Mobile databases

Handphone and mobile devices are becoming ubiquitous and more and more powerful. Even their software can support a large ecosystem of applications. That is where the applications vie for the limited resources on the mobile devices. As a text book says, some of the software problems which involve data management, transaction management and database recovery have their origin in distributed data systems and in mobile computing these problems become more difficult to solve –mainly because of the nature of bandwidth for the wireless communication channels, relatively short active life of the power supply and the changing locations of the required information (sometimes in cache, sometimes in the air, sometimes at the server)

The mobile computing architecture is also slightly different. It is one where mobile units and base stations communicate through wireless channels having bandwidths significantly lower than those of a wired network.  The mobile units operate within geographic mobility domain that is divided into smaller domains called cells. The mobile discipline requires that the movement of mobile units be unrestricted within the geographic mobility domain, while having information access contiguity during movement. The average duration of a user’s stay in the cell, referred to as the residence latency, is a parameter that is computed and continuously adjusted.

In this world, the mobile units suffer from the drawbacks that the database connectivity may be intermittent and may have limited power source. Clients slip into doze mode to conserve energy. They can be woken up when the server wants to communicate or the user keeps it active.

Moreover the type of data and data access is different for mobile applications. Data may be classified into 1) private data – one that is restricted to the user, 2) public data – one that can be accessed by anyone who can read it and 3) shared data – data that is accessed both in read and write modes by groups of users. Mobile databases can be distributed in one of two ways: The entire database may be distributed mainly among the wired components, possibly with full or partial replication. The database is distributed among wired and wireless components. If the database is local to the computer, the database may need synchronized with the remote. This environment, referred to as the intermittently synchronized database environment have the following characteristics: The communication between client and server is unicast or multicast. A server cannot connect to a client at will. Flakiness of the connection is immaterial. A client is free to manage its own data and transactions. A client has multiple ways of connecting to the server.

While today’s application and even mobile database providers are increasingly relying on wifi hot spots where internet ubiquity and connection speed and bandwidth have given rise to near wired connection environment, the classical problem of mobile databases has potential for untapped market of non-wifi coverages.

Snapshots can provide the last saved version enabling both recovery and file synchronization services. Since snapshots can be taken at any granularity, the low disk and memory footprint for mobile device usage can be maintained. This kind of service becomes external to the database.

#codingexercise

If you are given a number find if the number is colorful. A number is colorful when all the combinations of its digits result in a different product when multiplied.
Solution first we combine, then we find the duplicates in the products resulting from the combinations.
void Combine(ref string digits, ref stringbuilder b, int start, int level, ref List<int>Products)
{
for (int I =start; I < digits.length; I++)
{

if (b[level] != digits[I]){
     b[level] = digits[i];
     Products.add(calculateProduct(b.toString()));
}

if (I < digits.length)
    Combine(ref digits, ref b, start+1, level+1);

if (b[level] == digits[I]){
     b[level] = '/0';
}
}
}

void isColorful(List<int>Products)
{
 return (Products.Count == Products.distinct().count());
}

Observation the given sequence should not have the digit 1.

Observation the given sequence should not have duplicates.