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
Related
I have recently started learning docker. However when studying swarm mode I see that containers can be scaled up. What I would like to know is once you scale conatiner in replicated mode will the data within the container be replicated too ? or just fresh containers will be spawned ?
For example lets say I created mysql service initially only with 1 copy. I create and update tables in that mysql container. Later I scale it to 3, will newly spawned containers contain same table data ? Also will the data be continuously be replicated across 3 docker instances ?
A replicated service will use fresh container instances per container. Swarm does not take care about replication of persistent data to be stored in volumes.
Dependening on the volume plugin (e.g. local driver /w remote nfs shares) you are limited to read-write-once or read-write-many. Even if your volume allows read-write-many, the service replicas might not support that, for instance mysql will not work if you point n replicas to the same volume. You can leverage swarm service template variables for instance to point your volumes to different target folders of the same nfs share.
Also with swarm, you will want to have storage that needs to be reachable from all nodes, as a container can die and be re-spawned on a different node. So either you will need to use a remote share based on NFS or CIFS (see example usages nfs cifs), a storage cluster like Ceph or GlusterFS or a cloud native storage like Portworx. While you have to take care of HA for remote share solutions, data replication is build in for storage clusters and cloud native storage.
In case a containerized service itself is cluster/replica aware it is usualy better to not use the swarm replica mechanism - unless all instances can be started with the same set of parameters.
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.
I'm wondering whether there are any differences between the following docker setups.
Administrating two separate docker engines via the remote api.
Administrating two docker swarm nodes via one single docker engine.
I'm wondering if you can administrate a swarm with the ability run a container on a specific node are there any use cases to have separate docker engines?
The difference between the two is swarm mode. When a docker engine is running services in swarm mode you get:
Orchestration from the manager to continuously try to correct any differences between the current state and the target state. This can also include HA using the quorum model (as long as a majority of the managers are reachable to make decisions).
Overlay networking which allows containers on different hosts to talk to each other on their own container network. That can also involve IPSEC for security.
Mesh networking for published ports and a VIP for the service that doesn't change like container IP's do. The latter prevents problems from DNS caching. And the former has all nodes in the swarm publish the port and routes traffic to a container providing this service.
Rolling upgrades to avoid any downtime with replicated services.
Load balancing across multiple nodes when scaling up a service.
More details on swarm mode are available from docker's documentation.
The downside of swarm mode is that you are one layer removed from the containers when they run on a remote node. You can't run an exec command on a task to investigate a container, you need to do that on a container and be on the node it's currently using. Docker also removed some options from services like --volumes-from which don't apply when containers may be running on different machines.
If you think you may grow beyond running containers on a single node, need to communicate between the containers on different nodes, or simply want the orchestration features like rolling upgrades, then I would recommend swarm mode. I'd only manage containers directly on the hosts if you have a specific requirement that prevents swarm mode from being an option. And you can always do both, manage some containers directly and others as a service or stack inside of swarm, on the same nodes.
I'm trying to figure out whether Docker Swarm or Kubernetes are a good choice for my use case.
Basically, I want to build a small cluster of forward proxies (via squid, nginx or a custom nodejs script), and be able to deploy/start/stop/purge them all together.
I should be able to access the proxy cluster via a single IP address, manager should be able to load-balance the request to a node, and each proxy node must use a unique outgoing IP address.
I'm wondering:
Are Docker Swarm and/or Kubernetes the right way to go about it?
If so, should I set-up Docker Swarm and/or Kubernetes and its worker nodes (running the proxy) on a single dedicated server or separate virtual servers?
Is it also possible for all the cluster nodes to share a file system storage for caching, common config etc.
Any other tips to get this working.
Thanks!
Docker running in swarm mode should work well for this
Run docker on a single dedicated server; I see no need for virtual servers. You could also run the swarm across multiple dedicated servers.
https://docs.docker.com/engine/swarm/secrets/ work well for some settings and configurations. If you require significant storage, simply add a database service to your cluster
Docker swarm mode fits your requirements quite well; requests are automatically balanced across your swarm and each service instance can be configured to have a unique address. You should check out the swarm mode tutorial: https://docs.docker.com/engine/swarm/swarm-tutorial/
I have a swarm setup which has around 6 nodes. Whenever I execute a docker run or docker pull command from the swarm manager it downloads the new image on all the swarm nodes.
This is creating data redundancy and choking my network.
Is there any way I can avoid this ?
Swarm Nodes need Images available to them by design. That will help swarm to start the container on an available node immediately when current node hosting the container crashes or current hosting node goes into maintenance (Drain Mode).
On the other hand docker Images will be pulled one time only, and you can use them until you upgrade your service.
Another one, Docker is designed for microservices, If you Image getting too large, Maybe you should try to cut it down to multiple containers.