Migration of leases from one failover pair to another without outage

[Nick Urbanik]

Dear Folks,

In migrating a lease from one pair to another, we have a situation
like this:

    * we need to change the partners one at a time; here, from
      secondary-a.test to secondary-b.test.  However,
    * the configuration on the primary needs to change to point to the
      new secondary, and so it needs to restart to read this new
      configuration.

Since a restart takes some 15 minutes or so, we would like to avoid a
situation where service is disabled for such a length of time.

Any suggestions on avoiding an outage?

Problem of moving from here:

Step 1:

  +---------+                           +---------+
  | +-----+ |                           | +-----+ |
  | |testa| | <-----------------------> | |testa| |
  | +-----+ |                           | +-----+ |
  +---------+                           +---------+
  primary-a.test                        secondary-a.test

                                        +---------+
                                        |         |
                                        |         |
                                        |         |
                                        +---------+
                                        secondary-b.test
To here:
Step 2:

  +---------+                           +---------+
  | +-----+ |                           |         |
  | |testa| | <---------\               |         |
  | +-----+ |            \              |         |
  +---------+             \             +---------+
  primary-a.test           \            secondary-a.test
                            \
                             \          +---------+
                              \         | +-----+ |
                               \------> | |testa| |
                                        | +-----+ |
                                        +---------+
                                        secondary-b.test

Stage 1:
In /etc/dhcpd.conf on primary-a.test:

failover peer "testa" {
         primary;
         address primary-a.test;
         port 520;
         peer address secondary-a.test;
         peer port 520;
         max-response-delay 60;
         max-unacked-updates 10;
         mclt 3600;
         split 128;
         load balance max seconds 3;
}

In /etc/dhcpd.conf on secondary-a.test:

failover peer "testa" {
         primary;
         address dhca1.test;
         port 520;
         peer address secondary-a.test;
         peer port 520;
         max-response-delay 60;
         max-unacked-updates 10;
         load balance max seconds 3;
}

Stage 2:
failover peer "testa" {
         primary;
         address dhca1.test;
         port 520;
         peer address secondary-b.test; # CHANGED => need restart.
         peer port 520;
         max-response-delay 60;
         max-unacked-updates 10;
         mclt 3600;
         split 128;
         load balance max seconds 3;
}

In /etc/dhcpd.conf on secondary-b.test:

failover peer "testa" {
         primary;
         address dhca1.test;
         port 520;
         peer address secondary-b.test;
         peer port 520;
         max-response-delay 60;
         max-unacked-updates 10;
         load balance max seconds 3;
}

Or indeed, I'd love to hear any other suggestions for migrating leases
from one failover pair to another without an outage.
-- 
Nick Urbanik   RHCE         http://nicku.org        nicku at nicku.org
GPG: 7FFA CDC7 5A77 0558 DC7A 790A 16DF EC5B BB9D 2C24 ID: BB9D2C24
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