Stream Logs from Docker to Grafana Cloud with Alloy

#webdev #programming #beginners #devops

Setting up logging inside containers can be annoying — especially when logs vanish with the container or you have to mess with volume mounts just to see what's going on.

Here's how I made it super simple using Grafana Alloy to send logs from a Flask server running inside a Docker container to Grafana Cloud, without touching host volumes.

Quick Demo App: Flask Logger

Let's start with a super basic Flask app that just logs requests:

# app.py
from flask import Flask
import logging

app = Flask(__name__)

logging.basicConfig(filename="liveapi.log", level=logging.INFO)

@app.route("/")
def hello():
    app.logger.info("GET / was hit")
    return "Hello, World!"

if __name__ == "__main__":
    app.run(host="0.0.0.0", port=5000)

This will create a liveapi.log file in the current working directory.

☁️ Set Up Grafana Cloud + Loki

Go to Grafana Cloud.
Sign up or log in.
In the sidebar, go to Connections → Data sources.
Search for Loki and set it up.
Copy the Loki Push URL from the connection config. It'll look like:

https://<username>:<api-key>@logs-prod-000.grafana.net/api/prom/push

You'll use this in Alloy later.

🐳 Dockerfile: Flask + Alloy Without Volume Mounts

Here's the Dockerfile that installs everything, writes logs, and sends them to Grafana Loki using Alloy:

FROM debian:bookworm-slim

RUN apt-get update && apt-get install -y \
    python3 python3-pip gpg wget curl gnupg ca-certificates systemctl \
    && rm -rf /var/lib/apt/lists/*

WORKDIR /app

COPY requirements.txt .
RUN pip3 install --no-cache-dir --break-system-packages -r requirements.txt

# Install Alloy
RUN mkdir -p /etc/apt/keyrings/ && \
    wget -q -O - https://apt.grafana.com/gpg.key | gpg --dearmor | tee /etc/apt/keyrings/grafana.gpg > /dev/null && \
    echo "deb [signed-by=/etc/apt/keyrings/grafana.gpg] https://apt.grafana.com stable main" > /etc/apt/sources.list.d/grafana.list && \
    apt-get update && apt-get install -y alloy && \
    rm -rf /var/lib/apt/lists/*

COPY . .

RUN mkdir -p /etc/alloy/ && cp config.alloy /etc/alloy/config.alloy

EXPOSE 5000

CMD systemctl restart alloy && python3 app.py

No volume mounts. No host bind paths. Everything is self-contained inside the container.

⚙️ `config.alloy` (Log Pipeline for Alloy)

Create config.alloy in your repo:

local.file_match "local_files" {
  path_targets = [{ "__path__" = "/app/liveapi.log" }]
  sync_period = "5s"
}

loki.source.file "log_scrape" {
  targets       = local.file_match.local_files.targets
  forward_to    = [loki.process.filter_logs.receiver]
  tail_from_end = true
}

loki.process "filter_logs" {
  forward_to = [loki.write.grafana_loki.receiver]

  stage.static_labels {
    values = {
      job          = "liveapi"
      service_name = "liveapi"
    }
  }
}

loki.write "grafana_loki" {
  endpoint {
    url = "https://<your-username>:<your-api-key>@logs-prod-000.grafana.net/api/prom/push"
  }
}