# EVAL_RO

> Execute read-only Lua script.

Use `EVAL_RO` to run a Lua script that is not allowed to write.

It behaves like [`EVAL`](/redis/commands/scripting/eval), with `<numkeys>` splitting the arguments into the `KEYS` and `ARGV` tables, except that any write command called from the script fails with an error. Declaring the read-only intent lets the server serve the call on replicas and turns an accidental write into a clear error instead of an unexpected modification, which is worth doing for every script that only computes over existing data.

Being read-only is not by itself enough to run concurrently with other commands: like `EVAL`, this command takes the global lock unless the script's shebang sets the `allow-key-locking` flag. With the flag, the script takes shared read locks on the keys passed in `KEYS`, so several readers of the same key proceed together. Combine the two flags with a comma, as in `#!lua flags=no-writes,allow-key-locking`, and see [Key-Based Locking](/redis/features/key-locking) for the full rules.

## Syntax

```redis
EVAL_RO <script> <numkeys> [<key> [<key> ...]] [<arg> [<arg> ...]]
```

## Arguments

| Argument | Required | Repeatable | Description |
| --- | --- | --- | --- |
| `<script>` | Yes | No | Lua script source. |
| `<numkeys>` | Yes | No | Number of key arguments that follow. |
| `<key>` | No | Yes | Redis key targeted by the command. |
| `<arg>` | No | Yes | Additional argument, available to the script as `ARGV`. |

## Important points

- `numkeys` must equal the number of key arguments that immediately follow it; remaining arguments are available to the script or function as ordinary arguments.
- A read-only script still takes the global lock unless its shebang sets the `allow-key-locking` flag. See [Key-Based Locking](/redis/features/key-locking).
- Pass every key the script reads through `KEYS` whether or not `allow-key-locking` is set. A key built inside the script is read from disk under the lock when it is not in memory, and it is rejected outright when the flag is set. See [Dynamic Keys and Latency](/redis/features/key-locking#dynamic-keys-and-latency).

## Response

The reply reports the result of the operation. Error replies have the same shape in RESP2 and RESP3 and are surfaced as exceptions by the SDKs below.

| Protocol | Reply |
| --- | --- |
| RESP2 | Reply produced by the evaluated read-only script |
| RESP3 | Reply produced by the evaluated read-only script |

<Note>
  Client libraries often decode bulk strings, maps, sets, and numeric strings into language-native values. The table describes the Redis wire reply.
</Note>

## Examples

TCP examples use the TLS `REDIS_URL` from the Upstash console. REST examples use `UPSTASH_REDIS_REST_URL` and `UPSTASH_REDIS_REST_TOKEN`.

<AccordionGroup>

<Accordion title="Redis CLI" icon="terminal">

```bash
EVAL_RO "return ARGV[1]" 0 hello
```

</Accordion>

<Accordion title="@upstash/redis" icon="node-js" iconType="brands">

```ts
import { Redis } from "@upstash/redis";

const redis = Redis.fromEnv();

const script = `
    return ARGV[1]
`
const result = await redis.evalRo(script, [], ["hello"]);
console.log(result) // "hello"
```

</Accordion>

<Accordion title="upstash_redis" icon="python" iconType="brands">

```python
from upstash_redis import Redis

redis = Redis.from_env()
result = redis.eval_ro("return ARGV[1]", args=["value"])
print(result)
```

</Accordion>

<Accordion title="ioredis" icon="node-js" iconType="brands">

```ts
import Redis from "ioredis";

const redis = new Redis(process.env.REDIS_URL!);
const result = await redis.eval_ro("return ARGV[1]", "0", "value");
console.log(result);
```

</Accordion>

<Accordion title="node-redis" icon="node-js" iconType="brands">

```ts
import { createClient } from "redis";

const client = await createClient({ url: process.env.REDIS_URL })
  .on("error", console.error)
  .connect();
const result = await client.evalRo("return ARGV[1]", { arguments: ["value"] });
console.log(result);
```

</Accordion>

<Accordion title="redis-py" icon="python" iconType="brands">

```python
import os
import redis

client = redis.from_url(os.environ["REDIS_URL"])
result = client.eval_ro("return ARGV[1]", 0, "value")
print(result)
```

</Accordion>

<Accordion title="go-redis" icon="golang" iconType="brands">

```go
package main

import (
    "context"
    "fmt"
    "os"

    "github.com/redis/go-redis/v9"
)

func main() {
    opts, err := redis.ParseURL(os.Getenv("REDIS_URL"))
    if err != nil {
        panic(err)
    }
    client := redis.NewClient(opts)
    result, err := client.EvalRO(context.Background(), "return ARGV[1]", nil, "value").Result()
    if err != nil {
        panic(err)
    }
    fmt.Println(result)
}
```

</Accordion>

<Accordion title="jedis" icon="java" iconType="brands">

```java
import java.net.URI;

import redis.clients.jedis.Jedis;

try (Jedis jedis = new Jedis(new URI(System.getenv("REDIS_URL")))) {
  Object result = jedis.evalReadonly("return ARGV[1]", java.util.List.of(), java.util.List.of("value"));
  System.out.println(result);
}
```

</Accordion>

<Accordion title="redis-rs" icon="rust" iconType="brands">

```rust
fn main() -> redis::RedisResult<()> {
    let url = std::env::var("REDIS_URL").expect("REDIS_URL is not set");
    let client = redis::Client::open(url)?;
    let mut connection = client.get_connection()?;

    let mut command = redis::cmd("EVAL_RO");
    command.arg("return ARGV[1]");
    command.arg("1");
    let result: redis::Value = command.query(&mut connection)?;
    println!("{result:?}");
    Ok(())
}
```

</Accordion>

</AccordionGroup>