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How Caching Affects Request Handling

Applies To:
HTTP LargeThis platform has been optimized to cache and deliver static content (e.g., HTML, CSS, JavaScript, ISO, multimedia, and software downloads, etc.) over HTTP or HTTPS.
HTTP SmallLegacy. If you are currently serving traffic over this platform, then you may continue to do so. However, we recommend that you serve your traffic over our more robust HTTP Large platform.
ADNThe Application Delivery Network platform has been optimized to deliver dynamic content (e.g., login credentials, account information, etc.) over HTTP or HTTPS. Typically, user-specific and database-driven content are served over this platform.

The logic through which requests for content are handled by the CDN is illustrated below.

HTTP Large and HTTP Small:

ADN:

Upon receiving a request for content, an edge server will check to see whether it has a cached version. If it finds a cached version of the requested content, it will calculate the asset's time to live (TTLTime to Live. Refers to the amount of time that a cached asset is still considered fresh. Our edge servers will continue to serve a cached version of an asset while its TTL has not expired. An asset's TTL is calculated by the Cache-Control and Expires headers associated with the response sent by the origin server.) according to its Cache-Control and Expires headers. This TTL determines whether the cached version of the requested content is fresh or stale.

Cache State Action(s)
FreshIndicates that the requested content has been previously cached and its TTL has not expired. An edge server will immediately serve the requested content to the HTTP client.

StaleIndicates that the requested content has been previously cached and its TTL has expired.

HTTP Large and HTTP Small

An edge server will perform either a standard or conditional GET request to the origin server depending on whether the cached asset contains Last-Modified and/or ETag directives.

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  • An edge server performs a conditional GET request when an asset's headers contain a Last-Modified and/or ETag directive. A conditional GET request verifies whether the cached version of the asset matches the version stored on the origin server. It does this by checking the last modified date and/or the entity tag of the asset.

    • No Difference: An edge server will deliver the cached asset to the HTTP client and update its TTL when it contains the most recent version.
    • New Version: An edge server will retrieve the latest version of the requested content from the origin server and deliver it to the HTTP client. If the asset's HTTP headers do not specifically prohibit caching, then it will be cached by the edge server and the HTTP client.
  • If a Last-Modified or ETag directive is not found in the asset's headers, then a standard GET request will retrieve the entire asset from the origin server. If the asset's HTTP headers do not specifically prohibit caching, then it will be cached by the edge server and the HTTP client.

  • If the asset has been assigned a must-re-validate directive, then one of the following actions will take place:

    • If it can communicate with the origin server, then it will either use a conditional or a standard GET request (as described above).
    • If the edge server cannot communicate with the origin server, then a 504 Gateway Timeout will be returned to the client.

ADN

An edge server will perform either a standard or conditional GET request to the ADN gateway server depending on whether the cached asset contains Last-Modified and/or ETag directives.

  • If the ADN gateway server has it in cache and it is fresh, then it will serve the cached version to the client via the edge server.
  • If the ADN gateway server has it in cache and it is stale, then it will perform either a standard or conditional GET request to the origin server depending on whether the cached asset contains Last-Modified and/or ETag directives.
  • If the ADN gateway server does not have a cached version of it, then it will perform a standard GET request to the origin server.
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  • An edge server will request the content from the ADN gateway server. The type of request that will be submitted is determined by whether the asset cached on the edge server contains a Last-Modified and/or ETag directive.

    • Last-Modified and ETag: A conditional GET request will be performed using the If-Modified-Since and If-None-Match headers.
    • Last-Modified: A conditional GET request will be performed using the If-Modified-Since header.
    • ETag: A conditional GET request will be performed using the If-None-Match header.
    • Missing Directives: A standard GET request will be performed.

  • The ADN gateway server determines how to handle the request according to the cache state of the requested content.

