1 - Cloud Services

Tips and tricks for using ClickHouse with different cloud services.

1.1 - Altinity Cloud Access Management

Enabling access_management for Altinity.Cloud databases.

Organizations that want to enable administrative users in their Altinity.Cloud ClickHouse servers can do so by enabling access_management manually. This allows for administrative users to be created on the specific ClickHouse Cluster.

To add the access_management setting to an Altinity.Cloud ClickHouse Cluster:

  1. Log into your Altinity.Cloud account.

  2. For the cluster to modify, select Configure -> Settings.

    Cluster setting configure

  3. From the Settings page, select +ADD SETTING.

    Add cluster setting

  4. Set the following options:

    1. Setting Type: Select users.d file.

    2. Filename: access_management.xml

    3. Contents: Enter the following to allow the clickhouse_operator that controls the cluster through the clickhouse-operator the ability to set administrative options:


    access_management=1 means that users admin, clickhouse_operator are able to create users and grant them privileges using SQL.

  5. Select OK. The cluster will restart, and users can now be created in the cluster that can be granted administrative access.

  6. If you are running ClickHouse 21.9 and above you can enable storing access management in ZooKeeper. in this case it will be automatically propagated to the cluster. This requires yet another configuration file:

    1. Setting Type: Select config.d file

    2. Filename: user_directories.xml

    3. Contents:

        <user_directories replace="replace">

2 - MySQL

Integration Clickhouse with MySQL

Replication using MaterializeMySQL.

It reads mysql binlog directly and transform queries into something which clickhouse can support. Supports updates and deletes (under the hood implemented via something like ReplacingMergeTree with enforced FINAL and ‘deleted’ flag). Status is ’experimental’, there are quite a lot of known limitations and issues, but some people use it. The original author of that went to another project, and the main team don’t have a lot of resource to improve that for now (more important thing in the backlog)

The replication happens on the mysql database level.

Replication using debezium + Kafka

Debezium can read the binlog and transform it to Kafka messages. You can later capture the stream of message on ClickHouse side and process it as you like. Please remeber that currently Kafka engine supports only at-least-once delivery guarantees.

It’s used by several companies, quite nice & flexible. But initial setup may require some efforts.

Same as above but using https://maxwells-daemon.io/ instead of debezium.

Have no experience / feedback there, but should be very similar to debezium.

Replication using clickhouse-mysql

See https://altinity.com/blog/2018/6/30/realtime-mysql-clickhouse-replication-in-practice

That was done long time ago in altinity for one use-case, and it seem like it was never used outside of that. It’s a python application with lot of switches which can copy a schema or read binlog from mysql and put it to clickhouse. Not supported currently. But it’s just a python, so maybe can be adjusted to different needs.

Accessing MySQL data via integration engines from inside clickhouse.

MySQL table engine / table function, or MySQL database engine - clickhouse just connects to mysql server as a client, and can do normal selects.

We had webinar about that a year ago: https://www.youtube.com/watch?v=44kO3UzIDLI

Using that you can easily create some ETL script which will copy the data from mysql to clickhouse regularly, i.e. something like

INSERT INTO clickhouse_table SELECT * FROM mysql_table WHERE id > ...

Works great if you have append only table in MySQL.

In newer clickhouse versions you can query this was also sharded / replicated MySQL cluster - see ExternalDistributed

MySQL dictionaries

There are also MySQL dictionaries, which can be very nice alternative for storing some dimensions information in star schema.

3 - ODBC Driver for ClickHouse

ODBC Driver for ClickHouse

ODBC Driver for ClickHouse.

ODBC interface for ClickHouse RDBMS.

Licensed under the Apache 2.0.

Installation and usage


  1. Download the latest release. On 64bit system you usually need both 32 bit and 64 bit drivers.
  2. Install (usually you will need ANSI driver, but better to install both versions, see below).
  3. Configure ClickHouse DSN.

Note: that install driver linked against MDAC (which is default for Windows), some non-windows native applications (cygwin / msys64 based) may require driver linked agains unixodbc. Build section below.


