I'm trying to wrap my head around Docker from the point of deploying an application which is intended to run on the users on desktop. My application is simply a flask web application and mongo database. Normally I would install both in a VM and, forward a host port to the guest web app. I'd like to give Docker a try but I'm not sure how I'm meant to use more than one program. The documentations says there can only be only ENTRYPOINT so how can I have Mongo and my flask application. Or do they need to be in separate containers, in which case how do they talk to each other and how does this make distributing the app easy?
There can be only one ENTRYPOINT, but that target is usually a script that launches as many programs that are needed. You can additionally use for example Supervisord or similar to take care of launching multiple services inside single container. This is an example of a docker container running mysql, apache and wordpress within a single container.
Say, You have one database that is used by a single web application. Then it is probably easier to run both in a single container.
If You have a shared database that is used by more than one application, then it would be better to run the database in its own container and the applications each in their own containers.
There are at least two possibilities how the applications can communicate with each other when they are running in different containers:
Use exposed IP ports and connect via them.
Recent docker versions support linking.
I strongly disagree with some previous solutions that recommended to run both services in the same container. It's clearly stated in the documentation that it's not a recommended:
It is generally recommended that you separate areas of concern by using one service per container. That service may fork into multiple processes (for example, Apache web server starts multiple worker processes). It’s ok to have multiple processes, but to get the most benefit out of Docker, avoid one container being responsible for multiple aspects of your overall application. You can connect multiple containers using user-defined networks and shared volumes.
There are good use cases for supervisord or similar programs but running a web application + database is not part of them.
You should definitely use docker-compose to do that and orchestrate multiple containers with different responsibilities.
I had similar requirement of running a LAMP stack, Mongo DB and my own services
Docker is OS based virtualisation, which is why it isolates its container around a running process, hence it requires least one process running in FOREGROUND.
So you provide your own startup script as the entry point, thus your startup script becomes an extended Docker image script, in which you can stack any number of the services as far as AT LEAST ONE FOREGROUND SERVICE IS STARTED, WHICH TOO TOWARDS THE END
So my Docker image file has two line below in the very end:
COPY myStartupScript.sh /usr/local/myscripts/myStartupScript.sh
CMD ["/bin/bash", "/usr/local/myscripts/myStartupScript.sh"]
In my script I run all MySQL, MongoDB, Tomcat etc. In the end I run my Apache as a foreground thread.
source /etc/apache2/envvars
/usr/sbin/apache2 -DFOREGROUND
This enables me to start all my services and keep the container alive with the last service started being in the foreground
Hope it helps
UPDATE: Since I last answered this question, new things have come up like Docker compose, which can help you run each service on its own container, yet bind all of them together as dependencies among those services, try knowing more about docker-compose and use it, it is more elegant way unless your need does not match with it.
Although it's not recommended you can run 2 processes in foreground by using wait. Just make a bash script with the following content. Eg start.sh:
# runs 2 commands simultaneously:
mongod & # your first application
P1=$!
python script.py & # your second application
P2=$!
wait $P1 $P2
In your Dockerfile, start it with
CMD bash start.sh
I would recommend to set up a local Kubernetes cluster if you want to run multiple processes simultaneously. You can 'distribute' the app by providing them a simple Kubernetes manifest.
They can be in separate containers, and indeed, if the application was also intended to run in a larger environment, they probably would be.
A multi-container system would require some more orchestration to be able to bring up all the required dependencies, though in Docker v0.6.5+, there is a new facility to help with that built into Docker itself - Linking. With a multi-machine solution, its still something that has to be arranged from outside the Docker environment however.
With two different containers, the two parts still communicate over TCP/IP, but unless the ports have been locked down specifically (not recommended, as you'd be unable to run more than one copy), you would have to pass the new port that the database has been exposed as to the application, so that it could communicate with Mongo. This is again, something that Linking can help with.
For a simpler, small installation, where all the dependencies are going in the same container, having both the database and Python runtime started by the program that is initially called as the ENTRYPOINT is also possible. This can be as simple as a shell script, or some other process controller - Supervisord is quite popular, and a number of examples exist in the public Dockerfiles.
