HP 3000 Manuals HP 3000 Manuals
About Synchronization
From time to time, table definitions may lose synchronization with their
data sources. The discussion below covers:
* Why synchronization problems arise
* When to check synchronization
* What is checked for each kind of configured table
* How to correct synchronization problems
* How to avoid synchronization problems
Why Problems Arise
When Access Server accepts the definition of an IMAGE table, it means
that the table definition is synchronized with the current state of the
data source. (However, Access Server will accept the definition of a
file table whether or not the file source exists.)
When Access Server accepts the definition of a view table, it means that
the view table definition is synchronized with the current definitions of
the configured tables named in the Using Clause and with the current
state of the data sources from which those tables are derived.
* An IMAGE table definition loses synchronization when the remote
system (if any), the database, and/or the dataset named in its
definition change in such a way that the IMAGE table definition is
no longer correct.
* A file table definition loses synchronization when the remote
system (if any) or the files named in its definition change in
such a way that the file table definition is no longer correct.
Also, a file table definition loses synchronization if there are
problems with item definitions.
* A view table definition loses synchronization when the configured
tables named in its Using Clause (or the tables from which they
are derived) change their definitions in such away that the view
table definition is no longer correct.
When to Check
It's probably a good idea to run a synchronization check at regular
intervals. The frequency will depend on how changeable your data
environment is.
Specifically you ought to check tables:
* When a database changes its state (password change, name change,
location change, or item definition)
* When the items in a file change their state (name change, type
change, length change, or offset change)
* After defining a file table (particularly one derived from an MPE
flat file), to check the validity of items whose type is
free-format numeric, packed, or zoned, and to check for
overlapping item definitions
* When you change the definition of any configured table used in
defining view tables
What's Checked
What is checked depends on the kind of table involved.
IMAGE tables.
In trying to reach the IMAGE dataset from which the table is derived,
Access Server will uncover problems with the remote system definition or
the database definition.
Once the IMAGE dataset is reached successfully, the names and attributes
of each item used in the table definition are checked against the same
items in the dataset. Missing (or renamed) items or changes in item
attributes may be uncovered in this process.
File Tables.
In trying to open the file source from which the file table is derived,
Access Server will uncover problems with the remote system definition or
the file definition.
For all files, item definitions that overlap are reported. These may not
necessarily be in error. Overlap is checked regardless of whether or not
the file itself currently exists.
If the file source is an SD or BRW file, once it is opened successfully,
the names and attributes of each item used in the file table are checked
against the same items in the file source. If the file source is a KSAM
file, only the attributes of each key item used in the file table are
checked against the same key items in the file source. Since you can
alter names and since some files do not contain names, the initial basis
of the comparison is the byte offset from the beginning of the record.
If the file source is an MPE flat file, only sample values are checked
for the table definition. Because no item definitions are contained in
the file itself, no item attributes can be checked.
For all file tables, up to 12 records are checked to see if data types
are valid. However, only free-format numeric, packed, and zoned types
can be verified. If a suspect item is found, the record is identified at
the first "bad" byte or nibble within the item. All items in that record
are checked but no further records are checked.
View tables.
View table definitions are checked against the definitions of tables
found in the Using Clause of the view table. If the definitions of those
tables don't exist (they've been deleted or renamed), or if the item
names in the view table definition do not match the same items in the
"Using" tables, the synchronization check will uncover these problems.
NOTE Because no check is made against actual data, a view table
definition can be synchronized with the table definitions it draws
upon in its Using Clause, but those table definitions may not be
properly synchronized with their data sources. We recommend
checking tables closest to the data sources first (database and
file tables), then checking view tables derived from them, and so
forth.
Correcting Problems
Correct problems by using the appropriate Change screens (or Add and
Delete screens) in the Administrator Utility. In many cases, it won't be
necessary to type in a change; pressing Enter may be sufficient to update
the definition.
* For unsynchronized remote HP 3000 system definitions, use the
Change HP 3000 Systems (C3) screen.
* For unsynchronized data source definitions, use the Change IMAGE
Database (CIMD) screen or the Change File (CF) screen, as
appropriate.
* For unsynchronized table definitions, use the appropriate Change
Table (CT) screens. If you are correcting a table definition that
is used to define a view table, be sure you also check that view
table definition for synchronization.
Avoiding Problems
To avoid synchronization problems, try to be aware of:
* Changes to your data environment
* Changes to a definition on which other definitions depend
If, for example, a database definition is changed, you may then want to
check all tables derived from that database to make sure that the changed
database definition has not introduced synchronization problems.
If, for another example, one or more tables associated with a view table
have been changed, you should make any required changes to the view table
definitions.
NOTE If you change data type or data length specifications within a file
table, you might encounter 'Type mismatch' and 'Length mismatch'
messages when synchronizing view tables that reference those file
tables. These messages will go away if you use the Change View
Table screens on each of the view tables that reference the
modified file table. However, the view table will still reflect
the modified file table specifications, whether or not you use the
Change View Table screen.