# Caching and state

Source: https://docs.streamlit.io/develop/api-reference/caching-and-state

Optimize performance and add statefulness to your app!

## Caching

Streamlit provides powerful [cache primitives](https://docs.streamlit.io/develop/concepts/architecture/caching) for data and global resources. They allow your app to stay performant even when loading data from the web, manipulating large datasets, or performing expensive computations. With `refresh_mode="background"`, expired cache entries refresh in the background, so people don't wait for a slow query every time the cache expires. For more dashboard performance patterns, see [Build advanced dashboards](https://docs.streamlit.io/develop/concepts/design/dashboards).

**[Cache data](https://docs.streamlit.io/develop/api-reference/caching-and-state/st.cache_data)**: Function decorator to cache functions that return data (e.g. dataframe transforms, database queries, ML inference).

```python
@st.cache_data
def long_function(param1, param2):
  # Perform expensive computation here or
  # fetch data from the web here
  return data
```

**[Cache resource](https://docs.streamlit.io/develop/api-reference/caching-and-state/st.cache_resource)**: Function decorator to cache functions that return global resources (e.g. database connections, ML models).

```python
@st.cache_resource
def init_model():
  # Return a global resource here
  return pipeline(
    "sentiment-analysis",
    model="distilbert-base-uncased-finetuned-sst-2-english"
  )
```

## Browser and server state

Streamlit re-executes your script with each user interaction. Widgets have built-in statefulness between reruns, but Session State lets you do more!

**[Context](https://docs.streamlit.io/develop/api-reference/caching-and-state/st.context)**

`st.context` provides a read-only interface to access cookies, headers, locale, and other browser-session information.

```python
st.context.cookies
st.context.headers
```

**[Session State](https://docs.streamlit.io/develop/api-reference/caching-and-state/st.session_state)**: Save data between reruns and across pages.

```python
st.session_state["foo"] = "bar"
```

**[Query parameters](https://docs.streamlit.io/develop/api-reference/caching-and-state/st.query_params)**: Get, set, or clear the query parameters that are shown in the browser's URL bar.

```python
st.query_params[key] = value
st.query_params.clear()
```