Symfony logs to stdout inside Docker container

Question

I'm building a docker image for a Symfony application. In this image, I want to stream the Symfony logs to stdout. So, similar to how nginx logs are configured, I added this line to my Dockerfile:

ln -sf /dev/stdout /var/www/project/app/logs/prod.log

Inside the container, I can see this:

$ ls /var/www/project/app/logs/ -l
total 12
-rw-r--r-- 1 501 games 4473 Jul 21 08:36 dev.log
lrwxrwxrwx 1 501 games   11 Jul 21 08:35 prod.log -> /dev/stdout

However, the app throws following error:

PHP Fatal error: Uncaught exception 'UnexpectedValueException' with message 'The stream or file "/var/www/project/app/logs/prod.log" could not be opened: failed to open stream: No such file or directory' in /var/www/project/app/cache/prod/classes.php:5808
Stack trace:
#0 /var/www/project/app/cache/prod/classes.php(5746): Monolog\Handler\StreamHandler->write(Array)
#1 /var/www/project/app/cache/prod/classes.php(5917): Monolog\Handler\AbstractProcessingHandler->handle(Array)
#2 /var/www/project/app/cache/prod/classes.php(6207): Monolog\Handler\FingersCrossedHandler->handle(Array)
#3 /var/www/project/app/cache/prod/classes.php(6276): Monolog\Logger->addRecord(500, 'Fatal Error: Un...', Array)
#4 /var/www/project/app/cache/prod/classes.php(1978): Monolog\Logger->log('critical', 'Fatal Error: Un...', Array)
#5 /var/www/project/app/cache/prod/classes.php(2034): Symfony\Component\Debug\ErrorHandler->handleException(Object(Symfony\Component\Debug\Exception\FatalErrorException), Array)
#6 [internal function]: Symfony\Component\Debug\E in /var/www/project/app/cache/prod/classes.php on line 5808

Any suggestions ?

Solution

With the help of Monolog, it is very easy to send logs to stdout/stderr. My examples are using stderr, but I think it's the same with stdout.

Instead of defining a log file you just enter the preferred stream path

path:  "php://stderr"

BUT you are not done yet. You also have to configure PHP accordingly. The workers have to catch the output of their processes and log this output again to their stderr.

PHP Configuration

#/etc/php/7.0/fpm/php-fpm.conf
error_log = /proc/self/fd/2

#/etc/php/7.0/fpm/pool.d/www.conf
catch_workers_output = yes

Symfony Configuration

# app/config/config_prod.yml
monolog:
    handlers:
        main:
            type:         fingers_crossed
            action_level: error
            handler:      nested
        nested:
            type:  stream
            path:  "php://stderr"
            level: debug
        console:
            type:  console

If you are using any process control system in a fat docker container you have to make sure that this system also logs to stdout (or stderr).

Example with supervisor:

[supervisord]
nodaemon=true
;@see http://blog.turret.io/basic-supervisor-logging-with-docker/
;we need the output from the controlled processes
;but this is only possible with lowered loglevel
loglevel=debug

All in all make sure that:

• The application logs to stdout/stderr
• PHP catches workers output and logs to stderr
• optional: any process control system has to forward output of managed processes to stdout/stderr