My last two posts have described recent attempts at working with Hadoop, a map/reduce framework. As I noted, Hadoop for cheminformatics is quite trivial when working with SMILES files, which is line oriented but requires a bit more work when dealing with multi-line records such as in SD files. But now that we have a SDFInputFormat class that handles SD files we’re ready to throw a variety of tasks at it. In this post I describe a class to perform SMARTS substructure searching using the CDK and Hadoop.

Doing substructure searching in a map/reduce framework is conceptually quite simple. In the map step, the mapper gets a record (in this case a single entry from the SD file) and performs the match using the supplied SMARTS pattern. It emits a key/value pair of the form “molid 1” if the molecule matched the pattern, otherwise “molid 0” if it did not. In either case, molid is some identifier for the given molecule.

The reduce step simply examines each key/value pair it receives, and discards those with values equal to 0. The resultant output will contain the ID’s (in this case molecule titles, since we’re reading from SD files) of the files that matched the supplied pattern.

The source for this class is given below

The map method in MoleculeMapper does the job of performing the SMARTS matching.  If the molecule matches, it writes out the molecule title and the value 1. The reduce method in SMARTSMatchReducer simple examines each key/value and writes out those keys whose value equals 1.

Another important thing to note is that when we pass in the SMARTS pattern as a command line parameter, it doesn’t automatically become available to the mappers since they will, in general, be run on different nodes that the one you started the program. So naively storing a command line argument in a variable in main will result in a NPE when you run the program. Instead, we read in the argument and set it as a value for a (arbitrary) key in the Configuration object (line 90). The object can then be accessed via the Context object in the mapper class (lines 43-45), wherever the mapper is being run.

We compile this to a jar file, and then run it on a 100-molecule SD file from Pub3D:

The output is of the form

where each line lists the PubChem CID of the molecules that matched (27 in this case).

Postscript

While I’ve been working on these examples with relatively small inputs on my own laptop, it’d be useful to test them out with larger datasets on a real multi-node Hadoop cluster. If anybody has such a setup (using 0.20.0 of Hadoop), I’d love to try these examples out. I’ll provide a single jar file and the large datasets.