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    <p>I have had to solve a similar problem but very many years ago in the days of PEAR DB. In that particular case, there was a need to replicate the data across multiple databases.</p> <p>We did not have the problem of the different databases having different mappings though so it was a fair bit simpler.</p> <p>What we did was to facade the DB class and override the getResult function (or whatever it was called). This function then analysed the SQL and if it was a read - it would send it to just one backed and if it was a write, it would send it to all. </p> <p>This actually worked really well for a very heavily utilised site.</p> <p>From that background, I would suggest entirely facading all of the persistence operations. Once you have done that, the implementation details are less relevant and can be changed at any time.</p> <p>From this perspective, any of your implementation ideas seem like a reasonable approach. There are various things you will want to think about though.</p> <ul> <li>What if one of the backends throw an error?</li> <li>What is the performance impact of writing to three database servers?</li> <li>Can the writes be done asynchronously (if so, ask the first question again)</li> </ul> <p>There is potentially another way to solve this problem as well. That is to use stored procedures. If you have a primary database server, you could write a trigger which, on commit (or thereabouts) connects to the other database and sychronises the data.</p> <p>If the data update does not need to be immediate, you could get the primary database to log changes and have another script that regularly "fed" this data into the other system. Again, the issue of errors will need to be considered. </p> <p>Hope this helps.</p>
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    1. POPHP DataMapper with multiple persistence layers
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