Monday, 25 January 2010

Application Cache

If you frequently need to read the same data from the database in the same transaction (for example look-up table data), the DBMS cache/buffer should reduce the I/O to disk by having the data in memory. However there is still a considerable overhead involved with using SQL to obtain the data multiple times.

A technique that we have used to improve performance is an application cache. This uses a common action block to read the data. The action block checks to see if the data is cached, and if it is, avoids the need for a READ from the database. If it is not available, it reads the data and then stores it in the cache.The cache is implemented using uninitialised local views and you can store as many rows as you are prepared to allocate memory to.

The design needs to ensure that the cache is initialised at the start of the transaction (for C generated code when using the TE) and also to cater for the possibility of updates to the database, i.e. the cache needs refreshing or deleting if the data can be changed during the transaction.

In specific examples where we have used the cache for heavily accessed tables, we have found the cache improves the performance of the READ action block by 1000%.

Wednesday, 13 January 2010

Mapping Group Views

Gen allows you to view map group views with differing cardinalities on USE statements and dialog flows if the receiving view has a higher cardinality than the sending view.

However the view match remains intact even if you then subsequently change the cardinality of the sending view to a value that is greater than the receiving view, so you could end up with a sending view that is larger than the receiving view, which could then cause unexpected results, like loss of data without a runtime error. In this case, you could not establish the view match again, but the existing view match is still 'valid' in the model.

If the group view sizes were initially the same, the developer might not think that they need to add in any extra validation logic, but a subsequent change to one of the group views might then cause problems.

A new check in VerifIEr allows a quick check for differing group view cardinalities with a warning if they differ but are valid and an error if they differ and are invalid.