CODEWITHSUNDEEP
erlangmapreduceriak
user436605
user436605

Reputation: 421

how to sort documents using the erlang map reduce in riak

i'm using riak to store json documents right now, and i want to sort them based on some attribute, let's say there's a key, i.e

{ "someAttribute": "whatever", "order": 1 }

so i want to sort the documents based on the "order".

I am currently retrieving the documents in riak with the erlang interface. i can retrieve the document back as a string, but i dont' really know what to do after that. i'm thinking the map function just reduces the json document itself, and in the reduce function, i'd make a check to see whether the item i'm looking at has a higher "order" than the head of the rest of the list, and if so append to beginning, and then return a lists:reverse.

despite my ideas above i've had zero results after almost an entire day, i'm so confused with the erlang interface in riak. can someone provide insight on how to write this map/reduce function, or just how to parse the json document?

Upvotes: 2

Views: 1071

Answers (3)

user436605
user436605

Reputation: 421

so i "solved" the problem using javascript, still can't do it using erlang. here is my query

{"inputs":"test",
 "query":[{"map":{"language":"javascript",
                  "source":"function(value, keyData, arg){ var data = Riak.mapValuesJson(value)[0]; var obj = {}; obj[data.order] = data; return [ obj ];}"}},
         {"reduce":{"language":"javascript",
                    "source":"function(values, arg){ return [ values.reduce(function(acc, item){ for(var order in item){ acc[order] = item[order]; } return acc;  }) ];}",
                    "keep":true}}
         ]
}

so in the map phase, all i do is create a new array, obj, with the key as the order, and the value as the data itself. so visually, the obj is like this 

{"1":{"firstName":"John","order":1}

in the reduce phase, i'm just putting it in the accumulator, so basically that's the sort if you think about it, because when you're done, everything will be put in order for you. so i put 2 json documents for testing, one is above, the ohter is just firstName: Billie, order 2. and here is my result for the query above

[{"1":{"firstName":"John","order":1},"2":{"firstName":"Billie","order":2}}]

so it works! . but i still need to do this in ERLANG, any insights?

Upvotes: 1

Ben Ahlan
Ben Ahlan

Reputation: 507

This is not a map reduce solution but you should check out Riak Search.

Upvotes: 1

user425720
user425720

Reputation: 3598

As far as I know, You do not have access to Input list in Map. You emit from Map a document as 1 element list.

Inputs (all the docs to handle as {Bucket, Key}) -> Map (handle single doc) -> Reduce (whole list emitted from Map).

Maps are executed per each doc on many nodes whereas Reduce is done once on so called coordinator node (the one where query was called).

Solution:

  1. Define Inputs (as a list or bucket)
  2. Retrieve Value in Map and emit whole doc or {Id, Val_to_sort_by)
  3. Sort in Reduce (using regular list:keysort)

Upvotes: 2

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