  1. Install homebrew.
  2. Install driver
brew install https://raw.githubusercontent.com/proller/homebrew-core/chodbc/Formula/clickhouse-odbc.rb
  1. Add clickhouse DSN configuration into ~/.odbc.ini file. (sample)

Note: that install driver linked against iodbc (which is default for Mac), some homebrew applications (like python) may require unixodbc driver to work properly. In that case see Build section below.


  1. DEB/RPM packaging is not provided yet, please build & install the driver from sources.
  2. Add clickhouse DSN configuration into ~/.odbc.ini file. (sample)


On Linux / Max you configure DSN by adding new desctions in ~/.odbc.ini (See sample file: https://github.com/ClickHouse/clickhouse-odbc/blob/fd74398b50201ab13b535cdfab57bca86e588b37/packaging/odbc.ini.sample )

On Windows you can create/edit DSN using GUI tool through Control Panel.

The list of DSN parameters recognized by the driver is as follows:

ParameterDefault valueDescription
UrlemptyURL that points to a running ClickHouse instance, may include username, password, port, database, etc.
Protodeduced from Url, or from Port and SSLMode: https if 443 or 8443 or SSLMode is not empty, http otherwiseProtocol, one of: http, https
Server or Hostdeduced from UrlIP or hostname of a server with a running ClickHouse instance on it
Portdeduced from Url, or from Proto: 8443 if https, 8123 otherwisePort on which the ClickHouse instance is listening
Path/queryPath portion of the URL
UID or UsernamedefaultUser name
PWD or PasswordemptyPassword
DatabasedefaultDatabase name to connect to
Timeout30Connection timeout
SSLModeemptyCertificate verification method (used by TLS/SSL connections, ignored in Windows), one of: allow, prefer, require, use allow to enable <code>SSL_VERIFY_PEER</code> TLS/SSL certificate verification mode, <code>SSL_VERIFY_PEER | SSL_VERIFY_FAIL_IF_NO_PEER_CERT</code> is used otherwise
PrivateKeyFileemptyPath to private key file (used by TLS/SSL connections), can be empty if no private key file is used
CertificateFileemptyPath to certificate file (used by TLS/SSL connections, ignored in Windows), if the private key and the certificate are stored in the same file, this can be empty if PrivateKeyFile is specified
CALocationemptyPath to the file or directory containing the CA/root certificates (used by TLS/SSL connections, ignored in Windows)
DriverLogon if CMAKE_BUILD_TYPE is Debug, off otherwiseEnable or disable the extended driver logging
DriverLogFile\temp\clickhouse-odbc-driver.log on Windows, /tmp/clickhouse-odbc-driver.log otherwisePath to the extended driver log file (used when DriverLog is on)

Troubleshooting & bug reporting

If some software doesn’t work properly with that driver, but works good with other drivers - we will be appritiate if you will be able to collect debug info.

To debug issues with the driver, first things that need to be done are:

  • enabling driver manager tracing. Links may contain some irrelevant vendor-specific details.
    • on Windows/MDAC: 1, 2, 3
    • on Mac/iODBC: 1, 2
    • on Linux/unixODBC: 1, 2
  • enabling driver logging, see DriverLog and DriverLogFile DSN parameters above
  • making sure that the application is allowed to create and write these driver log and driver manager trace files
  • follow the steps leading to the issue.

Collected log files will help to diagnose & solve the issue.

Driver Managers

Note, that since ODBC drivers are not used directly by a user, but rather accessed through applications, which in their turn access the driver through ODBC driver manager, user have to install the driver for the same architecture (32- or 64-bit) as the application that is going to access the driver. Moreover, both the driver and the application must be compiled for (and actually use during run-time) the same ODBC driver manager implementation (we call them “ODBC providers” here). There are three supported ODBC providers:

  • ODBC driver manager associated with MDAC (Microsoft Data Access Components, sometimes referenced as WDAC, Windows Data Access Components) - the standard ODBC provider of Windows
  • UnixODBC - the most common ODBC provider in Unix-like systems. Theoretically, could be used in Cygwin or MSYS/MinGW environments in Windows too.
  • iODBC - less common ODBC provider, mainly used in Unix-like systems, however, it is the standard ODBC provider in macOS. Theoretically, could be used in Cygwin or MSYS/MinGW environments in Windows too.