Docker provides a couple of examples on how to do it. The lightweight option is to:
Put all of your commands in a wrapper script, complete with testing
and debugging information. Run the wrapper script as your CMD. This is
a very naive example. First, the wrapper script:
#!/bin/bash
# Start the first process
./my_first_process -D
status=$?
if [ $status -ne 0 ]; then
echo "Failed to start my_first_process: $status"
exit $status
fi
# Start the second process
./my_second_process -D
status=$?
if [ $status -ne 0 ]; then
echo "Failed to start my_second_process: $status"
exit $status
fi
# Naive check runs checks once a minute to see if either of the processes exited.
# This illustrates part of the heavy lifting you need to do if you want to run
# more than one service in a container. The container will exit with an error
# if it detects that either of the processes has exited.
# Otherwise it will loop forever, waking up every 60 seconds
while /bin/true; do
ps aux |grep my_first_process |grep -q -v grep
PROCESS_1_STATUS=$?
ps aux |grep my_second_process |grep -q -v grep
PROCESS_2_STATUS=$?
# If the greps above find anything, they will exit with 0 status
# If they are not both 0, then something is wrong
if [ $PROCESS_1_STATUS -ne 0 -o $PROCESS_2_STATUS -ne 0 ]; then
echo "One of the processes has already exited."
exit -1
fi
sleep 60
done
Next, the Dockerfile:
FROM ubuntu:latest
COPY my_first_process my_first_process
COPY my_second_process my_second_process
COPY my_wrapper_script.sh my_wrapper_script.sh
CMD ./my_wrapper_script.sh
I agree with the other answers that using two containers is preferable, but if you have your heart set on bunding multiple services in a single container you can use something like supervisord.
in Hipache for instance, the included Dockerfile runs supervisord, and the file supervisord.conf specifies for both hipache and redis-server to be run.
If a dedicated script seems like too much overhead, you can spawn separate processes explicitly with sh -c. For example:
CMD sh -c 'mini_httpd -C /my/config -D &' \
&& ./content_computing_loop
In docker, there are two ways you can run a program
CMD
ENTRYPOINT
If you want to know the difference between them, please refer here
In CMD/ENTRYPOINT, there are two formats to run a command
SHELL format
EXEC format
SHELL format:
CMD executable_first arg1; executable_second arg1 arg2
ENTRYPOINT executable_first arg1; executable_second arg1 arg2
This version will create a shell and executes above command. Here you can use any shell syntax such as ";", "&", "|", etc. So you can run any number of commands here. If you have complex set of commands to run, you can create separate shell script and use it.
CMD my_script.sh arg1
ENTRYPOINT my_script.sh arg1
EXEC format:
CMD ["executable", "parameter 1", "parameter 2", …]
ENTRYPOINT ["executable", "parameter 1", "parameter 2", …]
Here you can notice that only first parameter is an executable. From the second parameter, everything become an arguments/parameters for that executable.
To run multiple commands in EXEC format
CMD ["/bin/sh", "-c", "executable_first arg1; executable_second"]
CMD ["/bin/sh", "-c", "executable_first arg1; executable_second"]
In above command, we have used shell command as executable to run the command. This is the only way to run multiple commands in EXEC format.
Following are WRONG
CMD ["executable_first parameter", "executable_second parameter"]
ENTRYPOINT ["executable_first parameter", "executable_second parameter"]
CMD ["executable_first", "parameter", ";", "executable_second", "parameter"]
ENTRYPOINT ["executable_first", "parameter", ";", "executable_second", "parameter"]
Can I run multiple programs in a Docker container?
Yes. But with significant risks.
Below is the same answer as above. But with details and a recommended resolution. If you're interested in those.
Not Recommended
Warning. Using the same container for multiple services is not recommended by the Docker community, though. The Docker documentation reads: "It is generally recommended that you separate areas of concern by using one service per container." Source at:
• https://archive.ph/3Roa6#selection-307.2-307.100
• https://docs.docker.com/config/containers/multi-service_container/
If you choose to ignore the recommendation above, you container risk to be with weaker security, increasingly unstable, and in the future a painful growth.
If you are ok with those risks above, the documentation to use one container for multiple services is at:
• https://archive.ph/3Roa6#selection-335.0-691.1
• https://docs.docker.com/config/containers/multi-service_container/
Recommended
If you need a container(s) with stronger security, and more stability, and in the future, scale bigger, as well as better performance, then the Docker community recommends those two steps:
Use one service per Docker container. The end result is that you will have multiple containers.
