3 * Generator of database load balancing objects.
5 * This program is free software; you can redistribute it and/or modify
6 * it under the terms of the GNU General Public License as published by
7 * the Free Software Foundation; either version 2 of the License, or
8 * (at your option) any later version.
10 * This program is distributed in the hope that it will be useful,
11 * but WITHOUT ANY WARRANTY; without even the implied warranty of
12 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
13 * GNU General Public License for more details.
15 * You should have received a copy of the GNU General Public License along
16 * with this program; if not, write to the Free Software Foundation, Inc.,
17 * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
18 * http://www.gnu.org/copyleft/gpl.html
24 namespace Wikimedia\Rdbms
;
26 use Psr\Log\LoggerAwareInterface
;
27 use Psr\Log\LoggerInterface
;
28 use Psr\Log\NullLogger
;
29 use Wikimedia\WaitConditionLoop
;
33 * Class for ensuring a consistent ordering of events as seen by the user, despite replication.
34 * Kind of like Hawking's [[Chronology Protection Agency]].
36 class ChronologyProtector
implements LoggerAwareInterface
{
39 /** @var LoggerInterface */
42 /** @var string Storage key name */
44 /** @var string Hash of client parameters */
46 /** @var int|null Expected minimum index of the last write to the position store */
47 protected $waitForPosIndex;
48 /** @var int Max seconds to wait on positions to appear */
49 protected $waitForPosStoreTimeout = self
::POS_STORE_WAIT_TIMEOUT
;
50 /** @var bool Whether to no-op all method calls */
51 protected $enabled = true;
52 /** @var bool Whether to check and wait on positions */
53 protected $wait = true;
55 /** @var bool Whether the client data was loaded */
56 protected $initialized = false;
57 /** @var DBMasterPos[] Map of (DB master name => position) */
58 protected $startupPositions = [];
59 /** @var DBMasterPos[] Map of (DB master name => position) */
60 protected $shutdownPositions = [];
61 /** @var float[] Map of (DB master name => 1) */
62 protected $shutdownTouchDBs = [];
64 /** @var int Seconds to store positions */
65 const POSITION_TTL
= 60;
66 /** @var int Seconds to store position write index cookies (safely less than POSITION_TTL) */
67 const POSITION_COOKIE_TTL
= 60;
68 /** @var int Max time to wait for positions to appear */
69 const POS_STORE_WAIT_TIMEOUT
= 5;
72 * @param BagOStuff $store
73 * @param array[] $client Map of (ip: <IP>, agent: <user-agent>)
74 * @param int|null $posIndex Write counter index [optional]
77 public function __construct( BagOStuff
$store, array $client, $posIndex = null ) {
78 $this->store
= $store;
79 $this->clientId
= md5( $client['ip'] . "\n" . $client['agent'] );
80 $this->key
= $store->makeGlobalKey( __CLASS__
, $this->clientId
, 'v2' );
81 $this->waitForPosIndex
= $posIndex;
82 $this->logger
= new NullLogger();
85 public function setLogger( LoggerInterface
$logger ) {
86 $this->logger
= $logger;
90 * @param bool $enabled Whether to no-op all method calls
93 public function setEnabled( $enabled ) {
94 $this->enabled
= $enabled;
98 * @param bool $enabled Whether to check and wait on positions
101 public function setWaitEnabled( $enabled ) {
102 $this->wait
= $enabled;
106 * Initialise a ILoadBalancer to give it appropriate chronology protection.
108 * If the stash has a previous master position recorded, this will try to
109 * make sure that the next query to a replica DB of that master will see changes up
110 * to that position by delaying execution. The delay may timeout and allow stale
111 * data if no non-lagged replica DBs are available.
