---
title: Surrogate-Key
summary: null
url: https://www.fastly.com/documentation/reference/http/http-headers/Surrogate-Key
---

Setting a `Surrogate-Key` header on a response from a backend server tells Fastly to index that response against the specified key (or keys), as well as the cache key which is the subject of the current request.  These 'surrogate keys' cannot be used to find and serve the content in a request, but can be used to target the content for purging.

For example, consider a request to an e-commerce site for `/category/shoes`, which elicits a response from the backend server with the following headers:

```http
Cache-Control: max-age=86400
Surrogate-Key: product-62952 product-1043 product-14534
```

In the example above, the response will be cached, with the primary cache key being linked to "/category/shoes" (depending on the cache key configuration in `vcl_hash`), but will also be indexed against the keys "product-62952", "product-1043" and "product-14534" for purging purposes.  In this use case, this allows changes to a product to automatically purge all the pages on which that product appears, and not just the product page itself.

## Adding keys in VCL

While it's conventional to add surrogate keys to a response at the backend server, it's also possible to add them using VCL in `vcl_fetch`, by setting the value of the `Surrogate-Key` backend response header:

```vcl context="vcl_fetch { ... }"
set beresp.http.Surrogate-Key = "key1 key2 key3";
```

## Purging

To purge a group of resources that share the same surrogate key, [use the web interface](https://docs.fastly.com/en/guides/single-purges#purging-with-keys), or [the API](https://www.fastly.com/documentation/reference/api/purging/#purge-tag).

## Limitations

Individual keys are limited to 1024 bytes in length, and the total length of the `Surrogate-Key` header may not exceed 16,384 bytes.  If either of these limits are reached while parsing a `Surrogate-Key` header, the key currently being parsed and all keys following it within the same header will be ignored.

## Debugging

Fastly will automatically strip the `Surrogate-Key` header from responses unless the header `Fastly-Debug` is present in the request.  Setting `Fastly-Debug: 1` on a request is one way to see which surrogate keys, if any, are associated with a resource in the cache.