Use this Docker "Networking" feature to connect any of those containers to your liking.
If we are submitting a task to the compute engine through ssh from host machine and if we shut down the host machine is there a way that we can get hold of the output of the submitted task later on when we switch on the host machine?
From the Linux point of view ‘ssh’ and ‘gcloud compute ssh’ are commands like all the others, therefore it is possible to redirect their output to a file while the command is performed using for example >> to redirect and append stdout to a file or 2>> to store stderr.
For example if you run from the first instance 'name1':
$ gcloud compute ssh name2 --command='watch hostname' --zone=XXXX >> output.out
where 'name2' is the second instance, and at some point you shutdown 'name1' you will find stored into output.out the output provided by the command till the shutdown occurred.
Note that there is also the possibility to create shut down scripts, that in this scenario could be useful in order to upload output.out to a bucket or to perform any kind of clean-up operation.
In order to do so you can run the following command
$ gcloud compute instances add-metadata example-instance --metadata-from-file shutdown-script=path/to/script_file
Where the content of the script could be something like
#! /bin/bash
gsutil cp path/output.out gs://yourbucketname
Always keep in mind that Compute Engine only executes shutdown scripts on a best-effort basis and does not guarantee that the shutdown script will be run in all cases.
More Documentation about shutdown scrips if needed.
I wanted clarification on the possible scripts that can be added in the .s2i/bin directory in my project repo.
The docs say when you add these files they will override the default files of the same name when the project is built. For example, if I place my own "assemble" file in the .s2i/bin directory will the default assemble file run also or be totally replaced by my script? What If I want some of the behavior of the default file? Do I have to copy the default "assemble" contents into my file so both will be executed?
you will need to call out the original "assemble" script from your own. Similar to this
#!/bin/bash -e
# The assemble script builds the application artifacts from a source and
# places them into appropriate directories inside the image.
# Execute the default S2I script
source ${STI_SCRIPTS_PATH}/assemble
# You can write S2I scripts in any programming language, as long as the
# scripts are executable inside the builder image.
Using OpenShift, I want to execute my own run script (run).
So, I added in the src of my application a file in ./s2i/run
that slightly changes the default run file
https://github.com/sclorg/nginx-container/blob/master/1.20/s2i/bin/run
Here is my run file
#!/bin/bash
source /opt/app-root/etc/generate_container_user
set -e
source ${NGINX_CONTAINER_SCRIPTS_PATH}/common.sh
process_extending_files ${NGINX_APP_ROOT}/src/nginx-start ${NGINX_CONTAINER_SCRIPTS_PATH}/nginx-start
if [ ! -v NGINX_LOG_TO_VOLUME -a -v NGINX_LOG_PATH ]; then
/bin/ln -sf /dev/stdout ${NGINX_LOG_PATH}/access.log
/bin/ln -sf /dev/stderr ${NGINX_LOG_PATH}/error.log
fi
#nginx will start using the custom nginx.conf from configmap
exec nginx -c /opt/mycompany/mycustomnginx/nginx-conf/nginx.conf -g "daemon off;"
Then, changed the dockerfile to execute my run script as follows
The CMD command can be called once and dictates where is the script located that is executed when the Deployment pod starts.
FROM registry.access.redhat.com/rhscl/nginx-120
# Add application sources to a directory that the assemble script expects them
# and set permissions so that the container runs without root access
USER 0
COPY dist/my-portal /tmp/src
COPY --chmod=0755 s2i /tmp/
RUN ls -la /tmp
USER 1001
# Let the assemble script to install the dependencies
RUN /usr/libexec/s2i/assemble
# Run script uses standard ways to run the application
#CMD /usr/libexec/s2i/run
# here we override the script that will be executed when the deployment pod starts
CMD /tmp/run
I'm using the mysql image as an example, but the question is generic.
