ILP TCP Receiver

The TCP receiver is a high-throughput ingestion-only API for QuestDB. Here are some key facts about the service:

  • ingestion only, there is no query capability
  • accepts plain text input in a form on InfluxDB Line Protocol
  • implicit transactions, batching
  • supports automatic table and column creation
  • multi-threaded, non-blocking
  • supports authentication
  • encryption requires an optional external reverse-proxy


By default, QuestDB listens over TCP on The receiver consists of two thread pools, which is an important design feature to be aware of to configure the receiver for maximum performance. The io worker threads are responsible for parsing text input. The writer threads are responsible for persisting data in tables. We will talk more about these in capacity planning section.


Although the original protocol does not support it, we have added authentication over TCP. This works by using an elliptic curve P-256 JSON Web Token (JWT) to sign a server challenge. Details of authentication over ILP can be found in the authentication documentation

Insert data#

Follow this link for examples of sending data using ILP over TCP.

Error handling#

It is recommended that sending applications reuse TCP connections. If QuestDB receives an invalid message, it will discard invalid lines, produce an error message in the logs and forcibly disconnect the sender to prevent further data loss.

Data may be discarded because of:

  • missing new line characters at the end of messages
  • an invalid data format such as unescaped special characters
  • invalid column / table name characters
  • schema mismatch with existing tables
  • message size overflows on the input buffer
  • system errors such as no space left on the disk

Detecting malformed input can be achieved through QuestDB logs by searching for LineTcpMeasurementScheduler and LineTcpConnectionContext, for example:

2022-02-03T11:01:51.007235Z I i.q.c.l.t.LineTcpMeasurementScheduler could not create table [tableName=trades, ex=`column name contains invalid characters [colName=trade_%]`, errno=0]

The following input is tolerated by QuestDB:

  • a column is specified twice or more on the same line, QuestDB will pick the first occurrence and ignore the rest
  • missing columns, their value will be defaulted to null/0.0/false depending on the type of the column
  • missing designated timestamp, the current server time will be used to generate the timestamp
  • the timestamp is specified as a column instead of appending it to the end of the line
  • timestamp appears as a column and also present at the end of the line, the value sent as a field will be used

Commit strategy#

ILP transactions are implicit; the protocol is built to stream data at a high rate of speed and to support batching. There are three ways data is committed and becomes visible or partially visible. The commit method is chosen based on whichever occurs first.

Row-based commit#

Each table has a max uncommitted rows metadata property. The ILP server will issue a commit when the number of uncommitted rows reaches this value. The table commit implementation retains data under max uncommitted rows or newer than the commit lag (whichever is smallest) as uncommitted data. Committed data is visible to table readers.

This parameter is set using in the following server configuration property:

Commit when this number of uncommitted records is reached

Idle table timeout#

When there is no data ingested in the table after a set period, the ingested uncommitted data is fully committed, and table data becomes fully visible. The timeout value is server-global and can be set via the following server configuration property:

Minimum amount of idle time (millis) before table writer is released

The following server configuration property controls the interval to run idle table checks:

Setting maintenance interval (millis)

Interval-based commit#

A table's commit lag metadata property determines how much uncommitted data will need to remain uncommitted for performance reasons. This lag value is measured in time units from the table's data. Data older than the lag value will be committed and become visible. ILP derives the commit interval as a function of the commit lag value for each table. The difference is that the commit interval is a wall clock.

The commit interval is calculated for each table as a fraction of the commit lag

commitInterval=commitLagโˆ—fractioncommitInterval = commitLag * fraction

This fraction is 0.5 by default so if the table has a commit lag of 1 minute the commit interval will be 30 seconds. The fraction used to derive the commit interval can be set by the below configuration parameter.

Setting commit interval fraction

If there is no commit lag set for a table or the fraction is set to 0, the default commit interval of 2 seconds will be used. This can be changed in the configuration:

Setting the default commit interval in milliseconds

To ease understanding of how time interval interacts with commit lag, let's look at how real-time data stream will become visible. The wall clock is roughly aligned with time in the data stream in real-time data. Let's assume that table has a commit lag value of 60 seconds and a commit interval of 20 seconds. After the first 60 seconds of ingestion, ILP will attempt to commit 3 times. After each attempt, there will be no data visible to the application. This is because all the data will fall within the lag interval.

On the 4th commit, which would occur, 80 seconds after the data stream begins, the application will see the first 20 seconds of the data, with the remaining 60 seconds uncommitted. Each subsequent commit will reveal more data in 20-second increments. It should be noted that both commit lag and commit interval should be carefully chosen with both data visibility and ingestion performance in mind.

Capacity planning#

TCP receiver makes use of 3 logical thread pools:

  • I/O worker pool -, threads responsible for handling incoming TCP connections and parsing received ILP messages
  • writer pool - line.tcp.writer.worker.count, threads responsible for table writes
  • shared pool - shared.worker.count, threads responsible for handling O3 data

Depending on the number of concurrent TCP connections io worker pool size might need to be adjusted. The ideal ratio is 1:1 - a thread per connection. In less busy environments it is possible for single io worker thread to handle multiple connections simultaneously. We recommend starting with conservative ration, measure and increase ration up to 1:1. More threads than connections will be wasting server CPU.

Another consideration is the number of tables updates concurrently. writer pool should be tuned to increase concurrency. writer threads can also handle multiple tables concurrently. 1:1 ratio is the maximum required ratio between writer threads and tables. If 1:1 ratio is not an option, avoid writing to all tables from each connection. Instead, group connections and tables. For example, if there are 10 tables, 8 TCP connections and writer pool size is set to 2, 4 TCP connections may be used to write into tables 1-5, while 4 connections may write into tables 6-10.


Sending updates for multiple tables from a single TCP connection might be inefficient. Consider using multiple connections to improve performance. If a single connection is unavoidable, keep writer pool size set to 1 for optimal CPU resource utilization.

When ingesting data out of order (O3) shared pool accelerates O3 tasks. It is also responsible for SQL execution. shared pool size should be set to use the remaining available CPU cores.

Configuration reference#

The TCP receiver configuration can be completely customized using configuration keys. You can use this to configure the thread pools, buffer and queue sizes, receiver IP address and port, load balancing, etc.