Managing Deployed Services

This page explains some mechanisms that are common for the management of services deployed with the recipes.

Multisite Deployment

If you happen to have your cluster with nodes distributed in specific areas (A1, A2, ..., AN) and you would like for the deployment of the replicas of your service S to happen only in specific areas, you can achieve this by using placement constraints.

For example, imagine you have a cluster composed of VMS in Málaga, Madrid and Zurich, but you want the deployment of the replicas of your database to stay only within Spain boundaries due to legal regulations. (DISCLAIMER: This is just a simplification, you should always inform yourself on how to properly comply with data protection regulations.)

First, you need define a labeling for the nodes of your cluster where you want your deployment to happen. Your label in this case can be done by region. You connect to any of the swarm manager nodes and execute the following commands.

docker node update --label-add region=ES malaga-0
docker node update --label-add region=ES madrid-0
docker node update --label-add region=ES madrid-1

docker node update --label-add region=CH zurich-0
docker node update --label-add region=CH zurich-1

When you are about to deploy your database, you will have to add a constraint to the definition of the service. This means you will have to edit the recipe before deploying. For instance, in the case of MongoDB, it should have the deploy part looking something like this:

        - region == ES

This was just a simple example. You can have multiple tags, combine them so the placement only happens in nodes with all the tags, and many other combinations. For more details of this functionality, please refer to the official docker docs on service placement.

Note: For these features to work, you need access to a manager node of the swarm cluster and Docker 17.04+.


As you probably already know, each recipe deploys a bunch of services, and each service can be of any of two types: Stateless or Stateful. The way to differentiate which type a service is, is by inspecting the implementation of the service to tell if it needs data persistence within itself or not.

The point is, the way to scale services depends on the type of the service.

Scaling stateless services with Docker is pretty straightforward, you can simply increase the number of replicas. Assuming no constraints violations (see previous section), you will be able to dynamically set more or less replicas for each stateless service.

docker service scale orion=5

More info on the scaling process is documented here.

As regards scaling stateful services, there is no silver bullet and it will always depend on the service being discussed.

For example, Docker handles two types of service deployments: replicated and global, as explained here. A replicated service can be scaled as shown in the previous example, but the only way to scale a global service (which means there will be a maximum of one instance per node), is by adding nodes. To scale down a global service you can either remove nodes or apply constraints (see Multisite Deployment section above).

In any of the two cases, for a stateful service, someone will have to be responsible for coordinating the data layer among all the instances and deal with replication, partitioning and all sort of issues typically seen in distributed systems.

Finally, the recipes should properly document which of its services are stateless and which are not, so as to have these considerations for Scalability. They should include also, notes on how their stateful services could be scaled.