This document describes how ​Akamai​ caches content on its edge servers and how you can control this caching. Except the Downstream Caching section, it doesn’t discuss caching of objects on end-user clients such as browsers, although many of the underlying concepts are similar.

One of the core benefits of ​Akamai​ CDN is the ability to retrieve content (that is, HTML pages, images, fonts, and so on — also commonly referred to as “objects”) from your origin server and cache it in ​​Akamai​​ edge servers at the edge of the Internet, near the end-users. This feature improves performance by reducing both the load on your origin server and the time it takes to serve content to end-users.

What is cached and what is cacheable?

You may see discussions about caching requests. However, in fact, it’s the response to a request sent from your origin server that is cached. In practice, request and response have similar meanings, but in conjunction with the discussion of cache keys below, you can see that it is possible to design your caching strategy so that multiple similar but slightly different requests will all return the same object from cache.

By default, only some content is actually cacheable by edge servers — it must be requested using a GET method and any Vary response header sent with the object from the origin can only contain Accept-Encoding. If an object is cacheable, you can control for how long it is cached and for what requests it should be served to the requesting client.

Cache keys

When an object is cached on an edge server, it is identified internally by a cache key. The cache key is unique to this object and contains these elements:

You can use these attributes to decide whether similar requests should result in the same object being returned from the cache, or they should result in multiple objects being cached separately.

Separate caches per server

​Akamai​’s cache is not monolithic — each individual edge server has its own cache. When a request from an end user is routed to an edge server, content is cached on this specific edge server. Content is only cached as a result of a request from an end user.
For example, in many cases, requests for content come from end users in a particular geographic region, and therefore only the ​Akamai​ edge servers near those end users cache a copy of that content.
However, when content is purged from cache, the purge request is made to every edge server to remove the content from cache, whether it exists there or not.

How caching works in Property Manager

Property Manager automatically assigns a cache setting to your content. By default, edge servers cache content for a theoretically infinite time. Objects are removed from cache on a least recently used (LRU) basis. If an edge server’s cache is full, it can remove objects that are infrequently accessed from the cache even if those objects have not yet reached their Time-to-Live (TTL). In addition, you can use the Purge Cache application to completely remove objects from ​Akamai​ edge servers’ caches at any time.
To change the default settings, you can modify the TTL value through Property Manager behaviors, the most important of which is Caching, or through cache-related response headers sent by your origin when the object is retrieved.

What is TTL?

A Time-to-Live (TTL) is the amount of time an edge server can hold an object in its cache without checking with the origin server whether a newer version of the object is available. When you send a request, edge servers check whether the object has been in the edge server cache for longer than its defined TTL — this is known as the object being stale If the object is not stale, it is served to you from cache. If the object is stale, the edge server checks with the origin server using an If-Modified-Since (IMS) GET request to see if a newer version of the object is available. If so, it retrieves the newer object from the origin server before caching and serving it to you. If no newer object is available on the origin server, the currently-cached object is served to you and its TTL is reset. This is referred to as conditional revalidation.

How to set a TTL?

Your property configuration is the primary method for setting options for groups of objects, and it's the main method for setting a TTL. In rules within Property Manager, you use the Caching behavior to set the max-age value, which defines the maximum amount of time that objects can be retrieved from the cache before they are considered stale. Typically, you set it to the maximum acceptable value, based on which the remaining TTL is calculated. By applying matches to your rules, you can specify a variety of TTL values that best suit your needs. For example, you can match on file directories, file extensions, and other groupings to ensure the best possible caching for different objects.
If necessary, you can override configuration settings on selected objects with Edge Side Includes (ESI) attributes and specific headers coming from the origin server, such as Cache-Control, and Expires.

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The Edge-Control header

In addition to the normal Cache-Control and Expires caching response headers, there is an ​Akamai​-specific HTTP response header called Edge-Control. Edge-Control provides controls and parameters for content served over the edge network. On the ​​Akamai​​ network, the Edge-Control settings take precedence over any Cache-Control and Expires headers as well as over many caching-related configuration settings. The Edge-Control header is generally deprecated and is not described in this document. However, if you have specific complex caching requirements, your ​Akamai​ account team may suggest using the Edge-Control header and provide you with further details.

Cache-Control and Expires headers

By default, edge servers don’t honor these two response headers sent from the origin server. You can change this setting for specific objects by enabling each header individually, or both together with the Caching behavior. Once Honor origin Cache-Control and Expires is enabled, the edge network honors the Expires header value and the following Cache Control header directives:

• s-maxage (specific to shared caches)
• max-age
• no-store
• no-cache (behaves like setting a zero second max-age)

Potential conflicts

In case of conflicts between the Cache Control and Expires headers when both are returned from the origin, the Cache-Control value takes precedence for caching in ​Akamai​.
If the Cache-Control header contains both max-age and s-maxage directives, ​Akamai​ caches the content using the s-maxage value, and sends both directives downstream. If there are any caching proxies between ​Akamai​ and the client, they should also cache the content using the s-maxage value. Clients will ignore the s-maxage directive entirely and will only cache content using the max-age directive value or the Expires value, if passed.
Note that you can’t override a no-store setting defined in your configuration with a Cache-Control or Expires header.

