
Count requests received in last second, minute and hourProblem: Given an extremely busy server which receives thousands of requests per second. It is desired to find the number of requests received in the last second, minute and hour. What algorithm and datastructures can be used to do this as accurately as possible? Solution : Some of the first solutions that come to mind are:
Best approach: A good solution is one which allows concurrent updates and does not eat up too much memory. With this idea in mind, the following can be a good solution: 1) Create an array AtomicInteger frequencies[1000]; to store the number of requests received per second. 2) This array will store frequencies of requests for the current second i.e. from HH:MM:SS to HH:MM:SS+1 time 3) We will store current second in some variable, say currentSecond This can be retrieved in Java as: Calendar calendar = Calendar.getInstance(); int currentSecond = calendar.get(Calendar.SECOND); 4) frequencies array counts are incremented as: int newSecond = calendar.get(Calendar.SECOND); if (newSecond != currentSecond) { synchronized (...) { int requestsPerSecond = sumFrequenciesInTheSecond (frequencies); frequencies = new AtomicInteger[1000]; } } // frequencies points to current second at this point int requestMillisOfSecond = calendar.get(Calendar.MILLISECOND); frequencies[requestMillisOfSecond]++; 5) So we are able to get the frequencies per second by just using 1000 AtomicIntegers. 6) To get requests per minute, all we need to do is add the seconds in a minute. This can be done by storing just 60 seconds of data, i.e. just 60 integers. 7) Similarly, once we have minutes' data, we can get hours' data by adding 60 minutes. Again there is no need to store more than 60 minutes of data. Whenever 60 minutes complete for the current hour, we sum up those minutes, store the hour's requests per second and reset the minutes array. 8) Thus, all we need is the following: AtomicInteger frequencies[1000]; int secondsFrequencies[60]; // Every second gets its value by summing frequencies array int minutesFrequencies[60]; // Every minute gets its value by summing secondsFrequencies array int hoursFrequencies[60]; // Every hour gets its value by summing minutesFrequencies array 9) This way, we get our solution in just O(1) space 
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