193 lines
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193 lines
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<a href="intro.html"><h2>Session Module C Interface</h2></a><h2>Rebasing changesets</h2><blockquote><pre>typedef struct sqlite3_rebaser sqlite3_rebaser;
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</pre></blockquote><p><b>Important:</b> This interface is <a href="../c3ref/experimental.html">experimental</a> and is subject to change without notice.</p><p>
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Suppose there is a site hosting a database in state S0. And that
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modifications are made that move that database to state S1 and a
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changeset recorded (the "local" changeset). Then, a changeset based
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on S0 is received from another site (the "remote" changeset) and
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applied to the database. The database is then in state
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(S1+"remote"), where the exact state depends on any conflict
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resolution decisions (OMIT or REPLACE) made while applying "remote".
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Rebasing a changeset is to update it to take those conflict
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resolution decisions into account, so that the same conflicts
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do not have to be resolved elsewhere in the network. </p>
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<p>For example, if both the local and remote changesets contain an
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INSERT of the same key on "CREATE TABLE t1(a PRIMARY KEY, b)":</p>
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<p> local: INSERT INTO t1 VALUES(1, 'v1');
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remote: INSERT INTO t1 VALUES(1, 'v2');</p>
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<p>and the conflict resolution is REPLACE, then the INSERT change is
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removed from the local changeset (it was overridden). Or, if the
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conflict resolution was "OMIT", then the local changeset is modified
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to instead contain:</p>
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<p> UPDATE t1 SET b = 'v2' WHERE a=1;</p>
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<p>Changes within the local changeset are rebased as follows:</p>
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<p><dl>
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<dt>Local INSERT<dd>
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This may only conflict with a remote INSERT. If the conflict
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resolution was OMIT, then add an UPDATE change to the rebased
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changeset. Or, if the conflict resolution was REPLACE, add
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nothing to the rebased changeset.</p>
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<p><dt>Local DELETE<dd>
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This may conflict with a remote UPDATE or DELETE. In both cases the
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only possible resolution is OMIT. If the remote operation was a
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DELETE, then add no change to the rebased changeset. If the remote
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operation was an UPDATE, then the old.* fields of change are updated
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to reflect the new.* values in the UPDATE.</p>
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<p><dt>Local UPDATE<dd>
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This may conflict with a remote UPDATE or DELETE. If it conflicts
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with a DELETE, and the conflict resolution was OMIT, then the update
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is changed into an INSERT. Any undefined values in the new.* record
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from the update change are filled in using the old.* values from
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the conflicting DELETE. Or, if the conflict resolution was REPLACE,
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the UPDATE change is simply omitted from the rebased changeset.</p>
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<p> If conflict is with a remote UPDATE and the resolution is OMIT, then
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the old.* values are rebased using the new.* values in the remote
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change. Or, if the resolution is REPLACE, then the change is copied
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into the rebased changeset with updates to columns also updated by
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the conflicting remote UPDATE removed. If this means no columns would
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be updated, the change is omitted.
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</dl></p>
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<p>A local change may be rebased against multiple remote changes
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simultaneously. If a single key is modified by multiple remote
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changesets, they are combined as follows before the local changeset
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is rebased:</p>
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<p><ul>
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<li> If there has been one or more REPLACE resolutions on a
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key, it is rebased according to a REPLACE.</p>
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<p> <li> If there have been no REPLACE resolutions on a key, then
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the local changeset is rebased according to the most recent
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of the OMIT resolutions.
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</ul></p>
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<p>Note that conflict resolutions from multiple remote changesets are
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combined on a per-field basis, not per-row. This means that in the
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case of multiple remote UPDATE operations, some fields of a single
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local change may be rebased for REPLACE while others are rebased for
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OMIT.</p>
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<p>In order to rebase a local changeset, the remote changeset must first
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be applied to the local database using sqlite3changeset_apply_v2() and
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the buffer of rebase information captured. Then:</p>
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<p><ol>
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<li> An sqlite3_rebaser object is created by calling
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sqlite3rebaser_create().
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<li> The new object is configured with the rebase buffer obtained from
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sqlite3changeset_apply_v2() by calling sqlite3rebaser_configure().
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If the local changeset is to be rebased against multiple remote
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changesets, then sqlite3rebaser_configure() should be called
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multiple times, in the same order that the multiple
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sqlite3changeset_apply_v2() calls were made.
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<li> Each local changeset is rebased by calling sqlite3rebaser_rebase().
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<li> The sqlite3_rebaser object is deleted by calling
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sqlite3rebaser_delete().
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</ol>
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</p><p>See also lists of
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<a href="objlist.html">Objects</a>,
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<a href="constlist.html">Constants</a>, and
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<a href="funclist.html">Functions</a>.</p>
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