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Long Term Support Release
Differences Between: [Versions 310 and 401] [Versions 311 and 401] [Versions 39 and 401] [Versions 400 and 401]
1 <?php 2 // This file is part of Moodle - http://moodle.org/ 3 // 4 // Moodle is free software: you can redistribute it and/or modify 5 // it under the terms of the GNU General Public License as published by 6 // the Free Software Foundation, either version 3 of the License, or 7 // (at your option) any later version. 8 // 9 // Moodle is distributed in the hope that it will be useful, 10 // but WITHOUT ANY WARRANTY; without even the implied warranty of 11 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 12 // GNU General Public License for more details. 13 // 14 // You should have received a copy of the GNU General Public License 15 // along with Moodle. If not, see <http://www.gnu.org/licenses/>. 16 17 /** 18 * Trait that adds read-only slave connection capability 19 * 20 * @package core 21 * @category dml 22 * @copyright 2018 Srdjan Janković, Catalyst IT 23 * @license http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later 24 */ 25 26 defined('MOODLE_INTERNAL') || die(); 27 28 /** 29 * Trait to wrap connect() method of database driver classes that gives 30 * ability to use read only slave instances for SELECT queries. For the 31 * databases that support replication and read only connections to the slave. 32 * If the slave connection is configured there will be two database handles 33 * created, one for the master and another one for the slave. If there's no 34 * slave specified everything uses master handle. 35 * 36 * Classes that use this trait need to rename existing connect() method to 37 * raw_connect(). In addition, they need to provide get_db_handle() and 38 * set_db_handle() methods, due to dbhandle attributes not being named 39 * consistently across the database driver classes. 40 * 41 * Read only slave connection is configured in the $CFG->dboptions['readonly'] 42 * array. 43 * - It supports multiple 'instance' entries, in case one is not accessible, 44 * but only one (first connectable) instance is used. 45 * - 'latency' option: master -> slave sync latency in seconds (will probably 46 * be a fraction of a second). A table being written to is deemed fully synced 47 * after that period and suitable for slave read. Defaults to 1 sec. 48 * - 'exclude_tables' option: a list of tables that never go to the slave for 49 * querying. The feature is meant to be used in emergency only, so the 50 * readonly feature can still be used in case there is a rogue query that 51 * does not go through the standard dml interface or some other unaccounted 52 * situation. It should not be used under normal circumstances, and its use 53 * indicates a problem in the system that needs addressig. 54 * 55 * Choice of the database handle is based on following: 56 * - SQL_QUERY_INSERT, UPDATE and STRUCTURE record table from the query 57 * in the $written array and microtime() the event. For those queries master 58 * write handle is used. 59 * - SQL_QUERY_AUX queries will always use the master write handle because they 60 * are used for transaction start/end, locking etc. In that respect, query_start() and 61 * query_end() *must not* be used during the connection phase. 62 * - SQL_QUERY_AUX_READONLY queries will use the master write handle if in a transaction. 63 * - SELECT queries will use the master write handle if: 64 * -- any of the tables involved is a temp table 65 * -- any of the tables involved is listed in the 'exclude_tables' option 66 * -- any of the tables involved is in the $written array: 67 * * current microtime() is compared to the write microrime, and if more than 68 * latency time has passed the slave handle is used 69 * * otherwise (not enough time passed) we choose the master write handle 70 * If none of the above conditions are met the slave instance is used. 71 * 72 * A 'latency' example: 73 * - we have set $CFG->dboptions['readonly']['latency'] to 0.2. 74 * - a SQL_QUERY_UPDATE to table tbl_x happens, and it is recorded in 75 * the $written array 76 * - 0.15 seconds later SQL_QUERY_SELECT with tbl_x is requested - the master 77 * connection is used 78 * - 0.10 seconds later (0.25 seconds after SQL_QUERY_UPDATE) another 79 * SQL_QUERY_SELECT with tbl_x is requested - this time more than 0.2 secs 80 * has gone and master -> slave sync is assumed, so the slave connection is 81 * used again 82 */ 83 84 trait moodle_read_slave_trait { 85 86 /** @var resource master write database handle */ 87 protected $dbhwrite; 88 89 /** @var resource slave read only database handle */ 90 protected $dbhreadonly; 91 92 private $wantreadslave = false; 93 private $readsslave = 0; 94 private $slavelatency = 1; 95 private $structurechange = false; 96 97 private $written = []; // Track tables being written to. 98 private $readexclude = []; // Tables to exclude from using dbhreadonly. 