# 根据 CloudFront Functions 查看器请求事件的 KeyValueStore 配置重写请求 URI
<a name="example_cloudfront_functions_kvs_conditional_read_section"></a>

以下代码示例显示了如何根据 CloudFront Functions 查看器请求事件的 KeyValueStore 配置重写请求 URI。

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#### [ JavaScript ]

**适用于 CloudFront Functions 的 JavaScript 运行时 2.0**  
 查看 GitHub，了解更多信息。查找完整示例，并了解如何在 [CloudFront Functions examples](https://github.com/aws-samples/amazon-cloudfront-functions/tree/main/kvs-conditional-read) 存储库中进行设置和运行。

```
import cf from 'cloudfront'; 

// (Optional) Replace KVS_ID with actual KVS ID
const kvsId = "KVS_ID";
// enable stickiness by setting a cookie from origin or using another edge function
const stickinessCookieName = "appversion";
// set to true to enable console logging
const loggingEnabled = false;
   
// function rewrites the request uri based on configuration in KVS
// example config in KVS in key:value format
// "latest": {"a_weightage": .8, "a_url": "v1", "b_url": "v2"}
// given above key and value in KVS the request uri will be rewritten 
// for example http(s)://domain/latest/something/else will be rewritten as http(s)://domain/v1/something/else or http(s)://domain/v2/something/else depending on weightage
// if no configuration is found, then the request is returned as is
async function handler(event) {
    // NOTE: This example function is for a viewer request event trigger. 
    // Choose viewer request for event trigger when you associate this function with a distribution. 
    const request = event.request;
    const pathSegments = request.uri.split('/');
    const key = pathSegments[1];
    
    // if empty path segment or if there is valid stickiness cookie 
    // then skip call to KVS and let the request continue.
    if (!key || hasValidSticknessCookie(request.cookies[stickinessCookieName], key)) {
        return event.request;
    }

    try {
        // get the prefix replacement from KVS
        const replacement = await getPathPrefixByWeightage(key);
        if (!replacement) {
            return event.request;
        }
        //Replace the first path with the replacement 
        pathSegments[1] = replacement;
        log(`using prefix ${pathSegments[1]}`)
        const newUri = pathSegments.join('/');
        log(`${request.uri} -> ${newUri}`);
        request.uri = newUri;

        return request;
    } catch (err) {
        // No change to the path if the key is not found or any other error
        log(`request uri: ${request.uri}, error: ${err}`);
    }
    // no change to path - return request 
    return event.request;
}

// function to get the prefix from KVS
async function getPathPrefixByWeightage(key) {
    const kvsHandle = cf.kvs(kvsId);
    // get the weightage config from KVS
    const kvsResponse = await kvsHandle.get(key);
    const weightageConfig = JSON.parse(kvsResponse);
    // no configuration - return null    
    if (!weightageConfig || !isFinite(weightageConfig.a_weightage)) {
        return null;
    } 
    // return the url based on weightage
    // return null if no url is configured
    if (Math.random() <= weightageConfig.a_weightage) {
        return weightageConfig.a_url ? weightageConfig.a_url: null;
    } else {
        return weightageConfig.b_url ? weightageConfig.b_url : null;
    }
}

// function to check if the stickiness cookie is valid
function hasValidSticknessCookie(stickinessCookie, pathSegment) {
    // if the value exists and it matches pathSegment
    return (stickinessCookie && stickinessCookie.value === pathSegment)
}

function log(message) {
    if (loggingEnabled) {
        console.log(message);
    }
}
```

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有关 AWS SDK 开发人员指南和代码示例的完整列表，请参阅 [将 CloudFront 与 AWS SDK 配合使用](sdk-general-information-section.md) 本主题还包括有关入门的信息以及有关先前的 SDK 版本的详细信息。