EBI Index Level Table
Addressing within the database is managed by internal database entry numbers (LOID), that provide access to different kind of database entries. In this sense, a database entry is any contiguous data section in a database file. Since the size of database entries may change, database entries cannot be accessed via fixed addresses. Hence, ODABA assigns LOIDs to database entries (consecutive numbers with main base identifier), which are used to access an EBI that contains the current file position and data area number.
In the first step, the main base number is extracted from LOID. Than, the EBI index level is determined, which corresponds to the byte size of LOID after removing leading zeros (minus 1). E.g.LOID 517 (2/05) occupies two bytes an will start with level 1 in the level table. The level table points to the file location of entry 0 on that level. The level table is stored for each main base header:
EB_extHeader
|
Pos |
Size |
|
|
0 |
2 |
64-bit file access support |
|
2 |
4 |
Header identifier (e.g. EBRB for root base header) |
|
6 |
2 |
Modification count (uint16) |
|
8 |
2 |
Header size (int16) |
|
10 |
3 |
reserved for future use |
EB_extDAHeader (inherits EB_extHeader)
|
Pos |
Size |
|
|
13 |
2 |
Number of data area |
|
15 |
8 |
Maximum data area size |
|
23 |
4 |
reserved for future use |
|
27 |
2 |
Header version |
|
29 |
8 |
Next write position in data area file |
|
37 |
6 |
reserved for future use |
EB_extSBHeader (inherits EB_exDAtHeader)
|
Pos |
Size |
|
|
43 |
2 |
Number of sub base |
|
45 |
2 |
Number of data area currently in use |
|
47 |
2 |
Number of data areas |
|
49 |
4 |
Maximum entry size |
EB_extMBHeader (inherits EB_extSBHeader)
|
Pos |
Size |
|
|
53 |
2 |
main base number |
|
55 |
2 |
Number of sub bases (always 1) |
|
57 |
8 |
Last LOID for main base |
|
65 |
8 |
List of unused LOIDs (not yet supported) |
|
73 |
8 |
File position for level 0 index (EBI list) |
|
81 |
8 |
File position for level 1 index (64 bit file pointer) |
|
89 |
8 |
File position for level 2 index (64 bit file pointer) |
|
97 |
8 |
File position for level 3 index (64 bit file pointer) |
|
105 |
8 |
File position for level 4 index (64 bit file pointer) |
|
113 |
8 |
File position for level 5 index (64 bit file pointer) |
|
121 |
1 |
Platform independent storage format |
In order to get the EBI for LOID 517 (2/5), the database looks at level1 index + 2*8 and obtains the file position for the level 0 index entry. The EBI is now the 5th EBI in the index entry:
&EBI = :*(level1 index + 2*8) + 5*sizeof(EBI)
Now, when we found the EBI, we get file position and maximum size to read the database entry.