# Caching (/pipeline-sdk/caching)



Pass a cache policy to any step so its result is reused on the next run. A
policy decides when a cached step is still valid: for a fixed window
(`ttl`), until a tracked file changes (`on_change`), or until its command
changes (`forever`). Compose policies to require all of them.

<DslSignature name="ttl" />

<DslSignature name="on_change" />

**Python**

```python
import harmont as hm
from datetime import timedelta

deps = hm.sh(
    "make deps",
    cache=hm.compose(
        hm.on_change("requirements.txt"),
        hm.ttl(timedelta(hours=6)),
    ),
)
```

**TypeScript**

```typescript
import { sh, compose, onChange, ttl } from "@harmont/hm";

const deps = sh("make deps", {
  cache: compose(onChange("requirements.txt"), ttl(6 * 60 * 60)),
});
```

See the [cache reference](/pipeline-sdk/reference/cache) for every policy.