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SMILA/Documentation/Usage of Blackboard Service
What is the blackboard?
The blackboard holds the records while they are pushed through a pipeline. Pipelets are invoked with a blackboard instance and a list of IDs of records to process. The pipelet can then access the blackboard to get record metadata and attachments. The blackboard hides the handling of record persistence from the services. For example it can be configured to hold only the record metadata in memory, but to put the attachments in the BinaryStorage service to save memory, if large attachments are used or many records are processed at the same time. Or it could get a record from a RecordStorage service on demand and write it back if processing is done (however, in SMILA 1.0 we do not use the RecordStorage by default anymore).
The blackboard instance is released after the pipeline execution has been finished, for each pipeline execution a new blackboard instance is created. If the blackboard has storages attached is the choice of the creating component and can be configurable there, or it depends on if storage services are active in the SMILA application. For the user of the blackboard (the pipelet, usually), it should be not relevant, if the blackboard has storages attached or not.
For pipelet programmers, using the blackboard is usually trivial:
- Use getRecord(id) or getMetadata(id) to get the record metadata. Modify the returned object to change record metadata.
- To access record attachments, you should use the get/setAttachment methods of the blackboard. The Attachment objects of the Record object returned by getRecord(id) may not allow access to the attachment content, if the content has been swapped out to BinaryStorage. It's recommended to use streaming methods for attachments to keep memory consumption low.
For more details see the javadoc of these interfaces:
Notes are additional temporary data created by pipelets to be used in later pipelets in the same workflow, but not to be persisted in the storages. Notes can be either global or record specific (associated with a record ID). Record specific notes are copied on record splits and removed when the associated record is removed from the blackboard. Each Note has a String name and Serializable value. There are following methods for working with Notes:
- boolean hasGlobalNote(name);
- Serializable getGlobalNote(name);
- setGlobalNote(name, value);
- boolean hasRecordNote(id, name);
- getRecordNote(id, name);
- setRecordNote(id, name, value);