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\title{An Efficient Concurrency Control Technique for Mobile Database Environment}
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             \author[1]{Dr. Md. Anisur  Rahman}

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\date{\small \em Received: 6 December 2012 Accepted: 2 January 2013 Published: 15 January 2013}

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\begin{abstract}
        


Day by day, wireless networking technology and mobile computing devices are becoming more popular for their mobility as well as great functionality. Now it is an extremely growing demand to process mobile transactions in mobile databases that allow mobile users to access and operate data anytime and anywhere, irrespective of their physical positions. Information is shared among multiple clients and can be modified by each client independently. However, for the assurance of timely access and correct results in concurrent mobile transactions, concurrency control techniques (CCT) happen to be very difficult. Due to the properties of Mobile databases e.g. inadequate bandwidth, small processing capability, unreliable communication, mobility etc. existing mobile database CCTs cannot employ effectively. With the client-server model, applying common classic pessimistic techniques of concurrency control (like 2PL) in mobile database leads to long duration Blocking and increasing waiting time of transactions. Because of high rate of aborting transactions, optimistic techniques aren`t appropriate in mobile database as well. This paper discusses the issues that need to be addressed when designing a CCT technique for Mobile databases, analyses the existing scheme of CCT and justify their performance limitations. A modified optimistic concurrency control scheme is proposed which is based on the number of data items cached, amount of execution time and current load of the database server. Experimental results show performance benefits, such as increase in average response time and decrease in waiting time of the transactions.

\end{abstract}


\keywords{mobile database, optimistic concurrency control, mobile host, fixed host, base station, client-server.}

