In June 2011, I attended the Berlin Buzzwords conference. The main theme of the conference was undoubtedly the current paradigm shift in distributed processing, driven by the major success of Hadoop. Doug Cutting – founder of Apache projects such as Lucene, Nutch and Hadoop – held one of the keynotes. He focused on what he recognized as the new foundations for this paradigm shift:
- Commodity hardware
- Sequential file access
- Automated, high level reliability
- Open source
Distributed processing is done fairly well with Hadoop. Distributed search on the other hand is more or less limited to sharding and/or replicating the index. The downside of sharding is that you perform the same search on multiple servers and then need to combine the results. Due to the nature of algorithms in search such as tf/idf, tasks like ranking results suffers. Andrzej Białecki (another frequent Lucene committer) held a presentation on this topic, and his view can be summarized as: Use local search as long as you can, distribute only when the cost of local search limitations outweighs the cost of distributed search.
The setup of automated replication and sharding, with help from Zookeeper in the Solr Cloud project, is a major step in the right direction but the question on how to properly combine search results from different nodes still remains. One thing is sure though, there is a lot of interesting work being done in this area.