Automatically sever databases after prolonged unreachability

Summary:
Ref T4571. When a database goes down briefly, we fall back to replicas.

However, this fallback is slow (not good for users) and keeps sending a lot of traffic to the master (might be bad if the root cause is load-related).

Keep track of recent connections and fully degrade into "severed" mode if we see a sequence of failures over a reasonable period of time. In this mode, we send much less traffic to the master (faster for users; less load for the database).

We do send a little bit of traffic still, and if the master recovers we'll recover back into normal mode seeing several connections in a row succeed.

This is similar to what most load balancers do when pulling web servers in and out of pools.

For now, the specific numbers are:

  - We do at most one health check every 3 seconds.
  - If 5 checks in a row fail or succeed, we sever or un-sever the database (so it takes about 15 seconds to switch modes).
  - If the database is currently marked unhealthy, we reduce timeouts and retries when connecting to it.

Test Plan:
  - Configured a bad `master`.
  - Browsed around for a bit, initially saw "unrechable master" errors.
  - After about 15 seconds, saw "major interruption" errors instead.
  - Fixed the config for `master`.
  - Browsed around for a while longer.
  - After about 15 seconds, things recovered.
  - Used "Cluster Databases" console to keep an eye on health checks: it now shows how many recent health checks were good:

{F1213397}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4571

Differential Revision: https://secure.phabricator.com/D15677

src/__phutil_library_map__.php

@@ -2241,6 +2241,7 @@ phutil_register_library_map(array(
     'PhabricatorDashboardViewController' => 'applications/dashboard/controller/PhabricatorDashboardViewController.php',
     'PhabricatorDataCacheSpec' => 'applications/cache/spec/PhabricatorDataCacheSpec.php',
     'PhabricatorDataNotAttachedException' => 'infrastructure/storage/lisk/PhabricatorDataNotAttachedException.php',
+    'PhabricatorDatabaseHealthRecord' => 'infrastructure/cluster/PhabricatorDatabaseHealthRecord.php',
     'PhabricatorDatabaseRef' => 'infrastructure/cluster/PhabricatorDatabaseRef.php',
     'PhabricatorDatabaseSetupCheck' => 'applications/config/check/PhabricatorDatabaseSetupCheck.php',
     'PhabricatorDatasourceEditField' => 'applications/transactions/editfield/PhabricatorDatasourceEditField.php',
@@ -6697,6 +6698,7 @@ phutil_register_library_map(array(
     'PhabricatorDashboardViewController' => 'PhabricatorDashboardController',
     'PhabricatorDataCacheSpec' => 'PhabricatorCacheSpec',
     'PhabricatorDataNotAttachedException' => 'Exception',
+    'PhabricatorDatabaseHealthRecord' => 'Phobject',
     'PhabricatorDatabaseRef' => 'Phobject',
     'PhabricatorDatabaseSetupCheck' => 'PhabricatorSetupCheck',
     'PhabricatorDatasourceEditField' => 'PhabricatorTokenizerEditField',

src/applications/cache/PhabricatorCaches.php

@@ -174,6 +174,11 @@ final class PhabricatorCaches extends Phobject {
    * @task setup
    */
   private static function buildSetupCaches() {
+    // If this is the CLI, just build a setup cache.
+    if (php_sapi_name() == 'cli') {
+      return array();
+    }
+
     // In most cases, we should have APC. This is an ideal cache for our
     // purposes -- it's fast and empties on server restart.
     $apc = new PhutilAPCKeyValueCache();

src/applications/config/controller/PhabricatorConfigClusterDatabasesController.php

@@ -115,6 +115,29 @@ final class PhabricatorConfigClusterDatabasesController
         $replica_label,
       );
 
+      $health = $database->getHealthRecord();
+      $health_up = $health->getUpEventCount();
+      $health_down = $health->getDownEventCount();
+
+      if ($health->getIsHealthy()) {
+        $health_icon = id(new PHUIIconView())
+          ->setIcon('fa-plus green');
+      } else {
+        $health_icon = id(new PHUIIconView())
+          ->setIcon('fa-times red');
+      }
+
+      $health_count = pht(
+        '%s / %s',
+        new PhutilNumber($health_up),
+        new PhutilNumber($health_up + $health_down));
+
+      $health_status = array(
+        $health_icon,
+        ' ',
+        $health_count,
+      );
+
       $messages = array();
 
