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This paper mainly studies the snapshot change record emitter of debezium
SnapshotChangeRecordEmitter
debezium-v1.1.1.Final/debezium-core/src/main/java/io/debezium/relational/SnapshotChangeRecordEmitter.java
public class SnapshotChangeRecordEmitter extends RelationalChangeRecordEmitter { private final Object[] row; public SnapshotChangeRecordEmitter(OffsetContext offset, Object[] row, Clock clock) { super(offset, clock); this.row = row; } @Override protected Operation getOperation() { return Operation.READ; } @Override protected Object[] getOldColumnValues() { throw new UnsupportedOperationException("Can't get old row values for READ record"); } @Override protected Object[] getNewColumnValues() { return row; } }
- SnapshotChangeRecordEmitter inherits the RelationalChangeRecordEmitter. Its constructor receives row, its getOperation method returns Operation.READ, its getOldColumnValues method throws unsupportadoperationexception, and its getNewColumnValues returns row
RelationalChangeRecordEmitter
debezium-v1.1.1.Final/debezium-core/src/main/java/io/debezium/relational/RelationalChangeRecordEmitter.java
public abstract class RelationalChangeRecordEmitter extends AbstractChangeRecordEmitter<TableSchema> { protected final Logger logger = LoggerFactory.getLogger(getClass()); public RelationalChangeRecordEmitter(OffsetContext offsetContext, Clock clock) { super(offsetContext, clock); } @Override public void emitChangeRecords(DataCollectionSchema schema, Receiver receiver) throws InterruptedException { TableSchema tableSchema = (TableSchema) schema; Operation operation = getOperation(); switch (operation) { case CREATE: emitCreateRecord(receiver, tableSchema); break; case READ: emitReadRecord(receiver, tableSchema); break; case UPDATE: emitUpdateRecord(receiver, tableSchema); break; case DELETE: emitDeleteRecord(receiver, tableSchema); break; default: throw new IllegalArgumentException("Unsupported operation: " + operation); } } @Override protected void emitCreateRecord(Receiver receiver, TableSchema tableSchema) throws InterruptedException { Object[] newColumnValues = getNewColumnValues(); Object newKey = tableSchema.keyFromColumnData(newColumnValues); Struct newValue = tableSchema.valueFromColumnData(newColumnValues); Struct envelope = tableSchema.getEnvelopeSchema().create(newValue, getOffset().getSourceInfo(), getClock().currentTimeAsInstant()); if (skipEmptyMessages() && (newColumnValues == null || newColumnValues.length == 0)) { logger.warn("no new values found for table '{}' from create message at '{}'; skipping record", tableSchema, getOffset().getSourceInfo()); return; } receiver.changeRecord(tableSchema, Operation.CREATE, newKey, envelope, getOffset()); } @Override protected void emitReadRecord(Receiver receiver, TableSchema tableSchema) throws InterruptedException { Object[] newColumnValues = getNewColumnValues(); Object newKey = tableSchema.keyFromColumnData(newColumnValues); Struct newValue = tableSchema.valueFromColumnData(newColumnValues); Struct envelope = tableSchema.getEnvelopeSchema().read(newValue, getOffset().getSourceInfo(), getClock().currentTimeAsInstant()); receiver.changeRecord(tableSchema, Operation.READ, newKey, envelope, getOffset()); } @Override protected void emitUpdateRecord(Receiver receiver, TableSchema tableSchema) throws InterruptedException { Object[] oldColumnValues = getOldColumnValues(); Object[] newColumnValues = getNewColumnValues(); Object oldKey = tableSchema.keyFromColumnData(oldColumnValues); Object newKey = tableSchema.keyFromColumnData(newColumnValues); Struct newValue = tableSchema.valueFromColumnData(newColumnValues); Struct oldValue = tableSchema.valueFromColumnData(oldColumnValues); if (skipEmptyMessages() && (newColumnValues == null || newColumnValues.length == 0)) { logger.warn("no new values found for table '{}' from update message at '{}'; skipping record", tableSchema, getOffset().getSourceInfo()); return; } // some configurations does not provide old values in case of updates // in this case we handle all updates as regular ones if (oldKey == null || Objects.equals(oldKey, newKey)) { Struct envelope = tableSchema.getEnvelopeSchema().update(oldValue, newValue, getOffset().getSourceInfo(), getClock().currentTimeAsInstant()); receiver.changeRecord(tableSchema, Operation.UPDATE, newKey, envelope, getOffset()); } // PK update -> emit as delete and re-insert with new key else { Struct envelope = tableSchema.getEnvelopeSchema().delete(oldValue, getOffset().getSourceInfo(), getClock().currentTimeAsInstant()); receiver.changeRecord(tableSchema, Operation.DELETE, oldKey, envelope, getOffset()); envelope = tableSchema.getEnvelopeSchema().create(newValue, getOffset().getSourceInfo(), getClock().currentTimeAsInstant()); receiver.changeRecord(tableSchema, Operation.CREATE, newKey, envelope, getOffset()); } } @Override protected void emitDeleteRecord(Receiver receiver, TableSchema tableSchema) throws InterruptedException { Object[] oldColumnValues = getOldColumnValues(); Object oldKey = tableSchema.keyFromColumnData(oldColumnValues); Struct