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Manual Chapter: Configuring Lifetime Rules
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The length of time that the WebAcceleratorTM system keeps compiled content in its cache before refreshing it is called content lifetime. Content lifetime is expressed in the form of a time to live (TTL) value, and can vary for each cached response. When content is in cache longer than its TTL value, the WebAccelerator system considers the content expired. When the WebAccelerator system receives a request for expired content, it sends that request to the origin web servers for fresh content, replaces the expired cached content with the fresh response, and then responds to the request.
Lifetime rules are not relevant to requests. The WebAccelerator system applies lifetime rules only to responses that it receives from the origin web server. You can define how the WebAccelerator system manages cached content for responses, through the following lifetime rule options.
Header Lifetime Options
These options specify whether the WebAccelerator system honors the TTL values provided with the headers sent from the origin web server, and whether the WebAccelerator system should honor any existing no-cache directives. See Defining Header Lifetime Option settings, for more information.
WebAccelerator Cache Settings
These settings specify how long the WebAccelerator system retains cached content, how long the WebAccelerator system serves cached content if the origin web server is not available, and when to expire cached content. See Configuring the WebAccelerator Cache Settings, for more information.
Client Cache Settings
These settings specify whether the client browser should store content locally and if so, the maximum time the browser should store content. See Configuring the Client Cache Settings, for more information.
These settings you configure apply to any HTTP response that matches to a leaf node for a specific acceleration policy, for which the option is set.
The options and parameters that the WebAccelerator system uses to manage caching are collectively called lifetime mechanisms. The WebAccelerator system uses the lifetime mechanisms to determine the TTL value for compiled responses. Because the WebAccelerator system manages cached responses by using the parameters for different lifetime mechanism simultaneously, it obeys the following precedence if any of the lifetime mechanism parameters conflict.
If the Obey ESI max age if present setting is enabled, the TTL value indicated by the ESI Surrogate-Control headers max-age directive supersedes all other TTL values, provided that the ESI Surrogate-Control headers max-age directive does not violate the boundary conditions described in the previous bullet.
The Use HTTP lifetime headers if present setting is enabled
An ESI Surrogate-Control headers max-age directive is not included in the header
If the Use HTTP lifetime headers if present setting is not enabled, or if HTTP lifetime headers are not otherwise available in the response, then the WebAccelerator system uses the Maximum Age value specified in the WebAccelerator Cache Settings section of the lifetime rule.
Tip: Before you consider changing the WebAccelerator Cache Settings, determine whether using invalidations rules to invalidate specific content would be more appropriate for your application. For information about configuring invalidation rules, see Chapter 10, Configuring Invalidations Rules.
1.
In the navigation pane, expand WebAccelerator and click Policies.
2.
On the User-defined Acceleration Policies table, click the name of the acceleration policy that you want to edit.
5.
Click the Save save button.
For a new or modified acceleration policy to be in effect for your site, you must publish it. One way to do this is to click the Publish button.
You can specify whether the WebAccelerator system honors certain parameters contained in HTTP headers. To do this, enable or disable the following options from the Header Lifetime Options area of the lifetime screen.
1.
In the navigation pane, expand WebAccelerator and click Policies.
2.
On the User-defined Acceleration Policies table, click the name of the acceleration policy that you want to edit.
5.
Click the Save save button.
When responding to sites that use Edge Side Includes (ESI) for assembly, the origin web server sends the WebAccelerator system ESI Surrogate-Control headers, along with the HTTP response headers.
If you enable the HTTP lifetime header option, Obey ESI max-age headers if present, the WebAccelerator system uses the max-age directive included in the ESI Surrogate-Control headers as the TTL value for compiled responses.
The HTTP Cache-Control version 1.1 header specification identifies headers that are used to control web entities, such as the WebAccelerator system. Clients that are HTTP version 1.1 compliant are capable of providing request headers that contain directives that control caching behavior.
If the Use HTTP lifetime headers if present option is enabled, the WebAccelerator system obeys any HTTP Cache-Control header directives configured for the client. If this option is enabled and the client includes a HTTP Cache-Control header directive that indicates the client is not willing to accept content served from a cache, the WebAccelerator system is required to refresh the corresponding cached content for the request, even if that content has not yet expired.
If you want to use HTTP lifetime headers, but want to be able to serve valid content that the WebAccelerator system has in its cache when applicable, you can enable the associated option, Ignore no-cache HTTP headers in the request.
