<route url="/V1/customapi" method="POST">
        <service class="SarkCustomApiApiCustomRepositoryInterface" method="create"/>
        <resources>
             <resource ref="self"/>
        </resources>
    </route>

$objectManager = MagentoFrameworkAppObjectManager::getInstance();
    $indexerCollectionFactory = $objectManager->get("MagentoIndexerModelIndexerCollectionFactory");
    $indexerFactory = $objectManager->get("MagentoIndexerModelIndexerFactory");

    $indexerCollection = $indexerCollectionFactory->create();
    $allIds = $indexerCollection->getAllIds();
    foreach ($allIds as $id) {
        $indexer = $indexerFactory->create()->load($id);
        $indexer->reindexAll();
    }