Re: Revised formatv3 description

Hi Rony,

one more thing that just occured to me:

On Mon, 16 Jan 2006 16:01:25 -0500, Rony Shapiro <[email protected]> wrote:
>
> 2.8.1 HDR: The database header. All data in the header is written in
> fields, as defined in Section 3. The first field contains the version
> number of the database format. For this version, the value is 0x0300
> (stored in little-endian format, that is, 0x0003). The type of this field
> is zero.  The next field is the database's UUID, stored as 16 bytes. The
> type of this field is zero.  Following this, non-default user preferences
> are written as a string (as described below), with field type zero.
> Currently, no further data is written. To allow further enhancements, the
> database header is terminated by an empty field of type 'END'. This will
> allow older versions of the program to skip over records that may be  
> added over time to the header.
>

If the field types are zero for all header fields, then their sequence
becomes important, and it will be impossible to support only a subset of
fields -- you could support a prefix, but a software would not be able to
omit header fields from in the middle. E.g., in the above example, a
software could not omit the version but still have a UUID. This smells
like a potential future issue. So I suggest to clearly define field ids
and types, as you do for the records. I.e., something like:

Name          value  Type    Implemented Comments
-------------------------------------------------
Version       0x1    short   Y           [1]
UUID          0x2    UUID    Y           [2]
Preferences   0x3    Text    Y           [3]
End of Header 0xff   [empty] Y

[1] Stored as a 2 byte little-endian integer. The higher 8 bits contain
the major version number, the lower 8 bits the minor version number of
the database file format. This document corresponds to version 3.0.

[2] A 16-byte UUID value as defined by RFC 4122.

[3] Non-default preferences stored as described in formatV2.txt.

Frank

-- 
Frank Pilhofer, [email protected]


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