99 100 // Store original params. 101 private $pdbhost; 102 private $pdbuser; 103 private $pdbpass; 104 private $pdbname; 105 private $pprefix; 106 private $pdboptions; 107 108 /** 109 * Gets db handle currently used with queries 110 * @return resource 111 */ 112 abstract protected function get_db_handle(); 113 114 /** 115 * Sets db handle to be used with subsequent queries 116 * @param resource $dbh 117 * @return void 118 */ 119 abstract protected function set_db_handle($dbh): void; 120 121 /** 122 * Connect to db 123 * The real connection establisment, called from connect() and set_dbhwrite() 124 * @param string $dbhost The database host. 125 * @param string $dbuser The database username. 126 * @param string $dbpass The database username's password. 127 * @param string $dbname The name of the database being connected to. 128 * @param mixed $prefix string means moodle db prefix, false used for external databases where prefix not used 129 * @param array $dboptions driver specific options 130 * @return bool true 131 * @throws dml_connection_exception if error 132 */ 133 abstract protected function raw_connect(string $dbhost, string $dbuser, string $dbpass, string $dbname, $prefix, array $dboptions = null): bool; 134 135 /** 136 * Connect to db 137 * The connection parameters processor that sets up stage for master write and slave readonly handles. 138 * Must be called before other methods. 139 * @param string $dbhost The database host. 140 * @param string $dbuser The database username. 141 * @param string $dbpass The database username's password. 142 * @param string $dbname The name of the database being connected to. 143 * @param mixed $prefix string means moodle db prefix, false used for external databases where prefix not used 144 * @param array $dboptions driver specific options 145 * @return bool true 146 * @throws dml_connection_exception if error 147 */ 148 public function connect($dbhost, $dbuser, $dbpass, $dbname, $prefix, array $dboptions = null) { 149 $this->pdbhost = $dbhost; 150 $this->pdbuser = $dbuser; 151 $this->pdbpass = $dbpass; 152 $this->pdbname = $dbname; 153 $this->pprefix = $prefix; 154 $this->pdboptions = $dboptions; 155 156 if ($dboptions) { 157 if (isset($dboptions['readonly'])) { 158 $this->wantreadslave = true; 159 $dboptionsro = $dboptions['readonly']; 160 161 if (isset($dboptionsro['connecttimeout'])) { 162 $dboptions['connecttimeout'] = $dboptionsro['connecttimeout']; 163 } else if (!isset($dboptions['connecttimeout'])) { 164 $dboptions['connecttimeout'] = 2; // Default readonly connection timeout. 165 } 166 if (isset($dboptionsro['latency'])) { 167 $this->slavelatency = $dboptionsro['latency']; 168 } 169 if (isset($dboptionsro['exclude_tables'])) { 170 $this->readexclude = $dboptionsro['exclude_tables']; 171 if (!is_array($this->readexclude)) { 172 throw new configuration_exception('exclude_tables must be an array'); 173 } 174 } 175 $dbport = isset($dboptions['dbport']) ? $dboptions['dbport'] : null; 176 177 $slaves = $dboptionsro['instance']; 178 if (!is_array($slaves) || !isset($slaves[0])) { 179 $slaves = [$slaves]; 180 } 181 182 if (count($slaves) > 1) { 183 // Randomise things a bit. 184 shuffle($slaves); 185 } 186 187 // Find first connectable readonly slave. 188 $rodb = []; 189 foreach ($slaves as $slave) { 190 if (!is_array($slave)) { 191 $slave = ['dbhost' => $slave]; 192 } 193 foreach (['dbhost', 'dbuser', 'dbpass'] as $dbparam) { 194 $rodb[$dbparam] = isset($slave[$dbparam]) ? $slave[$dbparam] : $$dbparam; 195 } 196 $dboptions['dbport'] = isset($slave['dbport']) ? $slave['dbport'] : $dbport; 197 198 try { 199 $this->raw_connect($rodb['dbhost'], $rodb['dbuser'], $rodb['dbpass'], $dbname, $prefix, $dboptions); 200 $this->dbhreadonly = $this->get_db_handle(); 201 break; 202 } catch (dml_connection_exception $e) { // phpcs:ignore 203 // If readonly slave is not connectable we'll have to do without it. 204 } 205 } 206 // ... lock_db queries always go to master. 207 // Since it is a lock and as such marshalls concurrent connections, 208 // it is best to leave it out and avoid master/slave latency. 209 $this->readexclude[] = 'lock_db'; 210 // ... and sessions. 211 $this->readexclude[] = 'sessions'; 212 } 213 } 214 if (!$this->dbhreadonly) { 215 $this->set_dbhwrite(); 216 } 217 218 return true; 219 } 220 221 /** 222 * Set database handle to readwrite master 223 * Will connect if required. Calls set_db_handle() 224 * @return void 225 */ 226 private function set_dbhwrite(): void { 227 // Lazy connect to read/write master. 228 if (!$this->dbhwrite) { 229 $temptables = $this->temptables; 230 $this->raw_connect($this->pdbhost, $this->pdbuser, $this->pdbpass, $this->pdbname, $this->pprefix, $this->pdboptions); 231 if ($temptables) { 232 $this->temptables = $temptables; // Restore temptables, so we don't get separate sets for rw and ro. 233 } 234 $this->dbhwrite = $this->get_db_handle(); 235 } 236 $this->set_db_handle($this->dbhwrite); 237 } 238 239 /** 240 * Returns whether we want to connect to slave database for read queries. 