Some time ago we ran into strange behaviour of MongoDB’s GridFS which caused me creating a Bug Ticket for the MongoDB Java driver.
Today I found the link to the bug ticket in my browser bookmarks. The ticket isn’t solved at the current time so I thought it would be worth a short blog post in case someone else runs into this problem.
Let’s look at the following simplified Java service:
public class GridFsService {
private GridFS gridFs;
public void connect(String mongoDbHost, String databaseName) throws UnknownHostException {
DB db = Mongo.connect(new DBAddress(mongoDbHost, databaseName));
this.gridFs = new GridFS(db, "myBucket");
}
public void removeGridFsFile(String id) {
GridFSDBFile file = this.gridFs.findOne(new ObjectId(id));
this.gridFs.remove(file);
}
// .. other methods to create and update files
}This service uses the MongoDB Java driver to create, update and remove files from GridFS. However, there is a serious flaw in the removeGridFsFile() method. Guess what happens if an invalid id is passed to removeGridFsFile(). gridFs.findOne() returns null for non existent ids. So null is passed to gridFs.remove() which then removes all files in the current bucket.
Fixing this is easy. Just add a null check or use another GridFS remove() method that takes an ObjectId instead of a GridFsDBFile:
public void removeGridFsFile(String id) {
this.gridFs.remove(new ObjectId(id));
}Using this way everything works fine if an invalid id is passed to removeGridFsFile() (no file is removed). To make sure this won’t happen again I tested what happens if null is passed to any of the three different remove() methods:
gridFs.remove((String)null); // nothing happens gridFs.remove((ObjectId)null); // nothing happens gridFs.remove((DBObject)null); // all files from bucket are removed
I don’t know if this is intended behaviour. The Javadoc comment for gridFs.remove(DBObject query) tells me that it removes all files matching the given query. However, if it is intended I think it should be clearly stated in the javadoc comment that passing null removes all files in the bucket.
Thanks for the tips. GridFS is a very useful abstraction on top of MongodB. http://blog.mongodirector.com/when-to-use-gridfs/