Rails Logging to Stdout

Rails needs to log to stdout went running on AWS Lambda. Logs written to stdout end up in CloudWatch logs, and you get centralized logging baked in with jets logs -f.

Logging Error

If you’re seeing this error

Rails Error: Unable to access log file. Please ensure that /app/log/production.log exists and is writable (i.e. make it writable for user and group: chmod 0664 /app/log/production.log). The log level has been raised to WARN and the output directed to STDERR until the problem is fixed.

This means that your Rails logger is configured to write to log/production.log. However, the AWS Lambda filesystem is read-only, so it errors.


You should configure your logger to write to stdout instead of a file. Example:


Rails.application.configure do
  if ENV["RAILS_LOG_TO_STDOUT"].present?
    logger           = ActiveSupport::Logger.new(STDOUT)
    logger.formatter = config.log_formatter
    config.logger    = ActiveSupport::TaggedLogging.new(logger)

Note: The jets init generates a config/jets/env/.env with RAILS_LOG_TO_STDOUT=1. You can also remove the if check if you want.