Java Client Documentation

The QuestDB Java client is baked right into the QuestDB binary.

It requires no additional configuration steps.

The client provides the following benefits:

  • Automatic table creation: No need to define your schema upfront.
  • Concurrent schema changes: Seamlessly handle multiple data streams with on-the-fly schema modifications
  • Optimized batching: Use strong defaults or curate the size of your batches
  • Health checks and feedback: Ensure your system's integrity with built-in health monitoring
  • Automatic write retries: Reuse connections and retry after interruptions

Quick start

Add a QuestDB as a dependency in your project's build configuration file.


The code below creates an instance of a client configured to use HTTP transport to connect to a QuestDB server running on localhost on port 9000. It then sends two rows, each containing one symbol and two floating-point values. The client requests the server to assign a timestamp to each row based on the server's wall-clock time.

package com.example.sender;

import io.questdb.client.Sender;

public class HttpExample {
public static void main(String[] args) {
try (Sender sender = Sender.fromConfig("http::addr=localhost:9000;")) {
.symbol("id", "toronto1")
.doubleColumn("temperature", 23.5)
.doubleColumn("humidity", 0.49)
.symbol("id", "dubai2")
.doubleColumn("temperature", 41.2)
.doubleColumn("humidity", 0.34)

The configuration for the client is specified using a configuration string. This string follows the format:


The valid transport protocols are:

  • http: ILP/HTTP
  • https: ILP/HTTP with TLS encryption
  • tcp: ILP/TCP
  • tcps: ILP/TCP with TLS encryption

A transport protocol and the key addr=host:port are required. The key addr defines the hostname and port of the QuestDB server. If the port is not specified, it defaults to 9000 for HTTP(s) transports and 9009 for TCP(s) transports. For a complete list of options, refer to the Configuration Options section.

Example with TLS and Authentication enabled

This sample configures a client to use HTTP transport with TLS enabled for a connection to a QuestDB server. It also instructs the client to authenticate using HTTP Basic Authentication.

package com.example.sender;

import io.questdb.client.Sender;

public class HttpsAuthExample {
public static void main(String[] args) {
try (Sender sender = Sender.fromConfig("https::addr=localhost:9000;username=Aladdin;password=OpenSesame;")) {
.symbol("id", "toronto1")
.doubleColumn("temperature", 23.5)
.doubleColumn("humidity", 0.49)
.symbol("id", "dubai2")
.doubleColumn("temperature", 41.2)
.doubleColumn("humidity", 0.34)

Client instantiation

There are three ways to create a client instance:

  1. From a configuration string. This is the most common way to create a client instance. It describes the entire client configuration in a single string. See Configuration options for all available options. It allows sharing the same configuration across clients in different languages.
    try (Sender sender = Sender.fromConfig("http::addr=localhost:9000;auto_flush_rows=5000;retry_timeout=10000;")) {
    // ...
  2. From an environment variable. The QDB_CLIENT_CONF environment variable is used to set the configuration string. Moving configuration parameters to an environment variable allows you to avoid hard-coding sensitive information such as tokens and password in your code.
    export QDB_CLIENT_CONF="http::addr=localhost:9000;auto_flush_rows=5000;retry_timeout=10000;"
    try (Sender sender = Sender.fromEnv()) {
    // ...
  3. Using the Java builder API. This provides type-safe configuration.
    try (Sender sender = Sender.builder(Sender.Transport.HTTP)
    .build()) {
    // ...

General usage pattern

  1. Create a client instance via Sender.fromConfig().

  2. Use table(CharSequence) to select a table for inserting a new row.

  3. Use symbol(CharSequence, CharSequence) to add all symbols. You must add symbols before adding other column type.

  4. Use the following options to add all the remaining columns:

    • stringColumn(CharSequence, CharSequence)
    • longColumn(CharSequence, long)
    • doubleColumn(CharSequence, double)
    • boolColumn(CharSequence, boolean)
    • timestampColumn(CharSequence, Instant), or timestampColumn(CharSequence, long, ChronoUnit)
  5. Use at(Instant) or at(long timestamp, ChronoUnit unit) or atNow() to set a designated timestamp.

  6. Optionally: You can use flush() to send locally buffered data into a server.

  7. Go to the step no. 2 to start a new row.

  8. Use close() to dispose the Sender after you no longer need it.

Transport selection

Client supports the following transport options:

  • HTTP (default port 9000)
  • TCP (default port 9009)

The HTTP transport is recommended for most use cases. It provides feedback on errors, automatically retries failed requests, and is easier to configure. The TCP transport is kept for compatibility with older QuestDB versions. It has limited error feedback, no automatic retries, and requires manual handling of connection failures. However, while HTTP is recommended, TCP has a lower overhead than HTTP and may be useful in high-throughput scenarios in high-latency networks.


