A Real-Life Example - Getting Started with Storm - page 52

Therefore the emited tuples will be:

# Category
8 Players
8 Players
8 TVs
8 Mounts
0 Phones

2 Phones

17 Phones

21 Phones

Note that the relation between the products on the left and the categories on the right
should be incremented in one unit.
Now, let's explore the persistence used by the Bolt.
public class UserHistoryBolt extends BaseRichBolt {
...
private Set < String > getUserNavigationHistory ( String user ) {
Set < String > userHistory = usersNavigatedItems . get ( user );
if ( userHistory == null ) {
userHistory = jedis . smembers ( buildKey ( user ));
if ( userHistory == null )
userHistory = new HashSet < String >();
usersNavigatedItems . put ( user , userHistory );
}
return userHistory ;
}

private void addProductToHistory ( String user , String product ) {
Set < String > userHistory = getUserNavigationHistory ( user );
userHistory . add ( product );
jedis . sadd ( buildKey ( user ), product );
}
...
}

The getUserNavigationHistory method returns the set of products that the user has
visited. First, attempt to get the user's history from local memory with usersNavigate
dItems.get( user ) , but if it's not there, read from the Redis server using jedis.smem
bers(buildKey(user)) and add the entry to the memory structure usersNavigatedItems .
When the user navigates to a new product, call addProductToHistory to update both
the memory structure with userHistory.add(product) and the Redis server structure
with jedis.sadd( buildKey ( user ), product ) .