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 of
DBACC block (which manages
the index structures) and
DBTUP (which manages the
tuples). It is made up of the following files, found in
Dblqh.hpp: Contains the
Dblqhclass 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; connect
Start phase 4. Connect each
LQHwith every other
LQHin 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 of
LQHKEYREQmessages and all processing of operations on behalf of this request. This also involves reception of various types of
KEYINFOmessages, as well as communications with
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 in
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 orders
ACCto 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 in
ACC. It also interacts with
DIHto determine which global checkpoints are recoverable.
Global checkpoint module. Assists
DIHin discovering when GCPs are recoverable, and handles the
GCP_SAVEREQmessage requesting that
LQHsave 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:
System restart, Phase 1
System restart, Phase 2
System restart, Phase 3
System restart, Phase 4
Dblqhrecords and data. Also includes the
Dblqhclass destructor, used for deallocating these.
Dblqhfunctionality (class methods).
This directory also has the files listed here in a
redoLogReadersubdirectory containing the sources for the ndb_redo_log_reader utility:
This block also handles redo logging, and helps oversee the
BACKUP blocks. It is
implemented as the class
Dblqh, defined in
DBQLQH is a subtype of this block, used for
query and recovery threads, added in NDB 8.0.23.