If you don’t see a package that matches your platforms, or the version of your system is significantly different than those of the available packages, or maybe you want to try a bleeding edge version of the code that hasn’t been released yet, you can always build the driver manually from sources.

Note, that it is always a good idea to install the driver from the corresponding native package (.msi, etc., which you can also easily create if you are building from sources), than use the binaries that were manually copied to some folder.

Building from sources

The general requirements for building the driver from sources are as follows:

  • CMake 3.12 and later
  • C++17 and C11 capable compiler toolchain:
    • Clang 4 and later
    • GCC 7 and later
    • Xcode 10 and later
    • Microsoft Visual Studio 2017 and later
  • ODBC Driver manager (MDAC / unixodbc / iODBC)
  • SSL library (openssl)

Generic build scenario:

git clone --recursive git@github.com:ClickHouse/clickhouse-odbc.git
cd clickhouse-odbc
mkdir build
cd build
cmake -DCMAKE_BUILD_TYPE=RelWithDebInfo ..
cmake --build . -C RelWithDebInfo

Additional requirements exist for each platform, which also depend on whether packaging and/or testing is performed.


Execute the following in the terminal to install needed dependencies:

# on Red Hat/CentOS (tested on CentOS 7)
sudo yum groupinstall "Development Tools"
sudo yum install centos-release-scl
sudo yum install devtoolset-8
sudo yum install git cmake openssl-devel unixODBC-devel # You may use libiodbc-devel INSTEAD of unixODBC-devel
scl enable devtoolset-8 -- bash # Enable Software collections for that terminal session, to use newer versions of complilers

# on Ubuntu (tested on Ubuntu 18.10, for older versions you may need to install newer c++ compiler and cmake versions)
sudo apt install build-essential git cmake libpoco-dev libssl-dev unixodbc-dev # You may use libiodbc-devel INSEAD of unixODBC-devel

# MacOS: 
# You will need Xcode 10 or later and Command Line Tools to be installed, as well as [Homebrew](https://brew.sh/).
brew install git cmake make poco openssl libiodbc # You may use unixodbc INSTEAD of libiodbc 

Note: usually on Linux you use unixODBC driver manager, and on Mac - iODBC. In some (rare) cases you may need use other driver manager, please do it only if you clearly understand the differencies. Driver should be used with the driver manager it was linked to.

Clone the repo with submodules:

git clone --recursive git@github.com:ClickHouse/clickhouse-odbc.git

Enter the cloned source tree, create a temporary build folder, and generate a Makefile for the project in it:

cd clickhouse-odbc
mkdir build
cd build

# Configuration options for the project can be specified in the next command in a form of '-Dopt=val'
# For MacOS: you may also add '-G Xcode' to the next command, in order to use Xcode as a build system or IDE, and generate the solution and project files instead of Makefile.
cmake -DCMAKE_BUILD_TYPE=RelWithDebInfo ..

Build the generated solution in-place:

cmake --build . -C RelWithDebInfo
cmake --build . -C RelWithDebInfo --target package

…and, optionally, run tests (note, that for non-unit tests, preconfigured driver and DSN entries must exist, that point to the binaries generated in this build folder):

cmake --build . -C RelWithDebInfo --target test

For MacOS: if you configured the project with ‘-G Xcode’ initially, open the IDE and build all, package, and test targets manually from there

cmake --open .


CMake bundled with the recent versions of Visual Studio can be used.

An SDK required for building the ODBC driver is included in Windows SDK, which in its turn is also bundled with Visual Studio.

You will need to install WiX toolset to be able to generate .msi packages. You can download and install it from WiX toolset home page.

