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<head><title> db4o's Commit Process</title>
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<h1> db4o's Commit Process</h1>
<p>How does db4o make transactions safe, so that it can recover failures? Here's the short overview of the transaction-phases db4o uses.</p>
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<th style="text-align: left;" class="TableStyle_Basic_Body_0_0_RowSep_ColSep">Phase</th>
<th style="text-align: left;" class="TableStyle_Basic_Body_0_0_RowSep_ColEnd">In Case Of A Crash</th>
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<td class="TableStyle_Basic_Body_0_0_RowSep_ColSep">1. During the transactions. New and updated objects are written to a new Slot in the database-file. The id-mapping and freespace changes are kept in the transaction.</td>
<td class="TableStyle_Basic_Body_0_0_RowSep_ColEnd">The changes are lost, because the id-mapping and freespace changes weren't persisted. Therefore the changes are invisible to the database. The transaction is rolled back.</td>
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<td class="TableStyle_Basic_Body_0_0_RowSep_ColSep">2. Committing starts: The id-changes and free-space changes are written to a new slot, without damaging the old information.</td>
<td class="TableStyle_Basic_Body_0_0_RowSep_ColEnd">The changes are lost, because the id-mapping and freespace changes haven't been completely stored. The transaction is rolled back.</td>
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<td class="TableStyle_Basic_Body_0_0_RowSep_ColSep">3. Write the location of latest id-records, and free-space changes to the first location with and additional checksum.</td>
<td class="TableStyle_Basic_Body_0_0_RowSep_ColEnd">If the record write was completed, the transaction is resumed and completed. If not, the old information is used.</td>
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<td class="TableStyle_Basic_Body_0_0_RowEnd_ColSep">4. Write the of latest id-recods, and free-space changes to the backup location with and additional checksum.</td>
<td class="TableStyle_Basic_Body_0_0_RowEnd_ColEnd">If the record wasn't completely written, the transaction is resumed. </td>
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<p>Of course you don't need to worry about this. db4o ensures that a transaction either completes or is rolled back. Whenever you call commit and the call succeeds, all changes are persisted. If your application or db4o crashes before a successfully commit-call, all changes are undone. </p>
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