If you enable caching in a Lookup transformation, the Integration Service builds a cache in memory to store lookup data. If the lookup does not change between sessions, you can configure the transformation to use a persistent lookup cache. When you run the session containing persistent lookup cache, the Integration Service rebuilds the persistent cache if any cache file is missing or invalid. For details, please check How to Implement Informatica Persistent Cache.
Benefits of Persistent Cache Lookup
- Depending on your situation, persistent cache can give you huge performance boost during the session runtime since the Integration Service does not need to rebuild the lookup cache again, thereby saving time.
- The very fact that the data in a persistent cache file does not refresh or change in session run can be used to overcome major functional hurdles.
I would like to illustrate below one problematic situation that is often encountered in long running ETL while trying to load data in Fact tables and how the same can be overcome by using Persistent Cache lookups.
Problem: Ensuring Data Integrity in Long Running Load when underlying tables are changing
Suppose you have a requirement of producing cross subject area reports combining two fact tables from two different data marts – Sales and Marketing. The loading of both the facts are started at the 1st day of the month, however sales fact is loaded first and then the marketing fact is started.
Now suppose sales fact, being a very huge volume fact, takes more than a day to complete the load and there is a possibility that the surrogate keys flowing in to this fact from different SCD Type-II dimension tables get changed in the mean time during the daily refresh. Meaning the surrogate key of one customer populated in marketing fact will not be same with the surrogate key of the same customer populated in Sales Fact since they are loaded in two different days and mean time the key may have been changed.
Overcome using Persistent Cache Lookup
How to overcome this issue? If you think closely the issue basically boils down to ensuring that both facts get the same surrogate keys that can be easily achieved by using persistent cache lookup so that even if the underlying tables change data during daily loads, all the facts get the same set of keys.
Disadvantage of Persistent cache Lookup
Although persistent cache can give you considerable performance and other advantages, it comes with some hazards.
- Recovering sessions after failure in midway may not be possible. Consider this – you have a typical update-else-insert logic in a target table over which you also have persistent cache lookup. This PC lookup on target is used to determine if a certain record coming from source is already present in target (Update) or not (Insert). Suppose this session got failed after inserting a few record in target. If this was a normal lookup, you could simply restart the session after fixing the cause of the error. But if this is a persistent cache lookup, you can not restart this job directly as because the lookup cache will not be holding the records that got inserted in the first time and as a result the lookup will fail to determine that these records are already existing in target and your mapping will try to insert them once again.
- Persistent cache gives you performance boost by saving time in building the lookup while session run but it still takes the same time like a normal lookup for using the lookup data during session runtime. It is often observed that persistent cache shared across many sessions creates huge disk level I/O contention when the lookup is actually being used in the sessions. You need to monitor the disk IO performance using “iostat” or “vmstat” (UNIX) if you see huge transformation bottleneck without any apparent reason in sessions using persistent cache lookup.