241 * @return bool Want read only connection 242 */ 243 public function want_read_slave(): bool { 244 return $this->wantreadslave; 245 } 246 247 /** 248 * Returns the number of reads done by the read only database. 249 * @return int Number of reads. 250 */ 251 public function perf_get_reads_slave(): int { 252 return $this->readsslave; 253 } 254 255 /** 256 * On DBs that support it, switch to transaction mode and begin a transaction 257 * @return moodle_transaction 258 */ 259 public function start_delegated_transaction() { 260 $this->set_dbhwrite(); 261 return parent::start_delegated_transaction(); 262 } 263 264 /** 265 * Called before each db query. 266 * @param string $sql 267 * @param array|null $params An array of parameters. 268 * @param int $type type of query 269 * @param mixed $extrainfo driver specific extra information 270 * @return void 271 */ 272 protected function query_start($sql, ?array $params, $type, $extrainfo = null) { 273 parent::query_start($sql, $params, $type, $extrainfo); 274 $this->select_db_handle($type, $sql); 275 } 276 277 /** 278 * This should be called immediately after each db query. It does a clean up of resources. 279 * 280 * @param mixed $result The db specific result obtained from running a query. 281 * @return void 282 */ 283 protected function query_end($result) { 284 if ($this->written) { 285 // Adjust the written time. 286 array_walk($this->written, function (&$val) { 287 if ($val === true) { 288 $val = microtime(true); 289 } 290 }); 291 } 292 293 parent::query_end($result); 294 } 295 296 /** 297 * Select appropriate db handle - readwrite or readonly 298 * @param int $type type of query 299 * @param string $sql 300 * @return void 301 */ 302 protected function select_db_handle(int $type, string $sql): void { 303 if ($this->dbhreadonly && $this->can_use_readonly($type, $sql)) { 304 $this->readsslave++; 305 $this->set_db_handle($this->dbhreadonly); 306 return; 307 } 308 $this->set_dbhwrite(); 309 } 310 311 /** 312 * Check if The query qualifies for readonly connection execution 313 * Logging queries are exempt, those are write operations that circumvent 314 * standard query_start/query_end paths. 315 * @param int $type type of query 316 * @param string $sql 317 * @return bool 318 */ 319 protected function can_use_readonly(int $type, string $sql): bool { 320 if ($this->loggingquery) { 321 return false; 322 } 323 324 if (during_initial_install()) { 325 return false; 326 } 327 328 // Transactions are done as AUX, we cannot play with that. 329 switch ($type) { 330 case SQL_QUERY_AUX_READONLY: 331 // SQL_QUERY_AUX_READONLY may read the structure data. 332 // We don't have a way to reliably determine whether it is safe to go to readonly if the structure has changed. 333 return !$this->structurechange; 334 case SQL_QUERY_SELECT: 335 if ($this->transactions) { 336 return false; 337 } 338 339 $now = null; 340 foreach ($this->table_names($sql) as $tablename) { 341 if (in_array($tablename, $this->readexclude)) { 342 return false; 343 } 344 345 if ($this->temptables && $this->temptables->is_temptable($tablename)) { 346 return false; 347 } 348 349 if (isset($this->written[$tablename])) { 350 $now = $now ?: microtime(true); 351 352 if ($now - $this->written[$tablename] < $this->slavelatency) { 353 return false; 354 } 355 unset($this->written[$tablename]); 356 } 357 } 358 359 return true; 360 case SQL_QUERY_INSERT: 361 case SQL_QUERY_UPDATE: 362 foreach ($this->table_names($sql) as $tablename) { 363 $this->written[$tablename] = true; 364 } 365 return false; 366 case SQL_QUERY_STRUCTURE: 367 $this->structurechange = true; 368 foreach ($this->table_names($sql) as $tablename) { 369 if (!in_array($tablename, $this->readexclude)) { 370 $this->readexclude[] = $tablename; 371 } 372 } 373 return false; 374 } 375 return false; 376 } 377 378 /** 379 * Indicates delegated transaction finished successfully. 380 * Set written times after outermost transaction finished 381 * @param moodle_transaction $transaction The transaction to commit 382 * @return void 383 * @throws dml_transaction_exception Creates and throws transaction related exceptions. 384 */ 385 public function commit_delegated_transaction(moodle_transaction $transaction) { 386 if ($this->written) { 387 // Adjust the written time. 388 $now = microtime(true); 389 foreach ($this->written as $tablename => $when) { 390 $this->written[$tablename] = $now; 391 } 392 } 393 394 parent::commit_delegated_transaction($transaction); 395 } 396 397 /** 398 * Parse table names from query 399 * @param string $sql 400 * @return array 401 */ 402 protected function table_names(string $sql): array { 403 preg_match_all('/\b'.$this->prefix.'([a-z][A-Za-z0-9_]*)/', $sql, $match); 404 return $match[1]; 405 } 406 }
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