container.training/docs/firstservice.md

Running our first Swarm service

• How do we run services? Simplified version:

  docker run → docker service create

.exercise[

• Create a service featuring an Alpine container pinging Google resolvers:

  docker service create alpine ping 8.8.8.8
  

• Check the result:

  docker service ps <serviceID>
  

]

---

--detach for service creation

(New in Docker Engine 17.05)

If you are running Docker 17.05 or later, you will see the following message:

Since --detach=false was not specified, tasks will be created in the background.
In a future release, --detach=false will become the default.

Let's ignore it for now; but we'll come back to it in just a few minutes!

---

Checking service logs

(New in Docker Engine 17.05)

• Just like docker logs shows the output of a specific local container ...

• ... docker service logs shows the output of all the containers of a specific service

.exercise[

• Check the output of our ping command:

  docker service logs <serviceID>
  

]

Flags --follow and --tail are available, as well as a few others.

Note: by default, when a container is destroyed (e.g. when scaling down), its logs are lost.

---

class: extra-details

Before Docker Engine 17.05

• Docker 1.13/17.03/17.04 have docker service logs as an experimental feature
  (available only when enabling the experimental feature flag)

• We have to use docker logs, which only works on local containers

• We will have to connect to the node running our container
  (unless it was scheduled locally, of course)

---

class: extra-details

Looking up where our container is running

• The docker service ps command told us where our container was scheduled

.exercise[

• Look up the NODE on which the container is running:

  docker service ps <serviceID>
  

• If you use Play-With-Docker, switch to that node's tab, or set DOCKER_HOST

• Otherwise, ssh into tht node or use $(eval docker-machine env node...)

]

---

class: extra-details

Viewing the logs of the container

.exercise[

• See that the container is running and check its ID:

  docker ps
  

• View its logs:

  docker logs <containerID>
  

• Go back to node1 afterwards

]

---

Scale our service

• Services can be scaled in a pinch with the docker service update command

.exercise[

• Scale the service to ensure 2 copies per node:

  docker service update <serviceID> --replicas 10
  

• Check that we have two containers on the current node:

  docker ps
  

]

---

View deployment progress

(New in Docker Engine 17.05)

• Commands that create/update/delete services can run with --detach=false

• The CLI will show the status of the command, and exit once it's done working

.exercise[

• Scale the service to ensure 3 copies per node:

  docker service update <serviceID> --replicas 15 --detach=false
  

]

Note: --detach=false will eventually become the default.

With older versions, you can use e.g.: watch docker service ps <serviceID>

---

Expose a service

• Services can be exposed, with two special properties:

  • the public port is available on every node of the Swarm,

  • requests coming on the public port are load balanced across all instances.

• This is achieved with option -p/--publish; as an approximation:

  docker run -p → docker service create -p

• If you indicate a single port number, it will be mapped on a port starting at 30000
  (vs. 32768 for single container mapping)

• You can indicate two port numbers to set the public port number
  (just like with docker run -p)

---

Expose ElasticSearch on its default port

.exercise[

• Create an ElasticSearch service (and give it a name while we're at it):

  docker service create --name search --publish 9200:9200 --replicas 7 \
         --detach=false elasticsearch`:2`
  

]

Note: don't forget the :2!

The latest version of the ElasticSearch image won't start without mandatory configuration.

---

Tasks lifecycle

• During the deployment, you will be able to see multiple states:

  • assigned (the task has been assigned to a specific node)

  • preparing (this mostly means "pulling the image")

  • starting

  • running

• When a task is terminated (stopped, killed...) it cannot be restarted

  (A replacement task will be created)

---

class: extra-details

---

Test our service

• We mapped port 9200 on the nodes, to port 9200 in the containers

• Let's try to reach that port!

.exercise[

• Try the following command:

  curl localhost:9200
  

]

(If you get Connection refused: congratulations, you are very fast indeed! Just try again.)

ElasticSearch serves a little JSON document with some basic information about this instance; including a randomly-generated super-hero name.