    • Not Cached: If the ADN gateway server cannot find a cached version of the requested content, it will forward the request to the origin server.
    • Fresh Content (Standard GET Request): If the ADN gateway server finds a fresh version of the requested content, it will serve it to the user via an edge server.

    • Fresh Content (Conditional GET Request): A conditional GET request verifies whether the cached version of the asset matches the version stored on the ADN gateway server. It does this by checking the last modified date and/or the entity tag of the asset.

      • No Difference: If an edge server contains the latest version of the requested content, it will deliver the cached asset to the HTTP client and update the TTL for its cached version.
      • New Version: If an edge server contains an older version of the requested content, then the ADN gateway server will serve the requested content to the HTTP client via the edge server.
    • Stale Content: If the ADN gateway server contains an expired version of the requested content, it will request the asset from the origin server. The type of request that will be submitted is determined by whether the asset cached on the ADN gateway server contains a Last-Modified and/or ETag directive.

  • If the asset has been assigned a must-re-validate directive, then one of the following actions will take place:

    • If the ADN gateway server can communicate with the origin server, then it will either use a conditional or a standard GET request (as described above).
    • If the ADN gateway server cannot communicate with the origin server, then a 504 Gateway Timeout will be returned to the client.
Not CachedIndicates that the edge server did not find a cached version of the asset.

HTTP Large and HTTP Small:

  1. An edge server will forward the HTTP client's request to an origin server.

  2. The requested content is served to the HTTP client through the following path:

    Origin Server Edge server HTTP client

  3. If the asset's HTTP headers do not specifically prohibit caching, then it may be cached by the edge server and the HTTP client.

    Learn more.

ADN:

  1. An edge server will forward the HTTP client's request to the ADN gateway server.

  2. The ADN gateway server determines how to handle the request according to the cache state of the requested content.

    • Fresh Content: The ADN gateway server will respond with the cached version of the content to the user via an edge server. Skip to step 4.

    • Stale Content & Retrieval: The ADN gateway server will request a new version from the origin server when the following conditions are met:

    • Stale Content & Revalidation: The ADN gateway server will revalidateRefers to the process that occurs when a request for stale content requires that our edge servers check for a new version of the requested content on the origin server. with the origin server when the following conditions are met:

      Revalidation Result Description

      Match

      The following actions will take place when the cached version matches the version stored on the origin server:

      • The response headers for the asset cached on an ADN gateway server will be updated to reflect a new TTL.
      • The asset will be served to the requester via the edge server from step 1.

      Outdated

      If the cached version is older than the version stored on the customer origin server, then the following actions will take place:

      • The new version of the asset will be retrieved from the customer origin server.
      • The asset will be served to the requester via the edge server from step 1.

      Not Found

      If the requested content is not found on the origin server, then a 404 Not Found will be returned to the requester.

      Unknown

      If an ADN gateway server is unable to establish a connection with the origin server, then a stale version of the requested content may be served. Stale content may be served when both of the following conditions are true:

      • The Cache-Control:must-revalidate or Cache-Control:proxy-revalidate headers were not assigned to the cached version of the requested content.
      • The Cache-Control: max-stale header was assigned to the requested content and the max-stale time period has not expired.

      Learn more.

    • Not Cached: The ADN gateway server will forward the request to the origin server.

  3. The requested content is served to the HTTP client through the following path:

    Origin server ADN gateway server Edge server HTTP client

An edge server and the HTTP client will cache eligible requests. If the ADN gateway server requested the content from the origin server, then it will also cache the requested content.
Learn more.
ClosedOrigin Shield request handling workflow

This section assumes that content was requested from a customer origin protected by the Origin ShieldThis feature provides a layer of protection for a customer origin server by creating an intermediate caching layer between it and our edge servers. This caching layer resides on one or more of our point-of-presence (POPs). Requests that have not been previously cached on a POP will be channelled through the closest origin shield server. The origin shield server will then either serve a cached version of the requested content or retrieve it from a customer origin server. This feature shifts the network load from the customer origin server to the CDN, since most requests will be handled by the origin shield server. feature.

