ZeroMQ

High-Availability Pair (Binary Star Pattern)

The Binary Star pattern configures two servers as a primary/backup high-availability pair (Figure 4-6). At any given time, one of these (the active server) accepts connections from client applications. The other (the passive server) does nothing, but the two servers monitor each other. If the active one disappears from the network, after a certain time the passive one takes over as active.

Figure 4-6. High-availability pair, normal operation

We developed the Binary Star pattern at iMatix for our OpenAMQ server. We designed it:

• To provide a straightforward high-availability solution

• To be simple enough to actually understand and use

• To fail over reliably when needed, and only when needed

Assuming we have a Binary Star pair running, here are the different scenarios that will result in a failover (Figure 4-7):

• The hardware running the primary server has a fatal problem (power supply explodes, machine catches fire, or someone simply unplugs it by mistake), and disappears. Applications see this and reconnect to the backup server.

• The network segment on which the primary server sits crashes—perhaps because a router gets hit by a power spike—and applications start to reconnect to the backup server.

• The primary server crashes or is killed by the operator and does not restart automatically.

Figure 4-7. High-availability pair during failover

Recovery from failover ...