Client accumulates data into an internal buffer. Flushing the buffer sends the data to the server over the network and clears the buffer.

Flushing can be done explicitly or automatically.

Explicit flushing

An explicit flush can be done by calling the flush() method.

 try (Sender sender = Sender.fromConfig("http::addr=localhost:9000;")) {
.symbol("id", "toronto1")
.doubleColumn("temperature", 23.5)
.doubleColumn("humidity", 0.49)
.symbol("id", "dubai2")
.doubleColumn("temperature", 41.2)
.doubleColumn("humidity", 0.34)

Automatic flushing

To avoid accumulating very large buffers, the client will - by default - flush the buffer automatically.

HTTP auto-flushing is triggered when appending a row to the internal buffer and the buffer either:

  • Reaches 75,000 rows
  • Hasn't been flushed for 1 second.

Both parameters control batching and can be customized. Larger batches can improve throughput, but can increase lag between data ingestion and visibility in a target table. Smaller batches can reduce this lag, but can also reduce throughput.

A configuration string example that auto-flushes every 10 rows or every 10 seconds, whichever comes first:


An example with auto-flushing disabled:


TCP auto-flushing is triggered when appending a row to the internal sender buffer and the buffer is full.

Auto-flushing is also triggered when the client is being closed. Be aware that retrying of failed requests is disabled when flushing on close.

Error handling

The HTTP transport supports automatic retries for failed requests deemed recoverable. Recoverable errors include network errors, some server errors, and timeouts, while non-recoverable errors encompass invalid data, authentication errors, and other client-side errors.

Retrying is particularly beneficial during network issues or when the server is temporarily unavailable. The retrying behavior can be configured through the retry_timeout configuration option or via the builder API with retryTimeoutMillis(long timeoutMillis). The client continues to retry recoverable errors until they either succeed or the specified timeout is reached. Upon reaching the timeout, the client ceases retry attempts and throws LineSenderException.

When utilizing the HTTP transport, the client can be reused after receiving an error. Conversely, a client using TCP transport should be discarded after an error, necessitating the creation of a new client.

Retrying is disabled for failed requests when executing a flush upon closure.

The TCP transport lacks support for error propagation from the server. In such cases, the server merely closes the connection upon encountering an error, which manifests as a LineSenderException on the client side. Consequently, the client receives no additional error information from the server. This limitation significantly contributes to the preference for HTTP transport over TCP transport.

Exactly-once delivery vs at-least-once delivery

The retrying behavior of the HTTP transport can lead to some data being sent to the server more than once.

Example: Client sends a batch to the server, the server receives the batch, processes it, but fails to send a response back to the client due to a network error. The client will retry sending the batch to the server. This means the server will receive the batch again and process it again. This can lead to duplicated rows in the server.

The are two ways to mitigate this issue:

  • Use QuestDB deduplication feature to remove duplicated rows. QuestDB server can detect and remove duplicated rows automatically, resulting in exactly-once processing. This is recommended when using the HTTP transport with retrying enabled.
  • Disable retrying by setting retry_timeout to 0. This will make the client send the batch only once, failed requests will not be retried and the client will receive an error. This effectively turns the client into an at-most-once delivery.

Designated timestamp considerations

The concept of designated timestamp is important when ingesting data into QuestDB.

There are two ways to assign a designated timestamp to a row:

  1. User-assigned timestamp: The client assigns a specific timestamp to the row.

    java.time.Instant timestamp =; // or any other timestamp
    .symbol("id", "toronto1")
    .doubleColumn("temperature", 23.5)
    .doubleColumn("humidity", 0.49)

    The Instant class is part of the java.time package and is used to represent a specific moment in time. The method can accept a long timestamp representing the elapsed time since the beginning of the Unix epoch, as well as a ChronoUnit to specify the time unit. This approach is useful in high-throughput scenarios where instantiating an Instant object for each row is not feasible due to performance considerations.

  2. Server-assigned timestamp: The server automatically assigns a timestamp to the row based on the server's wall-clock time. Example:

    .symbol("id", "toronto1")
    .doubleColumn("temperature", 23.5)
    .doubleColumn("humidity", 0.49)

We recommended to use User-assigned timestamps when ingesting data into QuestDB. Using Server-assigned hinder the ability to deduplicate rows which is important for exactly-once processing.


