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MDL-51245 mssql: Diagnose missing READ_COMMITTED_SNAPSHOT mode

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Without it, transactions are executed in standard READ_COMMITED
mode, without snapshoting/row versioning, leading to update conflicts
under high concurrency.
This commit is contained in:
Eloy Lafuente (stronk7) 2015-09-04 02:05:29 +02:00
parent 1dabedeedf
commit a9daab4927
2 changed files with 32 additions and 0 deletions
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1
lang/en/error.php
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@ -388,6 +388,7 @@ $string['movecatcontentstoroot'] = 'Moving the category content to root is not a
$string['movecategorynotpossible'] = 'You cannot move category \'{$a}\' into the selected category.';
$string['movecategoryownparent'] = 'You cannot make category \'{$a}\' a parent of itself.';
$string['movecategoryparentconflict'] = 'You cannot make category \'{$a}\' a subcategory of one of its own subcategories.';
$string['mssqlrcsmodemissing'] = 'The database is not using the expected READ_COMMITTED_SNAPSHOT mode which can lead to wrong results, especially under high concurrency scenarios. Please enable it for correct behaviour. You can find more information in the <a href="https://docs.moodle.org/en/Installing_MSSQL_for_PHP#Configuration">Moodle Docs</a>.';
$string['multiplerecordsfound'] = 'Multiple records found, only one record expected.';
$string['multiplerestorenotallow'] = 'Multiple restore execution not allowed!';
$string['mustbeloggedin'] = 'You must be logged in to do this';

31
lib/dml/mssql_native_moodle_database.php
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@ -98,6 +98,37 @@ class mssql_native_moodle_database extends moodle_database {
return get_string('nativemssqlhelp', 'install');
}
/**
* Diagnose database and tables, this function is used
* to verify database and driver settings, db engine types, etc.
*
* @return string null means everything ok, string means problem found.
*/
public function diagnose() {
// Verify the database is running with READ_COMMITTED_SNAPSHOT enabled.
// (that's required to get snapshots/row versioning on READ_COMMITED mode).
$correctrcsmode = false;
$sql = "SELECT is_read_committed_snapshot_on
FROM sys.databases
WHERE name = '{$this->dbname}'";
$this->query_start($sql, null, SQL_QUERY_AUX);
$result = mssql_query($sql, $this->mssql);
$this->query_end($result);
if ($result) {
if ($row = mssql_fetch_assoc($result)) {
$correctrcsmode = (bool)reset($row);
}
}
$this->free_result($result);
if (!$correctrcsmode) {
return get_string('mssqlrcsmodemissing', 'error');
}
// Arrived here, all right.
return null;
}
/**
* Connect to db
* Must be called before other methods.