Logging

You are an expert in deploying applications with Kamal. Your goal is to configure where logs go: the Docker logging driver and options for the containers Kamal runs on your servers, and Kamal's output loggers for the command output it produces while deploying.

Two separate concerns

Kamal has two distinct, independent logging settings. Keep them straight before you touch the config — they solve different problems and live under different keys.

Use logging: when you care about your application's stdout/stderr inside Docker (rotation, size caps, shipping container logs to a driver). Use output: when you want a durable record of what Kamal itself did during a deploy.

Before you start

Inspect the existing configuration before asking the user questions:

• Read config/deploy.yml — check for an existing logging: or output: block, the service: name, and the roles under servers: (you may want logging per role).
• If the project uses destinations, check config/deploy.<destination>.yml for overrides.

Use what you find, then only ask for what's missing (which driver, what size cap, where to ship deploy logs).

Part 1: Docker container logging (logging:)

The logging: key controls the Docker logging driver and options. Kamal passes these straight through to Docker, so any driver and any option Docker accepts are valid here.

These settings can be specified at the root level or for a specific role.

Step 1: Set the driver

The logging driver is passed to Docker via --log-driver:

logging:
  driver: json-file

json-file is the driver used in the Kamal docs. Because the value is handed directly to Docker, see Docker's logging driver documentation for the full catalog of available drivers.

Step 2: Set the options

Any logging options are passed to the driver via --log-opt. Each entry under options: becomes one --log-opt key=value:

logging:
  driver: json-file
  options:
    max-size: 100m

This is the most common reason to reach for logging: — capping container log size so logs don't fill the disk. Which option keys are valid depends on the driver you chose (see the Docker logging docs linked above).

Step 3 (optional): Scope it to a role

Set logging: at the root to apply it everywhere, or place it under a specific role in servers: to override the logging for just that role:

servers:
  web:
    - 192.168.0.1
  workers:
    hosts:
      - 192.168.0.2
    logging:
      driver: json-file
      options:
        max-size: 100m

Viewing the resulting logs

Configuring the driver is separate from reading the logs. To tail or fetch logs from running containers, use kamal app logs (and kamal accessory logs for accessories). Those runtime commands belong to the app skill — see Related Skills.

Part 2: Output loggers (output:)

The output: key configures where Kamal sends its own command output — the deploy logs. There are two output loggers: otel and file. You can configure either, or both, under the same output: key.

Ship to OpenTelemetry (otel)

Ship deploy logs to an OpenTelemetry-compatible endpoint via OTLP HTTP:

output:
  otel:
    endpoint: http://otel-gateway:4318

Logs are sent as OTLP log records, with resource attributes derived from Kamal's deploy tags — service, version, performer, destination, and similar. This lets your observability backend slice deploy logs by who deployed, which version, and which environment.

Write to a file (file)

Write deploy logs to a file on the local machine (the machine running Kamal):

output:
  file:
    path: /var/log/kamal/

One log file is created per deploy, named with the timestamp and command. path is the directory those files are written into.

Both at once

Since otel and file are independent loggers under output:, you can enable both — for example, ship to a collector and keep a local copy:

output:
  otel:
    endpoint: http://otel-gateway:4318
  file:
    path: /var/log/kamal/

Quick reference

The keys above are the complete set. At a glance:

For the full table with types, examples, and the OTel resource-attribute detail, see [references/config-keys.md](references/config-keys.md).

Common pitfalls

• Confusing the two keys. logging: is for your app's container logs (Docker driver/options). output: is for Kamal's own deploy/command logs (OTel/file). Setting the wrong one won't do what you expect.
• Assuming an option works on any driver. options: is forwarded as --log-opt, and valid option keys depend on the chosen Docker driver. Check Docker's docs for the driver you picked.
• Expecting logging: to let you read logs. It only configures the driver. Viewing logs is a separate runtime command (kamal app logs) — see app.

Related Skills

• app: For viewing logs from running containers with kamal app logs (and kamal accessory logs), and other day-to-day app management commands.
• config: For where logging: and output: sit in config/deploy.yml, the other top-level options and their defaults, destinations (-d), and validating with kamal config.