Discovery with A Records

Configure service discovery

1. Update your DNS nameserver to resolve a hostname, such as myservice.example.local, to one or more IP addresses using A records.

2. Query the nameserver directly with the dig tool to ensure that it returns the correct records, as follows:

   $ dig @192.168.50.30 -p 53 A myservice.example.local
   
     ;; QUESTION SECTION:
     ;myservice.example.local.    IN  A
   
     ;; ANSWER SECTION:
     myservice.example.local. 90  IN  A  192.168.50.31
     myservice.example.local. 90  IN  A  192.168.50.32
     myservice.example.local. 90  IN  A  192.168.50.33

   Multiple IP addresses should return for the same hostname.

3. In your HAProxy Enterprise configuration, add a resolvers section.

   • Add one or more nameserver lines to specify the IP addresses and ports of your DNS nameservers.

   • Set the accepted_payload_size to 8192 to allow larger DNS payloads, which is required to receive more server IPs within a single DNS result.

   resolvers mydns
      nameserver dns1 192.168.50.30:53
      accepted_payload_size 8192

4. Use a server-template in a backend to set the template for the server lines. When HAProxy Enterprise queries your DNS servers, these will be populated with IP addresses.

   backend webservers
      balance roundrobin
      server-template web 5 myservice.example.local:80 check resolvers mydns init-addr none

   In this example, the server-template directive:

   • Adds the specified number of servers (5) to the backend.

   • Their names will have "web" as a prefix.

   • The hostname myservice.example.local is queried.

   • Port 80 is hardcoded.

   • The resolvers section mydns is specified.

   • The init-addr none argument means that HAProxy Enterprise can initialize without having to resolve the IP addresses at startup. They can be resolved during runtime.

   After converting the DNS records to server lines, it is equivalent to defining a backend that looks like this:

   backend webservers
      balance roundrobin
      server web1 192.168.50.31:80 check
      server web2 192.168.50.32:80 check
      server web3 192.168.50.33:80 check
      server web4 192.168.50.34:80 check disabled
      server web5 192.168.50.35:80 check disabled

   When you add more DNS records to your nameserver, they will automatically go into the backend to fill in the web4 and web5 slots.

Inspect servers with the Runtime API

Run the command show servers state [backend name] to see the servers loaded into memory:

$ echo "show servers state webservers" | sudo socat stdio /var/run/hapee-1.8/hapee-lb.sock

  # be_id  be_name  srv_id  srv_name  srv_addr       srv_fqdn                srv_port srvrecord
  3        servers  1       web1      192.168.50.33  myservice.example.local 80       -
  3        servers  2       web2      192.168.50.31  myservice.example.local 80       -
  3        servers  3       web3      192.168.50.32  myservice.example.local 80       -
  3        servers  4       web4      -              myservice.example.local 80       -
  3        servers  5       web5      -              myservice.example.local 80       -

This example shows five servers: web1, web2, web3, web4 and web5.

• The DNS records returned IP addresses for only three servers.

• When there are more servers generated from the template than there are DNS records, the extra servers do not get an assigned IP address; they show a dash where an IP address would normally go.

• The extra server lines start in maintenance mode and will pick up IP addresses as more DNS records return.