       $conn_message = $database->getConnectionMessage();
@@ -136,10 +159,12 @@ final class PhabricatorConfigClusterDatabasesController
         $database->getUser(),
         $connection,
         $replication,
+        $health_status,
         $messages,
       );
     }
 
+
     $table = id(new AphrontTableView($rows))
       ->setNoDataString(
         pht('Phabricator is not configured in cluster mode.'))
@@ -151,6 +176,7 @@ final class PhabricatorConfigClusterDatabasesController
           pht('User'),
           pht('Connection'),
           pht('Replication'),
+          pht('Health'),
           pht('Messages'),
         ))
       ->setColumnClasses(
@@ -161,6 +187,7 @@ final class PhabricatorConfigClusterDatabasesController
           null,
           null,
           null,
+          null,
           'wide',
         ));
 

src/infrastructure/cluster/PhabricatorDatabaseHealthRecord.php

@@ -0,0 +1,185 @@
+<?php
+
+final class PhabricatorDatabaseHealthRecord
+  extends Phobject {
+
+  private $ref;
+  private $shouldCheck;
+  private $isHealthy;
+  private $upEventCount;
+  private $downEventCount;
+
+  public function __construct(PhabricatorDatabaseRef $ref) {
+    $this->ref = $ref;
+    $this->readState();
+  }
+
+
+  /**
+   * Is the database currently healthy?
+   */
+  public function getIsHealthy() {
+    return $this->isHealthy;
+  }
+
+
+  /**
+   * Should this request check database health?
+   */
+  public function getShouldCheck() {
+    return $this->shouldCheck;
+  }
+
+
+  /**
+   * How many recent health checks were successful?
+   */
+  public function getUpEventCount() {
+    return $this->upEventCount;
+  }
+
+
+  /**
+   * How many recent health checks failed?
+   */
+  public function getDownEventCount() {
+    return $this->downEventCount;
+  }
+
+
+  /**
+   * Number of failures or successes we need to see in a row before we change
+   * the state.
+   */
+  public function getRequiredEventCount() {
+    return 5;
+  }
+
+
+  /**
+   * Seconds to wait between health checks.
+   */
+  public function getHealthCheckFrequency() {
+    return 3;
+  }
+
+
+  public function didHealthCheck($result) {
+    $now = microtime(true);
+    $check_frequency = $this->getHealthCheckFrequency();
+    $event_count = $this->getRequiredEventCount();
+
+    $record = $this->readHealthRecord();
+
+    $log = $record['log'];
+    foreach ($log as $key => $event) {
+      $when = idx($event, 'timestamp');
+
+      // If the log already has another nearby event, just ignore this one.
+      // We raced with another process and our result can just be thrown away.
+      if (($now - $when) <= $check_frequency) {
+        return $this;
+      }
+    }
+
+    $log[] = array(
+      'timestamp' => $now,
+      'up' => $result,
+    );
+
+    // Throw away older events which are now obsolete.
+    $log = array_slice($log, -$event_count);
+
+    $count_up = 0;
+    $count_down = 0;
+    foreach ($log as $event) {
+      if ($event['up']) {
+        $count_up++;
+      } else {
+        $count_down++;
+      }
+    }
+
+    // If all of the events are the same, change the state.
+    if ($count_up == $event_count) {
+      $record['up'] = true;
+    } else if ($count_down == $event_count) {
+      $record['up'] = false;
+    }
+
+    $record['log'] = $log;
+
+    $this->writeHealthRecord($record);
+
+    $this->isHealthy = $record['up'];
+    $this->shouldCheck = false;
+    $this->updateStatistics($record);
+
+    return $this;
+  }
+
+
+  private function readState() {
+    $now = microtime(true);
+    $check_frequency = $this->getHealthCheckFrequency();
+
+    $record = $this->readHealthRecord();
+
+    $last_check = $record['lastCheck'];
+
+    if (($now - $last_check) >= $check_frequency) {
+      $record['lastCheck'] = $now;
+      $this->writeHealthRecord($record);
+      $this->shouldCheck = true;
+    } else {
+      $this->shouldCheck = false;
+    }
+
+    $this->isHealthy = $record['up'];
+    $this->updateStatistics($record);
+  }
+
+  private function updateStatistics(array $record) {
+    $this->upEventCount = 0;
+    $this->downEventCount = 0;
+    foreach ($record['log'] as $event) {
+      if ($event['up']) {
+        $this->upEventCount++;
+      } else {
+        $this->downEventCount++;
+      }
+    }
+  }
+
+  private function getHealthRecordCacheKey() {
+    $ref = $this->ref;
+
+    $host = $ref->getHost();
+    $port = $ref->getPort();
+
+    return "cluster.db.health({$host}, {$port})";
+  }
+
+  private function readHealthRecord() {
+    $cache = PhabricatorCaches::getSetupCache();
+    $cache_key = $this->getHealthRecordCacheKey();
+    $health_record = $cache->getKey($cache_key);
+
+    if (!is_array($health_record)) {
+      $health_record = array(
+        'up' => true,
+        'lastCheck' => 0,
+        'log' => array(),
+      );
+    }
+
+    return $health_record;
+  }
+
+  private function writeHealthRecord(array $record) {
+    $cache = PhabricatorCaches::getSetupCache();
+    $cache_key = $this->getHealthRecordCacheKey();
+    $cache->setKey($cache_key, $record);
+  }
+
+}

