v4.1.0

KNOWLEDGE BASE

Release Notes v4.1.0

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 v4.1.0
* Copyright (C) 2015-2025 The Fluent Bit Authors
* Fluent Bit is a CNCF sub-project under the umbrella of Fluentd
* https://fluentbit.io

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

[2025/09/24 10:32:37.99970053] [ info] [fluent bit] version=4.1.0, commit=a8bd9e4cf6, pid=1153799
[2025/09/24 10:32:37.100078223] [ info] [storage] ver=1.5.3, type=memory, sync=normal, checksum=off, max_chunks_up=128
[2025/09/24 10:32:37.100095916] [ info] [simd    ] SSE2
[2025/09/24 10:32:37.100100885] [ info] [cmetrics] version=1.0.5
[2025/09/24 10:32:37.100111304] [ info] [ctraces ] version=0.6.6
[2025/09/24 10:32:37.100186423] [ info] [sp] stream processor started
[2025/09/24 10:32:37.100222029] [ info] [engine] Shutdown Grace Period=5, Shutdown Input Grace Period=2

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

https://docs.fluentbit.io/manual/4.1/installation/upgrade_notes

Introduction

Fluent Bit, a CNCF graduated project under the umbrella of Fluentd, announces the availability of v4.1.

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:

🔥 Highlights

Performance

• 🧠 JSON Parser: Up to 2.5x speedup using new backend based on yyjson
• 🦾 Tail Input: SIMD-enabled parsing yields 3-5% throughput improvement

New Features

• 📈 Multiline: Buffer truncation limit, metrics and diagnostics
• 🧩 Supervisor Mode: Parent process watches child for stability and graceful recovery
• 🕒 Hot Reload Watchdog: Timeout watchdog ensures reload safety
• 🔐 TLS API Enhancements: Session invalidation and ALPN support added
• 📦 Amazon S3 Output: Parquet Support
  • New Parquet output mode using Apache Arrow (pure C backend).
  • Now supports zstd compression as well.
• ☁️ Opentelemetry Output: AWS SigV4 authentication support

Metrics & Observability

• 🧠 New GPU Metrics collector (AMD)
• 📊 Prometheus Textfile Collector for .prom files

⚙️ General Improvements

• Plugin Lifecycle: Input plugins (e.g. tcp, event_type, splunk) now support pause/resume
• Unicode Control: escape_unicode config now respected across all JSON-outputting plugins
• TLS: ALPN replaces NPN; better session cleanup
• Windows & Platform Support: Certstore enhancements
• Compatibility for Debian Trixie, Rocky/AlmaLinux 10, CentOS Stream 10
• Bugfixes: Dozens of stability, memory, and logic improvements across HTTP, Winevtlog, compression, multiline, and more

Libraries

• ctraces: upgrade to v1.0.5
• monkey: upgrade to v1.8.5
• wamr: upgrade to v2.4.1

Join us

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

• Github: http://github.com/fluent/fluent-bit
• Slack: http://slack.fluentd.org
• Twitter: @fluentbit