Clickhouse Scala Client

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Clickhouse Scala Client that uses Pekko Http to create a reactive streams implementation to access the Clickhouse database in a reactive way.


  • read/write query execution
  • pekko streaming source for result parsing
  • pekko streaming sink for data insertion
  • streaming query progress (experimental)
  • all the http interface settings
  • load balancing with internal health checks (multi host and cluster aware host balancer)
  • ability to retry queries

We do not guarantee api-backwards compatibility, although the API has been very stable over the last years.

Scala version:

  • 2.13
  • 2.12


for sbt you can use

libraryDependencies += "com.crobox.clickhouse" %% "client" % "0.9.0"


When in doubt about the documentation please read the tests to find the truth.

Quick Setup


val config: Config
val queryDatabase: String = "default"
implicit val system:ActorSystem

val client = new ClickhouseClient(config, queryDatabase)
client.query("SELECT 1 + 1").map(result => {
    println(s"Got query result $result")


val config: Config
val client: ClickhouseClient

val sink = ClickhouseSink.insertSink(config, client)
sink.runWith(Source.single(Insert("clicks", "{some_column: 3 }")))


  • Client: All the configuration keys are under the prefix crobox.clickhouse.client
  • Indexer: All the configuration keys are under the prefix crobox.clickhouse.indexer. You can also provide specific overrides based on the indexer name by using the same configs under the prefix crobox.clickhouse.indexer.{indexer-name}

Client configuration

You can find all the configuration options in the reference file, with explanatory comments about their usage.

Connection configuration

Three different connection modes are supported.

  • single-host
  • balancing-hosts
  • cluster-aware

Health checks

The balancing-hosts and cluster-aware connections are setting up health checks for each host, by running a simple http request on clickhouse host as specified in the clickhouse docs. For the healthchecks we use separate Cached Host Connection Pools with a maximum of one connection to ensure we never run more than one health check at the same time for the same host. When a host fails the healthchecks we will no longer use it to run queries. If all the health checks are failing the queries will fail fast.

crobox.clickhouse.client.connection {
      health-check {
        interval = 5 seconds #minimum interval between two health checks
        timeout = 1 second #health check will fail if it exceed timeout

Single host connection

crobox.clickhouse.client {
    connection: {
        type = "single-host",
        host = "localhost",
        port = 8123

This will not setup a health check and will dispatch all queries to the configured host.

Multi host balancing connection

Round robin on the configured hosts.

crobox.clickhouse.client {
    connection: {
        type = "balancing-hosts"
        hosts: [
            host = "localhost",
            port = 7415

Cluster aware balancing connection

The host and the port will be used to continually update the list of clickhouse nodes by querying and using the host-name from the system.cluster clickhouse table. (check scanning-interval) You can specify a specific clickhouse cluster to run queries only on the respective cluster. Please do note that this connection type will default to using the port of 8123 for all nodes.

crobox.clickhouse.client {
    connection: {
        type = "cluster-aware"
        host = "localhost"
        port = 8123
        cluster = "cluster" # use only hosts which belong to the "cluster" cluster
        health-check {
              interval = 5 seconds
              timeout = 1 second
        scanning-interval = 10 seconds # min interval between running a new query to update the list of hosts from the system.cluster table 

Indexer configuration

Inserting into clickhouse is done using an pekko stream. All the settings are applied on a per table basis. We will do one insert when the maximum number of items batch-size or the maximum time has been exceeded flush-interval. Based on the number of concurrent-requests we can run multiple inserts in parallel for the same table.

crobox.clickhouse {
  indexer {
    batch-size = 10000
    concurrent-requests = 1
    flush-interval = 5 seconds
    fast-indexer {
        flush-interval = 1 second
        batch-size = 1000

Query settings

To set authentication or a settings profile for the client you can update the following configs. You can also set custom settings as presented in the clickhouse documentation

    settings {
      authentication {
        user = "default"
        password = ""
      profile = "default"
      http-compression = false
      custom {
           distributed_product_mode = "local"

Client API

Query execution

Read only queries

val client: ClickhouseClient
client.query("SELECT 1").map(result => println(result))

Write queries

val client: ClickhouseClient
client.execute("ALTER TABLE my_table DELETE WHERE id = 'deleted'").map(result => println(result))

Streaming delimited result (by new line)

val client: ClickhouseClient
client.source("SELECT * FROM my_table").runWith(Sink.foreach(line => println(line)))

Streaming raw result (ByteString)

val client: ClickhouseClient
client.sourceByteString("SELECT * FROM my_table").runWith(Sink.foreach(byteString => println(byteString)))

Sink streaming body

val client: ClickhouseClient
client.sink("INSERT INTO my_table", Source.single(ByteString("el1"))).map(result => println(result))

Query progress

@Experimental - might not be complete

We only expose progress when running read only queries. The current implementation is recommended to be used only for long running queries which return a result relatively small in size (fits easily in memory). The returned source is materialized with the query result.

When running queries with progress we set a custom client transport for the super pool used by client to run the queries. Due to limitation in the pekko implementation which does not allow for the headers to be streamed we are parsing the raw http output and intercept the http headers to receive the progress.

We expose multiple events for the progress:

  • QueryAccepted - clickhouse returned the http response with code 200 (query might still fail)
  • QueryRejected - clickhouse returned the http response with a code different than 200 (it has not started execution)
  • QueryFailed - clickhouse returned an exception in the body, after the query was accepted and it started execution
  • Progress - contains the numbers of rows read and the number of total rows
  • QueryRetry - the same query is being retried by the client
val client: ClickhouseClient
client.queryWithProgress("SELECT uniq(timestamps), uniq(mosquito_name) FROM mosquito_bites")
      .toMat(Sink.forEach(progress => println(progress)))(Keep.left)
      .map(result => println(result))

Query settings

Every call to the client accepts an implicit QuerySettings object which can override settings for that specific query.

  • You can set the query id so that you can track/kill/replace running queries.
  • You can mark the query as idempotent and it will be retried for all exceptions when running the ClickhouseSink(Indexer), or running queries using client.query/client.execute.
  • You can set specific clickhouse query settings to override the default ones
  • You can use a different clickhouse profile
  • You can run the query as a different user
val client: ClickhouseClient
implicit val settings = QuerySettings(queryId = Some("expensive_query"),settings = Map("replace_running_query" -> "1"))
client.query("SELECT uniq(expensive) FROM huge_table")//start query
client.query("SELECT uniq(expensive) FROM huge_table")//replaces existing query

Query retrying

Query retrying takes advantage of host balancing and will request another host for each retry.

The queries that use the client api source, sink are not going to be retried.

All the read only queries are considered idempotent and are retried up to a maximum number of configurable times. (3 times by default, so 4 total execution, 1 the initial execution and 3 retries)

crobox.clickhouse.retries = 3

By using the ClickhouseSink you can also retry inserts by setting the idempotent setting to true on the query settings.


Typed/composable DSL that is interpreted and parsed into queries, with ofcourse full seamless integration into the driver.

For more information see the wiki

Test Kit

We also expose an utility test kit which provider a helpful spec with testing utilities. It automatically creates a single use database before all tests and drops it afterwards.

libraryDependencies += "com.crobox.clickhouse" %% "testkit" % <latest_version>

Check the spec for more details.