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Configures how cached values should be retrieved at runtime.

This policy is intended for use in general purpose short-term caching. It is used in
conjunction with the [Populate
Cache policy](https://docs.apigee.com/api-platform/reference/policies/populate-cache-policy) (for writing entries) and the [Invalidate Cache policy](https://docs.apigee.com/api-platform/reference/policies/invalidate-cache-policy) (for invalidating entries).

For caching the responses of backend resources, see the
[Response Cache policy](https://docs.apigee.com/api-platform/reference/policies/response-cache-policy).

## Element reference

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

```
<LookupCache async="false" continueOnError="false" enabled="true" name="Lookup-Cache-1">
    <DisplayName>Lookup Cache 1</DisplayName>
    <Properties/>
    <CacheKey>
        <Prefix/>
        <KeyFragment ref=""/>
    </CacheKey>
    <!-- Omit this element if you're using the included shared cache. -->
    <CacheResource/>
    <CacheLookupTimeoutInSeconds/>
    <Scope>Exclusive</Scope>
    <AssignTo>flowVar</AssignTo>
</LookupCache>
```

A shared cache is included by default. To use the shared cache, omit the
`<CacheResource>` element in this policy configuration.

For more about the underlying data store, see [Cache internals](https://docs.apigee.com/api-platform/cache/cache-internals). For more about configuring
caches, see [Creating and editing an
environment cache](https://docs.apigee.com/api-platform/cache/manage-caches-environment).

## \<LookupCache\> 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. | 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 |
|---|---|

## \<AssignTo\> element

Specifies the variable where the cache entry is assigned after it has been retrieved from the
cache. The variable must be writable. If the cache lookup doesn't retrieve a value, the variable
will not be set.

```
<AssignTo>*variable_to_receive_cached_value*</AssignTo>
```

|---|---|
| **Default:** | N/A |
| **Presence:** | Required |
| **Type:** | String |

## \<CacheKey\> element

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

```
<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](https://docs.apigee.com/api-platform/reference/policies/working-cachekeys).

## \<CacheLookupTimeoutInSeconds\> element

Specifies the number of seconds after which an unsuccessful cache lookup will be considered a
cache miss. If this occurs, flow resumes along the cache-miss path.

```
<CacheLookupTimeoutInSeconds>30</CacheLookupTimeoutInSeconds>
```

|---|---|
| **Default:** | 30 |
| **Presence:** | Optional |
| **Type:** | Integer |

## \<CacheResource\> element

Specifies the cache where messages should be stored.

Omit this element completely if this policy (and your corresponding PopulateCache 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 [Creating and editing an environment
cache](https://docs.apigee.com/api-platform/cache/manage-caches-environment).

## \<CacheKey\>/\<KeyFragment\> element

Specifies a value that should be included in the cache key, creating a namespace for matching
requests to cached responses.

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

|---|---|
| **Default:** | N/A |
| **Presence:** | Optional |
| **Type:** | N/A |

This can be a key (a static name that you provide) or a value (a dynamic entry set by
referencing a variable). All specified fragments combined (plus the prefix) are concatenated to
create the cache key.

```
<KeyFragment>apiAccessToken</KeyFragment>
<KeyFragment ref="request.queryparam.client_id" />
```

You use the `<KeyFragment>` element in conjunction with
`<Prefix>` and `<Scope>`. For more information, see [Working with cache keys](https://docs.apigee.com/api-platform/reference/policies/working-cachekeys).

### 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 value to use as a cache key prefix.

