Difference between revisions of "GOCDB/PROM"
|Line 1:||Line 1:|
This page is part of the [[
This page is part of the [[|GOCDB4 Architecture Documentation]]
== PROM (Pseudo-Relational Object Model) ==
== PROM (Pseudo-Relational Object Model) ==
Revision as of 11:31, 23 December 2010
This page is part of the GOCDB4 Architecture Documentation
PROM (Pseudo-Relational Object Model)
The PROM is a database model implemented on top of a standard database. PROM's main goal is to facilitate schema changes, without affecting existing software that uses the database or the existing stored data.
This is achieved by storing the relations in a set of database tables and enforcing these relations through an external API rather than querying the database directly.
Objects: Groups of Data
PROM group data together in objects, rather than in tables. Each object is of a specific type and each object from this type is stored in a specified database table. There is no limit to the number of different object types that can be stored in a single table provided that the objects contain identically formatted fields. A complete list of all the objects stored in the database can be found in a table named #TOBJECTS. Each object type is associated with a unique type number. A list of object types, descriptions of the type and associated type numbers can be found in the table #TOBJECT_TYPES.
Each object is assigned a global ID in the local database known as an object ID. This ID is stored alongside the individual data for each object in the associated data table.
An example from the GOCDB4 schema is as follows:
This table stores a complete list of all the objects in the system by their unique object ID. It also contains a database ID to identify the object as belonging to this particular local database (allowing for distributed databases, more on that later), as well as an object type number. This table also stores the database ID that the type belongs to as well as date on and date off fields (more on these later too).
The following record refers to data about a particular user. This example will show how to find the type of an object and the associated data stored for this particular object.
|270||0||105||0||17-APR-09 09.31.16.000000 AM||-|
This record shows an object with an objectID of 270 of type 105. This object type identifier can be used to find information relating to the storage of this particular type of object by searching the TOBJECT_TYPES table.
|105||0||Registered users, operators and contacts||GRIDCORE||-||GOCDB_USERS||01-JAN-09 01.01.01.000000 AM||-|
In this table there is a tuple with a type ID of "105" with a description of "Registered users, operators and contacts". One of the fields of this tuple is CTABLENAME indicating the name of the table in which to store this type of data. The CTABLENAME field for this particular record contains "GOCDB_USERS", indicating the table name where user data is stored.
Notice that the object type has fields to store "date on" and "date off". These are used to help with schema changes and is discussed in more detail further on in this page.
The corresponding specific object table holding the user data (GOCDB_USERS) is as follows:
Here you can see a record with the object ID initially found in the TOBJECTS table along with the associated user record data.
Links between objects
In order to represent relations between objects, a parent-child link is created. A list of all of the parent-child links in the PROM database are stored in the table "TOBJECTLINKS". A link can only appear in this table if it's parameters (parent object type, child object type) appear in a list of link types. These link types are stored in a separate table named "TLINK_TYPES".
Here is an example record from the TOBJECTLINKS table:
|336||0||337||0||8||76||17-APR-09 03.27.39.000000 PM||-|
This example shows a link between two objects where the parent object has the object ID 336 and the child object has the object ID 337. The database ID (called GRID ID in GOCDB4) that each object belongs to is also present. Note that this table doesn't store any information about what type the objects are.
The "CLINKID" field specifies a link type value that corresponds to an entry in the "TLINK_TYPES" table.
What follows is the corresponding record from the TLINK_TYPES table:
|8||105||0||101||0||Y||user to site - homesite of a given user||01-JAN-09 01.01.01.000000 AM||-|
This entry shows the data used for link type 8. It specifies that the parent object must be of type 105 and that the child object type must be 101. A description of this link is given, in this case the link describes a user to site relationship. Note that the link includes a date on and date off field, more on this later.
The object IDs listed in the TOBJECTLINKS table correspond to a user object (the parent of the link) and a site object (the child of the link). Note that the API doesn't allow links to be created if an appropriate link type entry is not present in the TLINK_TYPES table.
Date On / Date Off
In order to facilitate schema changes, each object, object type, link and link type have associated date on and date off fields. These fields are used to indicate a time period during which the data stored is valid. By querying with an appropriate time stamp, the schema of the database at that particular time can be recalled while at the same time allowing queries to be executed on newer schema by specifying a different time stamp.