Content delivery best practices

This page provides best practices for optimizing and accelerating content delivery with Cloud CDN. The sections are divided into several key areas.

Cloud CDN uses an external Application Load Balancer as the origin for cacheable content. An external Application Load Balancer can deliver a mix of static and dynamically created content to users through one global IP address from the following types of backends:

Because of the seamless integration with Google Cloud, you have several options for deploying Cloud CDN and managing content. Use the best practices listed here to plan and refine your deployment. For more information, see Set up Cloud CDN.

Optimize the cache hit ratio

The following recommended practices help optimize the cache hit ratio.

Cache static content

As a best practice to improve performance, when you enable Cloud CDN, you need to choose the right cache mode for your application.

The most flexible and generally preferred method to manage cache rules is by using the cache control header. If you are not familiar with using origin cache-control headers, the best practice recommendation is to allow Cloud CDN to automatically cache static content.

To automatically cache static responses from your origin, you can use the --cache-mode=CACHE_ALL_STATIC setting (default). This setting lets Cloud CDN cache common static content types when the origin does not specify any caching directives in the response headers. Ensure that your content matches the categories outlined; otherwise, content is not cached.

Don't cache user-specific content

In some cases, browsers can cache user-specific content. Don't use Cloud CDN to cache user-specific content.

Use custom cache keys to improve cache hit ratio

For performance and scalability, it's important to optimize the cache hit ratio. By default, Cloud CDN uses the complete request URL to build the cache key. To help optimize your cache hit ratio, you can use custom cache keys so that Cloud CDN doesn't unnecessarily shard the cache.

Cloud CDN key-value store (click to enlarge).

Custom cache keys let you include or omit any combination of protocol, host, and query string. Following are some examples of when you might use custom cache keys:

  • You have two hosts that resolve to the same IP address and go to the same service. In this example, the entire website is the same on the two hosts. By default, Cloud CDN caches two copies because of the different Host: header in the HTTP requests. With a custom cache key, you can make Cloud CDN ignore the host part of the request and share the cache entries.

  • In a more specific example, you might have two websites on different domains that use the same logo. The website content is different, but you use the same company logo on both domains, and you have a dedicated backend service that holds shared content. When you enable Cloud CDN and customize the cache keys for the backend service that holds the logo, clear the Host checkbox so that the cache ignores the domain but caches the logo.

  • A logo needs to be cached whether displayed through HTTP or HTTPS. When you customize the cache keys for the backend service that holds the logo, clear the Protocol checkbox so that requests through HTTP and HTTPS count as matches for the logo's cache entry.

To learn how to customize cache keys, see Using cache keys.

Optimize performance

The following recommended practices help optimize performance.

Ensure that HTTP/3 and QUIC protocol support is enabled

HTTP/3 is a next-generation internet protocol. It is built on top of QUIC, a protocol developed from the original Google QUIC ) (gQUIC) protocol. HTTP/3 is supported between the external HTTP(S) load balancer, Cloud CDN, and clients.

To increase performance with Cloud CDN, ensure that HTTP/3 is enabled.

Use negative caching

Negative caching provides fine-grained control over caching for common errors or redirects. When Cloud CDN encounters specific response codes, it holds that response in cache for a set TTL. This can reduce the load on your origins and improve the end-user experience by reducing response latency.

Optimize security

The following recommended practices help optimize security.

Use Google Cloud Armor

Google Cloud Armor integrates with Cloud CDN for both cached and non-cached or cache-miss content. A best practice recommendation is to use Google Cloud Armor in conjunction with Cloud CDN wherever possible to increase the security of web applications.

Use signed URLs

If you're using signed URLs, note the following:

Authenticate private origins

Origin authentication offers a strong guarantee that the request comes only from your own configured backend service. It also offers in-transit data protection for the request and protects against reuse of the signed portion of the request.

We recommend using private origin authentication for Amazon S3 buckets or compatible object stores. Private origin authentication helps ensure that only trusted connections access content on your private origins and that users don't directly access them.

Additionally, if origin firewalls prevent access to the origin, use IP allowlisting to ensure that a request is from Cloud CDN or the external Application Load Balancer. However, this doesn't prevent other Media CDN customers from attempting to access your content by specifying your origin in their configuration.

Optimize the cache

The following recommended practices help optimize the cache.

Optimize cache TTLs

You can set or override the TTLs to fine tune how long Cloud CDN caches your responses and when Cloud CDN revalidates your responses.

You can also define a client-facing TTL to make the most of browser caches.

For more information, see Using TTL settings and overrides.

Set the expiration for time-sensitive content

Each piece of content in a Cloud CDN cache has an associated expiration time, and it's important to set an expiration that is appropriate for your use case. Because origin servers must resend content that expires on cache servers, you need to choose the expiration carefully.

