User's Manual

Chapter 9. Persistence Tutorial 89

9.6.3. Incorrect Attribute Mappings

When you receive an error indicating that an attribute cannot be found or an attribute name is invalid,

make sure that there are no typos. Simple typos cannot be caught by the compiler and cause many

errors.

9.6.4. Problems with Transactions

Typically, two types of mistakes occur when dealing with Transactions within the WAF.

The ﬁrst type of mistake is trying to access Data Objects (and hence the database) outside of a trans-

action. This typically happens when the developer tries to write a custom dispatcher that bypasses

the standard WAF dispatcher and does not remember to include transaction management. To ﬁx this,

make sure that the custom dispatcher properly opens and closes transactions using appropriate WAF

APIs.

The second type of mistake is the exact opposite of the ﬁrst: trying to nest transactions. Unless you

have written a custom dispatcher that bypasses the standard WAF dispatcher, you should never have

to open or close a transaction. Trying to do so within the standard framework of the WAF will result

in an error. In almost all cases, developers building on top of the WAF should not try to manage their

own transactions.

For more information, see the document on Section 9.7 Transaction Management.

9.6.5. Join Paths

Common problems:

• Non-continuous join paths -- the second column’s table in each join path element must match the

table of the next join path element’s ﬁrst column. That is, the path must look somthing like join

A.B to C.D, join C.E to E.F In this example, table C is the last table of the ﬁrst half and the

ﬁrst table of the second half.

• Join paths are too long -- MDSQL only supports join paths composed of 1 or 2 join path elements.

If paths are longer than that, it becomes impossible to generate events correctly given the current

implementation.

• Join path does not start in the primary table -- a join path must always start in the same table as the

reference key or object key. If it does not, there is a gap and the event cannot be generated.

9.6.6. Dynamic DDL Generation

Dyanmic DDL Generation works well when the PDL is speciﬁed correctly. However, it is easy to think

that the PDL is written to represent the model correctly when in fact you are missing a simple item to

two. Below are some symptoms that you may see with your generated DDL and some suggestions as

to how you may be able to ﬁx them.

Data Model Problem Likely Cause in PDL

Missing columns Properties were speciﬁed without a column or

join path