Changeset 34182 in project


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Timestamp:
06/17/17 17:18:51 (9 days ago)
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svnwiki
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Anonymous wiki edit for IP [64.134.25.154]: talked about keeping data edits in the flat files being carried over to the SDBM files too so that indexing is properly maintained. Also made mention of using a DELETE flag to mark records in the Flat Files and SDBM files (for later BATCH deletion) for an application program to recognize as a BYPASS indicator.

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  • wiki/eggref/4/sdbm

    r34181 r34182  
    2323=== Joint Database Technology
    2424
    25 Where external, binary, persistent, SDBM database files (tied to program hash tables) can really be made useful is in using the key/value pairs for random access indexing into a huge relational "text" flat file database composed of many flat files (with fixed-length records) exhibiting parent/child (1-to-many record) relationships. The key would be composed of a: single field, single partial field, or a compound key of multiple single and/or partial fields concatenated together (perhaps with a delimiter character between them such as a pipe "|"). The value in the key/value pair would the location offset (in bytes) to seek to (i.e. position the file pointer) in a flat file at the start of a specific record wished to be random accessed for: READ, READ/WRITE, or APPEND access.  Multiple SDBM files can be setup as alternate indexes into each of the Flat File database text files, each SDBM file containing a different key (composed of a: single field, single partial field, or a compound key of multiple single and/or partial fields concatenated together). When editing records, the changes are made "in place" overwriting existing data in the flat file record. In a multi-user environment, a manual record locking system could be designed to lock a specific record for editing by one user.  The username, flat file name, and record offset could be stored in an external SDBM database file (tied to a program hash table) at the time a user makes the request to edit a specific record. Once the record is released from EDIT (SAVE or CANCEL issued), then the lock is removed from the SDBM file.  Each time  a user makes a request to edit a record, the user-interface to the Flat File database would perform a Lookup to this Lock File to determine if the record in the Flat File was available for edit or already locked by another user.  If the user-interface was designed well, child records (in a 1-to-many, parent/child relationship) would not be directly editable, but only editable whenever the corresponding parent record was locked for edit.  This is a very stable relational database system. 
     25Where external, binary, persistent, SDBM database files (tied to program hash tables) can really be made useful is in using the key/value pairs for random access indexing into a huge relational "text" flat file database composed of many flat files (with fixed-length records) exhibiting parent/child (1-to-many record) relationships. The key would be composed of a: single field, single partial field, or a compound key of multiple single and/or partial fields concatenated together (perhaps with a delimiter character between them such as a pipe "|"). The value in the key/value pair would be the location offset (in bytes) to seek to (i.e. position the file pointer) in a flat file at the start of a specific record wished to be random accessed for: READ, READ/WRITE, or APPEND access.  Multiple SDBM files can be setup as alternate indexes into each of the Flat File database text files, each SDBM file containing a different key (composed of a: single field, single partial field, or a compound key of multiple single and/or partial fields concatenated together). An alternate key with duplicates can be created in the SDBM files by making as part of the key, an incremented number perhaps in the range 1-9999.  Example key:    LastName|IncNbr(perhaps in range 1-9999).
     26        Key example:  "Williams|1" ... "Williams|5745" 
    2627
    27 The binary SDBM files can easily be rebuilt from the Flat File database records. This is more desirable, then let's say, a MS-Access database, where the text data and indexes are stored together in a binary file which can become corrupted making it sometimes difficult to rescue your important textual data.  In MS-Access, the database Data and Objects: back-end Tables/Indexes/Data, and front-end Reports/Forms/Macros/etc. are often mistakenly stored in one file (in binary format) - although MS-Access does provide for the means to separate the back-end and front-end into separate files allowing for a much more stable DB system.
     28When editing Flat File records, the changes are made "in place" overwriting existing data in the flat file record. Be careful to design your user-interface so that any changes in the Flat File data are also made in any corresponding data in your SDBM file key/value pairs so that the indexing is properly maintained. A DELETE flag indicator field can be employed to mark records in both the Flat Files and SDBM files (for later BATCH deletion Server-side during off hours) for an application program to recognize as a BYPASS indicator.
     29
     30In a multi-user environment, a manual record locking system could be designed to lock a specific record for editing by one user.  The username, flat file name, and record offset could be stored in an external SDBM database file (tied to a program hash table) at the time a user makes the request to edit a specific record. Once the record is released from EDIT (SAVE or CANCEL issued), then the lock is removed from the SDBM file.  Each time  a user makes a request to edit a record, the user-interface to the Flat File database would perform a Lookup to this Lock File to determine if the record in the Flat File was available for edit or already locked by another user. 
     31
     32If the user-interface was designed well, child records (in a 1-to-many, parent/child relationship) would not be directly editable, but only editable whenever the corresponding parent record was locked for edit.   
     33
     34This is a very stable/safe database system. The binary SDBM files can easily be rebuilt from the Flat File database records. This is more desirable, then let's say, a MS-Access database, where the text data and indexes are stored together in a binary file which can become corrupted making it sometimes difficult to rescue your important textual data.  In MS-Access, the database Data and Objects: back-end Tables/Indexes/Data, and front-end Reports/Forms/Macros/etc. are often mistakenly stored in one file (in binary format) - although MS-Access does provide for the means to separate the back-end and front-end into separate files allowing for a much more stable DB system.
    2835
    2936One advantage joint/tandem/dual technology Flat File/SDBM databases have over MS-Access (for example) is that they require no MDAC (Microsoft Data Access Components) be installed to each client. ODBC-enabled MS-Access databases without the use of the MS-Access front-end software can be designed to create a huge database (perhaps to 1 Terabyte/5 Billion rows in practicality - depends on whether it is a READ ONLY Data Warehouse or a READ/WRITE Database) where each MDB file is used as a: single table, group of tables, or partial table (common to all the MDB files, and where the data is logically kept segregated for ease of random access to 1, or perhaps 2, MDB files - each MDB file containing as many as 10 million rows).
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