Preliminary Performance Evaluation of an ORDB-based IFC Server and an RDB-based IFC Server by Using the BUCKY Benchmark Method Jeong, J. Department of Architectural Engineering, Yonsei University (email: [email protected]) Lee, G. Department of Architectural Engineering, Yonsei University ([email protected]) Kang, H. Department of Architectural Engineering, Yonsei University (email: [email protected]) Abstract Many IFC servers, based on RDB systems, suffer from poor processing speed and performance. We suggest that an ORDB, which is based on a concept similar to EXPRESS, is a suitable alternative database management system (DBMS). This paper reports the results of a preliminary performance evaluation between an object-relational database (ORDB)-based IFC server and a relational database (RDB)-based IFC server by using the BUCKY benchmark method. The results showed that an ORDB-based IFC server was faster than an RDB-based IFC server. Since we have not yet conducted the full-model test, this result has not been confirmed. However, the benchmark test showed the potential for improvement in the performance of the IFC server. More reliable results should be observed through a full-model test in future work. Keywords: IFC server, ORDB, BUCKY Benchmark, IFC, BIM 192 1. Introduction The effective exchange of building information is an important aspect for BIM(building information modeling) throughout the entire life cycle of a building. For this reason, buildingSMART was developed with the goal of providing data interoperability. This study, which addresses the problem of data storage and management, also seeks to address this concern. Industry foundation classes (IFC) is a data schema that exchange relevant information between different software applications. A data schema comprises interdisciplinary building information that is used throughout the building’s life cycle (buildingSMART, 2009). Most projects that use IFC exchange data by using a conventional file-based system. File-based systems, however, have several problems, including inconsistency as well as poor accessibility, integrity, and authority. (Silberschatz et al., 2006). Database management systems (DBMS) have been developed for this reason. The aeronautical engineering and mechanical engineering fields already apply web-based database servers. Several recent studies have addressed the use of DBMS in architecture (Adachi 2002; Chen et al., 2005; Karola 2002; You et al., 2004). One problem to be resolved is the implementation of IFC into a physical model. IFC uses the EXPRESS language developed by the International Standard Organization (ISO). Since EXPRESS is an object-flavored data-modeling language that relies heavily on inheritance and aggregation, mapping to an object-oriented database (OODB) would be a logical approach, but few studies have been conducted. There has been some effort to map relational databases (RDB), such as IFCsvr by VTT in Finland, SECOM in Japan, and EXP2SQL at Georgia Technical University (Adachi 2002; You et al., 2004). RDB was mapped first because they are commonly used in many areas, have many users, and conform to a well-organized standard. Mapping EXPRESS to RDB is very complicated, and it cause bigger file sizes and processing delays. Some attempts have been made to store EXPRESS directly without mapping by using an EXPRESS repository, such as the EDM server (Jotnes EPM Technology) and EMS (Eurostep). Also, the speed and stability of the system can cause problems when mapping large models (Plume and Mitchell 2007). As buildings get larger and more complex, improved performance is essential in order to process very large files. Therefore, this paper examines the possibility of using an object-based IFC server as an alternative. 2. Research scope and methodology Since the system is still under development, we could not test the full model, but instead used a wall object. Thus a limited schema, objects related to the wall, was set up as a benchmark test. This paper presents alternative databases for IFC server, and shows the relationship between IFC and ORDB. Next, RDB and ORDB based on IFC server are developed by using previously reported research methods. The two systems are evaluated for performance by using the BUCKY benchmark method. 