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\let\tabcellsep& 	 	 		 
\section[{Introduction}]{Introduction}\par
In the modern decade, Because of having enhanced processing capability, easy and swift access to information as well as reduction in prices, mobile devices have achieved vast use in data processing services like mobile banking, traffic control, ecommerce, money transfers etc.\par
In these application and communication process of mobile devices client-server, peer to peer, and ad-hoc architectures are most recognized architectures of mobile database system. Fig. \hyperref[fig_0]{1} displays client-server architecture. Mobile clients/Host (MH) e.g. Palmtops, Laptops, PDA's, Cellular phones etc. and fix host (FH) e.g. Terminals, desktops, servers Author : Department of CSE, Dhaka University of Engineering \& Technology Gazipur, Bangladesh. E-mail : ansiur.rahman.duet@gmail.com etc and mobile base stations (BS) are three most important elements of this model. In client-server model, mobile clients are connected to fixed host through mobile base station. At present, using wireless 802. \hyperref[b11]{11} protocols, it is possible to have a synchronized and fast connection between mobile client and server. But, properties of mobile devices still impose some limitations upon assessment and data processing in mobile environment.\par
Therefore, for constant and continuous processing, required data are copied partially from server to mobile client. of databases while allowing them to run concurrently, so that the throughput and resource utilization of database systems are improved and waiting time of concurrent transactions is reduced. When designing a CC algorithm the factors that should be taken into considerations are mobility, low bandwidth, multi-hop communication, limited battery power, limited storage, frequent disconnections, wireless communication delay, less processing power, frequent disconnections and unbounded disconnection time etc. When the execution is prolonged, the probability of conflicts with other transactions becomes higher and, consequently, transactions are likely blocked if a pessimistic CC method applies or restarted if an optimistic CC method is in use. The existing CC techniques for traditional mobile network databases, in which only clients are mobile and battery-powered, cannot be directly, because of a number of factors related to its architecture, availability and sharing of hardware and software resources, distribution of data, and mobile client's processing capability.\par
In this paper we proposed a new scheme that is reduces number of abortion as well as minimize the connection time of client and server by enhancing concurrency. 
\section[{II.}]{II.} 
\section[{Related works}]{Related works}\par
Due to the boundaries, restrictions and specific properties of mobile devices cause traditional concurrency control techniques e.g. 2PL to exhibit a reduced amount of effectiveness in mobile database. A variety of researches have done in this field. These researches introduce new techniques of concurrency control or adapt the existing techniques of concurrency control with the requirements of mobile environment.\par
Because of the inherent limitations of mobile devices, constant and uninterrupted connection between client and server is not achievable over the transaction period. In locking-based pessimistic protocols, continuity of connection of mobile client with server is compulsory. If at the period of transaction, the connection is interrupted, the possibility of infinite blocking of transactions and deadlock is appeared. So, based on time out, many researchers have been done regarding methods of non-exclusive locks. In these techniques, if transaction doesn't end within the expected time, locks get caught by transaction will be free.\par
References \hyperref[b9]{[9,}\hyperref[b10]{10]} showed a technique of locking with non-exclusive lock based on time out. In reference \hyperref[b10]{[10]}, a dynamic timer was used to solve the problem of transactions` blocking. In this method, transactions should be finished in particular time, otherwise they would abort. In reference \hyperref[b11]{[11]}, for increasing the efficiency of \hyperref[b10]{[10]},transactions which don't finish in specified time and are near to final of transaction, wouldn't be aborted. They are allowed to continue execution for specified time. In methods basing on timer (time out) problems like long connection of mobile client with Server, estimated the amount of timer, and the length of transaction, do exist. Because of wrong estimation of the required time for completion of transactions, these problems could result in incorrect abortion of transactions. This suffers from the problem of frequent rollbacks due to regular expiry of the timer and wastage of computation. A protocol based on AVI is proposed in \hyperref[b12]{[12]}. This method has still the problem of transaction blocking and computational overhead. This problem has been mentioned in reference \hyperref[b13]{[13]}. Multi-version concurrency control based on MV2PL protocol and timestamp being introduced in \hyperref[b14]{[14]} are in fact an extension of method \hyperref[b15]{[15]}.\par
In \hyperref[b17]{[17,}\hyperref[b18]{18]}, combination of optimistic and pessimistic is performed according to the semantic of operators.\par
In \hyperref[b17]{[17]}, changing two phases locking (2PL) and considering the semantic of transaction`s operators, are executed for increasing the concurrency degree of transactions. If conflicted operators are compatible semantically, then, they can choose a resource simultaneously.\par
In this method, if compatible transactions lock a resource, there would be a resource scarcity for incompatible transaction. Moreover, there is the possibility of high rate of abortion through reconciliation process.\par
In reference \hyperref[b13]{[13]} optimistic method is utilized. At the time of completion of transactions, updated data are sent to other mobile transactions using these data. This transfer is in multi-cast status. This technique reduces abortion rate of transactions.\par
Reference \hyperref[b19]{[19]} puts emphasis on asymmetry of connecting bandwidth between mobile client and server. In \hyperref[b19]{[19]} optimistic method is under consideration in which in the time of data updating, timestamp of transactions is set dynamically and data is broadcast for mobile clients.\par
