diff --git a/Classes/Command/IndexCommand.php b/Classes/Command/IndexCommand.php
index a05e4dc..9a7419a 100644
--- a/Classes/Command/IndexCommand.php
+++ b/Classes/Command/IndexCommand.php
@@ -19,6 +19,7 @@
 use Psr\Http\Message\ServerRequestInterface;
 use Psr\Log\LoggerInterface;
 use Slub\LisztCommon\Common\ElasticClientBuilder;
+use Slub\LisztBibliography\Exception\TooManyRequestsException;
 use Slub\LisztBibliography\Processing\BibEntryProcessor;
 use Slub\LisztBibliography\Processing\BibElasticMapping;
 use Symfony\Component\Console\Command\Command;
@@ -41,6 +42,8 @@ class IndexCommand extends Command
     const API_TRIALS = 3;
 
     protected string $apiKey;
+    protected string $groupId;
+
     protected Collection $bibliographyItems;
     protected Collection $deletedItems;
     protected Collection $teiDataSets;
@@ -103,6 +106,16 @@ protected function initialize(InputInterface $input, OutputInterface $output): v
         $this->extConf = GeneralUtility::makeInstance(ExtensionConfiguration::class)->get('liszt_bibliography');
         $this->client = ElasticClientBuilder::getClient();
         $this->apiKey = $this->extConf['zoteroApiKey'];
+        if ($this->apiKey == '') {
+            $this->logger->info('Please set an API key in the extension configuration.');
+            throw new \Exception('Please set an API key in the extension configuration.');
+        }
+        $this->groupId = $this->extConf['zoteroGroupId'];
+        if ($this->groupId == '') {
+            $this->logger->info('Please set a group ID in the extension configuration.');
+            throw new \Exception('Please set a group ID in the extension configuration.');
+        }
+
         $this->io = new SymfonyStyle($input, $output);
         $this->io->title($this->getDescription());
     }
@@ -125,9 +138,9 @@ protected function execute(InputInterface $input, OutputInterface $output): int
 
     protected function fullSync(InputInterface $input): void
     {
-        $client = new ZoteroApi($this->extConf['zoteroApiKey']);
+        $client = GeneralUtility::makeInstance(ZoteroApi::class, $this->extConf['zoteroApiKey']);
         $response = $client->
-            group($this->extConf['zoteroGroupId'])->
+            group($this->groupId)->
             items()->
             top()->
             limit(1)->
@@ -160,7 +173,7 @@ protected function fullSync(InputInterface $input): void
             } else {
                 $this->io->error("Exception: " . $e->getMessage());
                 $this->logger->error('Bibliography sync unsuccessful. Error creating elasticsearch index.');
-                throw new \Exception('Bibliography sync unsuccessful.');
+                throw new \Exception('Bibliography sync unsuccessful. Error creating elasticsearch index.');
             }
         }
 
