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RON RDT: LWW-per-field

It seems very useful to implement not just plain last-write-wins (LWW) register, but with Cassandra-inspired extension, namely the field.

LWW-per-field is the same as LWW register but identified by the pair (object, ref) in terms of RON.

Plain LWW register may be expressed as a field of an LWW-per-field object with any specific op ref. Let it be 0.

From the other hand, an LWW-per-field object is a product (vector) of LWW registers.

So, with this type we can encode both a simple LWW register and a product (vector) of LWW registers.

Also, single LWW register isn't a monoid, but LWW-per-field object is.

1. Type UUID

RON text:

lww

Hexadecimal:

0x c3be c000 0000 0000
   0000 0000 0000 0000

2. Single op representation

  1. event encodes timestamp of write.
  2. ref encodes the object field.

2.1. Examples

  1. Write an integer to the default field

    *lww #38+foxtrot @40+golf :0 =35 ;

  2. Write a tuple of an integer and a string to the field alfa

    *lww #38+foxtrot @41+golf :alfa =42 'bravo' ;

  3. Simultaneous write to several fields of the same object

    *lww #38+foxtrot @43+golf :alfa    =42     ;
*lww #38+foxtrot @43+golf :charlie 'bravo' ;

    the same with compression

    *lww #38+foxtrot @43+golf :alfa    =42     ;
                          :charlie 'bravo' ;

3. Value representation

  1. Reduced chunks ops use the same format as raw ops.
  2. The order of fields doesn't matter. For reproducibility, ops should be sorted by ref.

3.1. Examples

  1. Empty object

    *lww #38+foxtrot @38+foxtrot :0 !

  2. Ops from example 2.1.3 applied to empty object

    *lww #38+foxtrot @43+golf :0               !
*lww #38+foxtrot @43+golf :alfa    =42     ,
*lww #38+foxtrot @43+golf :charlie 'bravo' ,

    the same with compression

    *lww #38+foxtrot @43+golf :0               !
                          :alfa    =42     ,
                          :charlie 'bravo' ,

  3. Write string "delta" to the field charlie of the object #38+foxtrot from the previous example. Op:

    *lww #38+foxtrot @98+hotel :charlie 'delta' ;

    Result value:

    *lww #38+foxtrot @98+hotel :0               !
                 @43+golf  :alfa    =42     ,
                 @98+hotel :charlie 'delta' ,

4. Patch representation

The same as the value encoding.

4.1. Examples

  1. Ops from example 2.1.3 combined

    *lww #38+foxtrot @43+golf :43+golf         !
*lww #38+foxtrot @43+golf :alfa    =42     ,
*lww #38+foxtrot @43+golf :charlie 'bravo' ,

    the same with compression

    *lww #38+foxtrot @43+golf :43+golf         !
                          :alfa    =42     ,
                          :charlie 'bravo' ,

  2. Write string "delta" to the field charlie of the object #38+foxtrot and merge this op into the patch from the previous example. Op:

    *lww #38+foxtrot @98+hotel :charlie 'delta' ;

    Result patch:

    *lww #38+foxtrot @98+hotel :43+golf         !
                 @43+golf  :alfa    =42     ,
                 @98+hotel :charlie 'delta' ,

5. Reduction

  1. For each field in a value or patch there must be exactly one op with corresponding ref if the field was set, and none otherwise.
  2. The result op for each field is selected by the LWW rule — which event is greater.