Nutch 2.0 is out (at last!)

Like pretty much any 2.0 release, Nutch 2.0 marks a radical change from the 1.x branch. I've mentioned 2.0 in previous posts but let's do a bit of history first. Nutch was initially started by Doug Cutting (Lucene's creator) and Mike Caffarella around 2002, then came the MapReduce paper from Google and in 2005 MapReduce was implemented as part of Nutch then became a sub project of Lucene at Apache. You know what happened to Hadoop after that : open source super-stardom, millions of dollars in investment, fierce competition between commercial distributions but also a myriad of related projects (HBase, ZooKeeper, Pig, Hive, Mahout etc...) with in the background the emergence of new concepts such as Big Data or NoSQL.

Meanwhile Nutch tagged along following the various releases of Hadoop but was based on the same architecture. It simply started relying on other projects more and more instead of implementing its own stuff, mainly Apache Tika (another offspring of Nutch) for parsing and extracting metadata from various document formats and Apache SOLR for indexing and searching documents. This made the code much lighter, easier to maintain and also up to date with all sorts of functionalities provided by these projects. However the way we stored and access data in Nutch remained the same since the beginning of Hadoop i.e. SequenceFiles and MapFiles.

Nutch 2.0 (a.k.a NutchGora) started in earnest 2 years ago when one of our clients decided to  invest in the development of a NoSQL-based version of Nutch. There had been a preliminary version called NutchBase developed by Dogacan Guney which was used as a basis except that instead of relying exclusively on HBase, we decided to implement our own Backend-Neutral-MapReduce-friendly-ORM which is now an Apache Top Level Project known as Apache GORA and serializes data with Apache AVRO. GORA provides us with a unified  access to various backends, NoSQL or not, an object-to-datastore mapping mechanism and utilities for MapReduce. This means that Nutch 2.0 can run on HBase, Cassandra, Accumulo or MySQL with just a few configuration files to modify.

One major change in 2.0 is that, instead of having a separation between the status of the URLs (crawlDB) separated from the data for these URLs (content and text in segments) and the webgraph (linkDB), we have a single table-like representation of the data where each entry contains everything we know about a URL, even the links that point to it or the various versions of its content (depending on the backend used). Not having separate segments is definitely good news. One of the side effects is that a fetch or parse step can be resumed. 

From a technical point of view this means that Nutch is not limited to the sequential processing of Hadoop data structures  but can operate at a more atomic level (GET, PUT). Most Nutch tasks are still MapReduce operations though but at least we can get the backends to filter the data and provide only what is needed for a specific task to the MapReduce operations.

The best example of this that I can think of is the update step in a Nutch crawl. Basically what this step does is to merge the information from a round of fetching with the rest of the CrawlDB, typically to change the status of the URLs we have fetched and add the new URLs we have discovered when parsing. With the 1.x branch this is done with a MapReduce operation which takes both the CrawlDB  and the segment as input, reduces on the URLs and updates the status of the CrawlDatum objects in the reduce step. All good. Except that as the crawlDB gets larger and larger, the time taken by the update step gets longer and longer up to a point where it ends up being the slowest part of the crawl. Think about a billion entries in the crawlDB and a single URL to update and you'll get the picture.

There are ways of alleviating this for 1.x (i.e. generate multiple segments in one go and update them with the crawldb at the same time) but the point is that with Nutch 2.0 the equivalent operation would be linear with the number of URLs modified, not the whole crawl dataset.

The change of paradigm between sequential datastructures to a table-like representation is a major change for Nutch which will certainly have many positive side-effects. Being the first release of 2.0, we can expect quite a few fixes to be needed and a massive overhaul of the documentation in the next months but the move seems to be positively welcomed by the Nutch community. Of course 1.x will continue to be the trunk for as long as necessary, i.e. until 2.0 is stable and has all the functionalities that  1.x has.

BTW my slides about 2.0 from last year's Berlin Buzzword are now here.

It is also a symbolic move, with Nutch being at the origin of many successful projects, it was about time it caught up with its famous offspring and the concepts which arose from it.