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From "Bhavani Iyer" <bhavan...@gmail.com>
Subject Delta CAS
Date Tue, 08 Jul 2008 13:58:59 GMT
We're planning to start  the implementation to support Delta CAS as
described here:
http://cwiki.apache.org/UIMA/reducing-overhead-for-remote-service-calls.html

The current thinking on the design is described below and we would like some
feedback.
*
CAS Activity Journal*
In order to be able to export as XMI only the updates to the CAS, we need to
maintain
a journal associated with the CAS to track these update activities.  The
journal will
contain the following information:

1) To identify new FSs, the max FS id at the start of processing is saved .
New FS added
     would have ids above this high water mark.
2) To identify which pre-existing FSs were modified,, a list of ids of
pre-existing FSs that have been changed.
3)  To track updates to the Views, a list of FS ids added, removed and
reindexed in the index repository of pre-existing Views.

The CAS APIs  that set feature values as well as APIs that add or remove
from the
index repository will be modified to update the CAS activity journal.
The overhead is expected to be be minimal since it will simply add an FS id
to the
appropriate list as mentioned above.

*Delta CAS XMI serialization*
A more compact representation of the delta CAS data than the proposed
XMI:Difference format
would be preferred for transmitting CAS data when making calls to a remote
service.

Instead, we propose that CAS updates  be serialized in the same format as
the XMI CAS with one modification.
Additional attributes will be defined in the View element to contain the
list of
ids of FSs that were added, deleted and reindexed:

Example of current cas:View
<cas:View sofa="1" members="8 13 20 26 42"/>

Example of proposed cas:View to support delta CAS
<cas:View sofa="1" members_added="32"  members_deleted="13 20"
members_reindexed="26 42" />

XMI deserialization of the delta CAS  will update the CAS as follows:
1) create new FS for those elements where xmi:id is above the high water
mark.
2) update the FSs feature values for those elements where xmi:id is below
the high water mark. Note, features
    missing in the XMI will be set to null.
3) Process the view element to add, remove or reindex in the specified VIew
index repository.

Please note that this proposed XMI representation itself does not identify
or mark the CAS as a delta CAS.
In the context of UIMA AS services, additional properties in the request and
reply messages will
specify that the XMI contains a delta CAS.
Applications should not use the API to export a delta CAS to a file for
later processing without
taking additional steps to retain the format information.


*Delta CAS for debugging*
To support debugging UIMA aggregates as described here
 http://cwiki.apache.org/UIMA/improving-uima-debug-capabilities.html
the delta CAS implementation will be extended as follows:
1) the CAS activity journal will be maintained for each component that is
called during aggregate processing.
2) an API to enable/disable this extended journaling by component.
3) define the XMI representation of the CAS activity journal. Details on
this will be posted shortly.

We would appreciate comments and suggestions on the proposed changes.

Thanks,

Bhavani

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