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The CODASYL Network Model

Oracle Tips by Burleson Consulting

A second database type is a network database.  Many computer systems have been implemented using the network Database Management System (DBMS) specifications developed by the Conference on Data Systems Languages (CODASYL).  Also involved were two subgroups of CODASYL:  the Database Task Group (DBTG), and the Data Description Language Committee (DDLC). 

CODASYL and it's subgroups are an organization of volunteer representatives of computer manufacturers and users. CODASYL began in 1959.  The first set of DBMS specifications was produced in 1969.  This set of specifications was revised, and the first real CODASYL DBTG specifications were issued in 1971.

Basically, all database management systems are based in some part of the CODASYL DBTG specifications.  From the early CODASYL DBTG specs, the data model was called a network data model.  The model that CODASYL DBTG developed became the basis for new database systems like IDMS from Cullinet in 1970.

The Data Base Task Group (DBTG) CODASYL Specifications included Schema definition, Device Media Control Language (DMCL) definition, Data Manipulation Language (DML) definition.  It also included the concept of a database "area" which referred to the Physical Structure of the data files.  The Logical Structure of the database was defined by a Data Definition Language (DDL), and a user view of the data was defined by a subschema.

The Data Manipulation Language (DML) commands were used to navigate through the linked-list structures that comprised the database, much the same as object-oriented databases are navigated in C++.  The CODASYL DML verbs included FIND, GET, STORE, MODIFY, and DELETE.

The Data Base Administrator (Remote DBA) functions included: data structure or schema, data integrity, security, and authorization.  Also a Data Base Manager (DBM) function was defined which included: operation, backup/recovery, performance, statistics, auditing.

The CODASYL model used two data storage methods, BDAM and linked-list data structure.  BDAM used a hashing algorithm to store and retrieve records.  Linked-list is a group of items, each of which points to the next item.  The pointers establish relationships between the items.

Because of the many choices that can be made in the design of a Network database, it is important to review the design with as many people as possible.  Charles Bachman developed a "diagram" that represented the data structures as required by CODASYL.  This diagram method became known as the Bachman diagram. (Figure 2-6)

Figure 2-6  Bachman diagram layout

The Bachman diagram describes the physical constructs of all record types in the database.  The rectangles of the Bachman diagram are subdivided into four rows.  The top row of the box contains the record name.  Row two contains the numeric identification ID number (each record is given a number which is associated with the record name), the length mode which is fixed or variable, the length of the records, and the location mode (CALC, or VIA). Row three contains for CALC, the field serving as the CALC key, and for VIA SET, the set name.  Row four contains the area designated.  The set type is shown by a Bachman arrow pointing from the owner record type to the member record type.  See figure 2-7.  Set name is the owner name hyphen member name.  Pointers are Next, Prior and Owner; the membership option is used for insertion and retention (MA, OA, MM, OM); the set order is (First, Last, Next, Prior, or Sorted); and the mode is (Chain or Index).

Figure 2-7 Bachman set type

Records are stored using hashing techniques, and records that are stored "CALC" use a symbolic key to determine the physical location of the record.  In a CODASYL database records are allowed to be clustered.  Records that have "VIA" indicate that they are stored on the same physical data blocks as their owner records.  Data relationships are established by using "sets", which link the relationships together.  For example, the ORDER-LINE records are physically clustered near their ITEM records.  This is indicated in the Bachman diagram (figure 2-8) where the ORDER-LINE box shows VIA as the location mode, and the ORDER-ITEM relationship as the cluster set. The Bachman diagram is still used today.  It is a very useful graphical picture of the database schema.

Figure 2-8 Bachman diagram example

The CODASYL model combined two data storage methods to create an engine which can and does process hundred of transactions per second.  The CODASYL model uses the basic direct access method (BDAM) which uses a hashing algorithm (sometimes called a CALC algorithm) to quickly store and retrieve records.  CODASYL also employs linked-list data structures that create embedded pointers in the prefix of each occurrence of a record.  These pointers, called NEXT, PRIOR, and OWNER, are used to establish relationships between data items and are referenced in the Data Manipulation Language (DML).  For example, the DML command OBTAIN NEXT ORDER WITHIN CUSTOMER-ORDER, would direct the network database to look in the prefix of the current ORDER record, and find the NEXT pointer for the CUSTOMER-ORDER set.  The database will then access the record whose address is found at this location. 

