I have a docker swarm cluster.
I have a pre-configured docker image in my docker hub and i would like to run a container of the image on demand.
I have written an api which can go to a host and run a container.
But now, i want to spin up that container on cluster in stead of pointing to a single host.
I have seen docker-compose and docker-stack.
But as per the documentation
If there are existing containers for a service, and the service’s configuration or image was changed after the container’s creation, docker-compose up picks up the changes by stopping and recreating the containers (preserving mounted volumes).
So, how can i start a container on demand on a cluster ?
In Swarm, you must create a service based on an image in order to create container. There is 2 mode, global to launch 1 container of this service on each machine, and replicated, load balancer of swarm will choose the machine which will host the container (of course you can add constraint with option --constraint).
docker service create --name=my_name --replicas=4 my_image
If you want to launch a container on demand, you must scale this service to the desire number of task (container).
docker service scale my_service=5
For instance you could use a SDK to get number of task of the service and scale it to number of task of service + 1.
In my own experience, Swarm is only good for stateless component (don't really know how to deal with database ...)
I hope i did not miss understand the question.
Related
I can create a docker container by command
docker run <<image_name>>
I can create a service by command
docker service create <<image_name>>
What is the difference between these two in behaviour?
When would I need to create a service over container?
docker service command in a docker swarm replaces the docker run. docker run has been built for single host solutions. Its whole idea is to focus on local containers on the system it is talking to. Whereas in a cluster the individual containers are irrelevant. We simply use swarm services to manage the multiple containers in a cluster. Swarm will orchestrate the containers of the services for us.
docker service create is mainly to be used in docker swarm mode. docker run does not have the concept of scaling up/down. With docker service create you can specify the number of replicas to be created using the --replicas command. This will create and manage multiple replicas of a containers in many different nodes. There are several such options for managing multiple containers using docker service create and other commands under docker service ...
One more note: docker services are for container orchestration systems(swarm). It has built in facility for failure recovery. ie. it recreates a container on failure. docker runwould never recreate a container if it fails. When the docker service commands are used we are not directly asking to perform action like "create a single container", rather we are saying to the orchestration system to "put this job in your queue and when you can get to it perform that action on the swarm". This means it has rollback facilities, failure mitigation and lots of intelligence built in.
You need to consider using docker service create when in swarm mode and docker run when not in swarm mode. You can lookup on docker swarms to understand docker services.
There is no real difference. In the official documentation you can read "Services are really just containers in production".
Services can be declared in "docker-compose.yml" and can be started from it. Once started, they will run as containers.
It is just a common way to name parts of your stack.
I have setup a 3 node cluster (with no Internet access) with 1 manager and 2 worker-nodes using the standard swarm documentation.
How does the swarm manager in swarm mode know about the images present in worker nodes?
Lets say I have image A in worker-node-1 and image B in worker-node-2 and no images in the manager-node.
Now how do I start container for image A using the manager?
Will it start in manager or node-1?
When I query manager for the list of images will it give the whole list with A and B in it?
Does anyone know how this works?
I couldn’t get the details from the documentation.
Docker Swarm manager node may to be a worker one by the second role but not strictly necessary.
Image deployment policy is mapped via docker-compose.yml which has an information like target nodes, networks, hostnames, volumes, etc. in relation of particular service. So, it will start either in specified node or in emptiest default one.
Swarm manager communicates with the worker nodes via Docker networks:
When you initialize a swarm or join a Docker host to an existing swarm, two new networks are created on that Docker host:
an overlay network called ingress, which handles control and data
traffic related to swarm services. When you create a swarm service and
do not connect it to a user-defined overlay network, it connects to
the ingress network by default
a bridge network called
docker_gwbridge, which connects the individual Docker daemon to the
other daemons participating in the swarm.
Reference
During Swarm deployment, the images of it's services are being propagated to worker nodes according to their deployment policy.
The manager node will contain images once the node is the worker one too (correct me, if it won't).
The default configuration with swarm mode is to pull images from a registry server and use pinning to reference a unique hash for those images. This can be adjusted, but there is no internal mechanism to distribute images within a cluster.
For an offline environment, I'd recommend a stand alone registry server accessible to the cluster. You can even run it on the cluster. Push your image there, and point your server l services to the registry for their images to pull. See this doc for details on running a stand alone registry, or any of the many 3rd party options (e.g. Harbor): https://docs.docker.com/registry/
The other option is to disable the image pinning, and manually copy images to each of your swarm nodes. You need to do this in advance of deploying any service changes. You'll also lose the benefit of reused image layers when you manually copy them. Because of all this issues it creates, overhead to manage, and risk of mistakes, I'd recommend against this option.
Run the docker stack deploy command with --with-registry-auth that will give the Workers access to pull the needed image
By default Docker Swarm will pull the latest image from registry when deploying
I am using Docker version 17.12.1-ce.
I have set up a swarm with two nodes, and I have a stack running on the manager, while I am to instantiate new nodes on the worker (not within a service, but as stand-alone containers).
So far I have been unable to find a way to instantiate containers on the worker specifically, and/or to verify that the new container actually got deployed on the worker.
