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2.3.11. The NdbEventOperation Class

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2.3.11.1. The NdbEventOperation::State Type
2.3.11.2. NdbEventOperation Methods

Abstract

This section describes the NdbEventOperation class, which is used to monitor changes (events) in a database. It provides the core functionality used to implement MySQL Cluster Replication.

Parent class.  None

Child classes.  None

Description.  NdbEventOperation represents a database event.

Creating an Instance of NdbEventOperation.  This class has no public constructor or destructor. Instead, instances of NdbEventOperation are created as the result of method calls on Ndb and NdbDictionary objects:

  1. There must exist an event which was created using Dictionary::createEvent(). This method returns an instance of the NdbDictionary::Event class. See Section 2.3.3.1.12, “Dictionary::createEvent()”.

    An NdbEventOperation object is instantiated using Ndb::createEventOperation(), which acts on instance of NdbDictionary::Event. See Section 2.3.8.1.11, “Ndb::createEventOperation”.

See also Section 2.3.4, “The Event Class”.

An instance of this class is removed by invoking Ndb::dropEventOperation. See Section 2.3.8.1.12, “Ndb::dropEventOperation()”, for more information.

Tip

A detailed example demonstrating creation and removal of event operations is provided in Section 2.4.7, “NDB API Event Handling Example”.

Known Issues.  The following issues may be encountered when working with event operations in the NDB API:

  • The maximum number of active NdbEventOperation objects is currently fixed at compile time at 2 * MaxNoOfTables. This may become a separately configurable parameter in a future release.

  • Currently, all INSERT, DELETE, and UPDATE events — as well as all attribute changes — are sent to the API, even if only some attributes have been specified. However, these are hidden from the user and only relevant data is shown after calling Ndb::nextEvent().

    Note that false exits from Ndb::pollEvents() may occur, and thus the following nextEvent() call returns zero, since there was no available data. In such cases, simply call pollEvents() again.

    See Section 2.3.8.1.13, “Ndb::pollEvents()”, and Section 2.3.8.1.14, “Ndb::nextEvent()”.

  • Event code does not check the table schema version. When a table is dropped, make sure that you drop any associated events.

    We intend to remedy this issue in a future release of MySQL Cluster and the NDB API.

  • If you have received a complete epoch, events from this epoch are not re-sent, even in the event of a node failure. However, if a node failure has occurred, subsequent epochs may contain duplicate events, which can be identified by duplicated primary keys.

    In the MySQL Cluster replication code, duplicate primary keys on INSERT operations are normally handled by treating such inserts as REPLACE operations.

Tip

To view the contents of the system table containing created events, you can use the ndb_select_all utility as shown here: 

ndb_select_all -d sys 'NDB$EVENTS_0'

Methods.  The following table lists the public methods of this class and the purpose or use of each method:

Method Purpose / Use
getState() Gets the current state of the event operation
getEventType() Gets the event type
getValue() Retrieves an attribute value
getPreValue() Retrieves an attribute's previous value
getBlobHandle() Gets a handle for reading blob attributes
getPreBlobHandle() Gets a handle for reading the previous blob attribute
getGCI() Retrieves the GCI of the most recently retrieved event
getLatestGCI() Retrieves the most recent GCI (whether or not the corresponding event has been retrieved)
getNdbError() Gets the most recent error
isConsistent() Detects event loss caused by node failure
tableNameChanged() Checks to see whether the name of a table has changed
tableFrmChanged() Checks to see whether a table .FRM file has changed
tableFragmentationChanged() Checks to see whether the fragmentation for a table has changed
tableRangeListChanged() Checks to see whether a table range partition list name has changed
mergeEvents() Allows for events to be merged
execute() Activates the NdbEventOperation

For detailed descriptions, signatures, and examples of use for each of these methods, see Section 2.3.11.2, “NdbEventOperation Methods”.

Types.  NdbEventOperation defines one enumerated type. See Section 2.3.11.1, “The NdbEventOperation::State Type”, for details.

Class diagram.  This diagram shows all the available methods and enumerated types of the NdbEventOperation class:

Public methods of the NdbEventOperation class.