193 3. IFC schema implementation 3.1 Types of database Relational database (RDB) generally consists of tables, and match data by using common characteristics found within the data set. RDB is the most widely used database due to its technical stability and reliability. It can easily extract the results needed by using a standardized query method, such as the structured query language (SQL). However, this approach is complicated, is limited in usefulness for large multimedia files, and does not adequately express such information at the object- based concept. Object-oriented databases (OODB) can better express realistic situations by relying on aggregation and inheritance. This means that a complicated data design can be simplified. Without “join” tables, data can be extracted by using its physical address, called object identifier (OID), which leads to good performance. Standard query methods and development support do not completely resolve the problems. Therefore, OODB was developed but was not widely accepted by users who were accustomed to RDBs. Object-relational database (ORDB) supports several object structures and rules, and add to conventional RDB services, including image, audio, and video data. ORBD also supports the encapsulation, inheritance, and aggregation that OODB supports. The combined advantages of RDB and OODB have resulted in a rising market share. As a result, ORDB is becoming a relevant research model. 3.2 Relationship between EXPRESS and ORDB IFC was developed as an exchange standard for building information (buildingSMART, 2009). IFC consists of EXPRESS (ISO 10303-21), a data modeling language. EXPRESS is also standard in STEP (ISO 10303) and has the following characteristics: An object-favored language, relevant for expressing hierarchical structure, including inheritance. Includes aggregation data type, such as set, list, array, and bag. Frequent references among different objects. Since most previous studies were RDB-based, it was necessary to create a number of tables to cover the above characteristics. It was also necessary to join the tables in order to extract the needed results. This led to complex queries and performance problems. Thus, this study applied an ORDB-based structure to implement on an IFC server. The advantages of ORDB, compared with RDB and OODB, are as follows: Since it can express object-flavored data like EXPRESS, a hierarchical data structure can be implemented. By using an OID, it is possible to access data connected directly without an additional “join” 194 process. Aggregation data can be expressed in single cell. User friendly by employing a pre-defined query. Possible to program in an object-flavored language like C++ or JAVA. However, there are few examples of mapping from IFC schema to ORDB. Therefore, this paper conducted a performance evaluation by using the developed ORDB-based IFC server. 3.3 Mapping process The mapping logic for ORDB is partially derived from Kang’s (2009) method. In this paper, the function and rule sections were not considered because they are still under development. Previous studies (Adachi 2002; You et al., 2004) were used to implement RDB with Cubrid as the platform. Cubrid, used as the platform was developed from the first object-oriented DBMS, which was called UniSQL. Both approaches, RDB and ORDB, can be supported in Cubrid, therefore it was considered to be a relevant DBMS for this study. 4. Performance evaluation 4.1.1 Preparation for test Considering IFC server developing status, specific object was selected for the test. Wall, selected object, includes several entities in IFC schema, and reflects major features of EXPRESS. Thus it is relevant preliminary performance evaluation. 4.1.1.1 Define subset of parts related to wall The subset of parts related to the wall, which is the range of the performance test, must first be defined. The subset was extracted based on the basic entity of wall schema “ifcwallstandardcase.” The Yonsei IFC MV Extractor was used as an extraction method. The Yonsei IFC MV Extractor automatically extracts a valid subset of the IFC model that is related to a certain IFC concept (Lee 2009). The minimum valid subset was defined as the range for the test by selecting all instances of ifcwallstandardcase in IFC2X3 (a total of 13 entities). Figure 1 shows the process. 195 Figure 1: Extraction subset of ifcwallstandardcase (Yonsei IFC MV Extractor) 4.1.1.2 Instance input Since the server system is not complicated, it is difficult to input an actual IFC2X3 file. Therefore, virtual files similar to real IFC data were inserted. The input method was “loaddb,” which can upload large files in a short time and is supported by Cubrid. For ORDB, attributes referencing other entities use the OID for directional access. “Table 1” shows that identical IFC instances were inserted to RDB and ORDB (a total of 9155 instances). The number of classes using an OID was 2000. Table 1: Virtual instances input Class Instance Ifcwallstandardcase 1153 Ifcwall 1002 Ifcownerhistory 1000 Ifcpersonandorganization 1000 Ifcapplication 1000 Ifcperson 1000 Ifcorganization 1000 Ifcactorrole 1000 Ifcaddress 1000 Total 9155 196 4.1.2 BUCKY benchmark method We use the BUCKY benchmark method for performance evaluation of RDB