> ## Documentation Index
> Fetch the complete documentation index at: https://docs.breakscale.tech/llms.txt
> Use this file to discover all available pages before exploring further.

# 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.

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

| 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% |

<Card title="Open this example" icon="play" href="https://breakscale.tech">
  Load **Read replicas** from the Examples menu and drag the traffic slider yourself.
</Card>