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Consul

Introduction

This page is currently my experimental log about consul on AL.

~~Downloaded package: consul-0.5.0-r0.apk from http://repos.mauras.ch/alpinelinux/x86_64/~~
Now in repo edge/testing

We will experiment to find the right spec to allocate the consul servers for 100 to 1000 agents. For now, we have a 52 nodes cluster.

We plan to orchestrate our VM fleet based on consul events, envconsul and consul-template. All with ash scripts and smart setup.

We combine our PXE servers and the consul ones to inherit from the consul resilence (consul leader) to offer high-availability (HA) (but still in ram only) to the PXE service.

Install

We will just focus on the consul installation and configuration parts.

apk add consul@testing

Web UI

One need to download and unpack https://dl.bintray.com/mitchellh/consul/0.5.0_web_ui.zip in some directory.

The contents of this zip file is a index.html with some js and css files in a subdirectory

Archive:  0.5.0_web_ui.zip
   creating: dist/
  inflating: dist/.gitkeep
  inflating: dist/index.html
   creating: dist/static/
  inflating: dist/static/application.min.js
  inflating: dist/static/base.css
  inflating: dist/static/base.css.map
  inflating: dist/static/bootstrap.min.css
  inflating: dist/static/consul-logo.png
  inflating: dist/static/favicon.png
  inflating: dist/static/loading-cylon-purple.svg

Tu use it, one has to specify "-ui-dir /path/to/ui" in /etc/conf.d/consul.
The UI is available at the /ui path on the same port as the HTTP API. By default this is http://localhost:8500/ui.

Configuration

Consul v0.5.0
Consul Protocol: 2 (Understands back to: 1)

By default, there are 4 files in /etc/consul

acl.json.sample      encrypt.json.sample  server.json          tls.json.sample

Consul Server

the default consul is for a stand alone server...

It is important to bootstrap the cluster properly before starting agents otherwise agents will never find the consul leader!

DNSmasq with consul

NIC="eth0" # PXEserver
NIC="lan"  # generic machine 
apk add dnsmasq
rc-update add dnsmasq
mkdir -p /etc/dnsmasq.d
echo "server=/consul/127.0.0.1#8600" > /etc/dnsmasq.d/10-consul
echo <<EOF > /etc/conf.d/dnsmasq
MASQ_OPTS="--user=dnsmasq --group=dnsmasq --interface=${NIC} --conf-dir=/etc/dnsmasq.d" 
EOF
rc-service dnsmasq start
DNS1=resolver.service.consul # need an IP here, not a fqdn...
DNS1=resolver.service.consul # should be different due to round-robin
echo "resolv-file=/etc/resolv.dnsmasq" > /etc/dnsmasq.d/20-resolv
echo "nameserver ${DNS1}" > /etc/resolv.dnsmasq
echo "nameserver ${DNS2}" >> /etc/resolv.dnsmasq
rc-service dnsmasq restart

Consul Agent

We need to lauch the consul service after being able to read the IP address of the PXEserver involved.
This particuliar address will be used to join the consul cluster as agent.

Whenever possible, a service will be referenced by its name (like collectd.service.consul for the collectd server).
Consul agent and dnsmasq are the 2 first services to be started on each machine.

This is now included in our generic default apkovl for PXE boot and in our LXC creation custom-made script..

If an agent (or server) has several private IP addresses, one has to specify the address to be used in /etc/conf.d/consul (with -bind=<IP>). That address has to be reachable by all consul members!

Securing consul

We will follow coredumb recommandations as on https://www.mauras.ch/securing-consul.html and based on sample config provided in the package.

With gossip encryption, we observe some flapping in the presence of nodes...

Usage

Be sure to have proxt http disabled if you want to use curl against localhost...

Machines discovery

List machines =

consul members

or

curl localhost:8500/v1/catalog/nodes

Services discovery

Register service =

echo '{"service": {"name": "pxe"}}' > /etc/consul/pxe.json ; consul reload

List services :

curl http://localhost:8500/v1/agent/services : Returns the services the local agent is managing 
curl http://localhost:8500/v1/catalog/services : Lists services in a given DC 
curl http://localhost:8500/v1/catalog/service/<service> : Lists the nodes in a given service
curl http://localhost:8500/v1/catalog/node/<node> : Lists the services provided by a node

For instance:

http_proxy= curl http://localhost:8500/v1/catalog/services?pretty

What if ?

What if I register a service service1 with tag tag1 on node node1,
then I register a service service1 with tag tag2 on node2.
then I register a service service1 with tag tag3 on node1?
will the tags just add?
NO, the lastest registration overwrite previous one.

I plan to advertize available NBD with {"service": "nbd", "tags": ["<name>"]} ans access it with nbd:name.nbd.service.consul:name.
but before that, need to use new nbd format with nammes on an unique port instead of differentiation by port (old scheme)

Is there a clean way to dynamically add a tag to a service on a given consul node without losing existing tags ?

Cheks

Key/Value storage

curl -X PUT -d 'value' http://localhost:8500/v1/kv/path/to/key

curl http://localhost:8500/v1/kv/path/to/key

curl -X DELETE http://localhost:8500/v1/kv/path/to/key?recurse

need a standard function to convert from json to ash and vice-versa...

Events

Define event

Watch event

Triggers

Open questions

How to know (from any node in the cluster) which is the active leader?
- an alternative is to define an applicative leader as in https://consul.io/docs/guides/leader-election.html

envconsul

I tried to build a package for it but without success till now...