1. MOLAP
Detailed Data, Aggregation and Metadata are stored on
Analysis Server in MOLAP. Analysis Services pulls complete detailed data from
relational database on analysis server and then creates aggregations. MOLAP
keep data (detailed data and aggregations) on same analysis server so query
response time is significantly better than other two approaches. MOLAP
has high latency because it remains disconnected from relational DB and
connects again for pulling new changed data. If data volume is huge then
latency will be higher. We can overcome at certain extent by using incremental
processing.
2. ROLAP
Detailed data and aggregations will reside in relational
database and metadata will be on Analysis server. In ROLAP, analysis server
sends request to relational database for each query. Among all storage modes
Query performance will be lowest in ROLAP (in most of the cases) because each
query needs to be addressed by relation DB and these will be needed extra time to
pass result over the network to OLAP server. Latency will be low in ROLAP
as data and aggregations are on relational database.
3. HOLAP
HOLAP is middle approach which is somewhere between MOLAP
and ROLAP. Detailed data reside in relational database but aggregations and
metadata reside in analysis server. If query hits aggregations (query can be
addressed by aggregations) then HOLAP query response time is similar to MOLAP
but if query needs to be addressed by relational database then Query response
time will be similar to ROLAP. Latency is somewhere between ROLAP and MOLAP.
Conclusion
If relational database size in few terabytes (<=5TB),
enough hardware (memory and CPU power) and incremental frequency in days, week
or month etc. then prefer MOLAP. It provides best Query response time.
ROLAP and HOLAP are mostly useful when latency is quite low
(in seconds, minutes and hours). Mostly project go with MOLAP and rarely with
ROLAP approach. I don’t have any industry stats but if I find something, I will
provide information in this post.
I have not seen HOLAP implantation related case
studies/articles so far on the net. If you have come across HOLAP
implementation then please share the details.
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