We have seen many use cases where one fires up a few Docker containers on one single host. To accommodate the growth of data or complexity in business, we would need to consider running the containerized tasks on multiple physical hosts. One of the challenges was how to maintain the communications among the distributed tasks as if they were on the same host.
Fortunately Docker provides a mechanism called Overlay Networking, which basically creates an
VXLAN layer 2 overlay tunnel on top of layer 3, i.e., TCP/IP. The details won’t be discussed here but interested readers can go here for more information. It is not hard to imagine that this would allow two containers, sitting on different hosts, to talk with each other. Cool!
This blog will walk through a simple example to create a Docker Swarm that spans two physical hosts, and we will create a Overlay Network to stitch together the distributed containers.
Since the version 1.12.0, Docker Engine natively includes Swarm mode, which makes bringing up a Swarm cluster much easier than using the previous standalone Swarm. Say now there are two nodes, node 1 and node 2. We decide to elect the node 1 to be the manager node. Note that one can have more than one manager nodes in a Swarm cluster but for the sake of simplicity, we just use node 1.
Initiate the Swarm
The following command on node 1 will initiate the Swarm and elect that node as the manager. This command will also spit out the command you would use on node 2 to join the cluster.
docker swarm init
Copy and paste the output from the above, run on node 2.
docker swarm join -- token ...
Come back on node 1 to verify 2 nodes are present in the cluster.
docker node ls ID HOSTNAME STATUS AVAILABILITY MANAGER STATUS 1gwudwxftloza3vldyr4p6p4y * indocgubt103 Ready Active Leader e9bcxw8vy1ow0jp80gopr2c58 indocgubt104 Ready Active
Create an Overlay Network
Now let’s create an Overlay Network called
es. On node 1 run the following:
docker network create -d overlay es
To verify, run the following on node 1. Please note that
es won’t show up on node 2 until there is actually a container uses the network.
docker network ls NETWORK ID NAME DRIVER SCOPE 43652a980910 bridge bridge local 1ac35860a4cb docker_gwbridge bridge local 912wlikzt94x es overlay swarm 5032a295b055 host host local 1my3c1fbunaq ingress overlay swarm 7687da500317 none null local
Attach the Service to the Overlay Network
Uses the example in this post, I would like to deploy an Elasticsearch service to
docker service create \ --network es \ --name es-master \ -p 9200:9200 \ --mount type=bind,source=/data/es,destination=/usr/share/elasticsearch/data \ elasticsearch
Now we bring up another test container to see if it can talk / ping the
docker service create \ --name test \ --network es \ busybox sleep 300000
docker service ps to find out on which node the
busybox is sent to, switch to that node, run
docker exec -it container_ID /bin/sh ping es-master
If nothing goes wrong, the ping should return the results. More information about service communication on Overlay network is here