Active-passive is a high-availability layout with one active node handling all requests and one or more passive nodes that stand ready. A failure detector promotes a passive node, which then becomes the active one.
Why it matters
It is simpler to reason about than active-active, because only one node writes data at a time. That makes it a natural fit for databases, firewalls and load balancers. The downside is that half your hardware does no work in normal operation, and recovery is not instant: detection, promotion and traffic redirection all take time. How fast depends on the standby style, from hot to cold.
Example
Two load balancers share a virtual IP using VRRP, for example with keepalived. The passive node watches advertisements from the active one:
vrrp_instance VI_1 {
state BACKUP
interface eth0
virtual_router_id 51
priority 100
virtual_ipaddress { 203.0.113.50 }
}If advertisements stop, the backup claims the virtual IP and the address keeps answering.
Common mistakes
- Running both nodes as active after a network partition (split brain).
- Detecting failure too aggressively, causing flapping failovers.
- Configuring the passive node differently from the active one and discovering it only during the incident.
Related
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