src/infrastructure/cluster/PhabricatorDatabaseRef.php

@@ -30,6 +30,7 @@ final class PhabricatorDatabaseRef
   private $replicaMessage;
   private $replicaDelay;
 
+  private $healthRecord;
   private $didFailToConnect;
 
   public function setHost($host) {
@@ -326,7 +327,7 @@ final class PhabricatorDatabaseRef
     return $this->newConnection(
       array(
         'retries' => 0,
-        'timeout' => 3,
+        'timeout' => 2,
       ));
   }
 
@@ -338,11 +339,24 @@ final class PhabricatorDatabaseRef
   }
 
   public function isSevered() {
-    return $this->didFailToConnect;
+    if ($this->didFailToConnect) {
+      return true;
+    }
+
+    $record = $this->getHealthRecord();
+    $is_healthy = $record->getIsHealthy();
+    if (!$is_healthy) {
+      return true;
+    }
+
+    return false;
   }
 
   public function isReachable(AphrontDatabaseConnection $connection) {
-    if ($this->isSevered()) {
+    $record = $this->getHealthRecord();
+    $should_check = $record->getShouldCheck();
+
+    if ($this->isSevered() && !$should_check) {
       return false;
     }
 
@@ -353,6 +367,10 @@ final class PhabricatorDatabaseRef
       $reachable = false;
     }
 
+    if ($should_check) {
+      $record->didHealthCheck($reachable);
+    }
+
     if (!$reachable) {
       $this->didFailToConnect = true;
     }
@@ -360,6 +378,26 @@ final class PhabricatorDatabaseRef
     return $reachable;
   }
 
+  public function checkHealth() {
+    $health = $this->getHealthRecord();
+
+    $should_check = $health->getShouldCheck();
+    if ($should_check) {
+      // This does an implicit health update.
+      $connection = $this->newManagementConnection();
+      $this->isReachable($connection);
+    }
+
+    return $this;
+  }
+
+  public function getHealthRecord() {
+    if (!$this->healthRecord) {
+      $this->healthRecord = new PhabricatorDatabaseHealthRecord($this);
+    }
+    return $this->healthRecord;
+  }
+
   public static function getMasterDatabaseRef() {
     $refs = self::getLiveRefs();
 
@@ -415,14 +453,26 @@ final class PhabricatorDatabaseRef
   }
 
   private function newConnection(array $options) {
+    // If we believe the database is unhealthy, don't spend as much time
+    // trying to connect to it, since it's likely to continue to fail and
+    // hammering it can only make the problem worse.
+    $record = $this->getHealthRecord();
+    if ($record->getIsHealthy()) {
+      $default_retries = 3;
+      $default_timeout = 10;
+    } else {
+      $default_retries = 0;
+      $default_timeout = 2;
+    }
+
     $spec = $options + array(
       'user' => $this->getUser(),
       'pass' => $this->getPass(),
       'host' => $this->getHost(),
       'port' => $this->getPort(),
       'database' => null,
-      'retries' => 3,
-      'timeout' => 15,
+      'retries' => $default_retries,
+      'timeout' => $default_timeout,
     );
 
     return PhabricatorEnv::newObjectFromConfig(

src/infrastructure/env/PhabricatorEnv.php

@@ -220,7 +220,10 @@ final class PhabricatorEnv extends Phobject {
     if (!$master) {
       self::setReadOnly(true, self::READONLY_MASTERLESS);
     } else if ($master->isSevered()) {
-      self::setReadOnly(true, self::READONLY_SEVERED);
+      $master->checkHealth();
+      if ($master->isSevered()) {
+        self::setReadOnly(true, self::READONLY_SEVERED);
+      }
     }
 
     try {