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Showing content from https://cloud.google.com/cdn/docs/using-ttl-overrides below:

Change TTL settings and overrides | Cloud CDN

This page describes how to change TTL overrides with Cloud CDN. TTL overrides give you fine-grained control over how long Cloud CDN caches your content before revalidating it.

The TTL settings are summarized in the following table.

Default TTL Max TTL Client TTL Reason to configure Improve cache hit rates for content that doesn't change often Force Cloud CDN to revalidate content more often than specified by an origin header Force clients to revalidate content more often against Cloud CDN Used when One of the following is true, for a successful response:

cache mode = FORCE_CACHE_ALL
cache mode = CACHE_ALL_STATIC, the content-type is statically cacheable, and a TTL isn't set by origin headers (max-age, s-maxage, or Expires)

All of the following are true:

cache mode = CACHE_ALL_STATIC
A TTL is set by origin headers (max-age, s-maxage, or Expires)
The TTL specified in the origin header is greater than the max TTL value

One of the following is true:

cache mode = FORCE_CACHE_ALL, and the client TTL value is less than Cloud CDN's TTL
cache mode = CACHE_ALL_STATIC, and the client TTL is less than the TTL set by the origin header (or default TTL if no origin header information is present)

Default value 3600 seconds (1 hour) 86,400 seconds (1 day) 3600 seconds (1 hour) Max allowed value 31,622,400 seconds (1 year) 31,622,400 seconds (1 year) 31,622,400 seconds (1 year) Notes Must be less than or equal to the max TTL

--default-ttl=0

forces the response to be revalidated at the origin Must be less than or equal to the max TTL Before you begin

Read about cache modes.
Ensure that Cloud CDN is enabled; for instructions, see Using Cloud CDN.
If necessary, update to the latest version of the Google Cloud CLI:
```
gcloud components update
```
Note: Make sure that you're using gcloud CLI version 309.0.0 or later.

Set the default TTL

To improve cache hit rates for content that doesn't change often, you can override the default TTL so that Cloud CDN revalidates content at the origin less frequently. Note that infrequently accessed objects might be evicted from the cache before the defined TTL.

When the cache mode is FORCE_CACHE_ALL, the default TTL overwrites the TTL set in all responses (including those that have a TTL set by origin headers). In this mode, the default TTL is visible to clients, because Cloud CDN sets the public and max-age attributes of the responses served to clients.

For CACHE_ALL_STATIC mode, the default TTL applies to cached content served by an origin for responses that don't have an existing valid TTL (max-age, s-maxage, or Expires header). In CACHE_ALL_STATIC mode, the default TTL doesn't modify the Cache-Control header served to clients. To modify the Cache-Control header in the CACHE_ALL_STATIC mode, you must set the client TTL.

When you set the cache mode to use origin headers (USE_ORIGIN_HEADERS), the default TTL value doesn't apply and cannot be set because Cloud CDN uses the max-age or s-maxage origin directive or the Expires header instead.

Console

In the Google Cloud console, go to the Load Balancing page.
Go to Load balancing
Click the name of your external Application Load Balancer.
Click Edit edit.
In Backend configuration, select a backend and click Edit edit.
Make sure that Enable Cloud CDN is selected.
Make sure that the cache mode is Cache static content (recommended) or Force cache all content. Overriding the TTL values isn't supported when the cache mode is Use origin settings based on Cache-Control headers.
Under Default time to live, select a value.
Click Save.

gcloud

For backend services, use the gcloud compute backend-services create or gcloud compute backend-services update command with the --default-ttl flag.

For backend buckets, use the gcloud compute backend-buckets create or gcloud compute backend-buckets update command with the --default-ttl flag.

gcloud compute backend-services (create | update) BACKEND_SERVICE_NAME
    --default-ttl=DEFAULT_TTL

gcloud compute backend-buckets (create | update) BACKEND_BUCKET_NAME
    --default-ttl=DEFAULT_TTL

Replace DEFAULT_TTL with a value up to 31,622,400 seconds (1 year).

API

For backend buckets, use the Method: backendBuckets.insert or Method: backendBuckets.update API call.

For backend services, use the Method: backendServices.insert or Method: backendServices.update API call.

Use one of the following API calls:

POST https://compute.googleapis.com/compute/v1/projects/PROJECT_ID/global/backendBuckets
PUT https://compute.googleapis.com/compute/v1/projects/PROJECT_ID/global/backendBuckets/BACKEND_BUCKET
POST https://compute.googleapis.com/compute/v1/projects/PROJECT_ID/global/backendServices
PUT https://compute.googleapis.com/compute/v1/projects/PROJECT_ID/global/backendServices/BACKEND_SERVICE

Add the following snippet to the JSON request body:

"cdnPolicy": {
  "defaultTtl": DEFAULT_TTL
}

Replace DEFAULT_TTL with a value up to 31,622,400 seconds (1 year).

Set the max TTL

The maximum (max) TTL specifies the maximum allowed TTL within Cloud CDN for cached content served by an origin.

The TTL of a response is capped by the max TTL if one of the following is true:

The response attempts to set a max-age or s-maxage higher than the max TTL value.
The response has an Expires header more than cdnPolicy.maxTtl seconds in the future.

The max TTL setting does not change the max-age value sent to the client; for more information, see Overriding the maximum client TTL. The max TTL setting only affects how long Cloud CDN attempts to cache the content.

This setting is only used when the cache mode is CACHE_ALL_STATIC. The maximum allowed value is 31,622,400 seconds (1 year). Note that infrequently accessed objects might be evicted from the cache before the defined TTL.

With FORCE_CACHE_ALL, the TTL is always set to the default TTL; you can't set a max TTL.

