Service Discovery with Marathon, Mesos-DNS and HAProxy

Problem

You have a bunch of microservices deployed in your Mesos cluster, and want to make them available under service specific URLs via HTTP so they can call each other or be accessed from the outside world.

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Overview

Components: Mesos-DNS, Marathon, HAProxy

• Marathon starts applications as tasks in the Mesos cluster and uses health checks to keep track of their status.
• Mesos-DNS polls Mesos Master for app/task - IP:PORT mapping
• HAProxy configuration is done via servicerouter.py following the default service discovery recipe for Marathon with Mesos-DNS.

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Pros

• Allows logical (DNS) names to IP:PORT mappings.
• DNS is mature and well understood.
• Allows matching other request characteristics like HTTP headers.
• Offers sticky sessions, HTTP to HTTPS redirection, SSL offloading, VHost support and templating capabilities.
• Battle tested code in HAProxy for the heavy lifting.

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Cons

• Depends on cron job.
• Some clients aggressively cache DNS entries (for example Java) which can lead to outdated IP addresses.
• SRV records might not be understood in your environment.

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Implementation steps

These steps assume you have already a running Mesos cluster with Marathon installed.

The rest of this guide focusses on Debian/Ubuntu, but can easily be adapted to any other OS.

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1. Install HAProxy and Mesos-DNS

HAProxy

HAProxy can usually be installed in a recent enough version via your OS package manager, e.g.

$ apt-get install haproxy

Version 1.5.x is recommended, which is not the default on e.g. Ubuntu 14.04 LTS.
Check http://haproxy.debian.net/ for instructions how to get a recent HAProxy on older Ubuntu and Debian releases.

Mesos-DNS

We will do a deployment of Mesos-DNS via Marathon as described in the following. You might want to launch Mesos-DNS outside of Mesos/Marathon and
monitor/re-start out of band, alternatively—see also Mesos-DNS using systemd.

First create a Mesos-DNS configuration file and place it, for example, in /etc/mesos-dns/:

$ cat /etc/mesos-dns/config.js
{
  "zk": "zk://127.0.0.1:2181/mesos",
  "refreshSeconds": 60,
  "ttl": 60,
  "domain": "mesos",
  "port": 53,
  "resolvers": ["10.0.2.3"],
  "timeout": 5,
  "email": "root.mesos-dns.mesos"
}

Here, I’ve set it up as primary nameserver, but you can also re-use an existing Bind server and use Mesos-DNS only for the .mesos domain.

The second step for a Marathon-based deployment of Mesos-DNS is to create a Marathon app specification that looks as follows:

{
    "args": [
        "/mesos-dns",
        "-config=/config.json"
    ],
    "container": {
        "docker": {
            "image": "mesosphere/mesos-dns",
            "network": "HOST"
        },
        "type": "DOCKER",
        "volumes": [
            {
                "containerPath": "/config.json",
                "hostPath": "/etc/mesos-dns/config.js",
                "mode": "RO"
            }
        ]
    },
    "cpus": 0.2,
    "id": "mesos-dns",
    "instances": 1,
}

Note that in above Marathon app specification it’s important to use the same path under the hostPath key where you stored the
Mesos-DNS configuration file in the previous step. I’ve stored this app spec at ~/marathon-mesosdns.json but it really doesn’t matter where 
you put it.

Now you can post it to the Marathon API via curl:

$ curl -X POST -H "Content-Type: application/json" http://localhost:8080/v2/apps -d@~/marathon-mesosdns.json

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2. Test Mesos-DNS setup

In order to test the setup, we now deploy a simple Python app to Marathon.
For that, we need a Marathon app spec which we call pyapp.json:

{
  "id": "peek",
  "cmd": "env >env.txt && python3 -m http.server 8080",
  "cpus": 0.5,
  "mem": 32.0,
  "container": {
    "type": "DOCKER",
    "docker": {
      "image": "python:3",
      "network": "BRIDGE",
      "portMappings": [
        { "containerPort": 8080, "hostPort": 0 }
      ]
    }
  }
}

Post it to the Marathon API via curl:

$ curl -X POST -H "Content-Type: application/json" http://localhost:8080/v2/apps [e-mail]

Check the Marathon UI, and you should now see both Mesos-DNS and the Python app running:

Test Mesos-DNS via dig:

$ dig _peek._tcp.marathon.mesos SRV
vagrant@mesos:~$ dig _peek._tcp.marathon.mesos SRV

; <<>> DiG 9.9.5-3ubuntu0.5-Ubuntu <<>> _peek._tcp.marathon.mesos SRV
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 57329
;; flags: qr aa rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; QUESTION SECTION:
;_peek._tcp.marathon.mesos.	IN	SRV

;; ANSWER SECTION:
_peek._tcp.marathon.mesos. 60	IN	SRV	0 0 31000 peek-27346-s0.marathon.mesos.

