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Expiration.md

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Specifying How Long Objects Stay in a CloudFront Edge Cache (Expiration)

You can control how long your objects stay in a CloudFront cache before CloudFront forwards another request to your origin. Reducing the duration allows you to serve dynamic content. Increasing the duration means your users get better performance because your objects are more likely to be served directly from the edge cache. A longer duration also reduces the load on your origin.

Typically, CloudFront serves an object from an edge location until the cache duration that you specified passes—that is, until the object expires. After it expires, the next time the edge location gets a user request for the object, CloudFront forwards the request to the origin server to verify that the cache contains the latest version of the object. The response from the origin depends on whether the object has changed:

  • If the CloudFront cache already has the latest version, the origin returns a 304 status code (Not Modified).
  • If the CloudFront cache does not have the latest version, the origin returns a 200 status code (OK) and the latest version of the object.

If an object in an edge location isn't frequently requested, CloudFront might evict the object—remove the object before its expiration date—to make room for objects that have been requested more recently.

By default, each object automatically expires after 24 hours. For web distributions, you can change the default behavior in two ways:

  • To change the cache duration for all objects that match the same path pattern, you can change the CloudFront settings for Minimum TTL, Maximum TTL, and Default TTL for a cache behavior. For information about the individual settings, see Minimum TTL, Maximum TTL, and Default TTL. To use these settings, you must choose the Customize option for the Object Caching setting.
  • To change the cache duration for an individual object, you can configure your origin to add a Cache-Control max-age or Cache-Control s-maxage directive, or an Expires header field to the object. For more information, see Using Headers to Control Cache Duration for Individual Objects.

For more information about how Minimum TTL, Default TTL, and Maximum TTL interact with Cache-Control max-age and Cache-Control s-maxage directives and the Expires header field, see Specifying the Amount of Time that CloudFront Caches Objects for Web Distributions.

You can also control how long errors (for example, 404, Not Found) stay in a CloudFront cache before CloudFront tries again to get the requested object by forwarding another request to your origin. For more information, see How CloudFront Processes and Caches HTTP 4xx and 5xx Status Codes from Your Origin.

Topics

Using Headers to Control Cache Duration for Individual Objects

You can use the Cache-Control and Expires headers to control how long objects stay in the cache. Settings for Minimum TTL, Default TTL, and **Maximum TTL ** also affect cache duration, but here's an overview of how headers can affect cache duration:

  • The Cache-Control max-age directive lets you specify how long (in seconds) that you want an object to remain in the cache before CloudFront gets the object again from the origin server. The minimum expiration time CloudFront supports is 0 seconds for web distributions and 3600 seconds for RTMP distributions. The maximum value is 100 years. Specify the value in the following format:

    Cache-Control: max-age=seconds

    For example, the following directive tells CloudFront to keep the associated object in the cache for 3600 seconds (one hour):

    Cache-Control: max-age=3600

    If you want objects to stay in CloudFront edge caches for a different duration than they stay in browser caches, you can use the Cache-Control max-age and Cache-Control s-maxage directives together. For more information, see Specifying the Amount of Time that CloudFront Caches Objects for Web Distributions.

  • The Expires header field lets you specify an expiration date and time using the format specified in RFC 2616, Hypertext Transfer Protocol -- HTTP/1.1 Section 3.3.1, Full Date, for example:

    Sat, 27 Jun 2015 23:59:59 GMT

We recommend that you use the Cache-Control max-age directive instead of the Expires header field to control object caching. If you specify values both for Cache-Control max-age and for Expires, CloudFront uses only the value of Cache-Control max-age.

For more information, see Specifying the Amount of Time that CloudFront Caches Objects for Web Distributions.

You cannot use the HTTP Cache-Control or Pragma header fields in a GET request from a viewer to force CloudFront to go back to the origin server for the object. CloudFront ignores those header fields in viewer requests.

For more information about the Cache-Control and Expires header fields, see the following sections in RFC 2616, Hypertext Transfer Protocol -- HTTP/1.1:

For an example of how to add Cache-Control and Expires header fields using the AWS SDK for PHP, see Upload an Object Using the AWS SDK for PHP in the Amazon Simple Storage Service Developer Guide. Some third-party tools are also able to add these fields.

Specifying the Amount of Time that CloudFront Caches Objects for Web Distributions

For web distributions, you can use Cache-Control or Expires headers, and CloudFront minimum, maximum, and default TTL values to control the amount of time in seconds that CloudFront keeps an object in the cache before forwarding another request to the origin. Headers values also determine how long a browser keeps an object in the cache before forwarding another request to CloudFront.