---

Test the load balancing

• If we repeat our curl command multiple times, we will see different names

.exercise[

• Send 10 requests, and see which instances serve them:

    for N in $(seq 1 10); do
      curl -s localhost:9200 | jq .name
    done
  

]

Note: if you don't have jq on your Play-With-Docker instance, just install it:

apk add --no-cache jq

---

Load balancing results

Traffic is handled by our clusters TCP routing mesh.

Each request is served by one of the 7 instances, in rotation.

Note: if you try to access the service from your browser, you will probably see the same instance name over and over, because your browser (unlike curl) will try to re-use the same connection.

---

Under the hood of the TCP routing mesh

• Load balancing is done by IPVS

• IPVS is a high-performance, in-kernel load balancer

• It's been around for a long time (merged in the kernel since 2.4)

• Each node runs a local load balancer

  (Allowing connections to be routed directly to the destination, without extra hops)

---

Managing inbound traffic

There are many ways to deal with inbound traffic on a Swarm cluster.

• Put all (or a subset) of your nodes in a DNS A record

• Assign your nodes (or a subset) to an ELB

• Use a virtual IP and make sure that it is assigned to an "alive" node

• etc.

---

class: btw-labels

Managing HTTP traffic

• The TCP routing mesh doesn't parse HTTP headers

• If you want to place multiple HTTP services on port 80, you need something more

• You can setup NGINX or HAProxy on port 80 to do the virtual host switching

• Docker Universal Control Plane provides its own HTTP routing mesh

  • add a specific label starting with com.docker.ucp.mesh.http to your services

  • labels are detected automatically and dynamically update the configuration

---

class: btw-labels

You should use labels

• Labels are a great way to attach arbitrary information to services

• Examples:

  • HTTP vhost of a web app or web service

  • backup schedule for a stateful service

  • owner of a service (for billing, paging...)

  • etc.

---

Pro-tip for ingress traffic management

• It is possible to use local networks with Swarm services

• This means that you can do something like this:

  docker service create --network host --mode global traefik ...
  

  (This runs the traefik load balancer on each node of your cluster, in the host network)

• This gives you native performance (no iptables, no proxy, no nothing!)

• The load balancer will "see" the clients' IP addresses

• But: a container cannot simultaneously be in the host network and another network

  (You will have to route traffic to containers using exposed ports or UNIX sockets)

---

class: extra-details

Using local networks (host, macvlan ...) with Swarm services

• Using the host network is fairly straightforward

  (With the caveats described on the previous slide)

• It is also possible to use drivers like macvlan

  • see this guide to get started on macvlan

  • see this PR for more information about local network drivers in Swarm mode

---

Visualize container placement

• Let's leverage the Docker API!

.exercise[

• Get the source code of this simple-yet-beautiful visualization app:

  cd ~
  git clone git://github.com/dockersamples/docker-swarm-visualizer
  

• Build and run the Swarm visualizer:

  cd docker-swarm-visualizer
  docker-compose up -d
  

]

---

Connect to the visualization webapp

• It runs a web server on port 8080

.exercise[

• Point your browser to port 8080 of your node1's public ip

  (If you use Play-With-Docker, click on the (8080) badge)

]

• The webapp updates the display automatically (you don't need to reload the page)

• It only shows Swarm services (not standalone containers)

• It shows when nodes go down

• It has some glitches (it's not Carrier-Grade Enterprise-Compliant ISO-9001 software)

---

Why This Is More Important Than You Think

• The visualizer accesses the Docker API from within a container

• This is a common pattern: run container management tools in containers

• Instead of viewing your cluster, this could take care of logging, metrics, autoscaling ...

• We can run it within a service, too! We won't do it, but the command would look like:

    docker service create \
      --mount source=/var/run/docker.sock,type=bind,target=/var/run/docker.sock \
      --name viz --constraint node.role==manager ...
  

Credits: the visualization code was written by Francisco Miranda.
Mano Marks adapted it to Swarm and maintains it.

---

Terminate our services

• Before moving on, we will remove those services

• docker service rm can accept multiple services names or IDs

• docker service ls can accept the -q flag

• A Shell snippet a day keeps the cruft away

.exercise[

• Remove all services with this one liner:

  docker service ls -q | xargs docker service rm
  

]