Read replicas

Reads that scale, and reads that go stale

Replicas scale reads but not writes, and a read can arrive before the write it should have seen.

What to watch

Copies give you read capacity and nothing else: 600 reads a second across three of them, but still only 200 writes against the one primary. Adding copies fixes one of those and does nothing at all for the other.

What it is made of

Component	Count
Client	1
Read replicas	1
Service	1

Under load

Measured by running this design through the simulator at multiples of its own offered rate of 300 requests a second. Twenty seconds of simulated time, one fixed seed, so the same numbers come out every time.

This design holds 600 requests a second with 0% errors. At 750 it is losing 5% of them.

Offered	Goodput	p99	Errors
300	308	50.1ms	0%
450	479	55.8ms	0%
600	608	58.6ms	0%
750	727	256.3ms	4.5%
900	703	262.9ms	23.6%
1200	773	271.9ms	35.4%
1800	796	694.9ms	55.9%
2400	799	715.1ms	67.9%

Open this example

Load Read replicas from the Examples menu and drag the traffic slider yourself.