All of the following commands have to be issued in Visual Studio Command Prompt:

  • use x86 Native Tools Command Prompt for VS 2019 or equivalent for 32-bit builds
  • use x64 Native Tools Command Prompt for VS 2019 or equivalent for 64-bit builds

Clone the repo with submodules:

git clone --recursive git@github.com:ClickHouse/clickhouse-odbc.git

Enter the cloned source tree, create a temporary build folder, and generate the solution and project files in it:

cd clickhouse-odbc
mkdir build
cd build

# Configuration options for the project can be specified in the next command in a form of '-Dopt=val'

# Use the following command for 32-bit build only.
cmake -A Win32 -DCMAKE_BUILD_TYPE=RelWithDebInfo ..

# Use the following command for 64-bit build only.
cmake -A x64 -DCMAKE_BUILD_TYPE=RelWithDebInfo ..

Build the generated solution in-place:

cmake --build . -C RelWithDebInfo
cmake --build . -C RelWithDebInfo --target package

…and, optionally, run tests (note, that for non-unit tests, preconfigured driver and DSN entries must exist, that point to the binaries generated in this build folder):

cmake --build . -C RelWithDebInfo --target test

…or open the IDE and build all, package, and test targets manually from there:

cmake --open .

cmake options

The list of configuration options recognized during the CMake generation step is as follows:

OptionDefault valueDescription
CMAKE_BUILD_TYPERelWithDebInfoBuild type, one of: Debug, Release, RelWithDebInfo
CH_ODBC_ENABLE_SSLONEnable TLS/SSL (required for utilizing https:// interface, etc.)
CH_ODBC_ENABLE_INSTALLONEnable install targets (required for packaging)
CH_ODBC_ENABLE_TESTINGinherits value of BUILD_TESTINGEnable test targets
CH_ODBC_PREFER_BUNDLED_THIRD_PARTIESONPrefer bundled over system variants of third party libraries
CH_ODBC_PREFER_BUNDLED_POCOinherits value of CH_ODBC_PREFER_BUNDLED_THIRD_PARTIESPrefer bundled over system variants of Poco library
CH_ODBC_PREFER_BUNDLED_SSLinherits value of CH_ODBC_PREFER_BUNDLED_POCOPrefer bundled over system variants of TLS/SSL library
CH_ODBC_PREFER_BUNDLED_GOOGLETESTinherits value of CH_ODBC_PREFER_BUNDLED_THIRD_PARTIESPrefer bundled over system variants of Google Test library
CH_ODBC_PREFER_BUNDLED_NANODBCinherits value of CH_ODBC_PREFER_BUNDLED_THIRD_PARTIESPrefer bundled over system variants of nanodbc library
CH_ODBC_RUNTIME_LINK_STATICOFFLink with compiler and language runtime statically
CH_ODBC_THIRD_PARTY_LINK_STATICONLink with third party libraries statically
CH_ODBC_DEFAULT_DSN_UNICODEClickHouse DSN (Unicode)Default Unicode DSN name
TEST_DSNinherits value of CH_ODBC_DEFAULT_DSN_ANSIANSI DSN name to use in tests
TEST_DSN_Winherits value of CH_ODBC_DEFAULT_DSN_UNICODEUnicode DSN name to use in tests

Packaging / redistributing the driver

You can just copy the library to another computer, in that case you need to

  1. install run-time dependencies on target computer
    • Windows:
      • MDAC driver manager (preinstalled on all modern Windows systems)
      • C++ Redistributable for Visual Studio 2017 or same for 2019, etc.
    • Linux
# CentOS / RedHat
sudo yum install openssl unixODBC

# Debian/Ubuntu
sudo apt install openssl unixodbc
  • MacOS (assuming you have Homebrew installed):
brew install poco openssl libiodbc
  1. register the driver so that the corresponding ODBC provider is able to locate it.

All this involves modifying a dedicated registry keys in case of MDAC, or editing odbcinst.ini (for driver registration) and odbc.ini (for DSN definition) files for UnixODBC or iODBC, directly or indirectly.

This will be done automatically using some default values if you are installing the driver using native installers.

Otherwise, if you are configuring manually, or need to modify the default configuration created by the installer, please see the exact locations of files (or registry keys) that need to be modified.

4 - ClickHouse + Spark


ClickHouse + Spark


The trivial & natural way to talk to ClickHouse from Spark is using jdbc. There are 2 jdbc drivers:

ClickHouse-Native-JDBC has some hints about integration with Spark even in the main README file.