One method for choosing the expiration is to categorize content based on how often you update the content; for example:

  • Near real-time updates such as live feeds for sporting events or traffic
  • Frequent updates such as weekly, daily, or hourly weather information or front-page news images
  • Infrequent updates such as a website logo or CSS or JavaScript files

Next, choose the expiration by content category. For example, a five-second expiration might be appropriate for near real-time sports scores, and a one-hour expiration could be used for weather updates. For content stored in Cloud Storage, set the expiration times by using Cache-Control metadata. When content is served by Compute Engine, you control expiration times by configuring your web server software.

Expiration times are specified by the max-age and s-maxage values in the Cache-Control header. This header is defined by the HTTP specification. For example, the following Cache-Control header makes the associated content publicly readable and cacheable with a cache expiration of 72 hours (259200 seconds):

  Cache-Control: public, max-age=259200

To maximize caching, follow the guidelines in the Caching overview. Remember that max-age and s-maxage values in the Cache-Control metadata field work together in the following ways:

  • The max-age and s-maxage values are measured in seconds.
  • The s-maxage value applies only to shared caches, not browser caches.
  • The max-age value applies to all caches unless s-maxage overrides it.

For content that changes infrequently or that must change along with related content, it's often appropriate to use a long expiration time in combination with versioned URLs.

Use versioned URLs to update content

Versioning content serves a different version of the same content, effectively removing it by showing users new content before the cache entry expires. Because versioning is free, we recommend that you use versioning as the default approach for updating cacheable content.

To version content, add a parameter to the URL, such as a version number. There are various ways to include parameters in URLs, such as:

  • Add a query string: file.ext?v=100.

    For backend buckets, any query strings used for versioning must be specified in the configuration for the backend bucket. For more information, see Query string include list for Cloud Storage cache keys.

  • Alter the filename: file.1.0.0.ext or file_v100.ext.

  • Alter the path: /v100/file.ext.

When you add the parameter, you change the name of the file and the URL. This change forces the cache to ignore any existing cache entry.

Use invalidation sparingly to remove content

Invalidation removes content from the Cloud CDN distributed cache servers before the cache entry expires. Invalidation is eventually consistent.

We recommend that you use invalidation sparingly and only as a last resort. For example, invalidation is useful when you must remove content for legal reasons or because of an accidental upload. Otherwise, we recommend that you use versioning whenever possible or wait until the content expires normally. Cloud CDN cache servers routinely evict infrequently accessed content to make room for new content. Content that is not accessed for 30 days is removed unconditionally.

Cache invalidations are rate limited.

To learn more about invalidation, see the Cache invalidation overview.

Optimize uploaded file consistency

The following recommended practices help optimize the consistency of file uploads.

Avoid updating existing files

Rather than update existing files, upload new versions.

For new files, use unique names that can include version numbers or dates. Appending a version number (for example, file_v2.css) or a date (for example, file_20230806.js) to the filename helps to ensure that Cloud CDN fetches the correct, updated version. Appending a parameter to the file URL (for example, file.css?v=2) to force a new version to be fetched isn't recommended because this approach doesn't address the risk of caching a nonatomic origin file update, where partial or incomplete files can still be cached.

It's crucial to upload new versions of dependencies before uploading the files that reference them. This practice helps to ensure that all references are to complete, updated files, thereby reducing the risk of serving partially updated or truncated files.

Make atomic updates to files

When updating existing files is necessary, do it atomically.

If a file is accessed and cached before an upload is complete, it might be cached as an incomplete or truncated file. For example, a file, such as /index.html, can't have a unique name but can point to other files that have unique names.

Uploading a file under its target name can result in incomplete files being cached when they are accessed during the upload. Instead, upload the file under a temporary name and rename it to the target name only after the upload is complete. This practice helps ensure that the file is fully and immediately available when referenced.

When existing files are updated, byte-range caching can result in Cloud CDN holding ranges of the earlier file after the new file has been uploaded. If Cloud CDN has cached ranges of the earlier file, requests for missing chunks can lead to partial responses. This happens because Cloud CDN detects that the origin file has changed (because etag or last-modified changes), deletes any stale content, disconnects any in-progress downloads, and generates an error, which prompts the client to retry. To mitigate this problem, issue invalidations for byte-range cached files that are being updated.

Optimize Monitoring and Logging

The following recommended practices help optimize Monitoring and Logging.

Ensure that logging is enabled for Cloud CDN

A best practice for managing Cloud CDN is to ensure that logging is enabled for all Cloud CDN enabled backends.

Use the custom monitoring dashboard for Cloud CDN

To ensure greater reliability and performance, a best practice is to regularly review monitoring metrics related to Cloud CDN. A great place to start is with the Cloud CDN custom monitoring dashboard.

Review third-party performance tests

Review reports from third-party providers, such as the availability, latency, and throughput reports provided by Citrix Radar.