6.2 Metrics Evaluating the Web Service Caching Efficiency

Real clients of a web service may benefit from the presence of network and browser caches, which can significantly reduce their perceived response time. However, none of the existing performance measurement techniques provide any information on the impact of caches on web services: what percentage of the files and bytes are delivered from the server comparing with the total files and bytes required for delivering the web service. This impact can only be partially evaluated from web server logs by checking response status code 304, whose corresponding requests are sent by the network caches to validate whether the cached object has been modified. If the status code 304 is set, the cached object is not expired and need not be retrieved again.

To evaluate the caching efficiency of a web service, we introduce two metrics: server file hit ratio and server byte hit ratio for each web page.

For a web page P, assume the objects composing the page are O₁, ..., O_n. Let Size(O_i) denote the size of object O_i in bytes. Then we define NumFiles(P) = n and $Size(P) = \sum_{j=1}^{n} {Size(O_j)}.$

Additionally, for each access P_accessⁱ of the page P, assume the objects retrieved in the access are O₁ⁱ, ..., O_kiⁱ, we define NumFiles(P_accessⁱ) = k_i and $Size(P_{access}^{i}) = \sum_{j=1}^{k_i} {Size(O_j^{i})}.$ First, we define file hit ratio and byte hit ratio for each page access in the following way:

FileHitRatio(P_accessⁱ) = NumFiles(P_accessⁱ) / NumFiles(P),

ByteHitRatio(P_accessⁱ) = Size(P_accessⁱ) / Size(P).

Let P_access¹, ..., P_access^N be all the accesses to the page P during the observed time interval. Then

$$ServerFileHitRatio(P) ={{1}\over{N}}{\sum_{k\leq N} {FileHitRatio(P_{access}^{k})}},$$

$$ServerByteHitRatio(P) ={{1}\over{N}}{\sum_{k\leq N}{ByteHitRatio(P_{access}^{k})}}.$$

The lower numbers for server file hit ratio and server byte hit ratio indicate the higher caching efficiency for the web service, i.e., more files and bytes are served from network and client browser caches.