Other considerations:

• If you set the configuration to honor the Cache-Control header, edge servers use this information for the lifetime of the object if the following conditions are met:
  1. You set edge servers to accept any of the s-maxage, max-age, no-store, and no-cache directives of the Cache-Control header.
  2. The edge servers receive the Cache-Control header in the response from the origin server and it contains any of the above directives.
• If you set the configuration to honor the Expires header, edge servers use the ‘implied’ max-age (Expires value - current time) as the lifetime for the object if the following conditions are met:
  1. Edge servers receive the Expires header in the response from the origin server.
  2. The expiration is in the future.
• If you set the configuration to honor the Expires header and an edge server receives an Expires header from the origin, the edge server uses the Date header from the origin to calculate the TTL of the object rather than its own internal clock, in case the origin’s clock isn't synchronized with the edge server’s clock.
• If you set the configuration to honor the Expires header and the received Expires value is in the past, the edge servers use their own current date stamp instead of the implied max-age value.

ESI tag attributes

This section may be of interest to ESI developers. When you use an esi:include statement to request a fragment for inclusion in an ESI template page, you can use the ttl or no-store tag attributes to control the caching properties for the requested object.

TTL best practices

The choice of an appropriate TTL depends on the time sensitivity and the nature of the objects. In general, you should choose the longest possible TTL that does not cause the end-user to receive stale content. The longer the TTL associated with an object, the greater the benefit of offloading content.

Prefresh

​Akamai​​ edge servers can asynchronously prefresh objects if a request is received during a specified time percentage of TTL, by default set to 90% of the TTL. This means that after 90% of the TTL has elapsed, a request for the object is immediately served from cache and also triggers an asynchronous IMS request to the origin to check for a newer version of the object.
For example, an object with a TTL of 60 minutes is requested 54 minutes after it’s added to the cache or later. The ​​Akamai​​ server immediately serves the object to the client and then sends an If-Modified-Since request to the origin server to conditionally revalidate the object and potentially retrieve an updated object to replace the existing object in the cache.
This asynchronous update reduces the waiting time even for popular objects with a short TTL and avoids a delay in serving the object on the next request. It also reduces potential spikes in traffic to the origin server when the TTL of the object expires.

To change the default setting, add the Cache Prefresh/Refresh behavior to your property configuration.

Time sensitivity

Consider the following question when selecting the TTL settings:
“How long do I want ​​Akamai​​​ to serve this cached version after I've already changed the content on my site?”

The answer to this question will inform the appropriate TTL for your content.

• Extreme time sensitivity. If an object is so time sensitive that it must be revalidated with each delivery, such as a breaking news story, then you should either not serve the object from cache at all and send each request to the origin server, or set TTL=0 (zero TTL) so that the object is cached but it is revalidated with each request. The decision of whether to prevent caching (no-store) or assign the zero-TTL depends primarily on the size of your object.
• Moderate time sensitivity. If you update the site once a day in the late evening, when request traffic is low, you may not mind stale content being served for some time. You can set the TTL for most objects to an hour or more. You can even assign longer TTLs and use Purge Cache in the publishing process to delete old content after each new publication. However, be aware that a change in content doesn't necessarily warrant purging the old content from the cache. Object sensitivity is the primary issue. For example, if you make superficial changes to an image and the user doesn’t necessarily need to see the updated version, you can let the changes take effect gradually as edge servers revalidate stale objects in the cache with the origin server when they receive requests.
• No time sensitivity. If the content isn't time sensitive at all, such as an archival magazine article, you can give it a very long TTL. It's probably most beneficial if the origin server isn't bothered with If-Modified-Since requests to confirm that the object is up-to-date. You can maintain the timeliness of the object by either changing the reference in the HTML file when the content changes, or using Purge Cache to delete the old content after a change.

Downstream caching

Downstream caching refers to the caching instructions that edge servers apply to the content they serve to clients (such as browsers, mobile devices, or client proxies). These instructions are sent to clients as response headers.

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Downstream no-store and bypass-cache instructions override other downstream TTL instructions. If the object can’t be cached, the TTL is irrelevant.