113 * @param ILoadBalancer $lb
116 public function initLB( ILoadBalancer
$lb ) {
117 if ( !$this->enabled ||
$lb->getServerCount() <= 1 ) {
118 return; // non-replicated setup or disabled
121 $this->initPositions();
123 $masterName = $lb->getServerName( $lb->getWriterIndex() );
125 isset( $this->startupPositions
[$masterName] ) &&
126 $this->startupPositions
[$masterName] instanceof DBMasterPos
128 $pos = $this->startupPositions
[$masterName];
129 $this->logger
->debug( __METHOD__
. ": LB for '$masterName' set to pos $pos\n" );
130 $lb->waitFor( $pos );
135 * Notify the ChronologyProtector that the ILoadBalancer is about to shut
136 * down. Saves replication positions.
138 * @param ILoadBalancer $lb
141 public function shutdownLB( ILoadBalancer
$lb ) {
142 if ( !$this->enabled
) {
143 return; // not enabled
144 } elseif ( !$lb->hasOrMadeRecentMasterChanges( INF
) ) {
145 // Only save the position if writes have been done on the connection
149 $masterName = $lb->getServerName( $lb->getWriterIndex() );
150 if ( $lb->getServerCount() > 1 ) {
151 $pos = $lb->getMasterPos();
153 $this->logger
->debug( __METHOD__
. ": LB for '$masterName' has pos $pos\n" );
154 $this->shutdownPositions
[$masterName] = $pos;
157 $this->logger
->debug( __METHOD__
. ": DB '$masterName' touched\n" );
159 $this->shutdownTouchDBs
[$masterName] = 1;
163 * Notify the ChronologyProtector that the LBFactory is done calling shutdownLB() for now.
164 * May commit chronology data to persistent storage.
166 * @param callable|null $workCallback Work to do instead of waiting on syncing positions
167 * @param string $mode One of (sync, async); whether to wait on remote datacenters
168 * @param int|null &$cpIndex DB position key write counter; incremented on update
169 * @return DBMasterPos[] Empty on success; returns the (db name => position) map on failure
171 public function shutdown( callable
$workCallback = null, $mode = 'sync', &$cpIndex = null ) {
172 if ( !$this->enabled
) {
176 $store = $this->store
;
177 // Some callers might want to know if a user recently touched a DB.
178 // These writes do not need to block on all datacenters receiving them.
179 foreach ( $this->shutdownTouchDBs
as $dbName => $unused ) {
181 $this->getTouchedKey( $this->store
, $dbName ),
187 if ( !count( $this->shutdownPositions
) ) {
188 return []; // nothing to save
191 $this->logger
->debug( __METHOD__
. ": saving master pos for " .
192 implode( ', ', array_keys( $this->shutdownPositions
) ) . "\n"
195 // CP-protected writes should overwhelmingly go to the master datacenter, so use a
196 // DC-local lock to merge the values. Use a DC-local get() and a synchronous all-DC
197 // set(). This makes it possible for the BagOStuff class to write in parallel to all
198 // DCs with one RTT. The use of WRITE_SYNC avoids needing READ_LATEST for the get().
199 if ( $store->lock( $this->key
, 3 ) ) {
200 if ( $workCallback ) {
201 // Let the store run the work before blocking on a replication sync barrier.
202 // If replication caught up while the work finished, the barrier will be fast.
203 $store->addBusyCallback( $workCallback );
207 $this->mergePositions(
208 $store->get( $this->key
),
209 $this->shutdownPositions
,
213 ( $mode === 'sync' ) ?
$store::WRITE_SYNC
: 0
215 $store->unlock( $this->key
);
221 $cpIndex = null; // nothing saved
222 $bouncedPositions = $this->shutdownPositions
;
223 // Raced out too many times or stash is down
224 $this->logger
->warning( __METHOD__
. ": failed to save master pos for " .