Important
If you configure CloudFront to forward all headers to your origin for a cache behavior, CloudFront never caches the associated objects. Instead, CloudFront forwards all requests for those objects to the origin. In that configuration, the value of minimum TTL must be 0. For more information, see Configuring CloudFront to Cache Objects Based on Request Headers.

To specify values for Minimum TTL, Maximum TTL, and Default TTL, you must choose the Customize option for the Object Caching setting.

Origin Configuration Minimum TTL = 0 Seconds Minimum TTL > 0 Seconds
The origin adds a Cache-Control max-age directive to objects CloudFront caching CloudFront caches objects for the lesser of the value of the Cache-Control max-age directive or the value of the CloudFront maximum TTL. Browser caching Browsers cache objects for the value of the Cache-Control max-age directive. CloudFront caching CloudFront caching depends on the values of the CloudFront minimum TTL and maximum TTL and the Cache-Control max-age directive: [See the AWS documentation website for more details] Browser caching Browsers cache objects for the value of the Cache-Control max-age directive.
The origin does not add a Cache-Control max-age directive to objects CloudFront caching CloudFront caches objects for the value of the CloudFront default TTL. Browser caching Depends on the browser. CloudFront caching CloudFront caches objects for the greater of the value of the CloudFront minimum TTL or default TTL. Browser caching Depends on the browser.
The origin adds Cache-Control max-age and Cache-Control s-maxage directives to objects CloudFront caching CloudFront caches objects for the lesser of the value of the Cache-Control s-maxage directive or the value of the CloudFront maximum TTL. Browser caching Browsers cache objects for the value of the Cache-Control max-age directive. CloudFront caching CloudFront caching depends on the values of the CloudFront minimum TTL and maximum TTL and the Cache-Control s-maxage directive: [See the AWS documentation website for more details] Browser caching Browsers cache objects for the value of the Cache-Control max-age directive.
The origin adds an Expires header to objects CloudFront caching CloudFront caches objects until the date in the Expires header or for the value of the CloudFront maximum TTL, whichever is sooner. Browser caching Browsers cache objects until the date in the Expires header. CloudFront caching CloudFront caching depends on the values of the CloudFront minimum TTL and maximum TTL and the Expires header: [See the AWS documentation website for more details] Browser caching Browsers cache objects until the date and time in the Expires header.
Origin adds Cache-Control: no-cache, no-store, and/or private directives to objects CloudFront and browsers respect the headers. For an exception to how CloudFront handles the Cache-Control: no-cache header, see Simultaneous Requests for the Same Object (Traffic Spikes). CloudFront caching CloudFront caches objects for the value of the CloudFront minimum TTL. Browser caching Browsers respect the headers.

For information about how to change settings for web distributions using the CloudFront console, see Viewing and Updating CloudFront Distributions. For information about how to change settings for web distributions using the CloudFront API, see UpdateDistribution.

Specifying the Minimum Time that CloudFront Caches Objects for RTMP Distributions

For RTMP distributions, CloudFront keeps objects in edge caches for 24 hours by default. You can add Cache-Control or Expires headers to your objects to change the amount of time that CloudFront keeps objects in edge caches before it forwards another request to the origin. The minimum duration is 3600 seconds (one hour). If you specify a lower value, CloudFront uses 3600 seconds.

Adding Headers to Your Objects Using the Amazon S3 Console

Note
Using the Amazon S3 console, you can only add headers to one object at a time, but with some third-party tools, you can add headers to multiple Amazon S3 objects at a time. For more information about third-party tools that support Amazon S3, perform a web search on AWS S3 third party tools.

To add a Cache-Control or Expires header field to Amazon S3 objects using the Amazon S3 console

  1. Sign in to the AWS Management Console and open the Amazon S3 console at https://console.aws.amazon.com/s3.

  2. In the Amazon S3 console, in the buckets list, choose the name of the bucket that contains the files.

  3. In the list of objects, select the check box for one or more objects that you want to add a header to.

  4. Choose More and choose Change metadata.

  5. In the Key list, choose Cache-Control or Expires, as applicable.

  6. In the Value field, type the applicable value:

    • For a Cache-Control field, type:

      max-age=number of seconds that you want objects to stay in a CloudFront edge cache

    • For an Expires field, type a date and time in HTML format.

  7. Choose Save.