTP protocol. To do this, you need to add the session_id GET parameter to the request. You can use any string as the session ID. By default, the session is terminated after 60 seconds of inactivity. To change this timeout (in seconds), modify the default_session_timeout setting in the server configuration, or add the session_timeout GET parameter to the request. To check the session status, use the session_check=1 parameter. Only one query at a time can be executed within a single session.

You can receive information about the progress of a query in X-ClickHouse-Progress response headers. To do this, enable send_progress_in_http_headers. Example of the header sequence:

Possible header fields:

  • read_rows — Number of rows read.
  • read_bytes — Volume of data read in bytes.
  • total_rows_to_read — Total number of rows to be read.
  • written_rows — Number of rows written.
  • written_bytes — Volume of data written in bytes.

Running requests do not stop automatically if the HTTP connection is lost. Parsing and data formatting are performed on the server-side, and using the network might be ineffective. The optional 'query_id' parameter can be passed as the query ID (any string). For more information, see the section "Settings, replace_running_query".

The optional 'quota_key' parameter can be passed as the quota key (any string). For more information, see the section "Quotas".

The HTTP interface allows passing external data (external temporary tables) for querying. For more information, see the section "External data for query processing".

Response Buffering​

You can enable response buffering on the server-side. The buffer_size and wait_end_of_query URL parameters are provided for this purpose. Also settings http_response_buffer_size and http_wait_end_of_query can be used.

buffer_size determines the number of bytes in the result to buffer in the server memory. If a result body is larger than this threshold, the buffer is written to the HTTP channel, and the remaining data is sent directly to the HTTP channel.

To ensure that the entire response is buffered, set wait_end_of_query=1. In this case, the data that is not stored in memory will be buffered in a temporary server file.

Example:

Use buffering to avoid situations where a query processing error occurred after the response code and HTTP headers were sent to the client. In this situation, an error message is written at the end of the response body, and on the client-side, the error can only be detected at the parsing stage.

Setting a role with query parameters​

This is a new feature added in ClickHouse 24.4.

In specific scenarios, setting the granted role first might be required before executing the statement itself. However, it is not possible to send SET ROLE and the statement together, as multi-statements are not allowed:

Which will result in an error:

To overcome this limitation, you could use the role query parameter instead:

This will be the equivalent of executing SET ROLE my_role before the statement.

Additionally, it is possible to specify multiple role query parameters:

In this case, ?role=my_role&role=my_other_role works similarly to executing SET ROLE my_role, my_other_role before the statement.

HTTP response codes caveats​

Because of limitation of HTTP protocol, HTTP 200 response code does not guarantee that a query was successful.

Here is an example:

The reason for this behavior is the nature of the HTTP protocol. The HTTP header is sent first with an HTTP code of 200, followed by the HTTP body, and then the error is injected into the body as plain text. This behavior is independent of the format used, whether it's Native, TSV, or JSON; the error message will always be in the middle of the response stream. You can mitigate this problem by enabling wait_end_of_query=1 (Response Buffering). In this case, the sending of the HTTP header is delayed until the entire query is resolved. However, this does not completely solve the problem because the result must still fit within the http_response_buffer_size, and other settings like send_progress_in_http_headers can interfere with the delay of the header. The only way to catch all errors is to analyze the HTTP body before parsing it using the required format.

Queries with Parameters​

You can create a query with parameters and pass values for them from the corresponding HTTP request parameters. For more information, see Queries with Parameters for CLI.

Example​

Tabs in URL Parameters​

Query parameters are parsed from the "escaped" format. This has some benefits, such as the possibility to unambiguously parse nulls as \N. This means the tab character should be encoded as \t (or \ and a tab). For example, the following contains an actual tab between abc and 123 and the input string is split into two values:

However, if you try to encode an actual tab using %09 in a URL parameter, it won't get parsed properly:

If you are using URL parameters, you will need to encode the \t as %5C%09. For example:

Predefined HTTP Interface​

ClickHouse supports specific queries through the HTTP interface. For example, you can write data to a table as follows:

ClickHouse also supports Predefined HTTP Interface which can help you more easily integrate with third-party tools like Prometheus exporter.

Example:

  • First of all, add this section to server configuration file:
  • You can now request the URL directly for data in the Prometheus format:

As you can see from the example if http_handlers is configured in the config.xml file and http_handlers can contain many rules. ClickHouse will match the HTTP requests received to the predefined type in rule and the first matched runs the handler. Then ClickHouse will execute the corresponding predefined query if the match is successful.

Now rule can configure method, headers, url, handler:

  • method is responsible for matching the method part of the HTTP request. method fully conforms to the definition of method in the HTTP protocol. It is an optional configuration. If it is not defined in the configuration file, it does not match the method portion of the HTTP request.

  • url is responsible for matching the URL part of the HTTP request. It is compatible with RE2's regular expressions. It is an optional configuration. If it is not defined in the configuration file, it does not match the URL portion of the HTTP request.

  • headers are responsible for matching the header part of the HTTP request. It is compatible with RE2's regular expressions. It is an optional configuration. If it is not defined in the configuration file, it does not match the header portion of the HTTP request.

  • handler contains the main processing part. Now handler can configure type, status, content_type, http_response_headers, response_content, query, query_param_name. type currently supports three types: predefined_query_handler, dynamic_query_handler, static.

    • query — use with predefined_query_handler type, executes query when the handler is called.

    • query_param_name — use with dynamic_query_handler type, extracts and executes the value corresponding to the query_param_name value in HTTP request parameters.

    • status — use with static type, response status code.

    • content_type — use with any type, response content-type.

    • http_response_headers — use with any type, response headers map. Could be used to set content type as well.

    • response_content — use with static type, response content sent to client, when using the prefix 'file://' or 'config://', find the content from the file or configuration sends to client.

Next are the configuration methods for different type.

predefined_query_handler​

predefined_query_handler supports setting Settings and query_params values. You can configure query in the type of predefined_query_handler.

query value is a predefined query of predefined_query_handler, which is executed by ClickHouse when an HTTP request is matched and the result of the query is returned. It is a must configuration.

The following example defines the values of max_threads and max_final_threads settings, then queries the system table to check whether these settings were set successfully.

note

To keep the default handlers such as query, play, ping, add the <defaults/> rule.

Example:

note

In one predefined_query_handler only one query is supported.

dynamic_query_handler​

In dynamic_query_handler, the query is written in the form of parameter of the HTTP request. The difference is that in predefined_query_handler, the query is written in the configuration file. You can configure query_param_name in dynamic_query_handler.

ClickHouse extracts and executes the value corresponding to the query_param_name value in the URL of the HTTP request. The default value of query_param_name is /query . It is an optional configuration. If there is no definition in the configuration file, the parameter is not passed in.

To experiment with this functionality, the example defines the values of max_threads and max_final_threads and queries whether the settings were set successfully.

Example:

static​

static can return content_type, status and response_content. response_content can return the specified content.

Example:

Return a message.

http_response_headers could be used to set content type instead of content_type.

Find the content from the configuration send to client.

Find the content from the file send to client.

Valid JSON/XML response on exception during HTTP streaming​

While query execution over HTTP an exception can happen when part of the data has already been sent. Usually an exception is sent to the client in plain text even if some specific data format was used to output data and the output may become invalid in terms of specified data format. To prevent it, you can use setting http_write_exception_in_output_format (enabled by default) that will tell ClickHouse to write an exception in specified format (currently supported for XML and JSON* formats).

Examples: