current/doc/Config.txt

accessing configuration data

*       overview

Conceptually, all configuration data is bound to a database.
Databases are organized in SCHEMAS,
with one schema representing local databases
and other schemas corresponding to remote server stubs.
While many configuration options may be treated likewise
for local and remote schemas, some options, such as file paths,
are meaningful only within the local schema
and should not be distributed by servers.


*       schema and database access

Schemas are accessed by either names or numbers (one byte),
the local schema has empty name and number 0.
A remote schema's name may be an URL like "host" or "host:port"
(possibly one day even with protocol like "z3950://...")
or a symbolic name, typically referring to some local
configuration source for host and other parameters.

Within each schema, databases are accessed by either
simple names like "cds" or numbers (two bytes).
A main entry with empty name and number 0 holds options which
are global to a schema and/or serve as defaults for databases.


In addition to the configuration data,
each schema entry may contain data related to the actual implementation,
e.g. file id's for a local database or sockets in a remote schema's
main entry.


*       configuration parameters

A database's configuration can for the most part be considered one record
with nested subrecords.
For efficient access, however, several logical subrecords may
be available as records on their own and/or in a parsed representation.

A configuration entry includes
-       general parameters record
        These include traditional standard system and database parameters,
        encoding names, named texts to be interpreted as "printformats" (PFT)
        and so on. This record's metadata is
>       Builtin
-       a parsed version of the FDT
-       a parsed version of the FST
-       optional character tables
        These are corresponding to the ISISAC.TAB,
>       http://www.cindoc.csic.es/isis/21-5.htm ISISUC.TAB, SRT-files
        and recoding tables as of
>       Syspar
        106, 107.
-       optional named views
        The sort of "printformat" acting as database views.
-       stopword list (STW)

Several of these are used only for indexing and thus
are usually not needed in a remote database description.

Some environments may use additional data like work sheets (FMT).


*       implementation notes

-       mapping of names to numbers is relative to each process.
        between processes, only names may be exchanged.
-       most ressources are accessed using defaulting,
        i.e. if a parameter is not found in the entry of a given database,
        it is searched for in the schema's main entry.
-       in the traditional (WinISIS) implementation of local schema
        configuration data, named PFTs, FMTs and other are looked up as files,
        which are not actually relative to a database, but to either
        the schema's standard directory or a tool directory as of dbname.par 10.
        While the sharing of such ressources in the standard directory
        corresponds to placing them in a schema's main entry,
        other --possibly shared-- tool directories will (some day)
        be implemented by additional database entries,
        which may be referred to by other databases.


*       synchronization

All configuration ressources can be modified only
by the default session.
Other sessions can assume them to be immutable
for the lifetime of one request.

If the default session wants to change a configuration,
the database has to be remounted in single user mode,
inaccessible to other sessions.
This might involve some waiting for other sessions
to finish their current request.


To support this, each database entry has a mount status and a
count of sessions using it (not counting the default session).

For details see the paper on
>       Concurrency
.


---
        $Id: Config.txt,v 1.3 2003/02/13 16:58:45 kripke Exp $
1	dpavlin	237	accessing configuration data
2
3			* overview
4
5			Conceptually, all configuration data is bound to a database.
6			Databases are organized in SCHEMAS,
7			with one schema representing local databases
8			and other schemas corresponding to remote server stubs.
9			While many configuration options may be treated likewise
10			for local and remote schemas, some options, such as file paths,
11			are meaningful only within the local schema
12			and should not be distributed by servers.
13
14
15			* schema and database access
16
17			Schemas are accessed by either names or numbers (one byte),
18			the local schema has empty name and number 0.
19			A remote schema's name may be an URL like "host" or "host:port"
20			(possibly one day even with protocol like "z3950://...")
21			or a symbolic name, typically referring to some local
22			configuration source for host and other parameters.
23
24			Within each schema, databases are accessed by either
25			simple names like "cds" or numbers (two bytes).
26			A main entry with empty name and number 0 holds options which
27			are global to a schema and/or serve as defaults for databases.
28
29
30			In addition to the configuration data,
31			each schema entry may contain data related to the actual implementation,
32			e.g. file id's for a local database or sockets in a remote schema's
33			main entry.
34
35
36			* configuration parameters
37
38			A database's configuration can for the most part be considered one record
39			with nested subrecords.
40			For efficient access, however, several logical subrecords may
41			be available as records on their own and/or in a parsed representation.
42
43			A configuration entry includes
44			- general parameters record
45			These include traditional standard system and database parameters,
46			encoding names, named texts to be interpreted as "printformats" (PFT)
47			and so on. This record's metadata is
48			> Builtin
49			- a parsed version of the FDT
50			- a parsed version of the FST
51			- optional character tables
52			These are corresponding to the ISISAC.TAB,
53			> http://www.cindoc.csic.es/isis/21-5.htm ISISUC.TAB, SRT-files
54			and recoding tables as of
55			> Syspar
56			106, 107.
57			- optional named views
58			The sort of "printformat" acting as database views.
59			- stopword list (STW)
60
61			Several of these are used only for indexing and thus
62			are usually not needed in a remote database description.
63
64			Some environments may use additional data like work sheets (FMT).
65
66
67			* implementation notes
68
69			- mapping of names to numbers is relative to each process.
70			between processes, only names may be exchanged.
71			- most ressources are accessed using defaulting,
72			i.e. if a parameter is not found in the entry of a given database,
73			it is searched for in the schema's main entry.
74			- in the traditional (WinISIS) implementation of local schema
75			configuration data, named PFTs, FMTs and other are looked up as files,
76			which are not actually relative to a database, but to either
77			the schema's standard directory or a tool directory as of dbname.par 10.
78			While the sharing of such ressources in the standard directory
79			corresponds to placing them in a schema's main entry,
80			other --possibly shared-- tool directories will (some day)
81			be implemented by additional database entries,
82			which may be referred to by other databases.
83
84
85			* synchronization
86
87			All configuration ressources can be modified only
88			by the default session.
89			Other sessions can assume them to be immutable
90			for the lifetime of one request.
91
92			If the default session wants to change a configuration,
93			the database has to be remounted in single user mode,
94			inaccessible to other sessions.
95			This might involve some waiting for other sessions
96			to finish their current request.
97
98
99			To support this, each database entry has a mount status and a
100			count of sessions using it (not counting the default session).
101
102			For details see the paper on
103			> Concurrency
104			.
105
106
107			---
108			$Id: Config.txt,v 1.3 2003/02/13 16:58:45 kripke Exp $