--- schema_version: "1" id: 869a22778b4a7846c9c590128c7dd62eb064bc7bb0c24e393c98f27192c62672 title: Conditional Request Headers collection: developer version: current locale: en-us portal_url: https://docs.salesforce.com/developer/current/en-us/platform/api-rest/guides/intro/intro-rest-conditional-requests.md canonical_url: https://developer.salesforce.com/docs/platform/api-rest/guide/intro-rest-conditional-requests.html source_site: developer.salesforce.com content_type: documentation content_hash: 864f90cede803cd29ece3ad2602a49336465e36f7632a9b7b4957638b8d31cd2 products: - REST API Developer Guide - Platform Developer Content --- # Conditional Request Headers Use a conditional request header to validate resources before accessing them. By setting a precondition in the header, you ensure that your request succeeds only if that precondition is met. This functionality helps you prevent mistakes and reject outdated requests when updating Salesforce data. You can implement a variety of techniques with conditional request headers, such as response caching. Two types of validation are available with conditional request headers: strong and weak. Strong validation checks whether the precondition exactly matches the resource in Salesforce. Strong validation headers include `If-Match` and `If-None-Match`, which use entity tags (ETags) to compare the precondition to the record in Salesforce. Weak validation checks a precondition against the resource in Salesforce, but it doesn’t guarantee that the two are identical. Weak validation headers include `If-Modified-Since` or `If-Unmodified-Since`, which compare the precondition to the last modified date of the record in Salesforce. REST API conditional headers follow the HTTP 1.1 specification with the following exceptions. - When you include an invalid header value for `If-Match`, `If-None-Match`, or `If-Unmodified-Since` on a PATCH or POST request, a `400 Bad Request` status code is returned. - The `If-Range` header isn’t supported. - DELETE requests aren’t supported ## ETag The `ETag` header is a response header that’s returned when you access the sObject Rows resource. It’s a hash of the content that’s used by the `If-Match` and `If-None-Match` request headers in subsequent requests to determine if the content has changed. This header is supported by [sObject Rows](https://developer.salesforce.com/docs/platform/api-rest/resources/resources-sobject-retrieve.md) (Account records only) resources. This example shows an `ETag` returned by REST API. ```nolang ETag: "U5iWijwWbQD18jeiXwsqxeGpZQk=-gzip" ``` You can find the HTTP 1.1 specification for the `ETag` header at [ www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.19 ](http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.19). ## If-Match The `If-Match` header is a request header for sObject Rows that includes a list of ETags. If the ETag of the record that you’re requesting matches an ETag specified as a precondition in the header, the request is processed. Otherwise, a 412 Precondition Failed status code is returned, and the request isn’t processed. This header supports [sObject Rows](https://developer.salesforce.com/docs/platform/api-rest/resources/resources-sobject-retrieve.md) (Account records only) resources. In this example an, `If-Match` header is included with a request. ```nolang If-Match: "Jbjuzw7dbhaEG3fd90kJbx6A0ow=-gzip", "U5iWijwWbQD18jeiXwsqxeGpZQk=-gzip" ``` You can find the HTTP 1.1 specification for the `If-Match` header at [ www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.24 ](http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.24). ## If-None-Match The `If-None-Match` header is a request header for sObject Rows that’s the inverse of `If-Match`. If the ETag of the record that you’re requesting matches an ETag specified in the header, the request isn’t processed. A 304 Not Modified status code is returned for GET or HEAD requests, and a 412 Precondition Failed status code is returned for PATCH requests. This header supports [sObject Rows](https://developer.salesforce.com/docs/platform/api-rest/resources/resources-sobject-retrieve.md) (Account records only) resources. In this example, an `If-None-Match` header is included with a request. ```nolang If-None-Match: "Jbjuzw7dbhaEG3fd90kJbx6A0ow=-gzip", "U5iWijwWbQD18jeiXwsqxeGpZQk=-gzip" ``` You can find the HTTP 1.1 specification for the `If-None-Match` header at [ www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.26 ](http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.26). ## If-Modified-Since The `If-Modified-Since` header is a time-based request header. The request is processed only if the data has changed since the date and time specified in the header. Otherwise, a 304 Not Modified status code is returned, and the request isn’t processed. This header supports [sObject Rows](https://developer.salesforce.com/docs/platform/api-rest/resources/resources-sobject-retrieve.md), [sObject Describe](https://developer.salesforce.com/docs/platform/api-rest/resources/resources-sobject-describe.md), [Describe Global](https://developer.salesforce.com/docs/platform/api-rest/resources/resources-describeglobal.md), and [Invocable Actions](https://developer.salesforce.com/docs/platform/api-rest/resources/resources-actions-invocable.md) resources. In this example an `If-Modified-Since` header is included with a request. ```nolang If-Modified-Since: Tue, 10 Aug 2015 00:00:00 GMT ``` You can find the HTTP 1.1 specification for the `If-Modified-Since` header at [ www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.25 ](http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.25). ## If-Unmodified-Since The `If-Unmodified-Since` header is a request header that’s the inverse of `If-Modified-Since`. If you make a request and include the If-Unmodified-Since header, REST API processes the request only if the data hasn’t changed since the specified date. Otherwise, a 412 Precondition Failed status code is returned, and the request isn’t processed. This header supports [sObject Rows](https://developer.salesforce.com/docs/platform/api-rest/resources/resources-sobject-retrieve.md), [sObject Describe](https://developer.salesforce.com/docs/platform/api-rest/resources/resources-sobject-describe.md), [Describe Global](https://developer.salesforce.com/docs/platform/api-rest/resources/resources-describeglobal.md), and [Invocable Actions](https://developer.salesforce.com/docs/platform/api-rest/resources/resources-actions-invocable.md) resources. In this example, an `If-Unmodified-Since` header is included in a request. ```nolang If-Unmodified-Since: Tue, 10 Aug 2015 00:00:00 GMT ``` You can find the HTTP 1.1 specification for the `If-Unmodified-Since` header at [ www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.28 ](http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.28).