The ANTARCTICA system provides new services such as the Locker Rent Service that allows mobile users to rent a dedicated disk space, located in the fixed network, where they can store and access their data. Taking into account that mobile computers can be more easily stolen, lost or accidentally damaged, this service offers an interesting help for mobile computer users.

EXPLANATION OF THE SERVICE

The Locker Rent Service allows mobile computer users to rent a dedicated disk space, located on a fixed network, where they can store and access their data. Besides extending the storage capacity of mobile computers with data, this service supports better utilization of the wireless medium, since many messages to and from the GSN can be avoided, delivered asynchronously or optimized. Moreover, the mobile client can stay disconnected for longer periods of time while its data are stored in a safe, secure and private space being readily available to be downloaded to the MU, or to be used by the agents that work in the GSN on behalf of the MU.

The flexibility and adaptability of the implementation of the Locker Rent Service using agents, facilitates the dynamic management of the space, the adaptation of the locker, and it supports the physical mobility of the locker following their users.

Advantages of the locker rent service

Independently of the specific mobile computing device or user, the Locker Rent Service offers several advantages to the users of mobile units:

• Storage space: The locker becomes an extension of the MU's disk placed at the fixed network. Thus, a mobile client can deposit in the locker all required data and results obtained (within the capacity limitation established).
• Data protection: The mobile client is the owner of the data stored in the locker. The data stored in the locker are protected against unauthorized accesses and modifications as well as any unexpected failures. This characteristic makes the locker an appealing option to maintain there a secure backup copy of some files of the MU, such as configuration files or files containing sensitive information, that need to be protected against losses.
• Battery power saving: The mobile client can stay disconnected for longer periods of time, while the data obtained for it are sent to its locker and stored there until it reconnects or explicitly asks for them.
• Wireless communications optimizations: The space in the locker can be used to store data until it is possible or desirable to send them to the MU. Before sending data to the MU, the data can be preprocessed, filtered or adapted to the needs of the mobile unit and its user. It can also be used to store data needed to perform some tasks that can be developed in the GSN and thus avoiding their transmission to the MU.
• Flexibility and adaptability: The implementation of the locker service using mobile agents provides for the dynamic adjustment of the space allocated to each locker. It also provides for the easy adaptation of the locker, by facilitating the dynamic creation of lockers tailored to the users needs. Furthermore, it physically supports mobility of the locker to follow the movements of the MUs.

Types of lockers

The Locker Rent Service offers a variety of types of lockers so users can choose the one that better fits their needs and preferences. Lockers can be rented by individual users (private locker) or by group of users (shared locker).

• A private locker is related to a single user; the data stored belongs to that particular user and can be accessed or modified only by authorized agents representing the user.
• A shared locker is a locker rented by a group of users. A shared locker distinguishes between data to be used by all users in the group and data for each particular user, by managing sub-lockers for each of them. Thus, a shared locker offers to each of its users the same services that a private locker offers to its single user. Moreover, the space in the locker that is shared by all users of the group constitute an encounter place for these users who are able to communicate, share data, interchange messages, write messages for the rest of the group in a blackboard service provided and collaborate together.

Implementation

The next figure shows the elements involved in the Locker Rent Service. Notice that there are 5 different types of agents involved.

The Locker Place is the specialist place in the GSN that offers the Locker Rent Service. The Locker Place offers disk space for renting, so it has mechanisms to administer and monitor the available space, to assign spaces and survey their use, and to register data necessary for billing for the service. In order to get those functionalities, the Locker Place has associated a database and a guardian agent.

When the Locker Place is created a static agent is created within it, the locker guardian agent. The locker guardian agent is always active, its mission is to control the population of agents in the Locker Place, check the authorization and authentication of incoming agents, create and dispose locker agents, maintain a register of the agents in the database (DB), monitor the use of the resources, and so on.

When the majordomo needs to use the Locker Rent Service, it creates an agent called locker rent agent and sends it to a Locker Place. Although it would be possible for the majordomo agent to use directly the Locker Rent Service, we chose to deal with a specialist agent because it would be difficult for the majordomo to use efficiently all the different services offered by the ANTARCTICA system heeding at the same time the different user requirements.

Lockers are implemented using a new kind of static agents that we call locker agents. The locker agents are created by the locker guardian agent in the Locker Place, and each one of them is assigned to a specific user or group of users, i.e., to their locker rent agents. This pair of the locker agent and the locker rent agent(s) constitutes the locker and takes care of storing the user's data, saving email messages, processing results and communicating with the MU's majordomo agent. The fact that both agents have to communicate with each other and interchange data incurs some overhead. However, this interaction is local as both agents reside in the same place, the Locker Place. Furthermore, by having both kinds of agents, we can take advantage of specializing them.

The locker agent is specialized in interacting with the place and the available resources in order to store and recover data, and assure the privacy and security of its locker. So it makes the characteristics of the place and its resources transparent to the locker rent agent and to its user. Moreover, the locker agent of a shared locker interacts with a group of locker rent agents, acting as intermediary and maintaining services such as a blackboard where the users can leave messages and data for other users. It also handles concurrent data accesses.

On the other hand, the locker rent agent is specialized on interacting with the user's majordomo agent. It has the knowledge about the user, as well as semantic knowledge about the data and how to utilize them. In that way, it makes transparent for the locker agent the characteristics of the specific user and its MU. For example, the locker agent knows how to store e-mails in the locker, but the locker rent agent knows how to ask for them and compose a summary list with the new e-mails received, specifying the date, subject and sender, that is sent to the user so that he can choose which e-mails he wants to read.