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Any replication schema must be well planned and well designed, including the ones you plan to use with your SQL Server CE applications Therefore, we take a moment to review the basic concepts of SQL Server s Merge Replication and then follow up with a detailed examination of design considerations In Merge Replication, as it is used with a SQL Server CE application, the SQL Server is referred to as the publisher with one or more defined publications on one or more databases, and the SQL Server CE databases are referred to as the subscribers Publications are made up of a defined set of tables, columns, and filters These chosen tables are referred to as the articles of a publication; and the definition of a subset, such as only columnA, columnB, columnC of TableX or only those rows of TableY where route code equals 'West92' are referred to as filters Filters allow you to replicate a vertical subset of a table (such as the first example in the previous sentence) or replicate a horizontal subset of a table (such as the second example) or both They also can be dynamic, such as only rows in the Orders table where EmployeeID equals the employee ID of the user performing the synchronization With this flexibility in publication design comes the need to ensure that changes made to this subset of the data residing on the device will synchronize correctly when propagated back to the server
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841 Using Good Design to Avoid Synchronization Failures Most mobile applications that have a Merge Replication component consist of three repetitively executed steps
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1 Subscribe to a publication located at the server and synchronize the data to the device 2 Make updates to that data as it resides on the device 3 Synchronize with the server to submit the changes made locally back to the server database All mobile application designers hope that step 3 succeeds so that modifications are applied at the server without errors and that those modifications do not overwrite any modifications made by other users Many mobile application designers end up wondering why step 3 failed The purpose of this subsection is to explain why publication design decisions can cause synchronization failures and to provide a blueprint on how to avoid those failures
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84 Using Merge Replication
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A mobile application that uses Merge Replication for the transfer of data between SQL Server and SQL Server CE does so by subscribing to a publication that resides on the server Since SQL Server CE provides only foranonymoussubscriptions,theentirepublicationmustbesubscribedto But mobile applications usually do not want to work with large amounts of data; rather, they want just the data that applies to the subject at hand and to the user of the device They almost always want less than the entire server database This means that almost all publications are defined as subsets of the database and therefore that all mobile applications work with a subset of the database Working with a subset of data always creates the possibility of data integrity problems occurring in the subset that will not be detected until the data is synchronized with the full set of data residing at the server For example, the Northwind database that comes with SQL Server contains two tables that are related by a foreign key constraint: Products, which contains the foreign key, and Categories, which contains the primary key Now suppose that you define a publication that includes the Products table but not the Categories table When you synchronize, the Products table is brought to the device That table has a CategoryID column both on the server and on the device At the server, the list of possible values that can be entered into that column is known; by definition it is the list of values in the Categories table s primary key column But on the device, that list is not known The local application cannot insert rows into the Products table or update the CategoryID field in rows that are already there because the application does not know what values will be accepted or rejected when the data is synchronized to the server Not all developers understand that each decision made during the design of the publication results in a specific and definable limitation that must be placed on the application for local data modifications to synchronize successfully at the server As we look at publication design decisions and the resulting restrictions that those decisions place on your local application, let us be clear about something: The design of a publication does notplaceautomaticrestrictionsonyourapplication;rather,itcreatessituations that force you to design limitations into your application as you develop it To illustrate this choice and impact world in which you must develop applications based on Merge Replication, we develop a sample publication and mobile application design For our server database, we pick the Northwind database Our Merge Replication based mobile
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