Similarly, if the origin web server provides content that contains an HTTP Cache-Control header that indicates the response should not be cached, the WebAccelerator system is required to send subsequent requests to the origin server for fresh content, even if the content is has cached is still valid. If you want to use HTTP lifetime headers, but want to be able to serve valid content that the WebAccelerator system has in its cache when applicable, you can enable the associated option, Ignore no-cache HTTP headers in the response.
You can specify how the WebAccelerator system manages cached content by configuring values for the following options, in the WebAccelerator Systems Cache Settings area of the lifetime screen.
1.
In the navigation pane, expand WebAccelerator and click Policies.
2.
On the User-defined Acceleration Policies table, click the name of the acceleration policy that you want to edit.
5.
Click the Save save button.
The value for the WebAccelerator Cache Settings Maximum Age option dictates how long the WebAccelerator system stores cached content. Once that limit is met, the WebAccelerator system requests fresh content from the origin web server.
If you have certain content that rarely changes, you can increase the amount of time that the WebAccelerator system stores that content. This reduces the load on your origin web server and increases the perceived performance of your site.
The value for the WebAccelerator Cache Settings Stand-in Period option identifies how long the WebAccelerator system continues to serve content from its cache, if the origin web server does not respond to the WebAccelerator systems requests for fresh content.
If the WebAccelerator system cannot retrieve fresh content from the origin web server after the stand-in period expires, it responds to subsequent requests for that content with an HTTP 404 (page not found) response. The default stand-in period is 0. If you do not specify a stand-in period, the WebAccelerator system immediately responds with an HTTP 404 response code, if content expires and the WebAccelerator system cannot obtain fresh content from the origin web servers.
The value for the WebAccelerator Cache Settings HTTP Lifetime Heuristic option specifies a percentage of time on which the WebAccelerator system calculates the lifetime for cached content from the last time it was refreshed. To determine the lifetime based on this setting, the WebAccelerator system reviews the value for the HTTP LAST_MODIFIED response header, and computes the cache expiration according to the percentage defined for the HTTP Lifetime Heuristic option. The formula for this calculation is:

((current time - HTTP LAST_MODIFIED response header = X) * (lifetime heuristic)) + (current time) = content expiration
For example if the HTTP LAST_MODIFIED response header specifies that the object was last modified at 9:00 a.m., the current time is 11:00 a.m., and the HTTP Lifetime Heuristic option is set to a value of 50%, then the content expiration is 12:00 p.m.
The HTTP Lifetime Heuristic option is only in effect if you are using HTTP headers to identify content lifetime. Use this setting only if you want to use the HTTP LAST_MODIFIED response header to set compiled response TTL values.
You can specify how the client browser caches content by configuring the following options, in the Client Cache Settings area of the lifetime screen.
1.
In the navigation pane, expand WebAccelerator and click Policies.
2.
On the User-defined Acceleration Policies table, click the name of the acceleration policy that you want to edit.
5.
Click the Save save button.
When you select the Client Cache Settings Do not change option, the WebAccelerator system directs the client browser to store content locally on the client, in accordance with the cache settings that are defined in the HTTP headers sent from the origin web server.
The value defined for the Client Cache Setting Maximum Age option dictates how long the client browser stores content locally on the client. The WebAccelerator system uses the Maximum Age value to override any HTTP Cache-Control Expires headers and max-age directives that the origin web server sends to the client. The WebAccelerator overrides the max-age directive only if the new value for the max-age is greater than the value supplied by the origin web server.
Note: When a user clicks the browsers Refresh button, most browsers update their cache with fresh content immediately, even before the value defined for the Maximum Age setting is met.
The Client Cache Settings Maximum Age value applies even if the WebAccelerator system has invalidated the compiled response in its cache (see Chapter 10, Configuring Invalidations Rules). Do not increase the Client Cache Settings Maximum Age value unless there is an acceptable trade off between the freshness of the content, and overall site performance.
Important: If the assembly rules Intelligent Browser Referencing option is enabled, then the WebAccelerator system ignores the Client Cache Settings Maximum Age value for any objects it loads through the Intelligent Browser referencing feature. The top-level HTML page still uses the Client Cache Settings Maximum Age value, if defined. See Using parameter value substitution, for more information.
When the Client Cache Settings Insert no-cache directive into header setting is selected, the WebAccelerator system inserts a no-cache directive into the HTTP Cache-Control header that is returned from the origin web server. This header instructs the client browser not to cache content.
This section of the chapter provides information about how to configure an example lifetime rule. For this example site, you have three top-level nodes on the Policy Tree as follows:
Home
This branch node specifies the rules related to the home page.
