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Automatically generated identifiers synching conflictsOfficial Content

This page makes recommendations regarding automatically generated identifiers, possible related behaviors and what to take into account.

It is very common that developers choose to use automatically generated identifiers for their transactions keys in order to solve enumeration problems.

In GeneXus, this can be performed by using features such as the

To avoid synchronization conflicts, use GUID data type (with Autogenerate). In that case, a unique identifier is generated when the record is created in the device, and that identifier remains unchanged when synchronizing.

If for some reason this is not possible, several conflicts may occur and the same for the related foreign keys.

GeneXus solves part of this problem automatically. The rest of the document briefly explains how GeneXus addresses this kind of problems and what needs to be taken into account if you use automatically generated identifiers (not autogenerated GUID).

The key mapping problem

For instance: An offline application about the classical Country-City example where both CountryID and CityID have the Autonumber property set in True.
This app is going to be installed on two devices, and there is also a server part to accumulate all the data.

Moment 0

Server: has no registers

DeviceA:

CountryID CountryName
1 Uruguay
2 Brasil

 

CityID

CityName CountryID
1 Porto Alegre 2
2 Montevideo 1

 


DeviceB:

CountryID CountryName
1 Argentina
2 Paraguay

 

CityID

CityName CountryID
1 Buenos Aires 1
2 Asunción 2

 


Moment 1

The DeviceB will sync with the server and send its data. Then the server and the DeviceB will be synched

Moment 2

After that, the DeviceA does the same. Which Id the DeviceA items will have?
Locally the CountryIDs were 1 and 2. But on the server, there's already a 1 and 2 ID for Country.

The solution is to enumerate on the server on its own. So the result of both synchronizations of Country will be:

CountryID CountryName
1 Argentina
2 Paraguay
3 Uruguay
4 Brasil


But what happens when CountryID is being used as a foreign key? Like in City table in this example?

Cascade updates have to be done to the subordinated tables as shown in the following table:

CityID CityName CountryID
1 Buenos Aires 1
2 Asunción 2
3 Porto Alegre 4
4 Montevideo 3


Note: CityIDs of Porto Alegre and Montevideo were modified the same way that CountryIDs of Uruguay and Brasil.

This problem seems simple on a basic example like this one but on bigger systems, it is much more complicated.

What problems are not solved when Identifiers change in synchronization time

There is a key problem that is not solved: Variables in memory are not renamed. 
That means that the following may happen:

  1. Record with ID = 1 is created in the device
  2. While some process occurs, data is synchronized and ID changed to value 2
  3. A variable with the ID still has value 1 in memory
  4. That variable is used to filter data ... Problem: It filters the record with value 1, which may not exist or at least for sure is not the one expected.

This is why you should use GUID Data Type (with Autogenerate).

Advanced Concepts

How does this synchronization key mapping works?
To learn more about this problem check out the Marcos Crispino session at the Genexus 23th event.

Availability

As from GeneXus X Evolution 3.

See also

The Data Synchronization Problem



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