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 Max time to wait for positions to appear */
67 const POS_STORE_WAIT_TIMEOUT
= 5;
70 * @param BagOStuff $store
71 * @param array[] $client Map of (ip: <IP>, agent: <user-agent>)
72 * @param int|null $posIndex Write counter index [optional]
75 public function __construct( BagOStuff
$store, array $client, $posIndex = null ) {
76 $this->store
= $store;
77 $this->clientId
= md5( $client['ip'] . "\n" . $client['agent'] );
78 $this->key
= $store->makeGlobalKey( __CLASS__
, $this->clientId
, 'v1' );
79 $this->waitForPosIndex
= $posIndex;
80 $this->logger
= new NullLogger();
83 public function setLogger( LoggerInterface
$logger ) {
84 $this->logger
= $logger;
88 * @param bool $enabled Whether to no-op all method calls
91 public function setEnabled( $enabled ) {
92 $this->enabled
= $enabled;
96 * @param bool $enabled Whether to check and wait on positions
99 public function setWaitEnabled( $enabled ) {
100 $this->wait
= $enabled;
104 * Initialise a ILoadBalancer to give it appropriate chronology protection.
106 * If the stash has a previous master position recorded, this will try to
107 * make sure that the next query to a replica DB of that master will see changes up
108 * to that position by delaying execution. The delay may timeout and allow stale
109 * data if no non-lagged replica DBs are available.
111 * @param ILoadBalancer $lb
114 public function initLB( ILoadBalancer
$lb ) {
115 if ( !$this->enabled ||
$lb->getServerCount() <= 1 ) {
116 return; // non-replicated setup or disabled
119 $this->initPositions();
121 $masterName = $lb->getServerName( $lb->getWriterIndex() );
123 isset( $this->startupPositions
[$masterName] ) &&
124 $this->startupPositions
[$masterName] instanceof DBMasterPos
126 $pos = $this->startupPositions
[$masterName];
127 $this->logger
->info( __METHOD__
. ": LB for '$masterName' set to pos $pos\n" );
128 $lb->waitFor( $pos );
133 * Notify the ChronologyProtector that the ILoadBalancer is about to shut
134 * down. Saves replication positions.
136 * @param ILoadBalancer $lb
139 public function shutdownLB( ILoadBalancer
$lb ) {
140 if ( !$this->enabled
) {
141 return; // not enabled
142 } elseif ( !$lb->hasOrMadeRecentMasterChanges( INF
) ) {
143 // Only save the position if writes have been done on the connection
147 $masterName = $lb->getServerName( $lb->getWriterIndex() );
148 if ( $lb->getServerCount() > 1 ) {
149 $pos = $lb->getMasterPos();
150 $this->logger
->info( __METHOD__
. ": LB for '$masterName' has pos $pos\n" );
151 $this->shutdownPositions
[$masterName] = $pos;
153 $this->logger
->info( __METHOD__
. ": DB '$masterName' touched\n" );
155 $this->shutdownTouchDBs
[$masterName] = 1;
159 * Notify the ChronologyProtector that the LBFactory is done calling shutdownLB() for now.
160 * May commit chronology data to persistent storage.
162 * @param callable|null $workCallback Work to do instead of waiting on syncing positions
163 * @param string $mode One of (sync, async); whether to wait on remote datacenters
164 * @param int|null &$cpIndex DB position key write counter; incremented on update
165 * @return DBMasterPos[] Empty on success; returns the (db name => position) map on failure
167 public function shutdown( callable
$workCallback = null, $mode = 'sync', &$cpIndex = null ) {
168 if ( !$this->enabled
) {
172 $store = $this->store
;
173 // Some callers might want to know if a user recently touched a DB.
174 // These writes do not need to block on all datacenters receiving them.
175 foreach ( $this->shutdownTouchDBs
as $dbName => $unused ) {
177 $this->getTouchedKey( $this->store
, $dbName ),
183 if ( !count( $this->shutdownPositions
) ) {
184 return []; // nothing to save
187 $this->logger
->info( __METHOD__
. ": saving master pos for " .
188 implode( ', ', array_keys( $this->shutdownPositions
) ) . "\n"
191 // CP-protected writes should overwhemingly go to the master datacenter, so get DC-local
192 // lock to merge the values. Use a DC-local get() and a synchronous all-DC set(). This
193 // makes it possible for the BagOStuff class to write in parallel to all DCs with one RTT.
194 if ( $store->lock( $this->key
, 3 ) ) {
195 if ( $workCallback ) {
196 // Let the store run the work before blocking on a replication sync barrier. By the
197 // time it's done with the work, the barrier should be fast if replication caught up.