oldValue = tableSchema.valueFromColumnData(oldColumnValues); if (skipEmptyMessages() && (oldColumnValues == null || oldColumnValues.length == 0)) { logger.warn("no old values found for table '{}' from delete message at '{}'; skipping record", tableSchema, getOffset().getSourceInfo()); return; } Struct envelope = tableSchema.getEnvelopeSchema().delete(oldValue, getOffset().getSourceInfo(), getClock().currentTimeAsInstant()); receiver.changeRecord(tableSchema, Operation.DELETE, oldKey, envelope, getOffset()); } /** * Returns the operation done by the represented change. */ protected abstract Operation getOperation(); /** * Returns the old row state in case of an UPDATE or DELETE. */ protected abstract Object[] getOldColumnValues(); /** * Returns the new row state in case of a CREATE or READ. */ protected abstract Object[] getNewColumnValues(); /** * Whether empty data messages should be ignored. * * @return true if empty data messages coming from data source should be ignored.</br> * Typical use case are PostgreSQL changes without FULL replica identity. */ protected boolean skipEmptyMessages() { return false; } }
- RelationalChangeRecordEmitter inherits AbstractChangeRecordEmitter, whose generic type is TableSchema. Its emitChangeRecords method executes different emit methods according to different operation s. These emit methods mainly construct key and envelope, and then execute receiver.changeRecord
AbstractChangeRecordEmitter
debezium-v1.1.1.Final/debezium-core/src/main/java/io/debezium/pipeline/AbstractChangeRecordEmitter.java
public abstract class AbstractChangeRecordEmitter<T extends DataCollectionSchema> implements ChangeRecordEmitter { private final OffsetContext offsetContext; private final Clock clock; public AbstractChangeRecordEmitter(OffsetContext offsetContext, Clock clock) { this.offsetContext = offsetContext; this.clock = clock; } @Override @SuppressWarnings({ "unchecked" }) public void emitChangeRecords(DataCollectionSchema schema, Receiver receiver) throws InterruptedException { Operation operation = getOperation(); switch (operation) { case CREATE: emitCreateRecord(receiver, (T) schema); break; case READ: emitReadRecord(receiver, (T) schema); break; case UPDATE: emitUpdateRecord(receiver, (T) schema); break; case DELETE: emitDeleteRecord(receiver, (T) schema); break; default: throw new IllegalArgumentException("Unsupported operation: " + operation); } } @Override public OffsetContext getOffset() { return offsetContext; } /** * Returns the clock of the change record(s) emitted. */ public Clock getClock() { return clock; } /** * Returns the operation associated with the change. */ protected abstract Operation getOperation(); /** * Emits change record(s) associated with a snapshot. * * @param receiver the handler for which the emitted record should be dispatched * @param schema the schema */ protected abstract void emitReadRecord(Receiver receiver, T schema) throws InterruptedException; /** * Emits change record(s) associated with an insert operation. * * @param receiver the handler for which the emitted record should be dispatched * @param schema the schema */ protected abstract void emitCreateRecord(Receiver receiver, T schema) throws InterruptedException; /** * Emits change record(s) associated with an update operation. * * @param receiver the handler for which the emitted record should be dispatched * @param schema the schema */ protected abstract void emitUpdateRecord(Receiver receiver, T schema) throws InterruptedException; /** * Emits change record(s) associated with a delete operation. * * @param receiver the handler for which the emitted record should be dispatched * @param schema the schema */ protected abstract void emitDeleteRecord(Receiver receiver, T schema) throws InterruptedException; }
- AbstractChangeRecordEmitter implements the ChangeRecordEmitter interface, which provides the emitChangeRecords method and encapsulates calls for different operation s. Colleagues define the emitCreateRecord, emitReadRecord, emitUpdateRecord and emitDeleteRecord methods for subclasses to implement
ChangeRecordEmitter
debezium-v1.1.1.Final/debezium-core/src/main/java/io/debezium/pipeline/spi/ChangeRecordEmitter.java
public interface ChangeRecordEmitter { /** * Emits the change record(s) corresponding to data change represented by this emitter. */ void emitChangeRecords(DataCollectionSchema schema, Receiver receiver) throws InterruptedException; /** * Returns the offset of the change record(s) emitted. */ OffsetContext getOffset(); public interface Receiver { void changeRecord(DataCollectionSchema schema, Operation operation, Object key, Struct value, OffsetContext offset) throws InterruptedException; } }
- The ChangeRecordEmitter interface defines the emitChangeRecords and getOffset methods, as well as the Receiver interface, which defines the changeRecord method
Summary
SnapshotChangeRecordEmitter inherits the RelationalChangeRecordEmitter. Its constructor receives row, its getOperation method returns Operation.READ, its getOldColumnValues method throws unsupportadoperationexception, and its getNewColumnValues returns row