Aws DynamoDb Counter

Increments or decrements a counter in Dynamodb. The counter is atomic and idempotent. It works by saving the id in a string set and only incrementing the counter if the id is present in the set and only decrementing the counter when the id is present in the set.

Example

Counter(table, hashKeyName, hashKeyValue, [rangeKey, rangeKeyValue])

table - dynamo table

hashKeyName - hash key in table to increment

hashKeyValue - hash key value

rangeKey - range key in table to increment

rangeKeyValue - range key value

    var Counter = require('./counter')('dbName', 'HASH_KEY', 'HASH_KEY_VALUE');

increment(fieldsToIncrement, id, callback)

fieldsToIncrement - object containing (field name):(increment by) key vals to increment by

id = unique id of event (md5 of the event object)

    UpdateExpression: ADD #id :ss, #counter :increment_by
    ConditionExpresssion: attribute_not_exists(#id) or (NOT contains(#id, :id))

    Counter.increment({ words:10, games:1 }, '55ad340609f4b302', function(err) {
        if (err) {
            throw err;
        }
 
        // Success!
    });
 

decrement(fieldsToDecrement, id, callback)

fieldsToDecrement - object containing (field name):(increment by) key vals to increment by

id = unique id of event (md5 of the event object)

   UpdateExpression: DELETE #id :ss SET #counter = #counter - :decrement_by
   ConditionExpresssion: attribute_exists(#id) and contains(#id, :id)

    Counter.decrement({ words:10, games:1 }, '55ad340609f4b302', function(err) {
        if (err) {
            throw err;
        }
 
        // Success!
    });