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<title>The Session Extension</title>
<tcl>
hd_keywords {session} {session extension}
</tcl>

<table_of_contents>

<h1>Introduction</h1>

<p>The session extension provide a mechanism for recording changes
to some or all of the [rowid tables] in an SQLite database, and packaging
those changes into a "changeset" or "patchset" file that can later
be used to apply the same set of changes to another database with
the same schema and compatible starting data.  A "changeset" may
also be inverted and used to "undo" a session.

<p>This document is an introduction to the session extension.
The details of the interface are in the separate
[Session Extension C-language Interface] document.

<h2>Typical Use Case</h2>

<p>Suppose SQLite is used as the [application file format] for a
particular design application.  Two users, Alice and Bob, each start
with a baseline design that is about a gigabyte in size.  They work
all day, in parallel, each making their own customizations and tweaks
to the design.  At the end of the day, they would like to merge their
changes together into a single unified design.

<p>The session extension facilitates this by recording all changes to
both Alice's and Bob's databases and writing those changes into
changeset or patchset files.  At the end of the day, Alice can send her
changeset to Bob and Bob can "apply" it to his database.  The result (assuming
there are no conflicts) is that Bob's database then contains both his
changes and Alice's changes.  Likewise, Bob can send a changeset of
his work over to Alice and she can apply his changes to her database.

<p>In other words, the session extension provides a facility for
SQLite database files that is similar to the unix
[https://en.wikipedia.org/wiki/Patch_(Unix)|patch] utility program,
or to the "merge" capabilities of version control systems such
as [https://www.fossil-scm.org/|Fossil], [https://git-scm.com|Git], 
or [http://www.mercurial-scm.org/|Mercurial].

<h2>Obtaining the Session Extension</h2>

<p> Since [version 3.13.0] ([dateof:3.13.0]), 
the session extension has been included in the SQLite
[amalgamation] source distribution. By default, the session extension is 
disabled. To enable it, build with the following compiler switches:

<codeblock>
  -DSQLITE_ENABLE_SESSION -DSQLITE_ENABLE_PREUPDATE_HOOK
</codeblock>

<p> Or, if using the autoconf build system, pass the --enable-session option to the configure script.

<h2>Limitations</h2>

<ul>

<li><p> Prior to SQLite version 3.17.0, the session extension only worked with
        [rowid tables], not [WITHOUT ROWID] tables. As of 3.17.0, both
        rowid and WITHOUT ROWID tables are supported.

<li><p> There is no support for [virtual tables]. Changes to virtual tables are
        not captured.

<li><p> The session extension only works with tables that have a declared
        PRIMARY KEY. The PRIMARY KEY of a table may be an INTEGER PRIMARY KEY
        (rowid alias) or an external PRIMARY KEY.

<li><p> SQLite allows <a href=nulls.html>NULL values</a> to be stored in
        PRIMARY KEY columns. However, the session extension ignores all
        such rows. No changes affecting rows with one or more NULL values
        in PRIMARY KEY columns are recorded by the sessions module.
</ul>

<h1>Concepts</h1>
<h2 id=changesets_and_patchsets>Changesets and Patchsets</h2>
<p> The sessions module revolves around creating and manipulating 
changesets. A changeset is a blob of data that encodes a series of 
changes to a database. Each change in a changeset is one of the 
following:

<ul>
  <li> <p>An <b>INSERT</b>. An INSERT change contains a single row to add to 
       a database table. The payload of the INSERT change consists of the
       values for each field of the new row.

  <li> <p>A <b>DELETE</b>. A DELETE change represents a row, identified by
       its primary key values, to remove from a database table. The payload
       of a DELETE change consists of the values for all fields of the 
       deleted row.

  <li> <p>An <b>UPDATE</b>. An UPDATE change represents the modification of
       one or more non-PRIMARY KEY fields of a single row within a database 
       table, identified by its PRIMARY KEY fields. The payload for an UPDATE
       change consists of:
   <ul>
     <li> The PRIMARY KEY values identifying the modified row, 
     <li> The new values for each modified field of the row, and
     <li> The original values for each modified field of the row.
   </ul>
       <p> An UPDATE change does not contain any information regarding
       non-PRIMARY KEY fields that are not modified by the change. It is not
       possible for an UPDATE change to specify modifications to PRIMARY 
       KEY fields. 
</ul>

<p> A single changeset may contain changes that apply to more than one 
database table. For each table that the changeset includes at least one change
for, it also encodes the following data:

<ul>
  <li> The name of the database table, 
  <li> The number of columns the table has, and
  <li> Which of those columns are PRIMARY KEY columns.
</ul>

<p> Changesets may only be applied to databases that contain tables 
matching the above three criteria as stored in the changeset.

