Hi,
Snapshotable [1] is the parent interface in Siddhi which should be
implemented by every processing element of a Siddhi event processing chain
to enable their state to be persisted and restored. Snapshotable
interface's getElementId()
method returns unique element identifiers (i.e., element ID) for each of
the elements in the Siddhi query.
@app:name('Test')
define stream StockStream ( symbol string, price float, volume int );
@info(name = 'query1')
from StockStream[price>10]#window.length(10)
select symbol, price, sum(volume) as totalVol
insert into OutStream
For a Siddhi query such as the one shown above, the length window may get
assigned an element ID such as "AbstractStreamProcessor-Test-1". The format
of this element ID is <Element_Type>-<Siddhi_App_Name>-<unique_integer>.
These unique element identifiers are unique only for that particular Siddhi
query. Such characteristics is completely fine as long as the Siddhi
application does not change. However, if new elements get added to the
Siddhi query, there is a rare possibility of loading invalid state into the
stream application. An example of such scenario has been reported in [2].
Similar issues would have to be faced in scenarios such as upgrades and
rollbacks, cross datacenter failover, state archiving, application
migration, etc. One solution for this issue is to keep track of the changes
made to the Siddhi application while maintaining a permanent unique ID
across the application's life cycle. If for example a new segment Y was
added to a Siddhi application X which creates a new application XY, we
could load the state of the already checkpointed segment (X) to the new
Siddhi application (XY) and then assign new IDs for the elements in the
newly added segment (Y). If we delete a segment Z from XY, then we will be
loading only elements pertaining to the remaining portion. The checkpoints
corresponding to the segment Z will be eventually cleaned by the state
management mechanism.
[1]
https://github.com/wso2/siddhi/blob/master/modules/siddhi-core/src/main/java/org/wso2/siddhi/core/util/snapshot/Snapshotable.java
[2] https://github.com/wso2/siddhi/issues/776
--
Thanks,
Miyuru Dayarathna
Senior Technical Lead
Mobile: +94713527783
Blog: http://miyurublog.blogspot.com
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