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Hydra is a distributed data processing and storage system originally developed at AddThis. It ingests streams of data (think log files) and builds trees that are aggregates, summaries, or transformations of the data. These trees can be used by humans to explore (tiny queries), as part of a machine learning pipeline (big queries), or to support live consoles on websites (lots of queries).

You can run hydra from the command line to slice and dice that Apache access log you have sitting around (or that gargantuan csv file). Or if terabytes per day is your cup of tea run a Hydra Cluster that supports your job with resource sharing, job management, distributed backups, data partitioning, and efficient bulk file transfer.

Documentation and References

The Hydra Documentation Page contains concepts, tutorials, guides, and the web api.

The Hydra User Reference is built automatically from the source code and contains reference material on hydra's configurable job components.

Getting Started With Hydra is a blog post that contains a nice self-contained introduction to hydra processing.

AddThis Java Code Style is the code style that hydra tries to adhere to.


Assuming you have Apache Maven installed and configured:

mvn package

Should compile and build jars. All hydra dependencies should be available on maven central but hydra itself is not yet published.

Berkeley DB Java Edition is used for several core features. The sleepycat license has strong copyleft properties that do not match the rest of the project. It is set as a non-transitive dependency to avoid inadvertently pulling it into downstream projects. In the future hydra should have pluggable storage with multiple implementations.

The hydra-uber module builds an exec jar containing hydra and all of it's dependencies. To include BDB JE when building with mvn package use -P bdbje. The main class of the exec jar launches the various components of a hydra cluster by name.

System dependencies

JDK 8 is required. Hydra has been developed on Linux (Centos 6) and should work on any modern Linux distro. Other unix-like systems should work with minor changes but have not been tested. Mac OSX should work for building and running local-stack (see below).

Hydra uses rabbitmq for low volume command and control message exchange. On a modern Linux systems apt-get install rabbitmq-server and running with the default settings is adequate in most cases.

To run efficiently Hydra needs a mechanism to take copy on write backups of the output of jobs. The is currently accomplished by adding the fl-cow library to LD_PRELOAD. Experimenting with other approaches such as ZFS or cp --reflink are under consideration.

Many components assume that there is a local user called hydra and that all minion nodes can ssh as that user to each other. This is used most prominently for rsync based replicas. The user hydra is not necessary when running a local-stack environment (see below).


On OS X several utilities are necessary to run the local-stack environment:

brew install coreutils
brew install wget


While hydra can be used for ad-hoc analysis of csv and other local files, it's most commonly used in a distributed cluster. In that case the following components are involved:

  • ZooKeeper
  • Spawn: Job control and execution
  • Minion: Task runner
  • QueryMaster: Handler for queries
  • QueryWorker: Handle scatter-gather requests from QueryMaster
  • Meshy: File server

A typical configuration is to have a cluster head with Spawn & QueryMaster backed by a homogeneous clusters of nodes running Minion, QueryWorker, and Meshy.

Local Stack

For local development all of the above components can run together in a single stack run out of hydra-local. There is a script to assist with this. To run the local stack:

  • You must be able to build hydra
  • Have rabbitmq installed
  • Allow your current user to ssh to itself

The first time the script is run a hydra-local directory will be created.

  • ./hydra-uber/local/bin/ start - start ZooKeeper
  • ./hydra-uber/local/bin/ start - start spawn, querymaster etc.
  • ./hydra-uber/local/bin/ seed - add some sample data

You can then navigate to http://localhost:5052/ and you should see the spawn web interface.

When done ./hydra-uber/local/bin/ stop will stop everything except ZooKeeper, and running stop a second time will bring that process down as well.

There are sample job configurations located in hydra-uber/local/sample/



Mailing list:

Freenode channel: #hydra


It's x.y.z where:

  • x: Something Big Happened
  • y: next release
  • z: strive for bug fix only


hydra is released under the Apache License Version 2.0. See Apache or the LICENSE file in this distribution for details.


Hydra logo by Appy Vohra.

Hydra Logo