225 implode( ', ', array_keys( $this->shutdownPositions
) ) . "\n"
227 } elseif ( $mode === 'sync' &&
228 $store->getQoS( $store::ATTR_SYNCWRITES
) < $store::QOS_SYNCWRITES_BE
230 // Positions may not be in all datacenters, force LBFactory to play it safe
231 $this->logger
->info( __METHOD__
. ": store may not support synchronous writes." );
232 $bouncedPositions = $this->shutdownPositions
;
234 $bouncedPositions = [];
237 return $bouncedPositions;
241 * @param string $dbName DB master name (e.g. "db1052")
242 * @return float|bool UNIX timestamp when client last touched the DB; false if not on record
245 public function getTouched( $dbName ) {
246 return $this->store
->get( $this->getTouchedKey( $this->store
, $dbName ) );
250 * @param BagOStuff $store
251 * @param string $dbName
254 private function getTouchedKey( BagOStuff
$store, $dbName ) {
255 return $store->makeGlobalKey( __CLASS__
, 'mtime', $this->clientId
, $dbName );
259 * Load in previous master positions for the client
261 protected function initPositions() {
262 if ( $this->initialized
) {
266 $this->initialized
= true;
268 // If there is an expectation to see master positions from a certain write
269 // index or higher, then block until it appears, or until a timeout is reached.
270 // Since the write index restarts each time the key is created, it is possible that
271 // a lagged store has a matching key write index. However, in that case, it should
272 // already be expired and thus treated as non-existing, maintaining correctness.
273 if ( $this->waitForPosIndex
> 0 ) {
275 $indexReached = null; // highest index reached in the position store
276 $loop = new WaitConditionLoop(
277 function () use ( &$data, &$indexReached ) {
278 $data = $this->store
->get( $this->key
);
279 if ( !is_array( $data ) ) {
280 return WaitConditionLoop
::CONDITION_CONTINUE
; // not found yet
281 } elseif ( !isset( $data['writeIndex'] ) ) {
282 return WaitConditionLoop
::CONDITION_REACHED
; // b/c
284 $indexReached = max( $data['writeIndex'], $indexReached );
286 return ( $data['writeIndex'] >= $this->waitForPosIndex
)
287 ? WaitConditionLoop
::CONDITION_REACHED
288 : WaitConditionLoop
::CONDITION_CONTINUE
;
290 $this->waitForPosStoreTimeout
292 $result = $loop->invoke();
293 $waitedMs = $loop->getLastWaitTime() * 1e3
;
295 if ( $result == $loop::CONDITION_REACHED
) {
296 $this->logger
->debug(
297 __METHOD__
. ": expected and found position index.",
299 'cpPosIndex' => $this->waitForPosIndex
,
300 'waitTimeMs' => $waitedMs
304 $this->logger
->warning(
305 __METHOD__
. ": expected but failed to find position index.",
307 'cpPosIndex' => $this->waitForPosIndex
,
308 'indexReached' => $indexReached,
309 'waitTimeMs' => $waitedMs
314 $data = $this->store
->get( $this->key
);
317 $this->startupPositions
= $data ?
$data['positions'] : [];
318 $this->logger
->debug( __METHOD__
. ": key is {$this->key} (read)\n" );
320 $this->startupPositions
= [];
321 $this->logger
->debug( __METHOD__
. ": key is {$this->key} (unread)\n" );
326 * @param array|bool $curValue
327 * @param DBMasterPos[] $shutdownPositions
328 * @param int|null &$cpIndex
331 protected function mergePositions( $curValue, array $shutdownPositions, &$cpIndex = null ) {
332 /** @var DBMasterPos[] $curPositions */
333 $curPositions = $curValue['positions'] ??
[];
334 // Use the newest positions for each DB master
335 foreach ( $shutdownPositions as $db => $pos ) {
337 !isset( $curPositions[$db] ) ||
338 !( $curPositions[$db] instanceof DBMasterPos
) ||
339 $pos->asOfTime() > $curPositions[$db]->asOfTime()
341 $curPositions[$db] = $pos;
345 $cpIndex = $curValue['writeIndex'] ??
0;
348 'positions' => $curPositions,
349 'writeIndex' => ++
$cpIndex