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Building your own Open SkyNode
Intro
This guide is designed to simply outline requirements for a full compliance
open skynode in the open skyquery project, or just osq. If you're building your
own skynode, it should give you an idea of what it will need to do, but will
not go into any implementation specific details; exactly how you accomplish
these items is up to you.
Overview
Open SkyQuery is consists of:
NVO Registry
All nodes must be registered in this registry. To view all nodes through the
registry, go to advanced search and look for "serviceType like 'skynode'".
Required fields for skynodes are 'ShortName', 'SkyNode Compliance' and
'ServiceUrl'.
Open SkyNodes
The meat of OSQ. Nodes are where the queries are actually run; this is where
the real work happens. They are not particullarly trivial to implement, but
they are not that difficult either. See this guide for more info.
Open SkyPortal
The starting point for a query is the portal. Here it is divied up organized
into sorted plan and sent off to the first node. This particular portal
provides asychronous execution and decent query status tracking, but these are
bonus features. The only thing a portal really has to do is split up a query
and ship it.
Execution Overview
After the portal has constructed an object containing node-specific queries it
then sends it to the first node. However, this node does not execute its
section yet, instead it passes it off to the next node and this continues until
it reaches the last node. Then this last node will run its section, return its
results to the previous node, and continue until it has reached the portal
again. This is less confusing than it sounds (no, really), and is a nice, easy
way to implement a distributed query without having to track state.
The ExecPlan
This is the actual object or document that the portal passes to the nodes. A
sample is provided below.
Nodes need only care about two sections in the plan; the 'Format' and the
'PlanElements'. Format is the specified transport for this particular plan, and
you should expect to receive and send results in that format. It is almost
always VOTABLE, but may occasionally be DataSet. VOTABLE is the only required
supported format for nodes.
The PlanElements object is an array of PlanElement objects. Each PlanElement is
the relevant section for a particular node; each node need only execute its own
PlanElement. A PlanElement consists of 'Statement', 'Hosts' and 'Target'.
Target is the shortName (from the registry) of the intended node, and Hosts is
a list of mirrors for that node (serviceURL in the registry). The PlanElements
array is sorted from lowest index to highest; so the the node at
PlanElements[0] would be the first to receive the plan.
Implementations
There's extensive information on building a skynode with SQLServer at
SkyServer.org.
To turn this into an open skynode you just need :
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Download
the latest node template
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extract it into the same directory.
There is also a template for the Basic SkyNode in Java. This was most recently
used at the NVO Summer school and is in the
summer school download. See the last part of
the Server Side tutorial
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