How to note web requests in concurrent environment?

Question

We have a web application which receives some million requests per day, we audit the request counts and response status using an interceptor, which intern calls a class annotated with @Async annotation of spring, this class basically adds them to a map and persists the map after a configured interval.
As we have fixed set of api we maintain ConcurrentHashMap map having API name as key and its count and response status object as value.So for every request for an api we check whether it exists in our map , if exist we fetch the object against it otherwise we create an object and put it in map. For ex

class Audit{
    CounterObject =null;
    if(APIMap.contains(apiname){
         // fetch existing object  
          CounterObject=APIMap.get(apiname);
    }
    else{
          //create new object and put it to the map
          CounterObject=new CounterObject();
    }
  // Increment count,note response status and other operations of the CounterObject recieved
}

Then we perform some calculation on the received object (whether from map or newly created) and update counters.
We aggreagate the map values for specific interval and commit it to database.
This works fine for less hits , but under a high load we face some issues. Like
1. First thread got the object and updated the count, but before updating second thread comes and gets the value which is not the latest one, by this time first thread has done the changes and commits the value , but the second threads updates the values it got previously and updated them. But as the key on which operation is performed is same for both the threads the counter is overwritten by the thread whichever writes last.
2. I don't want to put synchronized keyword over the block which has logic for updating the counter. As even if the processing is async and the user gets response even before we check apiname in map still the application resources consumed will be higher under high load if synchronized keyword is used , which can result in late response or in worst case a deadlock.

Can anyone suggest a solution which does can update the counters in concurrent way without having to use synchronized keyword.
Note :: I am already using ConcurrentHashMap but as the lock hold and release is so fast at high load by multiple threads , the counter mismatches.

Answer 1

In your case you are right to look at a solution without locking (or at least with very local locking). And as long as you do simple operations you should be able to pull this off.

First of all you have to make sure you only make one new CounterObject, instead of having multiple threads create one of their own and the last one overwriting earlier object.

ConcurrentHashMap has a very useful function for this: putIfAbsent. It will story an object if there is none and return the object that is in the map right after calling it (although the documentation doesn't state it as directly, the code example does). It works as follows:

CounterObject counter = APIMap.putIfAbsent("key", new CounterObject());
counter.countStuff();

The downside of the above is that you always create a new CounterObject, which might be expensive. If that is the case you can use the Java 8 computeIfAbsent which will only call a lambda to create the object if there is nothing associated with the key.

Finally you have to make sure you CounterObject is threadsafe, preferably without locking/sychronization (although if you have very many CounterObjects, locking on it will be less bad than locking the full map, because fewer threads will try to lock the same object at the same time).

In order to make CounterObject safe without locking, you can look into classes such as AtomicInteger which can do many simple operations without locking.

Note that whenever I say locking here it means either with an explicit lock class or by using synchronize.

Answer 2

The reason for counter mismatch is check and put operation in the Audit class is not atomic on ConcurrentHashMap. You need to use putIfAbsent method that performs check and put operation atomically. Refer ConcurrentHashMap javadoc for putIfAbsent method.