In order to update counters, we’ll need to store the actual counter information. For each counter and precision, like site hits and 5 seconds, we’ll keep a HASH that stores information about the number of site hits that have occurred in each 5-second time slice. The keys in the hash will be the start of the time slice, and the value will be the number of hits. Figure 5.1 shows a selection of data from a hit counter with 5-second time slices.
As we start to use counters, we need to record what counters have been written to so
that we can clear out old data. For this, we need an ordered sequence that lets us iterate
one by one over its entries, and that also doesn’t allow duplicates. We could use a LIST
combined with a SET, but that would take extra code and round trips to Redis. Instead,
we’ll use a ZSET, where the members are the combinations of precisions and names that
have been written to, and the scores are all 0. By setting all scores to 0 in a ZSET, Redis
will try to sort by score, and finding them all equal, will then sort by member name. This gives us a fixed order for a given set of members, which will make it easy to sequentially
scan them. An example ZSET of known counters can be seen in figure 5.2.
Figure 5.1A HASH that shows the number of web page hits over 5-second time slices around 7:40 a.m. on May 7, 2012Figure 5.2A ZSET that shows some known counters
Redis and object caching can vastly speed up your WordPress page load times with each subsequent visit. It’s also used by many popular websites like GitHub, Pinterest, StackOverflow and many others.