I have read the answer to this question which led me to run containers with the -e option specifying constraint:Role==worker, constraint:node==<nodeId> or constraint:<custom label>==<value>, and this github issue from 2016 showing the docker info command outputting just the information I would need (i.e. how many containers are on each node at any given time), however I am not sure if this is a feature of the stand-alone swarm, since docker info only the number of nodes, but no detailed info for each node. I have also tried with docker -D info.
Specifically, I need to:
Manually specify which node to deploy a stand-alone container to (i.e. not related to a service).
Check that a container is running on a specific swarm node, or check how many containers are running on a node.
Swarm commands will only care/show service-related containers. If you create one with docker run, then you'll need to use something like ssh node2 docker ps to see all containers on that node.
I recommend you do your best in a Swarm to have all containers as part of a service. If you need a container to run on nodeX, then you can create a service with a "node constraint" using labels and constraints. In this case you could restrict the single replica of that service to a node's hostname.
docker service create --constraint Node.Hostname==swarm2 nginx
To see all tasks on a node from any swarm manager:
docker node ps <nodename_or_id>
Lets say I want to edit a config file for an NGINX Docker service that is replicated across 3 nodes.
Currently I list the services using docker service ls.
Then get the details to find a node running a container for that service using docker serivce ps servicename.
Then ssh to a node where one of the containers is running.
Finally, docker exec -it containername bash. Then I edit the config file.
Two questions:
Is there a better way to do this rather than ssh to a node running a container? Maybe there is a swarm or service command to do so?
If I were to edit that config file on one container would that change be replicated to the other 2 containers in the swarm?
The purpose of this exercise would be to edit configuration without shutting down a service.
You should not be exec'ing into containers to change their configuration, and so docker has not created an easy way to do this within Swarm Mode. You could use classic swarm to avoid the need to ssh into the other host, but I still don't recommend this.
The correct way to do this is to migrate your configuration file into a docker config entry. Version your config name. Then when you want to update it, you create a new version with the desired changes, and do a rolling update of your service to use that new configuration.
Unless the config is mounted from an external source like NFS, changes to one config in one container will not apply to other containers running on other nodes. If that config is stored locally inside your container as part of it's internal copy-on-write filesystem, then no changes from one container will be visible in any other container.
I am having a problem trying to implement the best way to add new container to an existing cluster while all containers run in docker.
Assuming I have a docker swarm, and whenever a container stops/fails for some reason, the swarm bring up new container and expect it to add itself to the cluster.
How can I make any container be able to add itself to a cluster?
I mean, for example, if I want to create a RabbitMQ HA cluster, I need to create a master, and then create slaves, assuming every instance of RabbitMQ (master or slave) is a container, let's now assume that one of them fails, we have 2 options:
1) slave container has failed.
2) master container has failed.
Usually, a service which have the ability to run as a cluster, it also has the ability to elect a new leader to be the master, so, assuming this scenerio is working seemlesly without any intervention, how would a new container added to the swarm (using docker swarm) will be able to add itself to the cluster?
The problem here is, the new container is not created with new arguments every time, the container is always created as it was deployed first time, which means, I can't just change it's command line arguments, and this is a cloud, so I can't hard code an IP to use.
Something here is missing.
Maybe trying to declare a "Service" in the "docker Swarm" level, will acctualy let the new container the ability to add itself to the cluster without really knowing anything the other machines in the cluster...
There are quite a few options for scaling out containers with Swarm. It can range from being as simple as passing in the information via a container environment variable to something as extensive as service discovery.
Here are a few options:
Pass in IP as container environment variable. e.g. docker run -td -e HOST_IP=$(ifconfig wlan0 | awk '/t addr:/{gsub(/.*:/,"",$2);print$2}') somecontainer:latest
this would set the internal container environment variable HOST_IP to the IP of the machine it was started on.
Service Discovery. Querying a known point of entry to determine the information about any required services such as IP, Port, ect.
This is the most common type of scale-out option. You can read more about it in the official Docker docs. The high level overview is that you set up a service like Consul on the masters, which you have your services query to find the information of other relevant services. Example: Web server requires DB. DB would add itself to Consul, the web server would start up and query Consul for the databases IP and port.
Network Overlay. Creating a network in swarm for your services to communicate with each other.
Example:
$ docker network create -d overlay mynet
$ docker service create –name frontend –replicas 5 -p 80:80/tcp –network mynet mywebapp
$ docker service create –name redis –network mynet redis:latest
This allows the web app to communicate with redis by placing them on the same network.
Lastly, in your example above it would be best to deploy it as 2 separate containers which you scale individually. e.g. Deploy one MASTER and one SLAVE container. Then you would scale each dependent on the number you needed. e.g. to scale to 3 slaves you would go docker service scale <SERVICE-ID>=<NUMBER-OF-TASKS> which would start the additional slaves. In this scenario if one of the scaled slaves fails swarm would start a new one to bring the number of tasks back to 3.
https://docs.docker.com/engine/reference/builder/#healthcheck
Docker images have a new layer for health check.
Use a health check layer in your containers for example:
RUN ./anyscript.sh
HEALTHCHECK exit 1 or (Any command you want to add)
HEALTHCHECK check the status code of command 0 or 1 and than result as
1. healthy
2. unhealthy
3. starting etc.
Docker swarm auto restart the unhealthy containers in swarm cluster.