Reference Guide

}
}
The two methods shown read from the database in different ways. The first one issues a find query
using a filter object. The filter specifies the pivot around which the query results are read. The
second method reads a page of alerts and is used when there is a need to paginate results. This is
mostly used by a GUI where pages of Alerts are displayed instead of a single long list of Alerts.
The following is an example of filter object as defined in the demo application.
CassandraAlertFilter.java:
package com.hp.hm.model;
import com.hp.util.filter.EqualityCondition;
import com.hp.util.filter.SetCondition;
import com.hp.util.filter.StringCondition;
...
public class CasssandraAlertFilter {
private SetCondition<Severity> severityCondition;
private EqualityCondition<Boolean> stateCondition;
private StringCondition topicCondition;
private StringCondition originCondition;
...
// Implement setters and getters for all conditions.
// Good practice to override toString()
}
Every application needs to define its filter parameters as in the above code. In the demo
application, there is severity filter to “find Alerts where Severity = CRITICAL, WARNING” for
example. So, Severity is a Set condition. The find method returns the row if one of the values in a
set condition match. The other conditions in the demo follow similar principles.
They cater to various conditional queries that can be issued as a read query to the database. The
caller who wants to read from the database needs to create a filter object and fill it with
appropriate values before issuing a find query.
Data Access Object - DAO
In the previous information, the business logic called the DataStoreService API to perform any
persistence operation. The API performs the operation using a DAO. The DAO is a layer that acts
as a single point of communication between the business logic and the database. The
infrastructure provides generic abstractions of the DAO. However, each table needs to have a
table or a Column family specific DAO defined. For this Alerts Demo application there is a
CassandraAlertDao. The example code is illustrated in the following listing.
CassandraAlertDao.java:
package com.hp.demo.cassandra.dao.impl;
...
public class CassandraAlertDao extends
CassAbstractDao<String, String, CassandraAlert,
CassandraStorable<String, String>, CassandraAlertFilter,
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