Active-Active vs Active-Passive
The two ways to run redundant copies: one copy serves while a standby waits to take over, or every copy serves all the time. How each fails over, what capacity each needs, and how to choose layer by layer.
Overview
Every redundant system answers one question: what does the second copy do while everything is healthy?
In active-passive (also called primary-standby), one copy serves all traffic and the other waits. When the active copy fails, the standby is promoted and traffic moves to it. In active-active, every copy serves traffic all the time; when one fails, the others absorb its share.
The choice appears at every layer: a pair of load balancers, a fleet of app servers, a database, a whole region. It decides how long an outage lasts, how much data a failure can lose, how much hardware sits idle and how complicated writes become. The Redundancy and Replication topic introduces the two patterns and the RPO and RTO targets; Multi-Region Active-Active covers running writes in several regions. This topic goes deeper into the choice itself: the failover mechanics, the capacity math and how to decide.
What you will be able to do
- Describe the two topologies at each layer of a system, with a real example of each.
- Work out how much capacity each topology needs to survive losing one site.
- Walk through an active-passive failover step by step, with what can go wrong at each step.
- Explain the common hybrid: active-active reads with a single writer.
- Choose a topology for a component from its state, its RTO and RPO, and its budget.
Key concepts
- Active-passive
- Active-active
- Hot, warm and cold standby
- Failover steps
- Capacity headroom
- Single writer, many readers
- Fencing
- Automatic or manual failover
Interview questions
- What is the difference between active-active and active-passive?
- How much capacity do you need for active-active across three sites?
- Walk me through what happens when a database primary fails in an active-passive setup.
- Why not make everything active-active?
- What is the hybrid active-active reads, active-passive writes pattern, and what does it cost?
- Should failover be automatic or manual?
- Design high availability for an e-commerce checkout that must not lose orders.
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