Question: How to store the temporary data in Apache storm?

In storm topology, bolt needs to access the previously processed data.

```
Eg: if the bolt processes varaiable1 with result as 20 at 10:00 AM.

```

and again `varaiable1` is received as `50` at `10:15 AM` then the result should be `30 (50-20)` 

later if varaiable1 receives `70` then the result should be `20 (70-50)` at `10:30`.

How to achieve this functionality.

Answer: 
In short, you wanted to do micro-batching calculations with in storm’s running tuples.
First you need to define/find key in tuple set. 
Do field grouping(don't use shuffle grouping) between bolts using that key. This will guarantee related tuples will always send to same task of downstream bolt for same key.
Define class level collection List/Map to maintain old values and add new value in same for calculation, don’t worry they are thread safe between different executors instance of same bolt.
Uou can use CacheBuilder to remember your data within your extended BaseRichBolt (put this in the prepare method):

```
// init your cache.
this.cache = CacheBuilder.newBuilder()
                         .maximumSize(maximumCacheSize)
                         .expireAfterWrite(expireAfterWrite, TimeUnit.SECONDS)
                         .build();

```

Then in execute, you can use the cache to see if you have already seen that key entry or not. from there you can add your business logic:

```
// if we haven't seen it before, we can emit it.
if(this.cache.getIfPresent(key) == null) {
    cache.put(key, nearlyEmptyList);
    this.collector.emit(input, input.getValues());
}

this.collector.ack(input);

```