v4.0.0

Next generation Telemetry Agent for Logs, Metrics and Traces.

March 29, 2025

KNOWLEDGE BASE

Release Notes v4.0.0

Fluent Bit is a Fast and Lightweight Telemetry Agent for Linux, BSD, MacOS and Windows. We are proud to announce the availability of Fluent Bit v4.0.

Fluent Bit v4.0.0
* Copyright (C) 2015-2025 The Fluent Bit Authors
* Fluent Bit is a CNCF sub-project under the umbrella of Fluentd
* https://fluentbit.io

______ _                  _    ______ _ _             ___  _____
|  ___| |                | |   | ___ (_) |           /   ||  _  |
| |_  | |_   _  ___ _ __ | |_  | |_/ /_| |_  __   __/ /| || |/' |
|  _| | | | | |/ _ \ '_ \| __| | ___ \ | __| \ \ / / /_| ||  /| |
| |   | | |_| |  __/ | | | |_  | |_/ / | |_   \ V /\___  |\ |_/ /
\_|   |_|\__,_|\___|_| |_|\__| \____/|_|\__|   \_/     |_(_)___/


[2025/03/29 11:38:51] [ info] [fluent bit] version=4.0.0, commit=5251a768cd, pid=3973606
[2025/03/29 11:38:51] [ info] [storage] ver=1.5.2, type=memory, sync=normal, checksum=off, max_chunks_up=128
[2025/03/29 11:38:51] [ info] [simd    ] disabled
[2025/03/29 11:38:51] [ info] [cmetrics] version=0.9.9
[2025/03/29 11:38:51] [ info] [ctraces ] version=0.6.2
[2025/03/29 11:38:51] [ info] [sp] stream processor started

For people upgrading from previous versions you must read the Upgrading Notes section of our documentation:

https://docs.fluentbit.io/manual/4.0/installation/upgrade_notes

Introduction

Fluent Bit, a CNCF graduated project under the umbrella of Fluentd, announces the availability of v4.0.

In every release, there are many improvements and fixes, on this notes we will refer to the major changes that will make your infrastructure happier ;)

Below a list of the highlights of this release:

Processor: Conditional Processing for Logs

Introducing a powerful new capability allowing processors to conditionally modify logs based on specific field values. This enhancement provides granular control over log processing with a variety of conditional operators such as and, or, in, gt, lt, regex, and more.

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pipeline:
  processors:
    logs:
      - name: content_modifier
        match: '*'
        condition:
          operator: AND
          rules:
            - field: "$log[\"level\"]"
              operator: eq
              value: "error"
        action: insert
        context: log_body
        key: priority
        value: high

Trace Sampling Processor

Introducing a new trace sampling processor with a pluggable architecture for supporting various sampling strategies and backends. This includes both head sampling (probabilistic) and tail sampling with conditions.

  • Head Sampling (probabilistic): Allows probabilistic sampling of traces based on a specified sampling percentage.

  • Tail Sampling (conditional): Supports evaluating traces based on various conditions before making a sampling decision. Available conditions include:

    • Status Code
    • Latency
    • String Attribute
    • Numeric Attribute
    • Boolean Attribute
    • Span Count
    • Trace State
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pipeline:
  inputs:
    - name: opentelemetry
      port: 4318
      processors:
        traces:
          - name: sampling
            type: tail
            sampling_settings:
              decision_wait: 2s
            conditions:
              - type: boolean_attribute
                key: "user.logged"
                value: false



Enhanced Security Mechanisms

Fluent Bit introduces improved security and flexibility with the following features:

TLS Configuration Enhancements

Users can now configure specific TLS versions and define acceptable ciphers to strengthen communication security.

  • tls.min_version: Specifies the minimum allowed TLS version. Supported values include:
    • TLSv1.1
    • TLSv1.2
    • TLSv1.3
  • tls.max_version: Specifies the maximum allowed TLS version. Supported values include:
    • TLSv1.2
    • TLSv1.3
  • tls.ciphers: Allows users to define a specific set of TLS ciphers (up to TLSv1.2). This provides greater control over which ciphers are allowed during communication.
Environment Variable File Reference

Fluent Bit now supports referencing internal variables from the file system, making it easier to securely handle sensitive data like secrets or tokens.

  • This feature allows secrets or bearer tokens to be stored in files and referenced in configuration files using the file:// prefix.

example:

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env:
  FLUSH_INTERVAL: 1
  TOKEN: file://mysecret.txt

service:
  flush: ${FLUSH_INTERVAL}
  log_level: info

pipeline:
  inputs:
    - name: dummy
      dummy: '{"message": "Hello, World!", "secret": "${TOKEN}"}'

  outputs:
    - name: http
      match: '*'
      host: example.com
      port: 443
      uri: /ingest
      format: json
      tls: on
      header: Bearer ${TOKEN}

Experimental Features

The following experimental features, are still under active development and are not recommended for production use. However, we encourage you to try them out and provide feedback to help us improve them.

Note that experimental features are disabled by default, but can be enabled at build time.

Zig Language Support

Fluent Bit now includes experimental support for plugins written in the Zig programming language. This new capability allows developers to write high-performance plugins with Zig’s modern features and safety guarantees.

Libraries

  • ctraces: upgrade to v0.6.2
  • zstd: ugprade to v1.5.7
  • monkey: upgrade to v1.8.2

Plugins

Contributors

On every release, there are many people involved doing contributions on different areas like bug reporting, troubleshooting, documentation and coding, without these contributions from the community, the project won’t be the same and won’t be in the good shape that it is now. So THANK YOU! to everyone who takes part of this journey!

Join us

We want to hear about you, our community is growing and you can be part of it!, you can contact us at:

Latest Version

New release on March 24, 2026

Fluent Bit v5.0.0 is out!

Check out the Release Notes, read the Updated Documentation or jump directly to the Downloads Section.

Download Now

We are part of a wide community, no vendor lock-in.

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