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PopulateCache policy

Extensible policy This page applies to Apigee and Apigee hybrid.

View Apigee Edge documentation.

The PopulateCache policy configures how cached values should be written at runtime.

The Populate Cache policy is designed for writing entries in a short-term general-purpose cache. It's used in conjunction with the Lookup Cache policy (for reading cache entries) and the Invalidate Cache policy (for invalidating entries).

This policy is an Extensible policy and use of this policy might have cost or utilization implications, depending on your Apigee license. For information on policy types and usage implications, see Policy types.

For caching the responses of backend resources, see the Response Cache policy.

Element reference

The following lists the elements you can configure on this policy.

<PopulateCache async="false" continueOnError="false" enabled="true" name="Populate-Cache-1">
    <DisplayName>Populate Cache 1</DisplayName>
    <Properties/>
    <CacheKey>
        <Prefix/>
        <KeyFragment ref=""/>
    </CacheKey>
    <!-- Omit this element if you're using the included shared cache. -->
    <CacheResource/>
    <Scope>Exclusive</Scope>
    <ExpirySettings>
        <TimeoutInSeconds>300</TimeoutInSeconds>
    </ExpirySettings>
    <Source>flowVar</Source>
</PopulateCache>

<PopulateCache> attributes

The following table describes attributes that are common to all policy parent elements:

Attribute Description Default Presence
name

The internal name of the policy. The value of the name attribute can contain letters, numbers, spaces, hyphens, underscores, and periods. This value cannot exceed 255 characters.

Optionally, use the <DisplayName> element to label the policy in the management UI proxy editor with a different, natural-language name.

 N/A Required
continueOnError

Set to false to return an error when a policy fails. This is expected behavior for most policies.

Set to true to have flow execution continue even after a policy fails. See also:

 false Optional
enabled

Set to true to enforce the policy.

Set to false to turn off the policy. The policy will not be enforced even if it remains attached to a flow.

 true Optional
async

This attribute is deprecated.

 false Deprecated

<DisplayName> element

Use in addition to the name attribute to label the policy in the management UI proxy editor with a different, natural-language name.

<DisplayName>Policy Display Name</DisplayName>
Default

N/A

If you omit this element, the value of the policy's name attribute is used.
Presence Optional
Type String

<CacheKey> element

Configures a unique pointer to a piece of data stored in the cache.

Cache keys are limited to a size of 2 KB.

<CacheKey>
    <Prefix>string</Prefix>
    <KeyFragment ref="variable_name" />
    <KeyFragment>literal_string</KeyFragment>
</CacheKey>

Default:

N/A

Presence:

 Required

Type:

N/A

<CacheKey> constructs the name of each piece of data stored in the cache.

At runtime, <KeyFragment> values are prepended with either the <Scope> element value or <Prefix> value. For example, the following results in a cache key of UserToken__apiAccessToken__<value_of_client_id>:

<CacheKey>
    <Prefix>UserToken</Prefix>
    <KeyFragment>apiAccessToken</KeyFragment>
    <KeyFragment ref="request.queryparam.client_id" />
</CacheKey>

You use the <CacheKey> element in conjunction with <Prefix> and <Scope>. For more information, see Working with cache keys.

<CacheResource> element

Specifies the cache where messages should be stored.

Omit this element completely if this policy (and your corresponding LookupCache and InvalidateCache policies) is using the included shared cache.

<CacheResource>cache_to_use</CacheResource>

Default:

N/A

Presence:

 Optional

Type:

String

For more about configuring caches, see General purpose caching.

<CacheKey>/<KeyFragment> element

Specifies a value that should be included in the cache key. Specify a variable to de-reference with the ref attribute, or a fixed value.

<KeyFragment ref="variable_name"/>
<KeyFragment>literal_string</KeyFragment>

Default:

N/A

Presence:

 zero or more

Type:

N/A

At runtime, Apigee creates the cache key by prepending the value obtained from the <Scope> element or the <Prefix> element, to a concatenation of the resolved values of each of the <KeyFragment> elements. For more information, see Working with cache keys.

Attributes

Attribute Type Default Required Description
ref string No

The variable from which to get the value. Should not be used if this element contains a literal value.

<CacheKey>/<Prefix> element

Specifies a fixed value to use as a cache key prefix.

<Prefix>prefix_string</Prefix>

Default:

N/A

Presence:

 Optional

Type:

String

A <Prefix> element overrides any <Scope> element.

At runtime, Apigee creates the cache key by prepending the value obtained from the <Scope> element or the <Prefix> element, to a concatenation of the resolved values of each of the <KeyFragment> elements. For more information, see Working with cache keys.

<ExpirySettings> element

Specifies when a cache entry should expire.

<ExpirySettings>
  <!-- use exactly one of the following child elements -->
  <TimeoutInSeconds ref="duration_variable">seconds_until_expiration</TimeoutInSeconds>
  <ExpiryDate ref="date_variable">expiration_date</ExpiryDate>
  <TimeOfDay ref="time_variable">expiration_time</TimeOfDay>
</ExpirySettings>

Default:

N/A

Presence:

 Required

Type:

N/A

Child elements of <ExpirySettings>

Use exactly one child element. The following table provides a description of the child elements of <ExpirySettings>:

Child Element Description <TimeoutInSeconds>

The number of seconds after which a cache entry should expire. 

<ExpirySettings>
  <TimeoutInSeconds ref="var-containing-duration">expiry</TimeoutInSeconds>
</ExpirySettings>

This element replaces the now-deprecated TimeoutInSec element.

<ExpiryDate>

Specifies the date on which a cache entry should expire. Specify a string in the form mm-dd-yyyy.

