fluentbit

Fluent Bit

Fluent Bit is a lightweight log processor and forwarder.
It collects, parses, and routes logs to various destinations (stdout, Elasticsearch, Loki, S3, etc.).

How Fluent Bit works

flowchart LR
    App([Application]) -->|logs| Volume[./logs]
    Volume --> FB[Fluent Bit]
    FB --> Stdout[stdout]
    FB --> ES[Elasticsearch]
    FB --> Loki[Loki]
    FB --> S3[S3 / File]
    Monitor([Monitor]) -->|:2020| FB

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1. Applications write log files to a shared volume.
2. Fluent Bit tails log files using the tail input plugin.
3. Logs are parsed, filtered, and routed to configured outputs.
4. The built-in HTTP server on port 2020 exposes health and metrics endpoints.

Stack details in this repo

• Image: fluent/fluent-bit:latest
• Container name: fluent-bit
• Monitoring port: 2020
• Config mount: ./fluent-bit/fluent-bit.conf
• Log input: ./logs (read-only)

Environment variables

Set via .env (copy from .env.example):

• FLUENTBIT_PORT (default: 2020)

How to run

From the repository root:

cd fluentbit
cp .env.example .env
mkdir -p logs
docker compose up -d

Generate test logs:

echo '{"time":"2026-05-21T10:00:00.000","level":"info","msg":"hello fluent-bit"}' >> logs/app.log

Check health:

curl http://localhost:2020/api/v1/health

View metrics:

curl http://localhost:2020/api/v1/metrics

Useful commands:

docker compose ps
docker compose logs -f
docker compose restart
docker compose down

Use it effectively

• Edit fluent-bit/fluent-bit.conf to add outputs (Elasticsearch, Loki, S3, Kafka, etc.).
• Use filters to enrich, modify, or drop log records before forwarding.
• Mount additional log paths from other containers using shared volumes.
• Pair with Prometheus to scrape the /api/v1/metrics/prometheus endpoint.

Notes

• The default config outputs to stdout for quick testing — add real outputs for production use.
• Fluent Bit is extremely lightweight (~450KB) compared to Fluentd or Logstash.
• Place your application log files in ./logs/ and they will be tailed automatically.
• See Fluent Bit docs for full plugin and configuration reference.