Differences between DatabaseBridge and ISAM
DatabaseBridge allows you to keep the same approach used for ISAM files despite it works on a relational database. However there are few differences between the standard ISAM and the ISAM simulated by DatabaseBridge. These differences are discussed below.
Locks Management
To emulate the COBOL locks behavior, DatabaseBridge takes advantages of specific database features that are different from database to database.
The lock of a single record is fully supported when using Oracle, Informix, MS SQL Server, MySQL, MariaDB, DB2 and PostgreSQL.
The lock on multiple records is not officially supported. DatabaseBridge always takes one lock at a time because it re-uses the same ESQL cursor for reading a record with lock. If the program locks multiple records working, it depends by a database configuration that keeps locks alive.
The lock a whole file/table is supported when using Oracle, Informix, MS SQL Server, DB2 and PostgreSQL. MySQL and MariaDB don’t support the ability to lock a whole file/table.
The following operations are not currently supported by the DatabaseBridge. Contact Veryant if you need any of these capabilities:
• Lock between two programs in the same runtime session
• Unlock all records of all open files with UNLOCK ALL
• Unlock a record of a file with CLOSE (with lock on single record)
• Unlock all records of a file with CLOSE (with lock on multiple records)
• Unlock single record with UNLOCK (with lock on single record)
• Unlock single record with UNLOCK (with lock on multiple records)
• Unlock single record by reading another one (only with lock on single record)
• Unlock single record after REWRITE (with lock on single record)
• Unlock single record after REWRITE (with lock on multiple records)
Note: If you're working in a Application Server (Thin Client) or File Server environment and you wish to have a full support for locking features, then you may consider handling locks through the
Internal lock management.
Moving among duplicate key values
When you change the order of read (e.g. you perform a READ NEXT after a READ PREVIOUS or a READ PREVIOUS after a READ NEXT) among duplicated values of an alternate key, the EDBI routine retrieves the first record whose key value doesn't match with the current one.
A practical example follows.
Suppose to have a file with the following content:
Record |
|---|
Primary Key | Alt. Key |
|---|
1 | AAA |
2 | BBB |
3 | BBB |
4 | BBB |
The table below lists a series of operations and tells which record is read by ISAM file handlers like JIsam or c-tree versus the record read by DatabaseBridge:
Operation | Record read by ISAM | Record read by DatabaseBridge |
|---|
MOVE "BBB" TO Alt. Key | ... | ... |
START KEY NOT LESS Alt. Key | ... | ... |
READ NEXT | 2 BBB | 2 BBB |
READ NEXT | 3 BBB | 3 BBB |
READ PREVIOUS | 2 BBB | 1 AAA |