Streaming CTR Computation
Click-through rate (CTR) represents the ratio of the number of clicks divided by the number of impressions
When sufficient historical data is available, CTR is a good estimate of the probability that user will click on the item
S4 is used to measure CTR in real-time for each query-ad combination
To compute CTR in this manner - we need to route click and serve events using a key composed of the query and ad IDs
If the click payload doesnt include query and ad information, we need to do a join by serve ID prior to routing the events with query-ad as the key
Once joined, the events must pass-through a bot filter
Finally, serves and clicks are aggregated to compute CTR