4.7 The DBLQH Block

This is the local, low-level query handler block, which manages data and transactions local to the cluster's data nodes, and acts as a coordinator of 2-phase commits. It is responsible (when called on by the transaction coordinator) for performing operations on tuples, accomplishing this task with help ofDBACCblock (which manages the index structures) andDBTUP(which manages the tuples). It is made up of the following files, found instorage/ndb/src/kernel/blocks/dblqh:

  • Dblqh.hpp: Contains theDblqhclass definition. The code itself includes the following modules:

    • Start/Restart Module.This module handles the following start phases:

      • Start phase 1.Load block reference and processor ID

      • Start phase 2.Initiate all records within the block; connectLQHwithACCandTUP

      • Start phase 4.Connect eachLQHwith every otherLQHin the database system. For an initial start, create the fragment log files. For a system restart or node restart, open the fragment log files and find the end of the log files.

    • Fragment addition and deletion module.Used by the data dictionary to create new fragments and delete old fragments.

    • Execution module.This module handles the reception ofLQHKEYREQmessages and all processing of operations on behalf of this request. This also involves reception of various types ofATTRINFOandKEYINFOmessages, as well as communications withACCandTUP.

    • Log module.The log module handles the reading and writing of the log. It is also responsible for handling system restarts, and controls system restart inTUPandACCas well.

    • Transaction module.This module handles the commit and completion phases.

    • TC failure module.Handles failures in the transaction coordinator.

    • Scan module.This module contains the code that handles a scan of a particular fragment. It operates under the control of the transaction coordinator and ordersACCto perform a scan of all tuples in the fragment.TUPperforms the necessary search conditions to insure that only valid tuples are returned to the application.

    • Node recovery module.This is used when a node has failed, copying the effected fragment to a new fragment replica. It also shuts down all connections to the failed node.

    • LCP module.This module handles execution and control of local checkpoints inTUPandACC. It also interacts withDIHto determine which global checkpoints are recoverable.

    • Global checkpoint module.AssistsDIHin discovering when GCPs are recoverable, and handles theGCP_SAVEREQmessage requesting thatLQHsave a given GCP to disk and provide a notification of when this has been done.

    • File handling module.This includes a number of sub-modules:

      • Signal reception

      • Normal operation

      • File change

      • Initial start

      • System restart, Phase 1

      • System restart, Phase 2

      • System restart, Phase 3

      • System restart, Phase 4

      • Error

  • DblqhInit.cpp: InitialisesDblqhrecords and data. Also includes theDblqhclass destructor, used for deallocating these.

  • DblqhMain.cpp: ImplementsDblqhfunctionality (class methods).

  • This directory also has the files listed here in aredoLogReadersubdirectory containing the sources for thendb_redo_log_readerutility:

    • records.cpp

    • records.hpp

    • redoLogFileReader.cpp

This block also handles redo logging, and helps oversee theDBACC,DBTUP,LGMAN,TSMAN,PGMAN, andBACKUP块。它是实现类Dblqh, defined in the filestorage/ndb/src/kernel/blocks/dblqh/Dblqh.hpp.

DBQLQHis a subtype of this block, used for query and recovery threads, added in NDB 8.0.23.