TODO

  Documentation
    - Clarify the use of each error codes:
      - NOT_READY removed (connection kept opened until ready)
      - Split PROTOCOL_ERROR (BAD IDENTIFICATION, ...)
    - Add docstrings (think of doctests)

  Code

    Code changes often impact more than just one node. They are categorised by
    node where the most important changes are needed.

    General
    - Review XXX/TODO code tags (CODE)
    - When all cells are OUT_OF_DATE in backup mode, the one with most data
      could become UP_TO_DATE with appropriate backup_tid, so that the cluster
      stays operational. (FEATURE)
    - Finish renaming UUID into NID everywhere (CODE)
    - Delayed connection acceptation even when a storage node is not ready ?
      Currently, any node that connects too early to another that is busy for
      some reasons is immediately rejected with the 'not ready' error code.
      This is mainly the case for :
        - Client rejected before the cluster is operational
        - Empty storages rejected during recovery process
    - Implement transaction garbage collection API (FEATURE)
      NEO packing implementation does not update transaction metadata when
      deleting object revisions. This inconsistency must be made possible to
      clean up from a client application, much in the same way garbage
      collection part of packing is done.
    - Factorise node initialisation for admin, client and storage (CODE)
      The same code to ask/receive node list and partition table exists in too
      many places.
    - Clarify handler methods to call when a connection is accepted from a
      listening connection and when remote node is identified
      (cf. neo/lib/bootstrap.py).
    - Review PENDING/SHUTDOWN states, don't use notifyNodeInformation()
      to do a state-switch, use a exception-based mechanism ? (CODE)
    - Review handler split (CODE)
      The current handler split is the result of small incremental changes. A
      global review is required to make them square.
    - Review transactional isolation of various methods
      Some methods might not implement proper transaction isolation when they
      should. An example is object history (undoLog), which can see data
      committed by future transactions.
    - Add a 'devid' storage configuration so that master do not distribute
      replicated partitions on storages with same 'devid'.

    Storage
    - Use libmysqld instead of a stand-alone MySQL server.
    - In backup mode, 2 simultaneous replication should be possible so that:
      - outdated cells does not block backup for too long time
      - constantly modified partitions does not prevent outdated cells to
        replicate
      Current behaviour is undefined and the above 2 scenarios may happen.
    - Create a specialized PartitionTable that know the database and replicator
      to remove duplicates and remove logic from handlers (CODE)
    - Make listening address and port optional, and if they are not provided
      listen on all interfaces on any available port.
    - Make replication speed configurable (HIGH AVAILABILITY)
      In its current implementation, replication runs at lowest priority, to
      not degrade performance for client nodes. But when there's only 1 storage
      left for a partition, it may be wanted to guarantee a minimum speed to
      avoid complete data loss if another failure happens too early.
    - Find a way not to always start replication from the beginning. Currently,
      a temporarily down nodes can't replicate from where it was interrupted,
      which is an issue on big databases. (SPEED)
    - Pack segmentation & throttling (HIGH AVAILABILITY)
      In its current implementation, pack runs in one call on all storage nodes
      at the same time, which locks down the whole cluster. This task should
      be split in chunks and processed in "background" on storage nodes.
      Packing throttling should probably be at the lowest possible priority
      (below interactive use and below replication).
    - Verify data checksum on reception (FUNCTIONALITY)
      In current implementation, client generates a checksum before storing,
      which is only checked upon load. This doesn't prevent from storing
      altered data, which misses the point of having a checksum, and creates
      weird decisions (ex: if checksum verification fails on load, what should
      be done ? hope to find a storage with valid checksum ? assume that data
      is correct in storage but was altered when it travelled through network
      as we loaded it ?).
    - Check replicas: (HIGH AVAILABILITY)
      - Automatically tell corrupted cells to fix their data when a good source
        is known.
      - Add an option to also check all rows of trans/obj/data, instead of only
        keys (trans.tid & obj.{tid,oid}).

    Master
    - Implement back-channel for invalidations in read-only mode,
      so that clients of backup clusters are notified of new data.
    - Master node data redundancy (HIGH AVAILABILITY)
      Secondary master nodes should replicate primary master data (ie, primary
      master should inform them of such changes).
      This data takes too long to extract from storage nodes, and losing it
      increases the risk of starting from underestimated values.
      This risk is (currently) unavoidable when all nodes stop running, but this
      case must be avoided.
    - If the cluster can't start automatically because the last partition table
      is not operational, allow the user to select an older operational one,
      and truncate the DB.
    - Optimize operational status check by recording which rows are ready
      instead of parsing the whole partition table. (SPEED)

    Client
    - Race conditions on the partition table ?
      (update by the poll thread vs. access by other threads)
    - Merge Application into Storage (SPEED)
    - Optimize cache.py by rewriting it either in C or Cython (LOAD LATENCY)
    - Use generic bootstrap module (CODE)

    Admin
    - Make admin node able to monitor multiple clusters simultaneously
    - Send notifications (ie: mail) when a storage or master node is lost
    - Add ctl command to list last transactions, like fstail for FileStorage.

    Tests
    - Split neo/tests/threaded/test.py
    - Use another mock library: Python 3.3+ has unittest.mock, which is
      available for earlier versions at https://pypi.python.org/pypi/mock

  Later
    - Consider auto-generating cluster name upon initial startup (it might
      actually be a partition property).
    - Consider ways to centralise the configuration file, or make the
      configuration updatable automatically on all nodes.
    - Consider storing some metadata on master nodes (partition table [version],
      ...). This data should be treated non-authoritatively, as a way to lower
      the probability to use an outdated partition table.
    - Decentralize primary master tasks as much as possible (consider
      distributed lock mechanisms, ...)
    - Choose how to compute the storage size
    - Investigate delta compression for stored data
      Idea would be to have a few most recent revisions being stored fully, and
      older revision delta-compressed, in order to save space.