Redundancy and Failover

Redundancy is keeping extra copies of a part, so the system keeps working when one copy fails. Failover is switching work to a working copy when the first copy stops.

Server A answers while it is up

Two app servers sit behind the load balancer and server A is up. Every request lands on A.

a_alive = True
destinations = []
for request in range(3):
    destinations.append("A" if a_alive else "B")
print("destinations:", destinations)

After A stops, every request lands on B

A flag marks server A as stopped and the balancer switches the destination. Every later request lands on B.

a_alive = False
destinations = []
for request in range(3):
    destinations.append("A" if a_alive else "B")
print("destinations:", destinations)

Picking the first working server

Server A is stopped and server B is working. The picker skips A and chooses B.

servers = [("A", False), ("B", True)]

def pick(servers):
    for name, alive in servers:
        if alive:
            return name

print("chosen server:", pick(servers))

Example

a_alive = True
destinations = []
for number in range(1, 6):
    if number == 3:
        a_alive = False
    destinations.append("A" if a_alive else "B")
    print("request", number, "->", destinations[-1])
print("A answers:", destinations.count("A"))
print("B answers:", destinations.count("B"))

Key points

Redundancy: Keeping extra copies of a part beyond the one in use.
Failover: Switching work to a working copy when the first copy stops.
Spare copy: An extra copy that answers nothing until failover sends work to it.
Load balancer role: The load balancer sends each request to a server, so it is the part that can switch the destination.
No spare, no failover: Failover needs a working copy waiting, or there is nothing to switch to.

In this example, server A answers the first 2 requests and then stops. Failover sends the remaining 3 requests to B, so no request is lost when A stops.

Note

Redundancy supplies the spare copies, and failover is the switch that uses them.

Try it Yourself

Edit the code, then press Run.

Running is simulated

a_alive = True
destinations = []
for number in range(1, 6):
    if number == 3:
        a_alive = False
    destinations.append("A" if a_alive else "B")
    print("request", number, "->", destinations[-1])
print("A answers:", destinations.count("A"))
print("B answers:", destinations.count("B"))

Exercise

Server A stops and the load balancer starts sending every request to the waiting server B. What is that switch called?

Failover Pagination Idempotency Sharding