‘Official’ driver does support some conversion of complex data types (Roarring bitmaps) for Spark-Clickhouse integration: https://github.com/ClickHouse/clickhouse-jdbc/pull/596

But proper partitioning of the data (to spark partitions) may be tricky with jdbc.

Some example snippets:


via Kafka

ClickHouse can produce / consume data from/to Kafka to exchange data with Spark.

via hdfs

You can load data into hadoop/hdfs using sequence of statements like INSERT INTO FUNCTION hdfs(...) SELECT ... FROM clickhouse_table later process the data from hdfs by spark and do the same in reverse direction.

via s3

Similar to above but using s3.

via shell calls

You can call other commands from Spark. Those commands can be clickhouse-client and/or clickhouse-local.

do you really need Spark? :)

In many cases you can do everything inside ClickHouse without Spark help :) Arrays, Higher-order functions, machine learning, integration with lot of different things including the possibility to run some external code using executable dictionaries or UDF.

6 - CatBoost / MindsDB / Fast.ai

CatBoost / MindsDB / Fast.ai


  • It uses gradient boosting - a hard to use technique which can outperform neural networks. Gradient boosting is powerful but it’s easy to shoot yourself in the foot using it.
  • The documentation on how to use it is quite lacking. The only good source of information on how to properly configure a model to yield good results is this video: https://www.youtube.com/watch?v=usdEWSDisS0 . We had to dig around GitHub issues to find out how to make it work with ClickHouse.
  • CatBoost is fast. Other libraries will take ~5X to ~10X as long to do what CatBoost does.
  • CatBoost will do preprocessing out of the box (fills nulls, apply standard scaling, encodes strings as numbers).
  • CatBoost has all functions you’d need (metrics, plotters, feature importance)

It makes sense to split what CatBoost does into 2 parts:

  • preprocessing (fills nulls, apply standard scaling, encodes strings as numbers)
  • number crunching (convert preprocessed numbers to another number - ex: revenue of impression)

Compared to Fast.ai, CatBoost pre-processing is as simple to use and produces results that can be as good as Fast.ai.

The number crunching part of Fast.ai is no-config. For CatBoost you need to configure it, a lot.

CatBoost won’t simplify or hide any complexity of the process. So you need to know data science terms and what it does (ex: if your model is underfitting you can use a smaller l2_reg parameter in the model constructor).

In the end both Fast.ai and CatBoost can yield comparable results.

Regarding deploying models, CatBoost is really good. The model runs fast, it has a simple binary format which can be loaded in ClickHouse, C, or Python and it will encapsulate pre-processing with the binary file. Deploying Fast.ai models at scale/speed is impossible out of the box (we have our custom solution to do it which is not simple).

TLDR: CatBoost is fast, produces awesome models, is super easy to deploy and it’s easy to use/train (after becoming familiar with it despite the bad documentation & if you know data science terms).

Regarding MindsDB

The project seems to be a good idea but it’s too young. I was using the GUI version and I’ve encountered some bugs, and none of those bugs have a good error message.

  • It won’t show data in preview.

  • The “download” button won’t work.

  • It’s trying to create and drop tables in ClickHouse without me asking it to.

  • Other than bugs:

    • It will only use 1 core to do everything (training, analysis, download).
    • Analysis will only run with a very small subset of data, if I use something like 1M rows it never finishes.
  • Training a model on 100k rows took 25 minutes - (CatBoost takes 90s to train with 1M rows)

  • The model trained on MindsDB is way worse. It had r-squared of 0.46 (CatBoost=0.58)

    To me it seems that they are a plugin which connects ClickHouse to MySQL to run the model in Pytorch.

    It’s too complex and hard to debug and understand. The resulting model is not good enough.

    TLDR: Easy to use (if bugs are ignored), too slow to train & produces a bad model.

7 - Google S3 (GCS)

“Google S3 GCS”

GCS with the table function - seems to work correctly for simple scenarios.

Essentially you can follow the steps from the Migrating from Amazon S3 to Cloud Storage.

