Friday, March 26, 2010

Fortune Cookie Says

It is better to have beans and bacon in peace than cakes and ale in fear.

Friday, February 19, 2010

Fortune Cookie Says

Your dream must be bigger than your fear.

nosql - hypertable

I've been looking at some of the nosql databases and have been playing around with Hypertable a little. I had some difficulties getting my head around it, probably due more to my long experience with RDBMSs than Hypertable itself, so here are some notes about it.

Creating a table is analagous to creating a table in an RDBMS.

hypertable> create table foo ("bar", "bat");

Inserting data into the table is quite a bit different than what I am used to. First is that each 'row' inserted into the tab must have a row key, basically there is an implied row id column in every table. Second is that the insert is done as individual (row, column, value) tuples or cells. Third is that a single insert statement can insert into any number of cells for any number of rows for a single table. Fourth is that the columns are not really columns in the RDBMS sense but can hold data and multiple 'sub-columns' which can be dynamically defined.

hypertable> insert into foo values
         -> ("zap", "bar", "cat"),
         -> ("zap", "bat", "tap"),
         -> ("car", "bar:a", "nar"),
         -> ("car", "bar:b", "whal");
hypertable> select * from foo;
car    bar:a    nar
car    bar:b    whal
zap    bar    cat
zap    bat    tap
Each value inserted into the table is a tuple of '(' <row key>, <column name> [':'<column qualifier>], <value> ')'

There is no update statement, updates are simply inserts and Hypertable keeps history of all changes and timestamps each. Specifying an option of REVS=1 is analogous to a select in an RDBMS and gives the current revision of the row.

hypertable> insert into foo values
         -> ("zap", "bat", "lap"); 
hypertable> select * from foo where ROW="zap" DISPLAY_TIMESTAMPS;
2010-02-19 17:47:32.535534003    zap    bar    cat
2010-02-19 17:58:22.743904001    zap    bat    lap
2010-02-19 17:47:32.535534004    zap    bat    tap

hypertable> select * from foo where ROW="zap" DISPLAY_TIMESTAMPS REVS=1;
2010-02-19 17:47:32.535534003    zap    bar    cat
2010-02-19 17:58:22.743904001    zap    bat    lap
How the where clause is used  in Hypertable is quite different than in an RDBMS.  The value 'ROW' is the row key and can be used pretty much in the same way as a column in a boolean operation in SQL.
hypertable> select * from foo where ROW<="zap" DISPLAY_TIMESTAMPS REVS=1;
2010-02-19 17:47:32.535534001    car    bar:a    nar
2010-02-19 17:47:32.535534002    car    bar:b    whale
2010-02-19 17:47:32.535534003    zap    bar    cat
2010-02-19 17:58:22.743904001    zap    bat    lap
There is also a CELL option. I wanted to use this to select on column contents but it does not work that way. It allows one to select rows and columns by  row keys and column names. This probably has the most benefit if one uses column qualifiers ( dynamically created sub-columns).
hypertable> select * from foo where "a", "bar:a" <= CELL <= "zz", "bar:z";
car    bar:a    nar
car    bar:b    whale
zap    bar    cat
zap    bat    lap
zap    bat    tap
This basically sets up a range query where "a" <= ROW <= "zz" and there is a column such that "bar:a" <= column name <= "bar:z"

Wednesday, February 10, 2010

Unpacking 7-bit ASCII

To (finally) complete my series of posts on processing CDMA SMS bearer data and user data fields. In my cases the actual text of the SMS message is packed 7-bit ASCII which corresponds to messages with an encoding flag of ENCODE_7BIT, or 0x10. A packed message then looks something like the following.


Which packs 8 characters (a - h) into 7 octets. To unpack the message we have to pick out each individual character and make it an 8-bit ASCII character.  It helps if we reorganize the data to look at each character individually as follows.

|aaaaaaax| |xxxxxxxx|
|xxxxxxxb| |bbbbbbxx|
|xxxxxxcc| |cccccxxx|
|xxxxxddd| |ddddxxxx|
|xxxxeeee| |eeexxxxx|
|xxxfffff| |ffxxxxxx|
|xxgggggg| |gxxxxxxx|
|xhhhhhhh| |xxxxxxxx|

From this we see that a 7-bit ASCII character can be packed into octets in one of 8 patterns. So to unpack the characters we need to know the current octet, the next octet, and the packing pattern. With that we can apply some bit shifting and bit masks to create an unpacked 8-bit ASCII character. 

I coded this so that the pattern associated to the character 'a' above is pattern 0. And the user data headers cause the SMS message to start at packing pattern 3, which matches the character 'd' above. The routine to decode the SMS message then just becomes a simple loop over the 7-bit ASCII characters while keeping track of the packing pattern.

/* copyright (c) 2010 Steve Hill - All rights reserved */
decode_7bit_ascii( uint_8 *sms, uint_8 len, uint_8 startPat )
    uint_8 buffLen = len + 1;
    char *buff     = malloc( sizeof(char) * buffLen );

    memset( buff, 0, sizeof(char) * buffLen );
    char   *currChar = buff;        // current char in buff
    char   *lastChar = buff + len;  // last char in buff
    uint_8 *curr     = sms;         // current byte being converted
    uint_8 *next     = curr + 1;    // next byte
    uint_8  currPat  = startPat;    // conversion pattern

    while( currChar < lastChar )
        switch( currPat )
            case 0:     // aaaaaaax xxxxxxxx
                *currChar = ( *curr >> 1 ) & 0x7F;
            case 1:     // xxxxxxxa aaaaaaxx
                *currChar = (( *curr << 6 ) & 0x40 ) + 
                            (( *next >> 2 ) & 0x3F );
            case 2:     // xxxxxxaa aaaaaxxx
                *currChar = (( *curr << 5 ) & 0x60 ) + 
                            (( *next >> 3 ) & 0x1F );
            case 3:     // xxxxxaaa aaaaxxxx
                *currChar = (( *curr << 4 ) & 0x70 ) + 
                            (( *next >> 4 ) & 0x0F );
            case 4:     // xxxxaaaa aaaxxxxx
                *currChar = (( *curr << 3 ) & 0x78 ) + 
                            (( *next >> 5 ) & 0x07 );
            case 5:     // xxxaaaaa aaxxxxxx
                *currChar = (( *curr << 2 ) & 0x7C ) + 
                            (( *next >> 6 ) & 0x03 );
            case 6:     // xxaaaaaa axxxxxxx
                *currChar = (( *curr << 1 ) & 0x7E ) + 
                            (( *next >> 7 ) & 0x01 );
            case 7:     // xaaaaaaa xxxxxxxx
                *currChar = *curr & 0x7F;

        if( currPat ) // stay on current byte if pattern 0
        currPat = ++currPat % 8;

    return buff;

Fortune Cookie Says

You will make many changes before settling down happily.