QuestDB works best when rows are ingested in chronological order. This means rows with older timestamps are ingested before rows with newer timestamps.

Table and column auto-creation

When sending data to a table that does not exist, the server will create the table automatically. This also applies to columns that do not exist. The server will use the first row of data to determine the column types.

If the table already exists, the server will validate that the columns match the existing table. If the columns do not match, the server will return a non-recoverable error which is propagated to the client as a LineSenderException.

If you're using QuestDB Enterprise, you must grant further permissions to the authenticated user:

CREATE SERVICE ACCOUNT ingest_user; -- creates a service account to be used by a client
GRANT ilp, create table TO ingest_user; -- grants permissions to ingest data and create tables
GRANT add column, insert ON all tables TO ingest_user; -- grants permissions to add columns and insert data to all tables
-- OR
GRANT add column, insert ON table1, table2 TO ingest_user; -- grants permissions to add columns and insert data to specific tables

Read more setup details in the Enterprise quickstart and the role-based access control guides.

Configuration options

Client can be configured either by using a configuration string as shown in the examples above, or by using the builder API.

The builder API is available via the Sender.builder(Transport transport) method.

When using the configuration string, the following options are available:

HTTP transport authentication

  • username : Username for HTTP basic authentication.
  • password : Password for HTTP basic authentication.
  • token : Bearer token for HTTP authentication.

TCP transport authentication

  • username: Username for TCP authentication.
  • token: Token for TCP authentication.

TLS encryption

TLS in enabled by selecting the https or tcps protocol. The following options are available:

  • tls_roots : Path to a Java keystore file containing trusted root certificates. Defaults to the system default trust store.
  • tls_roots_password : Password for the keystore file. It's always required when tls_roots is set.
  • tls_verify : Whether to verify the server's certificate. This should only be used for testing as a last resort and never used in production as it makes the connection vulnerable to man-in-the-middle attacks. Options are on or unsafe_off. Defaults to on.


  • auto_flush : Global switch for the auto-flushing behavior. Options are on or off. Defaults to on.
  • auto_flush_rows : The number of rows that will trigger a flush. This option is supported for HTTP transport only. Defaults to 75,000.
  • auto_flush_interval : The time in milliseconds that will trigger a flush. Defaults to 1000. This option is support for HTTP transport only.

The TCP transport for a client automatically flushes when its buffer is full. The TCP transport utilizes a fixed-size buffer, and its maximum size is the same as the initial size. Thus, the option init_buf_size (see below) effectively controls the auto-flushing behavior of the TCP transport.


  • init_buf_size : The initial size of the buffer in bytes. Default: 65536 (64KiB)
  • max_buf_size : The maximum size of the buffer in bytes. Default: 104857600 (100MiB) This option is support for HTTP transport only. TCP transport uses a fixed-size buffer and its maximum size is the same as the initial size.

HTTP Transport

  • retry_timeout : The time in milliseconds to continue retrying after a failed HTTP request. The interval between retries is an exponential backoff starting at 10ms and doubling after each failed attempt up to a maximum of 1 second. Default: 10000 (10 seconds)
  • request_timeout : The time in milliseconds to wait for a response from the server. This is in addition to the calculation derived from the request_min_throughput parameter. Default: 10000 (10 seconds)
  • request_min_throughput : Minimum expected throughput in bytes per second for HTTP requests. If the throughput is lower than this value, the connection will time out. This is used to calculate an additional timeout on top of request_timeout. This is useful for large requests. You can set this value to 0 to disable this logic.

Other considerations

  • The Sender is not thread-safe. For multiple threads to send data to QuestDB, each thread should have its own Sender instance. An object pool can also be used to re-use Sender instances.
  • The Sender instance has to be closed after it is no longer in use. The Sender implements the java.lang.AutoCloseable interface, and therefore the try-with-resource pattern can be used to ensure that the Sender is closed.
  • The method flush() can be called to force sending the internal buffer to a server, even when the buffer is not full yet.

Health check

To monitor your active connection, there is a ping endpoint:

curl -I http://localhost:9000/ping

Returns (pong!):

HTTP/1.1 204 OK
Server: questDB/1.0
Date: Fri, 2 Feb 2024 17:09:38 GMT
Transfer-Encoding: chunked
Content-Type: text/plain; charset=utf-8
X-Influxdb-Version: v2.7.4

Determine whether an instance is active and confirm the version of InfluxDB Line Protocol with which you are interacting.