  1. Set up a GCS bucket.
  2. This bucket must be set as part of the default project for the account. This configuration can be found in settings -> interoperability.
  3. Generate a HMAC key for the account, can be done in settings -> interoperability, in the section for user account access keys.
  4. In ClickHouse, replace the S3 bucket endpoint with the GCS bucket endpoint This must be done with the path-style GCS endpoint: https://storage.googleapis.com/BUCKET_NAME/OBJECT_NAME.
  5. Replace the aws access key id and aws secret access key with the corresponding parts of the HMAC key.

s3 Disk on the top of GCS and writing to GSC may be NOT working because GCS don’t support some of bulk S3 API calls, see https://github.com/ClickHouse/ClickHouse/issues/24246

8 - Kafka

git log -- contrib/librdkafka | git name-rev --stdin
ClickHouse versionlibrdkafka version
21.10+ (#27883)1.6.1 + snappy fixes + boring ssl + illumos_build fixes + edenhill#3279 fix
21.6+ (#23874)1.6.1 + snappy fixes + boring ssl + illumos_build fixes
21.1+ (#18671)1.6.0-RC3 + snappy fixes + boring ssl
20.13+ (#18053)1.5.0 + msan fixes + snappy fixes + boring ssl
20.7+ (#12991)1.5.0 + msan fixes
20.5+ (#11256)1.4.2
20.2+ (#9000)1.3.0
19.11+ (#5872)1.1.0
19.5+ (#4799)1.0.0
19.1+ (#4025)1.0.0-RC5
v1.1.54382+ (#2276)0.11.4

8.1 - Adjusting librdkafka settings

Adjusting librdkafka settings

Some random example:

    <debug>all</debug><!-- only to get the errors -->

Authentication / connectivity

Amazon MSK





Inline Kafka certs

To connect to some Kafka cloud services you may need to use certificates.

If needed they can be converted to pem format and inlined into ClickHouse config.xml Example:

  RSA Private-Key: (3072 bit, 2 primes)

See xml



Azure Event Hub

See https://github.com/ClickHouse/ClickHouse/issues/12609


  <!-- Kerberos-aware Kafka -->

confluent cloud



How to test connection settings

Use kafkacat utility - it internally uses same library to access Kafla as clickhouse itself and allows easily to test different settings.

kafkacat -b my_broker:9092 -C -o -10 -t my_topic \
   -X security.protocol=SASL_SSL  \
   -X sasl.mechanisms=PLAIN \
   -X sasl.username=uerName \
   -X sasl.password=Password

Different configurations for different tables?

Is there some more documentation how to use this multiconfiguration for Kafka ?

The whole logic is here: https://github.com/ClickHouse/ClickHouse/blob/da4856a2be035260708fe2ba3ffb9e437d9b7fef/src/Storages/Kafka/StorageKafka.cpp#L466-L475

So it load the main config first, after that it load (with overwrites) the configs for all topics, listed in kafka_topic_list of the table.

Also since v21.12 it’s possible to use more straght-forward way using named_collections: https://github.com/ClickHouse/ClickHouse/pull/31691

So you can say something like

CREATE TABLE test.kafka (key UInt64, value UInt64) ENGINE = Kafka(kafka1, kafka_format='CSV');

And after that in configuration:


The same fragment of code in newer versions: https://github.com/ClickHouse/ClickHouse/blob/d19e24f530c30f002488bc136da78f5fb55aedab/src/Storages/Kafka/StorageKafka.cpp#L474-L496

8.2 - Error handling

Error handling

Pre 21.6

There are couple options:

Certain formats which has schema in built in them (like JSONEachRow) could silently skip any unexpected fields after enabling setting input_format_skip_unknown_fields

It’s also possible to skip up to N malformed messages for each block, with used setting kafka_skip_broken_messages but it’s also does not support all possible formats.

After 21.6

It’s possible to stream messages which could not be parsed, this behavior could be enabled via setting: kafka_handle_error_mode='stream' and clickhouse wil write error and message from Kafka itself to two new virtual columns: _error, _raw_message.

So you can create another Materialized View which would collect to a separate table all errors happening while parsing with all important information like offset and content of message.