```
<Prefix>prefix_string</Prefix>
```

|---|---|
| **Default:** | N/A |
| **Presence:** | Optional |
| **Type:** | String |

Use this value instead of `<Scope>` when you want to specify your own value
rather than a `<Scope>` -enumerated value. If defined,
`<Prefix>` prepends the cache key value for entries written to the cache. A
`<Prefix>` element value overrides a `<Scope>` element
value.

You use the `<Prefix>` element in conjunction with
`<CacheKey>` and `<Scope>`. For more information, see [Working with cache keys](https://docs.apigee.com/api-platform/reference/policies/working-cachekeys).

## \<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__**application**Name__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](https://docs.apigee.com/api-platform/reference/policies/working-cachekeys).

### 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__**application**Name*. |
| `Proxy` | ProxyEndpoint configuration is used as the prefix. Cache key is prepended in the form *orgName__envName__**application**Name__proxyEndpointName* . |
| `Target` | TargetEndpoint configuration is used as the prefix. Cache key prepended in the form *orgName__envName__**application**Name__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__**application**Name__proxyNameITargetName* For example, the full string might look like this: ``` apifactory__test__weatherapi__16__default__apiAccessToken ``` . |

## Usage notes

Use this policy for general-purpose caching. At runtime, the LookupCache policy retrieves a
value from the cache, assigning the value to the variable you specify with the AssignTo element
(if no value is retrieved, the variable will not be set). It looks for the value based on a cache
key created through configuration that combines the CacheKey and Scope elements. In other words,
to retrieve a particular value added to the cache by a PopulateCache policy, your LookupCache
policy must have cache key-related elements configured in the same way as the PopulateCache
policy.

General purpose caching with the [Populate Cache policy](https://docs.apigee.com/api-platform/reference/policies/populate-cache-policy),
LookupCache policy, and [InvalidateCache policy](https://docs.apigee.com/api-platform/reference/policies/invalidate-cache-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 the default cache, simply omit
the `<CacheResource>` element.

For more about configuring caches, see [Creating and editing an environment
cache](https://docs.apigee.com/api-platform/cache/manage-caches-environment). For more about the underlying data store, see [Cache internals](https://docs.apigee.com/api-platform/cache/cache-internals).

## Flow variables

Flow variables can be used to configure dynamic runtime behavior for policies and flows, based
on HTTP headers or message content, or the context available in the Flow. For more information
about flow variables, see [Variables
reference](https://docs.apigee.com/api-platform/reference/variables-reference).

The following predefined Flow variables are available after you customize the behavior of the
cache you define in a LookupCache policy.

| Variables | Type | Permission | Description |
|---|---|---|---|
| lookupcache.{policy-name}.cachename | String | Read-Only | Returns the cache name used in the policy. |
| lookupcache.{policy-name}.cachekey | String | Read-Only | Returns the key used. |
| lookupcache.{policy-name}.cachehit | Boolean | Read-Only | True if the policy found a value for the specified cache key. |
| lookupcache.{policy-name}.assignto | String | Read-Only | Returns the variable to which cache is assigned. |

## Error codes

This section describes the error messages and flow variables that are set when this policy
triggers an error. This information is important to know if you are developing fault rules for a
proxy. To learn more, see [What you need to know
about policy errors](https://docs.apigee.com/api-platform/fundamentals/what-you-need-know-about-policy-errors) and [Handling
faults](https://docs.apigee.com/api-platform/fundamentals/fault-handling).

### Error code prefix

N/A

### Runtime errors

This policy does not throw any runtime errors.

### Deployment errors

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

> [!IMPORTANT]
> Need help resolving an error? Click in the Fix column for detailed troubleshooting information.

| Error name | Cause | Fix |
|---|---|---|
| `InvalidCacheResourceReference` | This error occurs if the `<CacheResource>` element is set to a name which does not exist in the environment where the API proxy is being deployed. | [](https://docs.apigee.com/api-platform/troubleshoot/policies/deployment/lookup-cache-deployment-errors#invalidcacheresourcereference) |
| `InvalidTimeout` | If the `<CacheLookupTimeoutInSeconds>` element is set to a negative number, then the deployment of the API proxy fails. | [](https://docs.apigee.com/api-platform/troubleshoot/policies/deployment/lookup-cache-deployment-errors#invalidtimeout) |
| `CacheNotFound` | This error occurs if the specific cache mentioned in the error message has not been created on a specific Message Processor component. | [](https://docs.apigee.com/api-platform/troubleshoot/policies/deployment/lookup-cache-deployment-errors#cachenotfound) |

### Fault variables

N/A

### Example error response

N/A