[HBASE-4495] CatalogTracker has an identity crisis; needs to be cut-back in scope

• Type: Improvement
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 0.94.0
• Fix Version/s: 0.99.0
• Labels: 1.0.0
• Hadoop Flags: Reviewed
• Assignee: Mikhail Antonov
• Reporter: Michael Stack
• Created: 27/Sep/11 06:46
• Updated: 06/Apr/18 17:50
• Resolved: 03/Jul/14 02:06

Description

CT needs a good reworking. I'd suggest its scope be cut way down to only deal in zk transactions rather than zk and reading meta location in hbase (over an HConnection) and being a purveyor of HRegionInterfaces on meta and root servers and being an Abortable and a verifier of catalog locations. Once this is done, I would suggest it then better belongs over under the zk package and that the Meta* classes then move to client package.

Here's some messy notes I added to head of CT class in hbase-3446 where I spent some time trying to make out what it was CT did.

  // TODO: This class needs a rethink.  The original intent was that it would be
  // the one-stop-shop for root and meta locations and that it would get this
  // info from reading and watching zk state.  The class was to be used by
  // servers when they needed to know of root and meta movement but also by
  // client-side (inside in HTable) so rather than figure root and meta
  // locations on fault, the client would instead get notifications out of zk.
  // 
  // But this original intent is frustrated by the fact that this class has to
  // read an hbase table, the -ROOT- table, to figure out the .META. region
  // location which means we depend on an HConnection.  HConnection will do
  // retrying but also, it has its own mechanism for finding root and meta
  // locations (and for 'verifying'; it tries the location and if it fails, does
  // new lookup, etc.).  So, at least for now, HConnection (or HTable) can't
  // have a CT since CT needs a HConnection (Even then, do want HT to have a CT?
  // For HT keep up a session with ZK?  Rather, shouldn't we do like asynchbase
  // where we'd open a connection to zk, read what we need then let the
  // connection go?).  The 'fix' is make it so both root and meta addresses
  // are wholey up in zk -- not in zk (root) -- and in an hbase table (meta).
  //
  // But even then, this class does 'verification' of the location and it does
  // this by making a call over an HConnection (which will do its own root
  // and meta lookups).  Isn't this verification 'useless' since when we
  // return, whatever is dependent on the result of this call then needs to
  // use HConnection; what we have verified may change in meantime (HConnection
  // uses the CT primitives, the root and meta trackers finding root locations).
  //
  // When meta is moved to zk, this class may make more sense.  In the
  // meantime, it does not cohere.  It should just watch meta and root and
  // NOT do verification -- let that be out in HConnection since its going to
  // be done there ultimately anyways.
  //
  // This class has spread throughout the codebase.  It needs to be reigned in.
  // This class should be used server-side only, even if we move meta location
  // up into zk.  Currently its used over in the client package. Its used in
  // MetaReader and MetaEditor classes usually just to get the Configuration
  // its using (It does this indirectly by asking its HConnection for its
  // Configuration and even then this is just used to get an HConnection out on
  // the other end). St.Ack 10/23/2011.
  //

Issue Links

• is blocked by: HBASE-5573 Replace client ZooKeeper watchers by simple ZooKeeper reads
• is related to: HBASE-10856 Prep for 1.0