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erlangriakpost-commit-hookcrdt

dealing with riak datatypes in postcommit hooks

I am wanting to implement a postcommit hook for riak that decrements a counter in a map in another bucket. However I am having a bit of trouble dealing with the riak datatypes.

Here is me attempting from riak console:

([email protected])9>{ok, C} = riak:local_client().
{ok,{riak_client,['[email protected]',undefined]}}
([email protected])10>{ok, Obj} = C:get({<<"product">>, <<"default">>}, <<"1">>).
{ok,{r_object,{<<"product">>,<<"default">>},
          <<"1">>,
          [{r_content,{dict,5,16,16,8,80,48,
                            {[],[],[],[],[],[],[],[],[],[],[],[],...},
                            {{[],[],[],[],
                              [[<<"dot">>|{<<"#\tþù"...>>,{...}}]],
                              [],[],[],[],[],...}}},
                      <<69,2,0,0,0,11,114,105,97,107,95,100,116,95,109,97,112,
                        77,1,...>>}],
          [{<<35,9,254,249,108,41,151,242>>,{1,63593788980}}],
          {dict,1,16,16,8,80,48,
                {[],[],[],[],[],[],[],[],[],[],[],[],[],...},
                {{[],[],[],[],[],[],[],[],[],[],[],...}}},
          undefined}}
([email protected])11> Mp = riak_object:get_value(O3).
<<69,2,0,0,0,11,114,105,97,107,95,100,116,95,109,97,112,
  77,1,131,80,0,0,0,206,120,1,203,96,...>>
([email protected])12> MpP = riak_dt_map:from_binary(Mp).
{error,invalid_binary}

product bucket datatype is set to map. Each object stored should have a counter called quantity which I'd like to decrement.

However I cannot find any documentation or sample code dealing with datatypes in a pre or post commit context. (actually examples of any kind are few). I've been reading the source of riak_client and riak_dt_map but I am new to erlang so I'm making slow progress and would appreciate some help.

Solution

  • The r_object that you have there does not directly hold a riak_dt_map, rather it holds a riak_kv_crdt that contains a riak_dt_map that in turn contains your counter.

    To update the counter, you would need to first get the context from the map:

    {{Context,_},_}=riak_kv_crdt:value(Obj,riak_dt_map).

    Then build the operation to increment the counter named <<"name">> in the map contained in the CRDT:

    Op = {crdt_op,riak_dt_map,{update,[{update,{<<"name">>,riak_dt_emcntr},increment}]},Context}.

    And then apply that operation to the CRDT, providing the actor ID you want to use to update the vclock/version vector:

    NewObj = riak_kv_crdt:update(Obj,Actor,Op).

    The result should be another r_object that is ready to be sent back for storage.