<p> A patchset is similar to a changeset. It is slightly more compact than
a changeset, but provides more limited conflict detection and resolution
options (see the next section for details). The differences between a 
patchset and a changeset are that:

<ul>
  <li><p> For a <b>DELETE</b> change, the payload consists of the PRIMARY KEY 
          fields only. The original values of other fields are not stored as
          part of a patchset.

  <li><p> For an <b>UPDATE</b> change, the payload consists of the PRIMARY KEY 
          fields and the new values of modified fields only. The original
          values of modified fields are not stored as part of a patchset.
</ul>

<h2>Conflicts</h2>

<p> When a changeset or patchset is applied to a database, an attempt is 
made to insert a new row for each INSERT change, remove a row for each
DELETE change and modify a row for each UPDATE change. If the target 
database is in the same state as the original database that the changeset
was recorded on, this is a simple matter. However, if the contents of the
target database is not in exactly this state, conflicts can occur when
applying the changeset or patchset.

<p>When processing an <b>INSERT</b> change, the following conflicts can
occur:

<ul>
  <li> The target database may already contain a row with the same PRIMARY
       KEY values as specified by the INSERT change.

  <li> Some other database constraint, for example a UNIQUE or CHECK 
       constraint, may be violated when the new row is inserted.
</ul>

<p>When processing a <b>DELETE</b> change, the following conflicts may be
detected:

<ul>
  <li> The target database may contain no row with the specified PRIMARY 
       KEY values to delete.

  <li> The target database may contain a row with the specified PRIMARY
       KEY values, but the other fields may contain values that do not
       match those stored as part of the changeset. This type of conflict
       is not detected when using a patchset.
</ul>

<p>When processing an <b>UPDATE</b> change, the following conflicts may be
detected:

<ul>
  <li> The target database may contain no row with the specified PRIMARY 
       KEY values to modify.

  <li> The target database may contain a row with the specified PRIMARY
       KEY values, but the current values of the fields that will be modified
       by the change may not match the original values stored within the
       changeset. This type of conflict is not detected when using a patchset.

  <li> Some other database constraint, for example a UNIQUE or CHECK 
       constraint, may be violated when the row is updated.
</ul>

<p> Depending on the type of conflict, a sessions application has a variety
of configurable options for dealing with conflicts, ranging from omitting the
conflicting change, aborting the entire changeset application or applying
the change despite the conflict. For details, refer to the documentation for
the [sqlite3changeset_apply()] API.

<h2>Changeset Construction</h2>

<p> After a session object has been configured, it begins monitoring for 
changes to its configured tables. However, it does not record an entire
change each time a row within the database is modified. Instead, it records
just the PRIMARY KEY fields for each inserted row, and just the PRIMARY KEY 
and all original row values for any updated or deleted rows. If a row is 
modified more than once by a single session, no new information is recorded.

<p> The other information required to create a changeset or patchset is
read from the database file when [sqlite3session_changeset()] or
[sqlite3session_patchset()] is called. Specifically,

<ul>
  <li> <p>For each primary key recorded as a result of an INSERT operation, 
       the sessions module checks if there is a row with a matching primary
       key still in the table. If so, an INSERT change is added to the 
       changeset.

  <li> <p>For each primary key recorded as a result of an UPDATE or DELETE
       operation, the sessions module also checks for a row with a matching
       primary key within the table. If one can be found, but one or more
       of the non-PRIMARY KEY fields does not match the original recorded
       value, an UPDATE is added to the changeset. Or, if there is no row
       at all with the specified primary key, a DELETE is added to the 
       changeset. If the row does exist but none of the non-PRIMARY KEY
       fields have been modified, no change is added to the changeset.
</ul>

<p> One implication of the above is that if a change is made and then 
unmade within a single session (for example if a row is inserted and then
deleted again), the sessions module does not report any change at all. Or
if a row is updated multiple times within the same session, all updates
are coalesced into a single update within any changeset or patchset blob.

<h1>Using The Session Extension</h1>

<p> This section provides examples that demonstrate how to use the sessions
    extension.

<h2>Capturing a Changeset</h2>

<p> The example code below demonstrates the steps involved in capturing a
changeset while executing SQL commands. In summary:

<ol>
  <li> <p>A session object (type sqlite3_session*) is created by making a 
          call to the [sqlite3session_create()] API function.

       <p>A single session object monitors changes made to a single database 
          (i.e. "main", "temp" or an attached database) via a single 
          sqlite3* database handle.

  <li> <p>The session object is configured with a set of tables to monitor
          changes on.