  1. Upon receiving a request for content, an edge server will check to see whether it has a cached version. If it finds a cached version of the requested content, it will calculate the asset's time to live (TTLTime to Live. Refers to the amount of time that a cached asset is still considered fresh. Our edge servers will continue to serve a cached version of an asset while its TTL has not expired. An asset's TTL is calculated by the Cache-Control and Expires headers associated with the response sent by the origin server.) according to its Cache-Control and Expires headers. This TTL determines whether the cached version of the requested content is fresh or stale.

    • Fresh Content: If the asset's TTL has not expired, then an edge server will deliver it immediately to the requester.
    • Stale Content & Retrieval: If a cached version of the requested content with an expired TTL is found and it has not been assigned either a Last-Modified or Etag directive, then the edge server will request a new version from an Origin Shield server. See step 2.
    • Stale Content & Revalidation: If a cached version of the requested content with an expired TTL is found and it has been assigned a Last-Modified and/or Etag directive, then the edge server will revalidateRefers to the process that occurs when a request for stale content requires that our edge servers check for a new version of the requested content on the origin server. with an Origin Shield server. See step 2.
    • Not Found: If a cached version of the requested content is not found on the edge server, then it will request a new version from an Origin Shield server. See step 2.

  2. An Origin Shield server determines how to handle the request according to whether it has a cached version of the requested content.

    • Not Found: If the requested content is not found on an Origin Shield server, then please refer to the Origin Shield Request for New Content section.
    • Fresh Content: If a cached version of the requested content is found and its TTL has not expired, then it will be delivered to the requester via the edge server from step 1.
    • Stale Content & Retrieval: If a cached version of the requested content with an expired TTL is found and it has not been assigned either a Last-Modified or Etag directive, then an Origin Shield server will request a new version from the customer origin server. Please refer to the Origin Shield Request for New Content section.

    • Stale Content & Revalidation: If a cached version of the requested content with an expired TTL is found and it has been assigned a Last-Modified and/or Etag directive, then an Origin Shield server will revalidateRefers to the process that occurs when a request for stale content requires that our edge servers check for a new version of the requested content on the origin server. with the customer origin server.

      Revalidation Result Description

      Match

      If the cached version matches the version stored on the origin server, then the following actions will take place:

      • The response headers for the asset cached on an Origin Shield server will be updated to reflect a new TTL.
      • The asset will be served to the requester via the edge server from step 1.
      • The new version of the asset may be cached on that POP.
        Learn more.

      Outdated

      If the cached version is older than the version stored on the customer origin server, then the following actions will take place:

      • The new version of the asset will be retrieved from the customer origin server.
      • The asset will be served to the requester via the edge server from step 1.
      • The new version of the asset may be cached on the Origin Shield and edge servers that handled the request.
        Learn more.

      Not Found

      If the requested content is not found on the origin server, then a 404 Not Found will be returned to the requester.

      Unknown

      If an Origin Shield server is unable to establish a connection with the origin server, then a stale version of the requested content may be served. Stale content may be served when both of the following conditions are true:

      • The Cache-Control:must-revalidate or Cache-Control:proxy-revalidate headers were not assigned to the cached version of the requested content.
      • The Cache-Control: max-stale header was assigned to the requested content and the max-stale time period has not expired.

      Learn more.

Origin Shield Request for New Content

  1. An Origin Shield server will forward the request to the origin server.

  2. The origin server will provide a response to the client that requested it via the following path:

    Origin server Origin Shield server Edge server Client

  3. If eligible, the requested content may then be cached on the Origin Shield server, edge server, and the client.

    Learn more.

A default Origin Shield configuration behaves as described above. However, you may have configured your Origin Shield configuration to bypass Origin Shield for certain POPs. This type of configuration will cause an edge server on one of those POPs to perform the standard check for content freshness.

More Information