CREATE TABLE default.kafka_engine
    `i` Int64,
    `s` String
ENGINE = Kafka
SETTINGS kafka_broker_list = 'kafka:9092'
kafka_topic_list = 'topic',
kafka_group_name = 'clickhouse',
kafka_format = 'JSONEachRow',

CREATE MATERIALIZED VIEW default.kafka_errors
    `topic` String,
    `partition` Int64,
    `offset` Int64,
    `raw` String,
    `error` String
ENGINE = MergeTree
ORDER BY (topic, partition, offset)
SETTINGS index_granularity = 8192 AS
    _topic AS topic,
    _partition AS partition,
    _offset AS offset,
    _raw_message AS raw,
    _error AS error
FROM default.kafka_engine
WHERE length(_error) > 0

Table connections




8.3 - Exactly once semantics

Exactly once semantics

EOS consumer (isolation.level=read_committed) is enabled by default since librdkafka 1.2.0, so for ClickHouse - since 20.2


BUT: while EOS semantics will guarantee you that no duplicates will happen on the Kafka side (i.e. even if you produce the same messages few times it will be consumed once), but ClickHouse as a Kafka client can currently guarantee only at-least-once. And in some corner cases (connection lost etc) you can get duplicates.

We need to have something like transactions on ClickHouse side to be able to avoid that. Adding something like simple transactions is in plans for Y2022.

block-aggregator by eBay

Block Aggregator is a data loader that subscribes to Kafka topics, aggregates the Kafka messages into blocks that follow the Clickhouse’s table schemas, and then inserts the blocks into ClickHouse. Block Aggregator provides exactly-once delivery guarantee to load data from Kafka to ClickHouse. Block Aggregator utilizes Kafka’s metadata to keep track of blocks that are intended to send to ClickHouse, and later uses this metadata information to deterministically re-produce ClickHouse blocks for re-tries in case of failures. The identical blocks are guaranteed to be deduplicated by ClickHouse.


8.4 - Kafka main parsing loop

Kafka main parsing loop

One of the threads from scheduled_pool (pre 20.9) / background_message_broker_schedule_pool (after 20.9) do that in infinite loop:

  1. Batch poll (time limit: kafka_poll_timeout_ms 500ms, messages limit: kafka_poll_max_batch_size 65536)
  2. Parse messages.
  3. If we don’t have enough data (rows limit: kafka_max_block_size 1048576) or time limit reached (kafka_flush_interval_ms 7500ms) - continue polling (goto p.1)
  4. Write a collected block of data to MV
  5. Do commit (commit after write = at-least-once).

On any error, during that process, Kafka client is restarted (leading to rebalancing - leave the group and get back in few seconds).

Kafka batching

Important settings

These usually should not be adjusted:

  • kafka_poll_max_batch_size = max_block_size (65536)
  • kafka_poll_timeout_ms = stream_poll_timeout_ms (500ms)

You may want to adjust those depending on your scenario:

  • kafka_flush_interval_ms = stream_poll_timeout_ms (7500ms)
  • kafka_max_block_size = max_insert_block_size / kafka_num_consumers (for the single consumer: 1048576)

See also


8.5 - Kafka parallel consuming

Kafka parallel consuming

For very large topics when you need more parallelism (especially on the insert side) you may use several tables with the same pipeline (pre 20.9) or enable kafka_thread_per_consumer (after 20.9).

kafka_num_consumers = N,


  • the inserts will happen in parallel (without that setting inserts happen linearly)
  • enough partitions are needed.

Before increasing kafka_num_consumers with keeping kafka_thread_per_consumer=0 may improve consumption & parsing speed, but flushing & committing still happens by a single thread there (so inserts are linear).

8.6 - Rewind / fast-forward / replay

Rewind / fast-forward / replay

See also these configuration settings:


8.7 - SELECTs from engine=Kafka

SELECTs from engine=Kafka


What will happen, if we would run SELECT query from working Kafka table with MV attached? Would data showed in SELECT query appear later in MV destination table?


  1. Most likely SELECT query would show nothing.
  2. If you lucky enough and something would show up, those rows wouldn’t appear in MV destination table.

So it’s not recommended to run SELECT queries on working Kafka tables.

In case of debug it’s possible to use another Kafka table with different consumer_group, so it wouldn’t affect your main pipeline.