A concurrency control model for PDM systems

Abstract

In today's manufacturing environment, enterprises having work groups geographically dispersed are not uncommon. A product data management (PDM) system is therefore required for controlling the distribution and maintaining the integrity of the product data throughout its entire lifecycle; the efficiency of a PDM system is greatly affected by the concurrency control method it adopts. The paper proposes a concurrent control model for PDM that can also caters for version management and product architecture. The paper discusses how granularity and versioning are being embedded into a lock-based concurrency control model. The concurrent accessibility of an example product data is explained to illustrate the adjustability according to the actions taken by the users and the architecture of the corresponding entities.

Introduction

Product data management (PDM) systems have emerged over the last decade. These systems enable enterprises to conduct its business activities in a more efficient way via ingenious management of product information [1], [2]. The lack of communication among different product development stages often causes consistency problems in product lifecycle. The concept of concurrent engineering, integrated product and process development and others are introduced [3]. They are accompanied with the PDM system to manage all product-related data and provide data retrieval for product design and production. PDM system enhances collaborative work by online access and electronic interchange of product data [4]. The efficiency and quality of design and manufacturing processes can be greatly improved by product information sharing and visualization in the system. With the advent of the Internet- and web-based technologies, PDM systems can now be executed more effectively and efficiently. The development of web-based PDM system is essential for supporting collaborative design and manufacturing at geographically disperse sites [5], [6], [7]. The web-based PDM system not only facilitate the process of data exchanging and sharing but also increase the number of product data transaction.

Among the functions of the PDM system, concurrency control is essential to the checkout, release, obsolete, view, redlining and references. In order to provide the control over the data access, most PDM systems establish a set of access rules that determine what data can be accessed, in what mode and at what point in the product lifecycle. This is complicated when a part or drawing belongs to a certain subassembly. In such instance, the modification to the upper assembly can cause the part to be locked out for write access until the process is completed. Therefore, a concurrency control should be specifically designed with the consideration to the characteristics of PDM systems.

Design and manufacturing workflow can be streamlined by implementing suitable PDM system that manages all product-related data in an organized manner. One major function of PDM system is to maintain data integrity and to provide accurate data when required [8]. For example, building an assembly of a printer cartridge carriage (Fig. 1) involves the co-operation of many engineers from several departments. However, the large amount of interactions between the system and the users will give rise to concurrency problems, the lost update problem occurs when modifications are made in an uncontrollable manner is illustrated in Fig. 2. Let us denote T_i as the unit time i when a transaction T occurs, user A copies the assembly file of the carriage to his/her local workspace from the vault in T₁. User A places the lid to the carriage, the content of the local copy of file J in user A's workspace is updated to J^*. While user B copies file J to his/her workspace in T₂, file J is replaced by user A's local file with content J^* and user B inserts the cartridge to the carriage, hence the content of user B in his/her workspace is updated to J^** in T₃. Finally, file J in the vault, which has been previously updated by user A, is replaced by user B's local file with content J^** in T₄. Rather than updating the file based on user A's result, B is updating the file based on file J's original value; the work of user A (file J with content J^*) is unintentionally erased by the later update. The carriage assembly file in the vault now has three cartridges and without the lid installed. To avoid inconsistency in the vault, concurrency control mechanism is required to prevent database updates performed by one user from interfering with retrievals and updates performed by another [6].

Concurrency control is well studied in traditional database management systems (DBMSs). However, there are relatively few studies that address the issues in PDM systems. In this paper, a new concurrency model is proposed to cater for version management and product architecture in addition to concurrent information sharing required by distributed PDM system.