從 Amazon CloudFront 的公告看到的東西：「Amazon CloudFront now supports Common Media Client Data (CMCD) fields in real-time logs」。
CMCD (Common Media Client Data) 是一個私有的 log format，可以讓 client 透過 HTTP(S) header 回報狀態，所以 AWS 這邊有提到你可以設定讓 CloudFront 記錄下來，然後透過其他軟體分析：
Previously, CloudFront logged CMCD parameters as part of the full query string log field, or the HTTP headers log field. Now, you can simply select to include specific CMCD parameters in your real-time logs and save on compute needed to search and extract the CMCD key-value pairs, and reduce the data set for your log analysis.
現在 CloudFront 則是支援判讀做一些處理：
Starting today, you can enable Common Media Client Data (CMCD) fields in your CloudFront real-time logs. You can select key client-side performance parameters and CloudFront delivery performance parameters in the same log record. This can help you correlate variations in Quality of Experience (QoE) for your viewers to CloudFront performance at the granularity of single viewer sessions, simplifying the troubleshooting of QoE issues that impact your viewers engagement.
看了一下就是定義一些欄位，用 JSON 包起來後讓 client 打回伺服器端，可以利用這些資訊動態從伺服器端調整策略，而不像以前主要的調整策略都放在 client 端。
