Assuming I have a main application that runs by itself multiple sub-applications.
Is it possible to run that main application inside a container?
Currently only the main application starts but the others doesn't.
Docker containers are started from a single ENTRYPOINT but they can of course run multiple binaries. You could, for instance, have a shell script serve as an ENTRYPOINT and start call the other binaries from there. Depending on your application it might make sense to put the sub-applications you refer to into their own container.
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Is it ok to run docker from inside docker?
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We have app and which will spin the short term (short span) docker containers. Right now, it runs in Ubunut16.04 server (VM) and we installed docker, and nodejs in same server. We have nodejs app which runs in same server so whenever a request comes in, then the nodejs app will spin up a docker container and execute a user input inside the docker container. Once after the docker finish its job or if it runs out of admin defined resources then the docker container will be forcefully killed (docker kill) and removed (docker rm).
Now my question is, is it best practices to run the Ubunte16.04 docker container and run nodjes app and the short term docker containers inside the Ubunuter16.04 docker container.
In short run a docker inside other docker container.
Docker-in-Docker is generally considered fragile and hard to maintain and using it isn’t a best practice. https://hub.docker.com/_/docker/ has a little discussion on this.
A straightforward (but potentially dangerous) way to rearrange this is to give the server process access to the host’s Docker socket, with docker run -v /var/run/docker.sock:/var/run/docker.sock. Then it could launch its own Docker containers as needed. Note that if you do this, these sub-containers’ docker run -v options refer to the host’s filesystem, not the calling container’s filesystem, so if you’re trying to use the filesystem to transfer data this can get tricky. Also note that being able to run any Docker command this way gives unlimited access to the host, so you need to be extremely careful about how you launch containers.
A larger redesign would be to introduce some sort of message-queueing system; I’ve successfully used RabbitMQ in the past but there are many other options. Instead of the server process launching a subprocess directly, it writes a message to a queue. Instead of the workers being short-lived processes that start and stop frequently, you have a long-lived worker that reads jobs off the queue and does them. This puts you in a much more established Docker space where nothing needs to dynamically start and stop containers, and you can easily test the Node-Rabbit-worker stack in a non-Docker environment.
I have a simple question hopefully; I'm new to Docker and Linux.
Most of the articles/stackoverflow posts suggest installing cron INSIDE the docker container to get it to work as can be seen at this link
However, based on the picture attached below, we can see that the Docker Engine is an abstraction layer between the HOST OS's system and utility libraries and the application container.
Why are we not REUSING the system cron that comes with the HOST instead of installing cron INSIDE the container? It almost feels redundant.
My understanding of docker is you'd install application level libraries and packages like npm node modules INSIDE your nodejs app container for example but if you need a system utility like cron, then you would somehow call back out to the HOST OS's native cron utility; so why not use the HOST's cron within our container somehow, why reinstall cron inside the container?
Lastly, would you use docker-compose instead and separate out the cron service into its own container then somehow have the cron service talk to the application container and referencing its environment vars?
I mean the environment variables defined in the app container; making those accessible to the cron container?
So that we may follow best practice of one service per container?
Use cron on the host machine if you want, eg
0 0 * * * /usr/local/bin/docker run image
As far as I'm aware, modern container applications use some form of scheduling from the host (eco)system. You could use cron on the host machine to trigger docker run commands. You could use a general purpose scheduler like Airflow. You could use a full-fletched container platform like DC/OS, which come with built in scheduling services.
There is nothing wrong with running cron together with your application inside a container per se. However, if you trigger the application container from a scheduling service outside your application container, your container would terminate after the job is finished, thus releasing any resources to other applications.
Secondly, it's considered good practice to have one container per service. Cron is a service in itself.
I would like to use Supervisor to run multiple processes in my Docker container, as described here, in Docker docs.
It works but the doc does not say anything about what happens when one of the processes I start crashes.
Following docker behavior logic - when a process crashes - container should stop, and probably later it should be restarted by Docker according to restart policy.
But it does not happen, If one (or all) of application I start exits - container keeps working.
How can I tell Supervisor to exit (and stop the container in this way, because I run it in nodaemon=true mode) as well, when one of monitoring processes exits/crashes?
I found this article which describes that its sometimes valid to run multiple processes in one container.
He describes how to use honcho to create the behaviour you would like: stop the whole container when one of the processes fails.
I'am going to try this now, but I'm still a little bit in doubt because supervisord is used so much more in the docker world and is also described on their own site.
if you want to exit the container when your process stops, don't use supervisor (or any other process manager). just run the process in your container, directly.
but more importantly: don't run multiple critical applications in your container. the golden rule of Docker containers is not 1 process per container, but 1 concern per container. that way your container can properly shut down when that 1 concern (application) exits.
even in the example you cite, they are not running 2 critical processes. they are running 1 app process and then hosting sshd in the same container for ssh access. if sshd stops, it's probably not a big deal. if the apache server stops... well, they're using supervisor to handle that and automatically restart it.
to get what you want, separate your concerns into multiple containers and just run the app in the container directly.
I have three express servers written on nodejs. These servers are serving different purposes and hence running on different ports.
Eg: app1.js on 8000, app2.js on 5000 and app3.js on 5432.
I want to create a docker image using a docker file and run all these servers. Can we do so? If so, how can we do it? As per my knowledge we can run only one command from docker file.
The suggested mechanism by Ethan is correct for running multiple docker container at once, but does not explain why.
Just to explain a bit further, each docker container can spawn multiple processes (servers), but a docker container needs one of the processes to be in foreground, and docker the container lifecycle typically reflects the lifecycle of the foreground process.
Lot of the benefits of dockerization will be lost when you run all processes in one docker container. And hence it is recommended to have one docker container per process.
You may want to look into using Docker Compose.
Each server would have its own Dockerfile and your docker-compose.yml file would define the ports these expose and how they interact.
While it's not "recommended", sure you can. It's even documented.
Docker and Supervisord
Or you can use Runit
Lately I have been using s6
You may want to check out the phusion passenger nodejs image. You can configure it to run a single server that is serving data from multiple nodejs processes.
I have a Node.js application running inside a Docker Container.
I need to launch a new container from my Node.js application (via code; e.g. child_process.spawn()) with the sole purpose of running a Python script. I also need to pass one argument (a database record ID) to this Python script. So the command is:
python main.py 56fb661b7e51f80736d48113
Note that I do not want this container to run inside the current container but rather to be a separate container.
I understand an orchestration framework such as Swarm or Kubernates would be better suited for this task, but it has been requested that I use Docker Compose locally on my machine in my development environment, and then we will use Kubernates in production.
Is it possible to launch a new Docker container (just a container, not a whole new machine/VM) from within a running Docker container using Docker Compose, and if so, how might I go about doing so?
I haven't done it myself, but from what I gather if your have docker installed on your child container, if you make the docker socket of the host available in the child you are able to interact with it. i.e.
--volume=/var/run/docker.sock:/tmp/docker.sock
You'll need to config your child's docker process to point to that socket (presumably the DOCKER_HOST envvar should work?) but thats the basic idea. Running docker commands against that socket should work on the host.
https://github.com/gliderlabs/registrator use this method which might help give you some pointers.
Obviously, this method of using docker creates a number of issues, but if its best for your situation then go for it.