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<title>Berkeley DB Reference Guide: Defining application-specific log records</title>
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<td><dl><dt>Berkeley DB Reference Guide:<dd>Application Specific Logging and Recovery</dl></td>
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Defining application-specific log records
By convention, log records are described in files named XXX.src,
where "XXX" is typically a descriptive name for a subsystem or other
logical group of logging functions. These files contain interface
definition language descriptions for each type of log record that is
used by the subsystem.
All blank lines and lines beginning with a hash ("#") character in
the XXX.src files are ignored.
The first non-comment line in the file should begin with the keyword
PREFIX, followed by a string that will be prepended to every generated
function name. Frequently, the PREFIX is either identical or similar
to the name of the XXX.src file. For example, the Berkeley DB
application-specific recovery example uses the file
ex_apprec.src, which begins with the following PREFIX line:
<blockquote><pre>PREFIX ex_apprec</pre></blockquote>
Following the PREFIX line are the include files required by the
automatically generated functions. The include files should be listed
in order, prefixed by the keyword INCLUDE. For example, the Berkeley DB
application-specific recovery example lists the following include
files:
<blockquote><pre>INCLUDE #include "ex_apprec.h"</pre></blockquote>
The rest of the XXX.src file consists of log record descriptions. Each
log record description begins with one of the following lines:
<blockquote><pre>BEGIN RECORD_NAME DB_VERSION_NUMBER RECORD_NUMBER</pre></blockquote>
<blockquote><pre>BEGIN_COMPAT RECORD_NAME DB_VERSION_NUMBER RECORD_NUMBER</pre></blockquote>
and ends with the line:
<blockquote><pre>END</pre></blockquote>
The BEGIN line should be used for most record types.
The BEGIN_COMPAT is used for log record compatibility to facilitate
online upgrades of replication groups. Records created with this keyword will
produce reading and printing routines, but no logging routines. The recovery
routines are retrieved from older releases, so no recovery templates will be
generated for these records.
The DB_VERSION_NUMBER variable should be replaced with the
current major and minor version of Berkeley DB, with all punctuation removed.
For example, Berkeley DB version 4.2 should be 42, version 4.5 should be 45.
The RECORD_NAME variable should be replaced with a record
name for this log record. The RECORD_NUMBER variable should
be replaced with a record number.
The combination of PREFIX name and RECORD_NAME, and the
RECORD_NUMBER must be unique for the application, that is,
values for application-specific and Berkeley DB log records may not overlap.
Further, because record numbers are stored in log files, which are
usually portable across application and Berkeley DB releases, any change to
the record numbers or log record format or should be handled as
described in the <a href="../../ref/upgrade/process.html">Upgrading Berkeley DB
installations</a> section on log format changes. The record number space
below 10,000 is reserved for Berkeley DB itself; applications should choose
record number values equal to or greater than 10,000.
Between the BEGIN and END keywords there should be one optional
DUPLICATE line and one line for each
data item logged as part of this log record.
The DUPLICATE line is of the form:
<blockquote><pre>DUPLICATE RECORD_NAME DB_VERSION_NUMBER RECORD_NUMBER</pre></blockquote>
The DUPLICATE specifier should be used when creating a record
that requires its own record number but can use the argument structure,
reading and printing routines from another record. In this case, we will
create a new log record type, but use the enclosing log record type for
the argument structure and the log reading and printing routines.
The format of lines for each data item logged is as follows:
<blockquote><pre>ARG | DBT | POINTER	variable_name	variable_type	printf_format</pre></blockquote>
The keyword ARG indicates that the argument is a simple parameter of
the type specified. For example, a file ID might be logged as:
<blockquote><pre>ARG	fileID	int	d</pre></blockquote>
The keyword DBT indicates that the argument is a Berkeley DB DBT structure,
containing a length and pointer to a byte string. The keyword POINTER
indicates that the argument is a pointer to the data type specified (of
course the data type, not the pointer, is what is logged).
The variable_name is the field name within the structure that
will be used to refer to this item. The variable_type is
the C-language type of the variable, and the printf format is the
C-language format string, without the leading percent ("%") character,
that should be used to display the contents of the field (for example,
"s" for string, "d" for signed integral type, "u" for unsigned integral
type, "ld" for signed long integral type, "lu" for long unsigned
integral type, and so on).
For example, ex_apprec.src defines a single log record type, used to
log a directory name that has been stored in a DBT:
<blockquote><pre>BEGIN	mkdir		10000
DBT	dirname		DBT		s
END</pre></blockquote>
As the name suggests, this example of an application-defined log record
will be used to log the creation of a directory. There are many more
examples of XXX.src files in the Berkeley DB distribution. For example, the
file btree/btree.src contains the definitions for the log records
supported by the Berkeley DB Btree access method.
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