The Overseer isn't mentioned much because it's an implementation detail that the user doesn't have to really consider.
The Overseer first came about to handle writing the clusterstate.json file, as a suggestion by Ted Dunning. Originally, each node would try and update the custerstate.json file themselves - and use optimistic locking and retries. We decided that a cleaner method was to have an overseer and let new nodes register themselves and their latest state as part of a list - the Overseer then watches this list, and when things change, publishes a new clusterstate.json - no optimistic locking and retries needed. All the other nodes watch clusterstate.json and are notified to re-read it when it changes. Since, the Overseer has picked up a few other duties when it makes sense. For example, it handles the shard assignments if a user does not specify them. It also does the work for the collections api - eventually this will be beneficial in that it will use a distributed work queue and be able to resume operations that fail before completing. I think over time, there are lots of useful applications for the Overseer. He is elected in the same manner as a leader for a shard - if the Overseer goes down, someone simply takes his place. I don't think the Overseer is going away any time soon. - Mark On Mon, Nov 12, 2012 at 9:48 PM, Otis Gospodnetic <otis.gospodne...@gmail.com> wrote: > Hi, > > I was looking at http://wiki.apache.org/solr/SolrCloud and noticed that > while "overseer" is mentioned a handful of times, there is nothing there > that explains what exactly Overseer does. > > This 8-word Javadoc is the best I could find: > http://search-lucene.com/jd/solr/solr-core/org/apache/solr/cloud/Overseer.html > > The first diagram on http://wiki.apache.org/solr/SolrCloud shows 1 > Overseer. Does that make it a SPOF? If not, what happens when it goes > down? > > Also, is Overseer here to stay? > The other day, I saw an issue in JIRA questioning its use or something > along those line. > > Thanks, > Otis > -- > Search Analytics - http://sematext.com/search-analytics/index.html > Performance Monitoring - http://sematext.com/spm/index.html -- - Mark