Console

In the Google Cloud console, go to the Load Balancing page.
Go to Load balancing
Click the name of your external Application Load Balancer.
Click Edit edit.
In Backend configuration, select a backend and click Edit edit.
Make sure that Enable Cloud CDN is selected.
Make sure that the cache mode is Cache static content (recommended).
Under Maximum time to live, select a value.
Click Save.

gcloud

For backend services, use the gcloud compute backend-services create or gcloud compute backend-services update command with the --max-ttl flag.

For backend buckets, use the gcloud compute backend-buckets create or gcloud compute backend-buckets update command with the --max-ttl flag.

gcloud compute backend-services (create | update) BACKEND_SERVICE_NAME
    --max-ttl=MAX_TTL

gcloud compute backend-buckets (create | update) BACKEND_BUCKET_NAME
    --max-ttl=MAX_TTL

Replace MAX_TTL with a value up to 31,622,400 seconds (1 year).

API

For backend buckets, use the Method: backendBuckets.insert or Method: backendBuckets.update API call.

For backend services, use the Method: backendServices.insert or Method: backendServices.update API call.

Use one of the following API calls:

POST https://compute.googleapis.com/compute/v1/projects/PROJECT_ID/global/backendBuckets
PUT https://compute.googleapis.com/compute/v1/projects/PROJECT_ID/global/backendBuckets/BACKEND_BUCKET
POST https://compute.googleapis.com/compute/v1/projects/PROJECT_ID/global/backendServices
PUT https://compute.googleapis.com/compute/v1/projects/PROJECT_ID/global/backendServices/BACKEND_SERVICE

Add the following snippet to the JSON request body:

"cdnPolicy": {
  "maxTtl": MAX_TTL
}

Replace MAX_TTL with a value up to 31,622,400 seconds (1 year).

Override the client TTL

For all cache modes, Cloud CDN passes Cache-Control headers to the client.

The client TTL lets you set a maximum TTL for what is sent to browsers or clients so that clients revalidate content against Cloud CDN more often, without requiring revalidation at the origin. This way, content can be invalidated within Cloud CDN when necessary, and browsers can discover that the content is invalidated as soon as the client TTL expires.

In FORCE_CACHE_ALL mode, Cloud CDN normally passes the same max-age to the client that it uses internally for proxy caching; however, if a client TTL is specified and is a smaller value, the client TTL is passed to the client in the max-age directive instead. Similarly, in CACHE_ALL_STATIC mode, the client TTL serves as a clamp for whatever max-age is specified by the origin server so that the max-age sent to the browser or client won't be larger than the configured client TTL. If no max-age is specified by the origin, the lesser of Cloud CDN's TTL and the client TTL value is used as the max-age sent to the browser or client.

If an Expires header is present in the origin response, it is removed and replaced with a Cache-Control: max-age directive with the appropriate TTL. For error responses, if no negative caching TTL is set, the Cache-Control header is also removed.

Because the client TTL is treated as a maximum value for what is sent to browsers or clients, it can't be used to increase the max-age value that would otherwise be sent. If the max-age value that is sent to browsers and clients is lower than you would like, you must increase the max-age directive value in the response returned by the origin, or adjust the default TTL or negative caching settings appropriately.

Typically, a client TTL setting of about one day is a good practical upper bound. A one-day setting means that browsers check in with at least that frequency and can learn about cache invalidations that might have occurred in Cloud CDN. You can set the client TTL much higher (up to a year) as a way of allowing the origin and the configured TTLs to fully control what is sent to the client. This can be useful if you don't want the browser to perform more-frequent validations against Cloud CDN.

Console

In the Google Cloud console, go to the Load Balancing page.
Go to Load balancing
Click the name of your external Application Load Balancer.
Click Edit edit.
In Backend configuration, select a backend and click Edit edit.
Make sure that Enable Cloud CDN is selected.
Make sure that the cache mode is Cache static content (recommended) or Force cache all content. Overriding the TTL values isn't supported when the cache mode is Use origin settings based on Cache-Control headers.
Under Client time to live, select a value up to 1 year.
Click Save.

gcloud

For backend services, use the gcloud compute backend-services create or gcloud compute backend-services update command with the --client-ttl flag.

For backend buckets, use the gcloud compute backend-buckets create or gcloud compute backend-buckets update command with the --client-ttl flag.

gcloud compute backend-services (create | update) BACKEND_SERVICE_NAME
    --client-ttl=CLIENT_TTL

gcloud compute backend-buckets (create | update) BACKEND_BUCKET_NAME
    --client-ttl=CLIENT_TTL

Replace CLIENT_TTL with a value up to 31,622,400 seconds (1 year).

API

For backend buckets, use the Method: backendBuckets.insert or Method: backendBuckets.update API call.

For backend services, use the Method: backendServices.insert or Method: backendServices.update API call.

Use one of the following API calls:

POST https://compute.googleapis.com/compute/v1/projects/PROJECT_ID/global/backendBuckets
PUT https://compute.googleapis.com/compute/v1/projects/PROJECT_ID/global/backendBuckets/BACKEND_BUCKET
POST https://compute.googleapis.com/compute/v1/projects/PROJECT_ID/global/backendServices
PUT https://compute.googleapis.com/compute/v1/projects/PROJECT_ID/global/backendServices/BACKEND_SERVICE

Add the following snippet to the JSON request body:

"cdnPolicy": {
  "clientTtl": CLIENT_TTL
}

Replace CLIENT_TTL with a value up to 31,622,400 seconds (1 year).

What's next

To learn the reasons for serving stale, expired content, see Serve stale content.

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