# Functions

Run isolated Rust application code with explicit data, network, and compute capabilities.

Ferrite Functions run your application logic as isolated Rust processes. Each
function receives only the named platform handles declared in `ferrite.json`;
there are no ambient database credentials, arbitrary sockets, or hidden cloud
service clients.

## Create a function

```bash
ferrite new hello receipts-api
cd receipts-api
ferrite dev
```

The application implements `FerriteApp` and obtains resources from
`AppContext`:

```rust filename="src/main.rs"
#![forbid(unsafe_code)]

use std::io;
use ferrite_app::{app_main, AppContext, FerriteApp};

struct Receipts;

impl FerriteApp for Receipts {
    async fn run(ctx: AppContext) -> io::Result<()> {
        let mut requests = ctx.queue("requests")?;
        loop {
            let Some(lease) = requests.poll_wait(1_000).await? else { continue };
            // Decode, authorize, apply domain logic, then acknowledge.
            requests.ack(lease.token).await?;
        }
    }
}

fn main() { app_main::<Receipts>() }
```

## Lifecycle and scale

One OS process runs each app instance. Ferrite supervises the process, applies
authoritative timeouts, and kills the process group if it exceeds its boundary.
Scale adds competing instances against the same durable resources:

```bash
ferrite dev --instances 3
```

After deployment, scale from the Dashboard or call the Ferrite MCP `scale`
tool with the app name and desired instance count. Verify the result with
`ferrite status receipts-api`.

Use queue leases and idempotency for effects that may be retried after a crash.
Run `ferrite dev --chaos 5` to make this behavior visible before deployment.

## Compute limits

Declare memory and CPU in the app manifest. Admission rejects a configuration
that violates platform bounds or budget rules before it starts. Functions do
not own durable state; Store, Queue, Streams, Transactions, and Object Storage
survive function replacement.

Continue with [Routing](/developers/routing/), [Queues](/developers/queues/),
or [Deployments](/developers/deployments/).