Is it possible to hotcopy a docker container? or some sort of clustering with docker for HA purposes?
Can someone simplify this?
How to scale Docker containers in production
Docker containers are not designed to be VMs and are not really meant for hot-copies. Instead you should define your container such that it has a well-known start state. If the container goes down the alternate should start from the well-known start state. If you need to keep track of state that the container generates at run time this has to be done externally to docker.
One option is to use volumes to mount the state (files) on to the host filesystem. Then use RAID, NTFS or any other means, to share that file system with other physical nodes. Then you can mount the same files on to a second docker container on a second host with the same state.
Depending on what you are running in your containers you can also have to state sharing inside your containers for example using mongo replication sets. To reiterate though containers are not as of yet designed to be migrated with runtime state.
There is a variety of technologies around Docker that could help, depending on what you need HA-wise.
If you simply wish to start a stateless service container on different host, you need a network overlay, such as weave.
If you wish to replicate data across for something like database failover, you need a storage solution, such as Flocker.
If you want to run multiple services and have load-balancing and forget on which host each container runs, given that X instances are up, then Kubernetes is the kind of tool you need.
It is possible to make many Docker-related tools work together, we have a few stories on our blog already.
Related
I've been trying to devise a strategy for using Docker Swarm for managing a bunch of headless containers - don't need load balancer, exposing any ports, or auto scaling.
The only thing I want is the ability to update all of the containers (on all nodes), if any of the images are updated. Each container running will need to have a specific --hostname.
Is running docker service even viable for this? Or should I just do a normal docker run targeting specific nodes to specify the --hostname i want? The reason I'm even asking about docker service is because it allows you to do an update (forcing an update for all containers if there are updated images).
Was also thinking that Docker Swarm would make it a bit easier to keep an eye on all the containers (i.e. manage them from a central location).
The other option I was looking at was watchtower, to run on each server that is running one of the containers, as an alternative to swarm. My only issue with this is that it doesn't provide any orchestration, for centralized management.
Anyone have any ideas of what would be a better option given the scenario?
Docker swarm does not give you any advantage regarding rolling updates apart from the docker service command, swarm only provides the user horizontal scaling and places a load balancer in front of those replicas called "service", as well as some other goodies such as replicating the docker events across the swarm nodes.
docker service --force would work as expected.
However, you should probably use both, docker swarm for orchestration and watchtower for rolling updates.
I'm new to Docker Swarm. As I understand, Docker Swarm allows you to abstract from clustering. Means you don't care on which hardriwe container is deployed.
On the other hand, the standard way to handle database in Docker - is to write data outside Docker container (to avoid copy-on-write behaviour). That's achieved by mounting a Volume and write db-related data to it. The important thing here - are Volumes machine-specific? Are Docker & Docker Swarm clever enough to mount a Volume on the machine it's needed?
Example:
I have 3 machines and 3 microservices/containers. All of them are deployed through Docker Swarm. Only one microservice/container must connect to a database. So I need to mount Volume only on one machine. But on which?
Databases and similar stateful applications are still a hard thing to deal with when it comes to Docker swarm and other orchestration frameworks. Ideally, containers should be able to run on any node in the swarm, but the problem comes when you need to persist data beyond the container's lifecycle.
Mounting a volume is the Docker way to persist data, however this ties the container with a specific node as volumes are created on the specific nodes. There are many projects that try to solve this problem and provide some sort of distributed storage.
There was a project called Flocker that deals with the above problem (it’s no longer maintained). There is also a newer project called REXRAY.
Are Docker & Docker Swarm clever enough to mount a Volume on the machine it's needed?
By default, no. Docker swarm will choose one of the nodes and deploy the container on it. However, you can work around this problem:
First, you need to define a named volume in you Stackfile/Composefile under the service definition.
Second, you need to use node Placement Constraints to restrict where the database container should run.
If you do not you a distributed storage tool, then when it comes to databases and similar stateful containers that need volumes, you need to restrict the container to a specific nodes.
I run a complex app with a database backend and many other things all in one container. I notice that Docker images for different database systems are available. When would I want to move something like a DB server to its own container, instead of running everything in the same container? The advantage I have now is that I can deploy everything at once, and I don't have to configure more than one container to get things talking.
Docker or the Container Manager is using Linux container technology to provide a best abstraction, using docker container with multiple process is a bad idea; use docker container for isolating one process, use docker volume container for storing database data ( docker state is not persistent by default).
Use docker-compose or fig to attach two docker containers db and web app, it will ease your management in future!
I am new to docker. I have a doubt regarding docker. Based on the understanding of docker, Docker will help to create the container of the application we can to deploy along with application dependencies.
My question is that if i have web application inside docker container, is it possible to run multiple containers inside single host? If yes, How will i make sure the request be directed to each app?.
Will there be any change in performance depending on number of core of host?
Is it possible to run multiple containers inside single host?
Yes, you can run many.
If yes, How will direct requests to the right container?
You have many options, the simplest is just to run the container with port forwarding (which is built in to docker), but you could also run a load balancer or proxy on the host.
Will there be any change in performance depending on number of core of host?
There can be, of course. It depends on whether or not you're already reaching a performance bottleneck of some sort before adding another container. All the containers are making use of the same hardware.
I am having hard time figuring how I should combine Chef and Docker to get the best of them.
Right now I am using Chef to automatically pull a docker image and create a container.
But things get messy when I want to change the configuration inside the container.
I read about knife container but I didn't understand how one can bootstrap a container and a new vm (on Amazon for example) all together.
I would suggest that if all you want to do is manage Docker images/containers, that you don't really need Chef.
Docker provides tools like:
Fig (http://www.fig.sh/), which brings up multiple containers as one logical unit.
Swarm (https://github.com/docker/swarm/), which allows you to abstract away the machines you have for deployments. For example, "My app needs 2GB of RAM, 1 CPU, 10GB of HD, which machine has available resources?"
Machine (https://github.com/docker/machine), which allows you to create VMs in the cloud in pretty much any provider.
A REST API (https://docs.docker.com/reference/api/docker_remote_api/), which allows you to remotely start/stop containers etc.
In my opinion those suite of tools replace the need for Chef if all you're going to do is manage Docker images and containers.
As someone already noted, don't change configs after a container has started. Better to make a new image or restart the container. You could also mount the configs external to the container and modify them there, then restart the container.