Implementing PageRank using MapReduce

Question

I'm trying to get my head around an issue with the theory of implementing the PageRank with MapReduce.

I have the following simple scenario with three nodes: A B C.

The adjacency matrix is here:

A { B, C }
B { A }

The PageRank for B for example is equal to:

(1-d)/N + d ( PR(A) / C(A) ) 

N     = number of incoming links to B
PR(A) = PageRank of incoming link A
C(A)  = number of outgoing links from page A

I am fine with all the schematics and how the mapper and reducer would work but I cannot get my head around how at the time of calculation by the reducer, C(A) would be known. How will the reducer, when calculating the PageRank of B by aggregating the incoming links to B will know the number of outgoing links from each page. Does this require a lookup in some external data source?

Answer 1

Here is a pseudocode:

map( key: [url, pagerank], value: outlink_list )
    for each outlink in outlink_list
        emit( key: outlink, value: pagerank/size(outlink_list) )

    emit( key: url, value: outlink_list )

reducer( key: url, value: list_pr_or_urls )
    outlink_list = []
    pagerank = 0

    for each pr_or_urls in list_pr_or_urls
        if is_list( pr_or_urls )
            outlink_list = pr_or_urls
        else
            pagerank += pr_or_urls

    pagerank = 1 - DAMPING_FACTOR + ( DAMPING_FACTOR * pagerank )

    emit( key: [url, pagerank], value: outlink_list )

It is important that in the reduce you should output outlinks and not inlinks, as some articles on the intenret suggests. This way the consecutive iterations will also have outlinks as input of the mapper.

Pay attention that multiple outlinks with the same address from the same page count as one. Also, don't count loops (page linking to itself).

The damping factor is traditionally 0.85, although you can play around with other values, too.