Two advantages to the CODASYL approach were performance and the ability to represent complex data relationships.  The following example shows how BDAM is invoked for the OBTAIN CALC CUSTOMER statement, and linked lists are used in the statement OBTAIN NEXT CUSTOMER WITHIN CUSTOMER-ORDER.  

This example shows how to navigate a one-to-many relationship, (i.e. to get all of the orders for a customer), a CODASYL programmer would enter:

            MOVE 'MS' to CUST-DESC.






            WRITE OUT-REC.

Figure 2-9  Set occurrence diagram

The set occurrence diagram figure 2-9, used as a visual tool has great potential for use in object-oriented databases.  Relationships between objects become readily apparent, and a programmer can easily envision the navigation paths.  For example, in figure 2-9, you can easily see that order 1202 is for 24 carrots, 6 oranges, and 8 apples.  Now look at the "item" side of the diagram, and you can easily see which orders include apples.  When you are working with systems that physically link objects, the set occurrence diagram is an extremely useful visual tool.

The design of the CODASYL network model was very elaborate, but there were serious problems with implementation.  Network databases, very much like hierarchical databases, are very difficult to navigate.  The Data Manipulation Language (DML), especially for complex navigation, was a skill that required months of training.

Structural changes are like a bad dream with network databases, data relationships are "hard linked" with embedded pointers, to add an index or a new relationship requires special utility programs that will "sweep" each and every effected record in the database.  As records are located, the prefix is restructured to accommodate the new pointers.  Object-oriented databases will encounter this same problem if a class hierarchy needs to be modified. 

Even with these shortcomings, CODASYL databases were still far superior to any other technology of the day, and many corporations began to implement their mission-critical systems on IDMS platforms.  However, as soon as relational databases solved their speed problems and became stable enough to support mission-critical systems, the awkward and inflexible CODASYL systems were deserted.

CODASYL and the Object Database Management Group (ODMG)

Even though the CODASYL model is more than 25 years old, it is fascinating to note that there is a remarkable similarity between the CODASYL model and the internal models of today's state-of-the-art object databases.

Just as the CODASYL model required a CALC to uniquely identify a record, object databases require an "Object ID" or OID (pronounced oy-id) to identify an object.

Unfortunately, relational database models cannot address the high overhead and potential problems involved in generating OBJECT ID's.  Some theoreticians have proposed a data model which allow a single field to contain multiple values, or even another table, such as Dr. Won Kim's UniSQL database.   In a procedural language such as C++, the problem of recursion is addressed very elegantly with pointers to structures. 

The ODMG standard for object-oriented databases requires unique object ID's to identify each object, and they have deliberately not addressed the ability to access a row, based on the data contents of the row.

Many researchers have noted remarkable similarities between the CODASYL Network Model (NWM), and the requirements for object-oriented databases.  The CODASYL model supports the declaration of abstract "sets" to relate classes together, and CODASYL also supports the notion of "currency", whereby a record may be accessed without any reference to its data attributes.  CODASYL databases provide currency tables that allow the programmer to "remember" the most recently accessed record of each type, and the most recently accessed record within a set. 

Schek and Scholl, two object database researchers, state, "This shows that some of the essential features of the object model can be found in the CODASYL; CODASYL records are instances of abstract types manipulated by a limited set of functions (called FIND's), mostly for navigational access.  CONNECT and DISCONNECT are used to add or remove objects to or from relationships.  Finally, GET retrieves data about the objects into a pre-defined communications area." 

Of all of the existing database models, the CODASYL network model most closely matches the requirements for object-oriented databases, and with some refinement (such as the support of "cyclic" data relationships), the CODASYL model may re-emerge in a new form, as the standard data model for object-oriented modeling.

Some vendors are already using the CODASYL model as the architecture for object-oriented databases.  For example, the C-Data Manager from Database Technologies is an object-oriented database and programming environment which is based on the CODASYL Network Data Model, and uses ISAM file structures to index its data records.


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