Default downstream caching behavior

By default, ​​Akamai​​ sets the cacheability in the browser to no longer than the remaining lifetime for the content stored in the edge server cache. If the cacheability headers sent by the origin server suggest a shorter time, that shorter time is used.
When delivering content to clients, ​​Akamai​​ servers by default send the smaller of the Cache-Control: max-age and/or Expires values received from the origin server and the remaining lifetime of the object in the edge server cache. This means that the client max-age is always equal to or lower than the edge max-age.

Exceptions

By default, edge servers update only the values in the Cache-Control and Expires headers. They don't add these headers if the origin server didn't send them. However, there are certain exceptions to this default behavior:

• If you use the no-store or bypass-cache settings in your property configuration to indicate that an object shouldn't be cached, edge servers send "cache-busting" headers downstream to the client. Any Cache-Control: max-age or s-maxage directives received from the origin server are discarded.
• You can set the edge server configuration to compute the downstream caching itself.
• If you use ESI, the downstream cacheability is calculated as the lowest value of all fragments that make up the output page served to the client.

Custom TTL downstream settings

If you want to set the downstream TTL directly, without relying on the default behavior, you can use either:

• The Downstream Cacheability behavior.
• ESI configuration file or ESI language controls. With ESI Downstream Time-to-Live, you can only set a specific value in the response. There are no controls to select the headers to send or the method to use to calculate the value of those headers.

Cache prevention

You must specify at least one default caching rule for your object in the property configuration. If you select no-store, the object isn't cached, and any downstream caching is also prevented.
To bust downstream caches, edge servers send these HTTP headers in response to the client:

• Expires: <current_time>
• Cache-Control: max-age=0
• Cache-Control: no-store
• Pragma: no-cache

If the Cache-Control header is enabled in your settings, the cache-busting behavior also applies when edge servers receive Cache-Control: no-cache or Cache-Control: no-store headers in a response from the origin server. This is true even if a prior origin response contained the Expires header but no Cache-Control: max- age.
If you want to cache in edge servers but only prevent downstream caching, you can:
Add the Downstream Cacheability behavior, with Caching Option set to Allow caching and Cache Lifetime set to Fixed value, set the max-age value to -1.
Use the ESI function $add_cachebusting_header(), or set the ESI downstream TTL configuration attribute to -1.

TTL alternatives

In some cases, setting a TTL duration may not meet your needs. Imagine you need fresh content the moment you publish it, such as a market quote that needs to reflect changes in real time. Or you need to authenticate for each request, so the requests need to be forwarded to the origin. There are some alternatives to TTL that you can apply to your objects.

Zero TTL

If you set TTL to zero, edge servers can cache your object, but are required to revalidate the object with an If-Modified-Since (IMS) request to the origin each time the object is requested. This option may be appropriate for large, time-sensitive objects. If the object is large, the benefit of serving it from the edge of the Internet may outweigh the cost of revalidating the object for each client request.
You may also find zero TTL useful if you need real-time logging of client requests, which is a viable alternative to using no-store to provide real-time logging. However, to ensure fresh content and minimal latency, you should specify an appropriate TTL for the object and set the prefresh option to 0%. This way, the edge servers contact the origin server for each request only after they have served the content to the client, to avoid increased latency due to real-time logging.

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If the content changes for every request, use no-store instead.

No-store

Use no-store if you don't want ​​Akamai​​ to cache the object. In general, if the object is likely to change more often than it’s requested — for example, if it's different with each request — it's best to simply disallow caching. Without caching the benefits of persistent TCP connections and route optimization between ​​Akamai​​ servers and the origin server are still realized.
If you need to pass requests and responses transparently, no-store is sometimes the only solution. You can specify no-store using the Cache-Control: no-store response header directives from your origin.
For dynamically generated content, it may be essential to use no-store. However, ​​Akamai​​ also provides ESI (Edge Side Includes) that allow dynamic content to be served at the edge, close to the end user.

Bypass-cache

Bypass-cache is similar to no-store, except that the request goes directly through the edge server to the origin and the origin response is returned to the requesting client without removing the underlying object from the cache if it's already there. Bypass-cache can be useful when the returned object is an alternative for the normally delivered content that is usually delivered — for example, when the requested page is already in the cache, but the user must be redirected to a separate sign-in page first. In this case, you can keep the requested page in the cache (for use by signed-in users), but use the bypass-cache for this particular end user request.

TTL and Purge Cache

To optimize your delivery and have better control over your content on ​​Akamai​​ servers, you can combine the caching settings with the use of Purge Cache. You would typically use TTL control in the Caching behavior as the primary content management solution. However, in some cases where content is time-sensitive but rarely changed, you may want to set long TTLs to reduce the If-Modified-Since requests to your origin and purge the old content after it is changed. You can also use Purge Cache when an object has changed and you need strict consistency — that is, you can’t afford to serve the stale object for a period of time. For more information, see the Purge Cache documentation.