[dancer-users] dancer-users Digest, Vol 124, Issue 4

Tue Jul 27 13:30:44 EDT 2021

The alternatives suggested all have shortcomings in my view.

A script which creates config_local.yaml on the fly and then starts 
Dancer2 app would otherwise work but it means that container won't be 
immutable. Immutable is one of the architectural best practices listed 
by Google.

As it happens, this container is planned to run on Google Container 
Engine. You mentioned there are other ways to do this in Kubernetes. How?

Using a tmpfs is also not an option because it would mean that container 
is dependent on the way it is being run (the runtime environment).

Substituting variables in config.yaml is not overly complicated in my 
mind. Many platforms use environment variables to pass information to 
containers, docker or others, e.g. Java jar. I have used Openshift 
platform and it gave me a very good example of how env vars are governed 
in order to provide secrets to the container.

By stating the env vars in the configuration, we make it clear to the 
programmer and to operations what is expected from the runtime 
environment. Besides this, it also makes local development easier if you 
"practice dotenv" - what The Twelve-Factor App recommends. I keep my 
environment in a .env file and I also use the same file when testing the 
container with local docker.

See:
* 
https://cloud.google.com/architecture/best-practices-for-operating-containers
* https://docs.docker.com/storage/tmpfs/
* https://12factor.net/config

On 27/07/2021 13:00, dancer-users-request at lists.preshweb.co.uk wrote:
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> Today's Topics:
> 
>     1. Re: Passing environment to dockerized Dancer2 (Adam Clarke)
>     2. Re: Passing environment to dockerized Dancer2 (Adam Clarke)
> 
> 
> ----------------------------------------------------------------------
> 
> Message: 1
> Date: Mon, 19 Jul 2021 10:51:08 +1000
> From: Adam Clarke <adamc at strategicdata.com.au>
> To: Perl Dancer users mailing list <dancer-users at lists.preshweb.co.uk>
> Subject: Re: [dancer-users] Passing environment to dockerized Dancer2
> Message-ID:
> 	<FF6A5F5E-448C-4F0D-B829-1DA2BDAE3BAC at strategicdata.com.au>
> Content-Type: text/plain;	charset=utf-8
> 
> 
>> On 19 Jul 2021, at 2:54 am, Mikko Johannes Koivunalho <mikko.koivunalho at iki.fi> wrote:
>>> Problem:
>>> How do I pass the environment dependent parts of the configuration to
>>> the docker? I cannot use file `config_local.yaml` because then I would
>>> need to copy that file somehow into the container when it starts.
>>>
>>> Possible solution:
>>> I would pass parts of the configuration as environment variables, e.g.
>>> /DB_HOST/  and/SITE_URL/, and then apply these over the values in
>>> config.yaml when I am creating the endpoints.
>>>
>>> Suggestion:
>>> Change the command `/config/` (or the underlying file reader function)
>>> so that it can transpose environment variables into config strings, e.g.
>>> plugins->database->connections->main->host = '${DB_HOST}'.
>>
>> I have built a solution to this.
>> I have not yet submitted a Pull Request.
>> I am using this solution in my build and web application.
>> Please comment. Would this be a good pull request?
> 
> I?m not clear that this complexity is required give that the solution can be
> achieved using a startup script as per Racke?s previous reply. Also I?m
> assuming you?re not running your containers under Docker swarm or
> Kubernetes but if you are there are more advanced ways to achieve what I
> describe below.
> 
> In case Racke?s approach wasn?t clear, what you do is add an entry point
> script to your Docker image build using the ENTRYPOINT instruction.
> e.g. startup.sh. This script will always run at container start up.
> 
> The startup script would use the environment you had set to build an
> appropriate config_local.yaml inside your container before Dancer is
> invoked. There are lots of ways to do this, one simple and way from a
> shell script is to use envsubst. The last thing the startup script would do
> is start your Dancer app.
> 
> If you want or need your container to run in read only mode then you
> can combine this technique with a tempfs mount for config_local.yaml.
> 
> See:
> * https://docs.docker.com/develop/develop-images/dockerfile_best-practices/#entrypoint
> * https://www.gnu.org/software/gettext/manual/html_node/envsubst-Invocation.html
> 
> 
> 
> ------------------------------
> 
> Message: 2
> Date: Tue, 20 Jul 2021 17:52:48 +1000
> From: Adam Clarke <adamc at strategicdata.com.au>
> To: Perl Dancer users mailing list <dancer-users at lists.preshweb.co.uk>
> Subject: Re: [dancer-users] Passing environment to dockerized Dancer2
> Message-ID:
> 	<EF3743C7-C62A-477D-BD13-D82F0E7714DB at strategicdata.com.au>
> Content-Type: text/plain;	charset=utf-8
> 
> 
>> On 19 Jul 2021, at 2:54 am, Mikko Johannes Koivunalho <mikko.koivunalho at iki.fi> wrote:
>>> Problem:
>>> How do I pass the environment dependent parts of the configuration to
>>> the docker? I cannot use file `config_local.yaml` because then I would
>>> need to copy that file somehow into the container when it starts.
>>>
>>> Possible solution:
>>> I would pass parts of the configuration as environment variables, e.g.
>>> /DB_HOST/  and/SITE_URL/, and then apply these over the values in
>>> config.yaml when I am creating the endpoints.
>>>
>>> Suggestion:
>>> Change the command `/config/` (or the underlying file reader function)
>>> so that it can transpose environment variables into config strings, e.g.
>>> plugins->database->connections->main->host = '${DB_HOST}'.
>>
>> I have built a solution to this.
>> I have not yet submitted a Pull Request.
>> I am using this solution in my build and web application.
>> Please comment. Would this be a good pull request?
> 
> I?m not clear that this complexity is required give that the solution can be
> achieved using a startup script as per Racke?s previous reply. Also I?m
> assuming you?re not running your containers under Docker swarm or
> Kubernetes but if you are there are more advanced ways to achieve what I
> describe below.
> 
> In case Racke?s approach wasn?t clear, what you do is add an entry point
> script to your Docker image build using the ENTRYPOINT instruction.
> e.g. startup.sh. This script will always run at container start up.
> 
> The startup script would use the environment you had set to build an
> appropriate config_local.yaml inside your container before Dancer is
> invoked. There are lots of ways to do this, one simple and way from a
> shell script is to use envsubst. The last thing the startup script would do
> is start your Dancer app.
> 
> If you want or need your container to run in read only mode then you
> can combine this technique with a tempfs mount for config_local.yaml.
> 
> See:
> * https://docs.docker.com/develop/develop-images/dockerfile_best-practices/#entrypoint
> * https://www.gnu.org/software/gettext/manual/html_node/envsubst-Invocation.html
> 
> 
> 
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> 
> End of dancer-users Digest, Vol 124, Issue 4
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