Started in summer semester 2012 at the University of Applied Science, Wedel
Created by: Christian Kulpa chris@gethydrated.org, Hanno Sternberg hanno@gethydrated.org
Overseen by: Prof. Dr. Ulrich Hoffmann
Hydra is a distributed computing network, which acts as a service plattform.
The name originates from the creature out of the greek mythology the Lernaean Hydra. It was described as a creature with nine heads, one immortal in the center and eight mortal heads around. Killing this creature was one of the twelve Labours of Heracles. Whenever Hercules sliced off one head, two new heads appeared. He could only overcome the creature, because of his nephew Iolaus, who cauterized the stumps of the heads after Hercules decapitated them.
This distributed computing network with its nodes is like a hydra with its heads.
Like every other network, the hydra is build from different interconnected nodes. A node can either be a dedicated or a virtual machine. It is even possible to connected two JVMs on the same host machine.
The network is fully interconnected, allowing every node to directly communicate with the others. By monitoring the TCP connections, the nodes can easily detect, if a node is still up and reachable.
The nodes share a global registry, which holds information about a network structure. Each time a new nodes connects into the network, it will receive a eplica of the registry. It is seen as consensus, that the registry is always in sync. The ability to force a re-sync will be added later.
The network shares an eventstream, acting as an abstraction layer for the interprocess communication.
A single Node inside the net has two main layers. On the bottom layer, their is an actor system, encapsulating every process, so they behave like being atomic.
Each actor can listen to specific messages inside the eventstream and can send messages in response. The system will also generate eventmessages, if for example a new node connects to the network.
In the layer above the actor system, their are services. A service can be everything, from a single (low level) database query to a complex business application, build from other lower level services.
This layer holds the "userland" - the application forming code. As a result, it should by atypical to have the need to write an actor as part of a user service or application.
Hydra uses the gradle buildsystem.
It comes bundled with a wrapper script gradlew.
Assuming the repository is already checked out, open a command line and type:
gradlew build
This will start the build process. The initial build will take a bit longer as usual as gradle downloads all the dependencies needed for the build.
Hydra can be run from the command line by executing the
build\assemble\bin\hydra startup script. This script can also be executed
from a graphical file browser.
Inside the tools\typhon directory is a python script (typhon.py)
which provides a small command line interface to hydra.
It will try to locate the gradle wrapper and offers the ability to build and
start a local hydra instance.
After successfully starting the hydra instance the builtin command line interface will open up. This serves a the main administration tool.
To connect to another network you need to set up your own network connector interface. This is done by typing
port <PORT>
into the command line. <PORT> is the network port, on which the hydra will
listen to incoming packets.
A connection to a remote node can be established using the connect command.
connect <REMOTE_IP> <REMOTE_PORT>
<REMOTE_IP> is the address of the remote node; <REMOTE_PORT> refers to the
port on which the remote node is listening.
After the connection is established, the two nodes will exchange information about their peer network and start interconnecting to every other node inside the network.
It is possible to switch between the nodes inside the network, from the builtin command line interface.
Type
nodes
to get a list of all nodes in the network.
To switch to another node type
node <Number>
where <Number> is the number of the node in the nodes list.
To switch back to your local node type
local
To shutdown your hydra instance, type
shutdown
The complete reference for all commands can be found in the wiki.