       <p> By default a session object does not monitor changes on any 
           database table. Before it does so it must be configured. There 
           are three ways to configure the set of tables to monitor changes
           on:
       <ul>
         <li> By explicitly specifying tables using one call to
              [sqlite3session_attach()] for each table, or

         <li> By specifying that all tables in the database should be monitored
              for changes using a call to [sqlite3session_attach()] with a
              NULL argument, or

         <li> By configuring a callback to be invoked the first time each table
              is written to that indicates to the session module whether or
              not changes on the table should be monitored.
       </ul>
        <p> The example code below uses the second of the methods enumerated
            above - it monitors for changes on all database tables.

  <li> <p> Changes are made to the database by executing SQL statements. The
           session object records these changes.

  <li> <p> A changeset blob is extracted from the session object using a call
           to [sqlite3session_changeset()] (or, if using patchsets, a call to
           the [sqlite3session_patchset()] function).

  <li> <p> The session object is deleted using a call to the 
           [sqlite3session_delete()] API function.

       <p> It is not necessary to delete a session object after extracting
           a changeset or patchset from it. It can be left attached to the
           database handle and will continue monitoring for changes on the
           configured tables as before. However, if 
           [sqlite3session_changeset()] or [sqlite3session_patchset()] is
           called a second time on a session object, the changeset or patchset
           will contain <em>all</em> changes that have taken place on the connection
           since the session was created. In other words,
           a session object is not reset or
           zeroed by a call to sqlite3session_changeset() or
           sqlite3session_patchset().
</ol>

<codeblock class=C>
  /*
  ** Argument zSql points to a buffer containing an SQL script to execute 
  ** against the database handle passed as the first argument. As well as
  ** executing the SQL script, this function collects a changeset recording
  ** all changes made to the "main" database file. Assuming no error occurs,
  ** output variables (*ppChangeset) and (*pnChangeset) are set to point
  ** to a buffer containing the changeset and the size of the changeset in
  ** bytes before returning SQLITE_OK. In this case it is the responsibility
  ** of the caller to eventually free the changeset blob by passing it to
  ** the sqlite3_free function.
  **
  ** Or, if an error does occur, return an SQLite error code. The final
  ** value of (*pChangeset) and (*pnChangeset) are undefined in this case.
  */
  int sql_exec_changeset(
    sqlite3 *db,                  /* Database handle */
    const char *zSql,             /* SQL script to execute */
    int *pnChangeset,             /* OUT: Size of changeset blob in bytes */
    void **ppChangeset            /* OUT: Pointer to changeset blob */
  ){
    sqlite3_session *pSession = 0;
    int rc;

    /* Create a new session object */
    rc = sqlite3session_create(db, "main", &pSession);

    /* Configure the session object to record changes to all tables */
    if( rc==SQLITE_OK ) rc = sqlite3session_attach(pSession, NULL);

    /* Execute the SQL script */
    if( rc==SQLITE_OK ) rc = sqlite3_exec(db, zSql, 0, 0, 0);

    /* Collect the changeset */
    if( rc==SQLITE_OK ){
      rc = sqlite3session_changeset(pSession, pnChangeset, ppChangeset);
    }

    /* Delete the session object */
    sqlite3session_delete(pSession);

    return rc;
  }
</codeblock>

<h2>Applying a Changeset to a Database</h2>

<p> Applying a changeset to a database is simpler than capturing a changeset.
Usually, a single call to [sqlite3changeset_apply()], as depicted in the
example code below, suffices.

<p> In cases where it is complicated, the complications in applying a 
changeset lie in conflict resolution. Refer to the API documentation linked
above for details.

  <codeblock class=C>
    /*
    ** Conflict handler callback used by apply_changeset(). See below.
    */
    static int xConflict(void *pCtx, int eConflict, sqlite3_changset_iter *pIter){
      int ret = (int)pCtx;
      return ret;
    }

    /*
    ** Apply the changeset contained in blob pChangeset, size nChangeset bytes,
    ** to the main database of the database handle passed as the first argument.
    ** Return SQLITE_OK if successful, or an SQLite error code if an error
    ** occurs.
    **
    ** If parameter bIgnoreConflicts is true, then any conflicting changes 
    ** within the changeset are simply ignored. Or, if bIgnoreConflicts is
    ** false, then this call fails with an SQLTIE_ABORT error if a changeset
    ** conflict is encountered.
    */
    int apply_changeset(
      sqlite3 *db,                  /* Database handle */
      int bIgnoreConflicts,         /* True to ignore conflicting changes */
      int nChangeset,               /* Size of changeset in bytes */
      void *pChangeset              /* Pointer to changeset blob */
    ){
      return sqlite3changeset_apply(
          db, 
          nChangeset, pChangeset, 
          0, xConflict, 
          (void*)bIgnoreConflicts
      );
    }
</codeblock>

<h2>Inspecting the Contents of a Changeset</h2>

<p> The example code below demonstrates the techniques used to iterate 
through and extract the data related to all changes in a changeset. To
summarize:

<ol>
  <li><p> The [sqlite3changeset_start()] API is called to create and
          initialize an iterator to iterate through the contents of a
          changeset. Initially, the iterator points to no element at all.

