code: public function commit($flush = false)
      {
          $payload = $flush ?
              '<commit/>' :
              '<commit softCommit="true"/>';
  
          foreach ($this->endpointResolver->getEndpoints() as $endpointName) {
              $result = $this->client->request(
                  'POST',
                  $this->endpointRegistry->getEndpoint($endpointName),
                  '/update',
                  new Message(
                      array(
                          'Content-Type' => 'text/xml',
                      ),
                      $payload
                  )
              );
  
              if ($result->headers['status'] !== 200) {
                  throw new RuntimeException(
                      'Wrong HTTP status received from Solr: ' .
                      $result->headers['status'] . var_export($result, true)
                  );
              }
          }
      }
docs: Commits the data to the Solr index, making it available for search.
  
  This will perform Solr 'soft commit', which means there is no guarantee that data
  is actually written to the stable storage, it is only made available for search.
  Passing true will also write the data to the safe storage, ensuring durability.
  
  @param bool $flush