Optimistic concurrency control based on timestamp ordering in \hyperref[b20]{[20]} is adapted for broadcast environments. Also the introduced method in [21] is suitable for broadcast environments.\par
[3] OPCOT algorithm is introduced by assigned timestamp to operators and based on optimistic method according to concept of commitment ordering schedulers. So, reducing the waiting time as well as abortion rate, providing high concurrency, secure from deadlock and starvation, reducing computational are objectives to be considered in concurrency control protocol in mobile database environment. 
\section[{III.}]{III.} 
\section[{Proposed Method}]{Proposed Method}\par
Some concurrency control scheme performs better in some particular database systems e.g. Time  
\section[{C}]{C}\par
stamp based multi-version protocols is better in real time databases. So according to the demand of the mobile database, we can define some specific method for a defined type of transactions and consider its condition to set best performed rule for it. We can categorize transaction operations as:\par
Transactions that only Read an item (TR), Transactions that only Write an item (TW), Transactions that Read as well as write an item that is Update (TU), Transactions that inserts an item (TI), Transactions that delete an item (TD) etc.\par
In my proposed method, I have used Timestamp based multi-version protocol for TR and TW. So TR always gets the most recent version of the data elements and TW just creates a new version of data element and the old version remains intact and hence possibility of occurring conflict is zero.\par
For the TU, writing conflict may happen. So it is essential to resolve the conflicts among the TUs effectively to maintain integrity of the data item.\par
In my proposed method, when a client MH requests for the data items e.g. Xj. On which the MH going to have write operation(s), Server stores an entry for that MH including the client Id, Time of the beginning of transaction execution (TB), the list of data items Xi for that client, maximum validation period PV, Rank RC.\par
T B is maximum duration to execute and send cache to server which is supplied by MH for the use of server to achieve more concurrency. PV is a time limit within which MH have to perform its operation and send cache to server to commit else request will be discarded from the transaction list of the server. PV is determined by the TB and the bandwidth of the connection to that client. RC is an integer that keeps track of the commit attempt of a transaction.\par
As all transactions are executed locally affecting the cache of the client, client sends partial update of the transaction for a data item that has no further update operation within this transaction for the partial commitment to the server if connection between mobile client and server be available. It would increase the concurrency significantly. In this case server checks for the conflicts and resolves by commitment algorithm. If connection is not available during execution then at the end of all updates in client cache, updated cache is sent to server to be committed. All kind of update is done according to the commit algorithm described below:\par
When any client MH i sends cache to server, server finds the clients MH a , MH b with which MHi conflicts. Server performs following operations:\par
If MH i have expired maximum validation period of MH i , Server cancels the commit operation and causes to restart the transaction of MH i .\par
If Executed Time Tex of MH i < ?j Tex of MH j , for all j X i Then If RC > ?j RC for all j Xi MH i commits; invalidating the conflicting clients causing to restart their execution immediately with updated value of the data item. Else MH i aborts and restarts its execution. Else MH i commits and causes to restart their execution immediately with updated value of the data item.\par
IV. 
\section[{Performance Analysis}]{Performance Analysis}\par
The proposed concurrency control scheme is assessed in this section. To perform assessment, it is compared with optimistic concurrency control methods as proposed technique is based on optimistic methods. In order to assess and compare performance of proposed technique, optimistic technique is simulated as well. I have used of network simulator NS-3 [22, 23] version 3.12 to implement my protocols.\par
Simulation is done using C + + in application layer protocols.\par
A circular-shaped area (D=1000 meters) space, a fix host (server), 5 mobile base stations, and 100 mobile clients are used in simulation environment. Mobile hosts can move randomly in various directions and communicate their requirements by mobile base station to the server.\par
Simulation is done for getting the comparison result values for the optimistic method and proposed scheme. Since response time and waiting time of a transaction are two important parameters of performance, the graph is drawn and studied only for the two parameters. The load of the system is increased incrementally. V. 
\section[{Conclusions}]{Conclusions}\par
In this paper, a new concurrency control technique based on optimistic method in mobile database environment with client-server model is introduced. This method is based on Optimistic method, so there is no blocking or deadlock in transactions. Moreover, concurrency Degree of transactions is high. To reduce the rate of abortion, proposed algorithm, based on broadcast commit, applies the technique of partial update as well as assigning priority depending on some parameters. This leads to increase in the rate of commitment of transactions. There some overhead for the determining execution time of transaction which is worthless and is tolerable. There is plan to improve this method by introducing time stamp in it. \begin{figure}[htbp]
\noindent\textbf{1}\includegraphics[]{image-2.png}
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\caption{\label{fig_1}An}\end{figure}
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\noindent\textbf{23}\includegraphics[]{image-4.png}
\caption{\label{fig_2}Figure 2 :Figure 3 :}\end{figure}
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\caption{\label{figure5}}\end{figure}
 			\footnote{© 2013 Global Journals Inc. (US)} 		 		\backmatter  			  				\begin{bibitemlist}{1}
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\end{bibitemlist}
 			 		 	 
\end{document}
