Scalable Outlier/Anomaly Detection
I would take a look at t-digest algorithm. It's been merged into mahout and also a part of some other libraries for big data streaming. You can get more about this algorithm particularly and big data anomaly detection in general in next resources:


Practical machine learning anomaly detection book.
Webinar: Anomaly Detection When You Don't Know What You Need to Find
Anomaly Detection in Elasticsearch.
Beating Billion Dollar Fraud Using Anomaly Detection: A Signal Processing Approach using Argyle Data on the Hortonworks Data Platform with Accumulo