Caching Responses

Overview

IBM Application Gateway (IAG) can cache static web content received from resource servers to increase the performance and response time of requests. By default this caching is disabled, but can be selectively enabled for particular resources.

Before configuring the cache you must understand the key concepts, configuration variables, conditions that affect content caching and the impact of HTTP headers. Caching is always a trade-off between performance and memory utilization. It needs to be tuned for each individual environment to maximize the benefits.

The following diagrams illustrate how the IAG manages the cache during a request flow:

Key caching concepts

Caching web content gives you the flexibility of serving static documents locally from IAG rather than retrieving the content from a back-end resource server for every request. With the content caching feature, you can store commonly accessed web document types in the memory of the IAG server. Clients can experience much faster response to follow up requests for documents that are cached in the IAG server.

Cached content can include static text documents and graphic images. Dynamically generated documents, such as database query results, cannot be cached.

Caching is based on MIME type. Separate caches can be configured for different MIME types to give fine control over the caching of each type of document. The following settings can be used to configure the cache based on MIME type:

• Document MIME type;
• Type of storage (Note: only 'memory' is currently supported on the IAG);
• Size of storage;
• Maximum age if expiry information is missing from the original response.

Configuration of content caching

The IAG can be configured to enable content caching, as shown in the following YAML snippet template:

advanced:
  configuration:
    - stanza: content-cache
      operation: set
      entry: <mime-type>
      value:
        - <cache-type>[:<cache-size>:<def-max-age>]

Please note:

• The asterisk is a wildcard only for a MIME-type directory and its contents.

Example

The following example caches:

• HTML documents ('text/html');
• Images of any type ('image/*');
• A default cache ('*/*') for other object types. The default cache expires after 600 seconds (10 minutes).

advanced:
  configuration:
    - stanza: content-cache
      operation: set
      entry: "text/html"
      value:
        - "memory:2000"
    - stanza: content-cache
      operation: set
      entry: "image/*"
      value:
        - "memory:5000"
    - stanza: content-cache
      operation: set
      entry: "*/*"
      value:
        - "memory:1000:600"

Conditions affecting content caching configuration

The content caching mechanism observes the following conditions:

• Content caching occurs only when a cache is defined in the IAG YAML file;

  • By default, no content caching is enabled by the IAG;
• If you do not specify a default content cache, documents that do not match any explicit cache are not cached;
• Authorization is still performed on all requests for cached information;
• The content caching mechanism does not cache responses to requests that contain query strings.

Impact of HTTP headers on IAG content caching

The following table describes how HTTP Request headers from the client affect whether IAG retrieves the static content from the cache, or proxies the request to the resource server:

The following table describes how HTTP Response headers from the resource server affect the caching of responses:

Note: IAG does not process <meta> tags with http-equiv attributes for Response headers.

Impact of other conditions on IAG content caching

Other conditions that affect IAG content caching include:

• IAG does not cache content if the response from the resource server does not have the status '200 OK';
• IAG does not cache content if the request URL contains a query string;
• IAG does not cache content if the value of the 'Date' header is greater than the value of the 'Expires' header;
• IAG flushes a cache entry if a PUT, POST, or DELETE method is used in the URL;
• IAG only returns values from the cache for HEAD and GET requests;
• IAG regularly sends HEAD requests to resource servers to check that they are running. The response to these health checks are not cached;
• IAG does not cache the response if user-identifying data is passed to the resource server, unless the response has an Expires header.