<ExpirySettings>
  <ExpiryDate ref="var-containing-date">expiry</ExpiryDate>
</ExpirySettings>

If the date specified is in the past, the policy will apply the maximum time-to-live to the cached entry. This maximum is 30 days.

<TimeOfDay>

Specifies the time of day on which a cache entry should expire. Specify a string in the form HH:mm:ss, where HH represents the hour on a 24-hour clock, in the UTC time zone. For example, 14:30:00 implies 2:30 in the afternoon.

<ExpirySettings>
  <TimeOfDay ref="var-containing-time">expiry</TimeOfDay>
</ExpirySettings>

You should specify only one of the possible child elements. If you specify multiple elements, the order of precedence is:TimeoutInSeconds, ExpiryDate, TimeOfDay.

With each of the above child elements of <ExpirySettings>, if you specify the optional ref attribute on the child element, the policy will retrieve the expiry value from the named context variable. If the variable is not defined, the policy uses the literal text value of the child element.

<Scope> element

Enumeration used to construct a prefix for a cache key when a <Prefix> element is not provided in the <CacheKey> element.

<Scope>scope_enumeration</Scope>

Default:

"Exclusive"

Presence:

 Optional

Type:

String

The <Scope> setting determines a cache key that is prepended according to the <Scope> value. For example, a cache key would take the following form when scope is set to Exclusive:

orgName__envName__apiProxyName__proxy|TargetName__ [ serializedCacheKey ]

If a <Prefix> element is present in <CacheKey>, it supersedes a <Scope> element value. Valid values include the enumerations below.

You use the <Scope> element in conjunction with <CacheKey> and <Prefix>. For more information, see Working with cache keys.

Acceptable values

Global

Cache key is shared across all API proxies deployed in the environment. Cache key is prepended in the form orgName __ envName __.

If you define a <CacheKey> entry with the <KeyFragment> apiAccessToken and a <Global> scope, each entry is stored as orgName__envName__apiAccessToken, followed by the serialized value of the access token. For an API proxy deployed in an environment called 'test' in an organization called 'apifactory', access tokens would be stored under the following cache key: apifactory__test__apiAccessToken.
Application

API proxy name is used as the prefix.

Cache key is prepended in the form orgName__envName__apiProxyName.
Proxy

ProxyEndpoint configuration is used as the prefix.

Cache key is prepended in the form orgName__envName__apiProxyName__proxyEndpointName .
Target

TargetEndpoint configuration is used as the prefix.

Cache key prepended in the form orgName__envName__apiProxyName__targetEndpointName .
Exclusive

Default. This is the most specific, and therefore presents minimal risk of namespace collisions within a given cache.

Prefix is one of two forms:

  • If the policy is attached to the ProxyEndpoint flow, prefix is of the form ApiProxyName_ProxyEndpointName.
  • If the policy is attached at TargetEndpoint, prefix is of the form ApiProxyName_TargetName.

Cache key prepended in the form orgName__envName__apiProxyName__proxyNameITargetName

For example, the full string might look like this:

apifactory__test__weatherapi__default__apiAccessToken
.

<Source> element

Specifies the variable whose value should be written to the cache.

<Source>source_variable</Source>

Default:

N/A

Presence:

 Required

Type:

String

Usage notes

Use this policy for general purpose caching. At runtime, the <PopulateCache> policy writes data from the variable you specified in the <Source> element to the cache you specified in the <CacheResource> element. You can use the <CacheKey>, <Scope>, and <Prefix> elements to specify a key that you can use from the <LookupCache> policy to retrieve the value. Use the <ExpirySettings> element to configure when the cached value should expire.

General purpose caching with the PopulateCache policy, LookupCache policy, and InvalidateCache policy uses either a cache you configure or a shared cache that's included by default. In most cases, the underlying shared cache should meet your needs. To use this cache, simply omit the <CacheResource> element.

Cache limits: Various cache limits apply, such as name and value size, total number of caches, the number of items in a cache, and expiration.

For more about the underlying data store, see Cache internals.

About cache encryption

Apigee and Apigee hybrid (version 1.4 and later): Cache and KVM data are always encrypted.

Error codes

This section describes the fault codes and error messages that are returned and fault variables that are set by Apigee when this policy triggers an error. This information is important to know if you are developing fault rules to handle faults. To learn more, see What you need to know about policy errors and Handling faults.

Runtime errors

These errors can occur when the policy executes.

Fault code HTTP Status Occurs when
policies.populatecache.EntryCannotBeCached 500 An entry cannot be cached. The message object being cached is not an instance of a class that is Serializable.

Deployment errors

These errors can occur when you deploy a proxy containing this policy.

Error name Cause Fix
InvalidCacheResourceReference This error occurs if the <CacheResource> element in the PopulateCache policy is set to a name that does not exist in the environment where the API proxy is being deployed.
CacheNotFound The cache specified in the <CacheResource> element does not exist.

Fault variables

These variables are set when this policy triggers an error. For more information, see What you need to know about policy errors.

Variables Where Example
fault.name="fault_name" fault_name is the name of the fault, as listed in the Runtime errors table above. The fault name is the last part of the fault code. fault.name = "EntryCannotBeCached"
populatecache.policy_name.failed policy_name is the user-specified name of the policy that threw the fault. populatecache.POP-CACHE-1.failed = true

Example error response

{
  "fault": {
    "faultstring": "[entry] can not be cached. Only serializable entries are cached.",
    "detail": {
      "errorcode": "steps.populatecache.EntryCannotBeCached"
    }
  }
}

Example fault rule

<FaultRule name="Populate Cache Fault">
    <Step>
        <Name>AM-EntryCannotBeCached</Name>
        <Condition>(fault.name Matches "EntryCannotBeCached") </Condition>
    </Step>
    <Condition>(populatecache.POP-CACHE-1.failed = true) </Condition>
</FaultRule>