# 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:
  ```bash
  docker service create --name pingpong alpine ping 8.8.8.8
  ```

- Check the result:
  ```bash
  docker service ps pingpong
  ```

]

---

class: extra-details

## `--detach` for service creation

(New in Docker Engine 17.05)

If you are running Docker 17.05 to 17.09, 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.
```

You can ignore that 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:
  ```bash
  docker service logs pingpong
  ```

]

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
  <br/>(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
  <br/>(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:
  ```bash
  docker service ps pingpong
  ```

- 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:
  ```bash
  docker ps
  ```

- View its logs:
  ```bash
  docker logs containerID
  ```

  <!-- ```wait No such container: 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:
  ```bash
  docker service update pingpong --replicas 10
  ```

- Check that we have two containers on the current node:
  ```bash
  docker ps
  ```

]

---

## Monitoring deployment progress with `--detach`

(New in Docker Engine 17.05)

- The CLI can monitor commands that create/update/delete services

- `--detach=false`

  - synchronous operation
  - the CLI will monitor and display the progress of our request
  - it exits only when the operation is complete

- `--detach=true`

  - asynchronous operation
  - the CLI just submits our request
  - it exits as soon as the request is committed into Raft

---

## To `--detach` or not to `--detach`

- `--detach=false`

  - great when experimenting, to see what's going on

  - also great when orchestrating complex deployments
    <br/>(when you want to wait for a service to be ready before starting another)

- `--detach=true`

  - great for independent operations that can be parallelized

  - great in headless scripts (where nobody's watching anyway)

.warning[`--detach=false` does not complete *faster*. It just *doesn't wait* for completion.]

---

class: extra-details

## `--detach` over time

- Docker Engine 17.10 and later: the default is `--detach=false`

- From Docker Engine 17.05 to 17.09: the default is `--detach=true`

- Prior to Docker 17.05: `--detach` doesn't exist

 (You can watch progress with e.g. `watch docker service ps <serviceID>`)

---

## `--detach` in action

.exercise[

- Scale the service to ensure 3 copies per node:
  ```bash
  docker service update pingpong --replicas 15 --detach=false
  ```

- And then to 4 copies per node:
  ```bash
  docker service update pingpong --replicas 15 --detach=true
  ```

]

---

## 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
  <br/>(vs. 32768 for single container mapping)

- You can indicate two port numbers to set the public port number
  <br/>(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):
  ```bash
  docker service create --name search --publish 9200:9200 --replicas 7 \
         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

![diagram showing what happens during docker service create, courtesy of @aluzzardi](images/docker-service-create.svg)

---

## 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:
  ```bash
  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:
  ```bash
    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](
https://docs.docker.com/engine/swarm/ingress/).

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 set up NGINX or HAProxy on port 80 to do the virtual host switching

- Docker Universal Control Plane provides its own [HTTP routing mesh](
  https://docs.docker.com/datacenter/ucp/2.1/guides/admin/configure/use-domain-names-to-access-services/)

  - 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:
  ```bash
  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` ...)

- It is possible to connect services to local networks

- Using the `host` network is fairly straightforward

  (With the caveats described on the previous slide)

- Other network drivers are a bit more complicated

  (IP allocation may have to be coordinated between nodes)

- See for instance [this guide](
  https://docs.docker.com/engine/userguide/networking/get-started-macvlan/
  ) to get started on `macvlan`

- See [this PR](https://github.com/moby/moby/pull/32981) 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:
  ```bash
  cd ~
  git clone git://github.com/dockersamples/docker-swarm-visualizer
  ```

- Build and run the Swarm visualizer:
  ```bash
  cd docker-swarm-visualizer
  docker-compose up -d
  ```

  <!-- ```longwait Creating dockerswarmvisualizer_viz_1``` -->

]

---

## 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)

  <!-- ```open http://node1:8080``` -->

]

- 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:

  ```bash
    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](https://github.com/maroshii).
<br/>
[Mano Marks](https://twitter.com/manomarks) 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:
  ```bash
  docker service ls -q | xargs docker service rm
  ```

]