  <li><p> The first call to [sqlite3changeset_next()] on the iterator moves
          it to point to the first change  in the changeset (or to EOF, if 
          the changeset is completely empty). sqlite3changeset_next() returns
          SQLITE_ROW if it moves the iterator to point to a valid entry,
          SQLITE_DONE if it moves the iterator to EOF, or an SQLite error
          code if an error occurs.

  <li><p> If the iterator points to a valid entry, the [sqlite3changeset_op()]
          API may be used to determine the type of change (INSERT, UPDATE or
          DELETE) that the iterator points to. Additionally, the same API 
          can be used to obtain the name of the table the change applies to
          and its expected number of columns and primary key columns.

  <li><p> If the iterator points to a valid INSERT or UPDATE entry, the
          [sqlite3changeset_new()] API may be used to obtain the new.* values
          within the change payload.

  <li><p> If the iterator points to a valid DELETE or UPDATE entry, the
          [sqlite3changeset_old()] API may be used to obtain the old.* values
          within the change payload.

  <li><p> An iterator is deleted using a call to the 
          [sqlite3changeset_finalize()] API. If an error occured while
          iterating, an SQLite error code is returned (even if the same error
          code has already been returned by sqlite3changeset_next()). Or,
          if no error has occurred, SQLITE_OK is returned.
</ol>

  <codeblock class=C>
    /*
    ** Print the contents of the changeset to stdout.
    */
    static int print_changeset(void *pChangeset, int nChangeset){
      int rc;
      sqlite3_changeset_iter *pIter = 0;

      /* Create an iterator to iterate through the changeset */
      rc = sqlite3changeset_start(&pIter, nChangeset, pChangeset);
      if( rc!=SQLITE_OK ) return rc;

      /* This loop runs once for each change in the changeset */
      while( SQLITE_ROW==sqlite3changeset_next(pIter) ){
        const char *zTab;           /* Table change applies to */
        int nCol;                   /* Number of columns in table zTab */
        int op;                     /* SQLITE_INSERT, UPDATE or DELETE */
        sqlite3_value *pVal;

        /* Print the type of operation and the table it is on */
        rc = sqlite3changeset_op(pIter, &zTab, &nCol, &op, 0);
        if( rc!=SQLITE_OK ) goto exit_print_changeset;
        printf("%s on table %s\n",
          op==SQLITE_INSERT?"INSERT" : op==SQLITE_UPDATE?"UPDATE" : "DELETE",
          zTab
        );

        /* If this is an UPDATE or DELETE, print the old.* values */
        if( op==SQLITE_UPDATE || op==SQLITE_DELETE ){
          printf("Old values:");
          for(i=0; i&lt;nCol; i++){
            rc = sqlite3changeset_old(pIter, i, &pVal);
            if( rc!=SQLITE_OK ) goto exit_print_changeset;
            printf(" %s", pVal ? sqlite3_value_text(pVal) : "-");
          }
          printf("\n");
        }

        /* If this is an UPDATE or INSERT, print the new.* values */
        if( op==SQLITE_UPDATE || op==SQLITE_INSERT ){
          printf("New values:");
          for(i=0; i&lt;nCol; i++){
            rc = sqlite3changeset_new(pIter, i, &pVal);
            if( rc!=SQLITE_OK ) goto exit_print_changeset;
            printf(" %s", pVal ? sqlite3_value_text(pVal) : "-");
          }
          printf("\n");
        }
      }

      /* Clean up the changeset and return an error code (or SQLITE_OK) */
     exit_print_changeset:
      rc2 = sqlite3changeset_finalize(pIter);
      if( rc==SQLITE_OK ) rc = rc2;
      return rc;
    }
</codeblock>

<h1>Extended Functionality</h1>

<p> Most applications will only use the session module functionality described
in the previous section. However, the following additional functionality is
available for the use and manipulation of changeset and patchset blobs:

<ul>
  <li> <p>Two or more changeset/patchsets may be combined using the 
       [sqlite3changeset_concat()] or [sqlite3_changegroup] interfaces.

  <li> <p>A changeset may be "inverted" using the [sqlite3changeset_invert()]
       API function. An inverted changeset undoes the changes made by the
       original. If changeset C<sup>+</sup> is the inverse of changeset C, then
       applying C and then C<sup>+</sup> to a database should leave
       the database unchanged.