Weighted capacity selection

Figure 5.3: Average response time with dynamic Weighted policy used at DNS

In weighted capacity selection, each cluster is assigned either a static weight measured off-line (for example server 2 is twice as powerful as server 1 and 3) or may dynamically report about free capacity of clusters to DNS. DNS returns IP address of particular server in proportion to its weight or free capacity as reported by cluster. We implemented dynamic status reporting based weighted selection. To return IP addresses in proportion to their weight following algorithm is used:

1. Generate running sum of weights associated with each cluster
2. Generate random number between one and sum of weights
3. Return cluster having least running sum of weights and having running sum of weights greater than or equal to the generated random number.

This algorithm is used for servers having different capacity and if used with dynamic capacity reporting, it can deal with load skew due to uneven request rate easily.

Average response times reported by webstone is plotted in Figure 5.3.