# Caching (/caching)

Every successful `GET` carries:

```http
ETag: "3f2a9c..."
Cache-Control: private, max-age=60
Vary: Authorization, OpenGrid-Version
```

- **`ETag`** identifies the data and the API version. It ignores `as_of` and `request_id`, so it only changes when the data does.
- **`Cache-Control: private, max-age=60`**: your client may reuse a response for 60 seconds. Shared caches and CDNs
  must not store it.

## Conditional requests [#conditional-requests]

Send the last `ETag` back in `If-None-Match`. If nothing changed, you get `304 Not Modified` with no body:

```bash
curl -i https://data.tryopengrid.com/v1/markets \
  -H "Authorization: Bearer $OPENGRID_API_KEY" \
  -H 'If-None-Match: "3f2a9c..."'
# HTTP/2 304
```

```python
etag, cached = None, None

def markets(session):
    global etag, cached
    r = session.get("https://data.tryopengrid.com/v1/markets",
                    headers={"If-None-Match": etag} if etag else {})
    if r.status_code == 304:
        return cached
    r.raise_for_status()
    etag, cached = r.headers.get("ETag"), r.json()
    return cached
```

A `304` still counts against your [rate limit](/rate-limits), but it is faster and carries no body.

## Exports [#exports]

A completed day's [bulk export](/bulk-exports) never changes, so it carries `Cache-Control: private, max-age=86400`.