198 $store->addBusyCallback( $workCallback );
202 $this->mergePositions(
203 $store->get( $this->key
),
204 $this->shutdownPositions
,
208 ( $mode === 'sync' ) ?
$store::WRITE_SYNC
: 0
210 $store->unlock( $this->key
);
213 $cpIndex = null; // nothing saved
217 $bouncedPositions = $this->shutdownPositions
;
218 // Raced out too many times or stash is down
219 $this->logger
->warning( __METHOD__
. ": failed to save master pos for " .
220 implode( ', ', array_keys( $this->shutdownPositions
) ) . "\n"
222 } elseif ( $mode === 'sync' &&
223 $store->getQoS( $store::ATTR_SYNCWRITES
) < $store::QOS_SYNCWRITES_BE
225 // Positions may not be in all datacenters, force LBFactory to play it safe
226 $this->logger
->info( __METHOD__
. ": store may not support synchronous writes." );
227 $bouncedPositions = $this->shutdownPositions
;
229 $bouncedPositions = [];
232 return $bouncedPositions;
236 * @param string $dbName DB master name (e.g. "db1052")
237 * @return float|bool UNIX timestamp when client last touched the DB; false if not on record
240 public function getTouched( $dbName ) {
241 return $this->store
->get( $this->getTouchedKey( $this->store
, $dbName ) );
245 * @param BagOStuff $store
246 * @param string $dbName
249 private function getTouchedKey( BagOStuff
$store, $dbName ) {
250 return $store->makeGlobalKey( __CLASS__
, 'mtime', $this->clientId
, $dbName );
254 * Load in previous master positions for the client
256 protected function initPositions() {
257 if ( $this->initialized
) {
261 $this->initialized
= true;
263 // If there is an expectation to see master positions from a certain write
264 // index or higher, then block until it appears, or until a timeout is reached.
265 // Since the write index restarts each time the key is created, it is possible that
266 // a lagged store has a matching key write index. However, in that case, it should
267 // already be expired and thus treated as non-existing, maintaining correctness.
268 if ( $this->waitForPosIndex
> 0 ) {
270 $loop = new WaitConditionLoop(
271 function () use ( &$data ) {
272 $data = $this->store
->get( $this->key
);
273 if ( !is_array( $data ) ) {
274 return WaitConditionLoop
::CONDITION_CONTINUE
; // not found yet
275 } elseif ( !isset( $data['writeIndex'] ) ) {
276 return WaitConditionLoop
::CONDITION_REACHED
; // b/c
279 return ( $data['writeIndex'] >= $this->waitForPosIndex
)
280 ? WaitConditionLoop
::CONDITION_REACHED
281 : WaitConditionLoop
::CONDITION_CONTINUE
;
283 $this->waitForPosStoreTimeout
285 $result = $loop->invoke();
286 $waitedMs = $loop->getLastWaitTime() * 1e3
;
288 if ( $result == $loop::CONDITION_REACHED
) {
289 $msg = "expected and found pos index {$this->waitForPosIndex} ({$waitedMs}ms)";
290 $this->logger
->debug( $msg );
292 $msg = "expected but missed pos index {$this->waitForPosIndex} ({$waitedMs}ms)";
293 $this->logger
->info( $msg );
296 $data = $this->store
->get( $this->key
);
299 $this->startupPositions
= $data ?
$data['positions'] : [];
300 $this->logger
->info( __METHOD__
. ": key is {$this->key} (read)\n" );
302 $this->startupPositions
= [];
303 $this->logger
->info( __METHOD__
. ": key is {$this->key} (unread)\n" );
308 * @param array|bool $curValue
309 * @param DBMasterPos[] $shutdownPositions
310 * @param int|null &$cpIndex
313 protected function mergePositions( $curValue, array $shutdownPositions, &$cpIndex = null ) {
314 /** @var DBMasterPos[] $curPositions */
315 $curPositions = isset( $curValue['positions'] ) ?
$curValue['positions'] : [];
316 // Use the newest positions for each DB master
317 foreach ( $shutdownPositions as $db => $pos ) {
319 !isset( $curPositions[$db] ) ||
320 !( $curPositions[$db] instanceof DBMasterPos
) ||
321 $pos->asOfTime() > $curPositions[$db]->asOfTime()
323 $curPositions[$db] = $pos;
327 $cpIndex = isset( $curValue['writeIndex'] ) ?
$curValue['writeIndex'] : 0;
330 'positions' => $curPositions,
331 'writeIndex' => ++
$cpIndex