;; ADDITIONAL SECTION:
peek-27346-s0.marathon.mesos. 60 IN	A	10.141.141.10

;; Query time: 4 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Sat Oct 24 23:21:15 UTC 2015
;; MSG SIZE  rcvd: 160

This tells us that the app with the Marathon ID /peek and the logical FQHN peek.marathon.mesos
is running on host 10.141.141.10 and is available via port 31000.

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3. Use servicerouter to configure HAProxy

To generate an HAProxy configuration from Marathon running at localhost:8080 with servicerouter.py do the following:

$ wget https://raw.githubusercontent.com/mesosphere/marathon/master/bin/servicerouter.py
$ sudo python servicerouter.py --marathon http://localhost:8080 --haproxy-config /etc/haproxy/haproxy.cfg
servicerouter: fetching apps
servicerouter: GET http://10.141.141.10:8080/v2/apps?embed=apps.tasks
servicerouter: got apps [u'/mesos-dns', u'/peek']
servicerouter: generating config
servicerouter: setting default value for HAPROXY_HEAD
servicerouter: setting default value for HAPROXY_HTTP_FRONTEND_HEAD
servicerouter: setting default value for HAPROXY_HTTP_FRONTEND_APPID_HEAD
servicerouter: setting default value for HAPROXY_HTTPS_FRONTEND_HEAD
servicerouter: setting default value for HAPROXY_FRONTEND_HEAD
servicerouter: setting default value for HAPROXY_BACKEND_REDIRECT_HTTP_TO_HTTPS
servicerouter: setting default value for HAPROXY_BACKEND_HEAD
servicerouter: setting default value for HAPROXY_HTTP_FRONTEND_ACL
servicerouter: setting default value for HAPROXY_HTTP_FRONTEND_APPID_ACL
servicerouter: setting default value for HAPROXY_HTTPS_FRONTEND_ACL
servicerouter: setting default value for HAPROXY_BACKEND_HTTP_OPTIONS
servicerouter: setting default value for HAPROXY_BACKEND_HTTP_HEALTHCHECK_OPTIONS
servicerouter: setting default value for HAPROXY_BACKEND_TCP_HEALTHCHECK_OPTIONS
servicerouter: setting default value for HAPROXY_BACKEND_STICKY_OPTIONS
servicerouter: setting default value for HAPROXY_BACKEND_SERVER_OPTIONS
servicerouter: setting default value for HAPROXY_BACKEND_SERVER_HTTP_HEALTHCHECK_OPTIONS
servicerouter: setting default value for HAPROXY_BACKEND_SERVER_TCP_HEALTHCHECK_OPTIONS
servicerouter: setting default value for HAPROXY_FRONTEND_BACKEND_GLUE
servicerouter: configuring app /mesos-dns
servicerouter: frontend at *:10000 with backend mesos-dns_10000
servicerouter: backend server at 10.141.141.10:31421
servicerouter: trying to resolve ip address for host 10.141.141.10
servicerouter: configuring app /peek
servicerouter: frontend at *:10001 with backend peek_10001
servicerouter: backend server at 10.141.141.10:31000
servicerouter: reading running config from /etc/haproxy/haproxy.cfg
servicerouter: running config is different from generated config - reloading
servicerouter: writing config to temp file /tmp/tmp_32M3e
servicerouter: moving temp file /tmp/tmp_32M3e to /etc/haproxy/haproxy.cfg
servicerouter: trying to find out how to reload the configuration
servicerouter: we seem to be running on a sysvinit based system
servicerouter: reloading using /etc/init.d/haproxy reload

This will refresh haproxy.cfg, and if there were any changes, then it will automatically reload HAProxy.
In order to keep HAProxy up to date, you can use a cron job that executes the servicerouter.py, say, every minute.

 

http://programmableinfrastructure.com/guides/service-discovery/mesos-dns-haproxy-marathon/