Seq

Seq is a log aggregation tool that's perfect for browsing logs during local development.

You can see a full example of using Batect with Seq in the Seq sample project.

Simple configuration

You can launch an instance of Seq with the following Batect configuration:

danger

This simple configuration has a flaw explained below.

batect.yml

containers:
  sqelf:
    image: datalust/sqelf:2.0.288
    ports:
      - 12201:12201/udp
    environment:
      SEQ_ADDRESS: http://seq:5341
    dependencies:
      - seq

  seq:
    image: datalust/seq:2020.3.4761
    ports:
      - 9000:80
    environment:
      ACCEPT_EULA: Y

This will make the Seq UI available at http://localhost:9000 on your local machine, and allows submitting logs to Seq using the GELF protocol on UDP port 12201.

You can combine this with Docker's GELF log driver to automatically send logs from your application to Seq. For example, to send all logs from the app container to Seq:

batect.yml

containers:
  sqelf:
    # ... configuration as above

  seq:
    # ... configuration as above

  app:
    # ... other configuration
    log_driver: gelf
    log_options:
      gelf-address: udp://localhost:12201
    dependencies:
      - sqelf

caution

Docker does not support streaming logs to the console when using the GELF log driver. Instead, you'll receive an error message such as Error attaching: configured logging driver does not support reading.

This does not affect the operation of your containers, only the ability to see the output in the console.

Preventing the loss of logs on startup

If you use the configuration above, depending on how quickly your application starts, you may lose some of the first log messages it emits.

The solution to this is to specify a healthcheck for the Seq container. For example, you can replace the image with this Dockerfile to add a healthcheck:

.batect/seq/Dockerfile

FROM datalust/seq:2020.3.4761

RUN apt-get update && apt-get install -y --no-install-recommends curl

HEALTHCHECK --interval=2s CMD curl "http://localhost/api" --fail --show-error --silent

And then reference that Dockerfile (saved as .batect/seq/Dockerfile) in your Batect configuration:

batect.yml

containers:
  # ... other containers

  seq:
    build_directory: .batect/seq
    ports:
      - 9000:80
    environment:
      ACCEPT_EULA: Y