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Synchronizing client-server databases

Asked 2010-08-04T15:06:45.130
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I'm looking for some general strategies for synchronizing data on a central server with client applications that are not always online.

In my particular case, I have an android phone application with an sqlite database and a PHP web application with a MySQL database.

Users will be able to add and edit information on the phone application and on the web application. I need to make sure that changes made one place are reflected everywhere even when the phone is not able to immediately communicate with the server.

I am not concerned with how to transfer data from the phone to the server or vice versa. I'm mentioning my particular technologies only because I cannot use, for example, the replication features available to MySQL.

I know that the client-server data synchronization problem has been around for a long, long time and would like information - articles, books, advice, etc - about patterns for handling the problem. I'd like to know about general strategies for dealing with synchronization to compare strengths, weaknesses and trade-offs.

sql database design-patterns client-server data-synchronization
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I would recommend that you have a timestamp column in every table and every time you insert or update, update the timestamp value of each affected row. Then, you iterate over all tables checking if the timestamp is newer than the one you have in the destination database. If it´s newer, then check if you have to insert or update.

Observation 1: be aware of physical deletes since the rows are deleted from source db and you have to do the same at the server db. You can solve this avoiding physical deletes or logging every deletes in a table with timestamps. Something like this: DeletedRows = (id, table_name, pk_column, pk_column_value, timestamp) So, you have to read all the new rows of DeletedRows table and execute a delete at the server using table_name, pk_column and pk_column_value.

Observation 2: be aware of FK since inserting data in a table that´s related to another table could fail. You should deactivate every FK before data synchronization.

answered 2012-04-24T18:30:23.813

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