Encyclopedia of Computer Science and Technology

tree 479Some applications process transactions in batches. Forexample, a company may run a program once a month thatgenerates paychecks and withholding stubs from employeerecords that include hours worked, number of dependentsclaimed, and so on. Indeed, in the ATM example, theaccount balance is typically not updated during the on-linetransaction, but instead a batch transaction is stored. Overnightthat transaction will be processed together with othertransactions affecting that account (such as checks), andthe balance will then be officially updated. (The programmodule that keeps track of the progress of transactions iscalled a transaction monitor.)There are several considerations that are important indesigning transaction systems. While some transactionsmay simply involve a request for information and do notupdate any files, many transactions may require that severalfiles or database records be updated. For example, a transferof funds from a saving account to a checking account willrequire that both accounts be updated: the first with a debitand the second with a credit. What happens if the computerperforms the savings debit but then goes down before thechecking account can be credited? The result would be anupset customer whose money seems to have disappeared.The solution to this problem is to design a process wherethe various updates are done not to the actual databasesbut to associated temporary databases. Once these potentialtransactions are posted, the system issues a “commit” commandto the databases. Each database must send a replyacknowledging that it’s ready to perform the actual update.Only if all databases reply affirmatively is the commit commandgiven, which updates all the databases simultaneously.stored in a binary tree, a program can use two pointers, oneto the current node’s left child and one to its right child(see pointers and indirection). The pointers can then beused to trace the paths through the nodes. If the tree representsa file that has been sorted (see sorting and searching),comparing nodes to the desired value and branchingaccordingly quickly leads to the desired record.Alternatively, the data can be stored directly in contiguousmemory locations corresponding to the successivenumbers of the nodes. This method is faster than havingto trace through successive pointers, and a binary searchalgorithm can be applied directly to the stored data. On theother hand it is easier to insert new items into a linked list(see list processing).A common solution is to combine the two structures,storing the linked list in a contiguous range of memory bystoring its root in the middle of the range, its left child atthe beginning of the range, its right child at the end, andthen repeatedly splitting each portion of the range to storeeach level of children. Intuitively, one can see that algorithmsfor processing such stored trees will take a recursiveapproach (see recursion).Further ReadingLewis, Philip M., Arthur Bernstein, and Michael Kifer. Databasesand Transaction Processing: An Application-Oriented Approach.Upper Saddle River, N.J.: Addison-Wesley, 2002.Little, Mark, Jon Maron, and Greg Pavlik. Java Transaction Processing:Design and Implementation. Upper Saddle River, N.J.:Prentice Hall PTR, 2004.treeThe tree is a data structure that consists of individual intersectionscalled nodes. The tree normally starts with a singleroot node. (Unlike real trees, data trees have their root atthe top and branch downward.) The root connects to oneor more nodes, which in turn branch into additional nodes,often through a number of levels. (A node that branchesdownward from another node is called that node’s childnode.) A node at the bottom that does not branch any furtheris called a terminal node or sometimes a leaf.Trees are useful for expressing many sorts of hierarchicalstructures such as file systems where the root of a diskholds folders that in turn can hold files or additional folders,and so on down many levels. (A corporate organizationchart is a noncomputer example of a hierarchical tree.)The most common type of tree used as a data structureis the binary tree. A binary tree is a tree in which nonode has more than two child nodes. To move through dataIn a binary tree each node either has two branch (child) nodes oris a terminal node. Here a binary tree is shown with the equivalentrepresentation in an array of memory locations. Notice that thenumbers are stored level by level (50 in the top level, 25 and 75 inthe second level, and so on.) Null would be a special value (suchas -1) representing an empty node.