Applications
This branch node specifies the rules related to the applications for the site, with the following leaf nodes:
Default
This leaf node specifies the rules related to non-search related applications.
Search
This leaf node specifies the rules related to your sites search application.
Images
This branch node specifies the rules related to graphics images.
For this example, you configure the acceleration policys lifetime rules for the specified nodes on the Policy Tree, as described in Table 9.1.
You change your sites content approximately every 4 hours. You want the WebAccelerator system to cache content for no longer than 24 hours. If the origin web servers are not responding for request for fresh content, you are willing to allow the WebAccelerator system to serve content that is 8 hours old (or twice the age of the content).
To ensure that you can manage content invalidation, you do not want to rely solely on the browsers local cache settings for the home node, so you do not have a minimum time set for content residing in the browser cache before performing a check for content freshness.
Create a lifetime rule for the Home node and in the WebAccelerator Cache Settings section, specify a Maximum Age of 24 hours and a Stand-in Period of 8 hours. Leave all other options at the default settings.
The content served for your general applications changes about once every 4 hours. You use an invalidations rule to force a refresh when content changes, but you do not want content to remain in the WebAccelerator systems cache for more than 5 hours without a refresh
If the origin web servers are not responding to the WebAccelerator systems refresh requests, you are willing to allow the WebAccelerator system to serve content that is 8 hours old (or twice the age of the content).
Create a lifetime rule for the Default leaf node and in the WebAccelerator Cache Settings section, specify a Maximum Age of 5 hours and a Stand-in Period of 8 hours. Leave all other options at the default settings.
Your search application returns data that has various expiration times; some content expires in as little as 10 minutes, and some content expires at 8 hours. You intend to use the HTTP Cache-Control Expire header max-age directive to identify the cache time for content served by the search application.
Since search applications change more frequently than the rest of the site content, you are willing to allow the WebAccelerator system to serve search content that is 2 hours old.
Create a lifetime rule for the Search leaf node and in Header Lifetime Options section, enable the Use HTTP lifetime headers if present option. In the WebAccelerator Cache Settings section, specify a Maximum Age of 8 hours and a Stand-in Period of 10 hours. Leave all other options at the default settings
You change the images for your applications approximately every 4 hours. You want the WebAccelerator system to cache images for no longer than 24 hours and if the origin web servers are not responding to the WebAccelerator systems refresh requests, you are willing to allow the WebAccelerator system to serve images that are 8 hours old (or twice the age of the image).
Create a lifetime rule for the Image node and and in the WebAccelerator Cache Settings section, specify a Maximum Age of 24 hours and a Stand-in Period of 8 hours. In the Client Cache Setting section, specify a Maximum Age of 4 days. Leave all other options at the default settings.
1.
In the navigation pane, expand WebAccelerator and click Policies.
3.
Click the Home node.
4.
From the Matching Rules list on the Policy Editor menu bar, select Acceleration Rules.
5.
Click Lifetime.
a)
Clear the Obey ESI max age if present check box.
b)
Clear the Use HTTP lifetime headers if present check box.
a)
In the Maximum Age box, type 24 and select Hours from the associated list.
b)
In the Stand-in Period box, type 8 and select Hours from the associated list.
8.
Click the Save button.
1.
Click the Default leaf node.
a)
Clear the Obey ESI max age if present box.
b)
Clear the Use HTTP lifetime headers if present box.
a)
In the Maximum Age box, type 5 and select Hours from the associated list.
b)
In the Stand-in Period box, type 8 and select Hours from the associated list.
4.
Click the Save button.
1.
Click the Search leaf node.
a)
Clear the Obey ESI max age if present box.
b)
Clear the Ignore no-cache HTTP headers in the request box.
c)
Clear the Ignore no-cache HTTP headers in the response box.
a)
In the Maximum Age box, type 8 and select Hours from the associated list.
b)
In the Stand-in Period box, type 2 and select Hours from the associated list.
4.
Click the Save button.
1.
Click the Image node.
a)
Clear the Obey ESI max age if present box.
b)
Clear the Use HTTP lifetime headers if present box.
a)
Select Maximum Age.
The Maximum Age options display.
b)
In the Maximum Age box, type 4 and select Days from the associated list.
a)
In the Maximum Age box, type 5 and select Hours from the associated list.
b)
In the Stand-in Period box, type 8 and select Hours from the associated list.
5.
Click the Save button.
For a new or modified acceleration policy to be in effect for your site, you must publish it. One way to do this is to click the Publish button.
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