The password used to launch mysqld in docker is not visible in docker ps however it's visible in docker inspect:
sudo docker run --name mysql-5.7.7 -e MYSQL_ROOT_PASSWORD=12345 -d mysql:5.7.7
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
b98afde2fab7 mysql:5.7.7 "/entrypoint.sh mysq 6 seconds ago Up 5 seconds 3306/tcp mysql-5.7.7
sudo docker inspect b98afde2fab75ca433c46ba504759c4826fa7ffcbe09c44307c0538007499e2a
"Env": [
"MYSQL_ROOT_PASSWORD=12345",
"PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
"MYSQL_MAJOR=5.7",
"MYSQL_VERSION=5.7.7-rc"
]
Is there a way to hide/obfuscate environment parameters passed when launching containers. Alternatively, is it possible to pass sensitive parameters by reference to a file?
Weirdly, I'm just writing an article on this.
I would advise against using environment variables to store secrets, mainly for the reasons Diogo Monica outlines here; they are visible in too many places (linked containers, docker inspect, child processes) and are likely to end up in debug info and issue reports. I don't think using an environment variable file will help mitigate any of these issues, although it would stop values getting saved to your shell history.
Instead, you can pass in your secret in a volume e.g:
$ docker run -v $(pwd)/my-secret-file:/secret-file ....
If you really want to use an environment variable, you could pass it in as a script to be sourced, which would at least hide it from inspect and linked containers (e.g. CMD source /secret-file && /run-my-app).
The main drawback with using a volume is that you run the risk of accidentally checking the file into version control.
A better, but more complicated solution is to get it from a key-value store such as etcd (with crypt), keywhiz or vault.
You say "Alternatively, is it possible to pass sensitive parameters by reference to a file?", extract from the doc http://docs.docker.com/reference/commandline/run/ --env-file=[] Read in a file of environment variables.
I'm launching instances using the following command:
gcutil addinstance \
--image=debian-7 \
--persistent_boot_disk \
--zone=us-central1-a \
--machine_type=n1-standard-1 \
--metadata_from_file=startup-script:install.sh \
instance-name
How can I detect when this instance has completed it's install script? I'd like to be able to place this launch command in a larger provisioning script that then goes on to issue commands to the server that depend on the install script having been successfully completed.
There is a number of ways: sending yourself an email, uploading to Cloud Storage, sending a jabber message, ...
One simple, observable way IMHO is to add a logger entry at the end of your install.sh script (I also tweak the beginning for symmetry). Something like:
#!/bin/bash
/usr/bin/logger "== Startup script START =="
#
# Your code goes here
#
/usr/bin/logger "== Startup script END =="
You can check then if the script started or ended in two ways:
From your Developer's Console, select "Projects" > "Compute" > "VM Instances" > your instance > "Serial console" > "View Output".
From CLI, by issuing a gcutil getserialportoutput instance-name.
I don't know of a way to do all of this within gcutil addinstance.
I'd suggest:
Adding the instance via gcutil addinstance, making sure to use the --wait_until_running flag to ensure that the instance is running before you continue
Copying your script over to the instance via something like gcutil push
Using gcutil ssh <instance-name> </path-to-script/script-to-run> to run your script manually.
This way, you can write your script in such a way that it blocks until it's finished, and the ssh command will not return until your script on the remote machine is done executing.
There really are a lot of ways to accomplish this goal. One that tickles my fancy is to use the metadata server associated with the instance. Have the startup script set a piece of metadata to "FINISHED" when the script is done. You can query the metadata server with a hanging GET that will only return when the metadata updates. Just use gcutil setmetadata
from within the script as the last command.
I like this method because the hanging GET just gives you one command to run, rather than a poll to run in a loop, and it doesn't involve any services besides Compute Engine.
One more hacky way:
startup_script_finished=false
while [[ "$startup_script_finished" = false ]]; do
pid=$(gcloud compute ssh $GCLOUD_USER#$GCLOUD_INSTANCE -- pgrep -f "\"/usr/bin/python /usr/bin/google_metadata_script_runner --script-type startup\"")
if [[ -z $pid ]]; then
startup_script_finished=true
else
sleep 2
fi
done
One possible solution would be to have your install script create a text file in a cloud storage bucket, as the last thing it does, using the host name as the filename.
Your main script that did the original gcutil addinstance command could then be periodically polling the contents of the bucket (using gsutil ls) until it sees a file with a matching name and then it would know the install had completed on that instance.