@@ -175,6 +188,10 @@ protected function fullSync(InputInterface $input): void
                 $advanceBy = min($remainingItems, $this->bulkSize);
                 $this->io->progressAdvance($advanceBy);
                 $cursor += $this->bulkSize;
+            } catch (TooManyRequestsException $e) {
+                $this->io->note('Received a 429 status from Zotero API. Too many requests. Please try again later.');
+                $this->logger->error('Received a 429 status from Zotero API. Too many requests. Please try again later.');
+                throw new TooManyRequestsException('Bibliography sync unsuccessful.');
             } catch (\Exception $e) {
                 $this->io->newline(1);
                 $this->io->caution($e->getMessage());
@@ -182,7 +199,7 @@ protected function fullSync(InputInterface $input): void
                 if ($apiCounter == 0) {
                     $this->io->note('Giving up after ' . self::API_TRIALS . ' trials.');
                     $this->logger->error('Bibliography sync unsuccessful. Zotero API sent {trials} 500 errors.', ['trials' => self::API_TRIALS]);
-                    throw new \Exception('Bibliography sync unsuccessful.');
+                    throw new \Exception('Bibliography sync unsuccessful. Zotero API sent {trials} 500 errors.', ['trials' => self::API_TRIALS]);
                 } else {
                     $this->io->note('Trying again. ' . --$apiCounter . ' trials left.');
                 }
@@ -206,7 +223,7 @@ protected function versionedSync(int $version): void
                 if ($apiCounter == 0) {
                     $this->io->note('Giving up after ' . self::API_TRIALS . ' trials.');
                     $this->logger->warning('Bibliography sync unsuccessful. Zotero API sent {trials} 500 errors.', ['trials' => self::API_TRIALS]);
-                    throw new \Exception('Bibliography sync unsuccessful.');
+                    throw new \Exception('Bibliography sync unsuccessful. Zotero API sent {trials} 500 errors.', ['trials' => self::API_TRIALS]);
                 } else {
                     $this->io->note('Trying again. ' . --$apiCounter . ' trials left.');
                 }
@@ -268,16 +285,16 @@ protected function getVersion(InputInterface $input): int
                 return 0;
             } else {
                 $this->io->error("Exception: " . $e->getMessage());
-                throw new \Exception('Bibliography sync unsuccessful.');
+                throw new \Exception('Bibliography sync unsuccessful.' . $e->getMessage());
             }
         }
     }
 
     protected function fetchBibliography(int $cursor, int $version): void
     {
-        $client = new ZoteroApi($this->extConf['zoteroApiKey']);
+        $client = GeneralUtility::makeInstance(ZoteroApi::class, $this->extConf['zoteroApiKey']);
         $response = $client->
-            group($this->extConf['zoteroGroupId'])->
+            group($this->groupId)->
             items()->
             top()->
             start($cursor)->
@@ -285,6 +302,13 @@ protected function fetchBibliography(int $cursor, int $version): void
             setSince($version)->
             send();
 
+        if (isset($response->getHeaders()['Backoff'])) {
+            $this->logger->warning('Received a Backoff header: '. $response->getHeaders()['Backoff']);
+        }
+        if ($response->getStatusCode() == 429) {
+            throw new TooManyRequestsException('Too many requests issued. Try again later.');
+        }
+
         $this->bibliographyItems = Collection::wrap($response->getBody())->
             pluck('data');
     }
@@ -297,10 +321,10 @@ protected function fetchCitations(int $cursor, int $version): void
 
     protected function fetchCitationLocale(string $locale, int $cursor, int $version): void
     {
-        $client = new ZoteroApi($this->extConf['zoteroApiKey']);
+        $client = GeneralUtility::makeInstance(ZoteroApi::class, $this->extConf['zoteroApiKey']);
         $style = $this->extConf['zoteroStyle'];
         $response = $client->
-            group($this->extConf['zoteroGroupId'])->
+            group($this->groupId)->
             items()->
             top()->
             start($cursor)->
@@ -320,9 +344,9 @@ protected function fetchCitationLocale(string $locale, int $cursor, int $version
 
     protected function fetchTeiData(int $cursor, int $version): void
     {
-        $client = new ZoteroApi($this->extConf['zoteroApiKey']);
+        $client = GeneralUtility::makeInstance(ZoteroApi::class, $this->extConf['zoteroApiKey']);
         $response = $client->
-            group($this->extConf['zoteroGroupId'])->
+            group($this->groupId)->
             items()->
             top()->
             start($cursor)->
diff --git a/Classes/Exception/TooManyRequestsException.php b/Classes/Exception/TooManyRequestsException.php
new file mode 100644
index 0000000..623ce32
--- /dev/null
+++ b/Classes/Exception/TooManyRequestsException.php
@@ -0,0 +1,5 @@
+<?php
+
+namespace Slub\LisztBibliography\Exception;
+
+class TooManyRequestsException extends \Exception { }
diff --git a/Tests/Unit/Command/IndexCommandTest.php b/Tests/Unit/Command/IndexCommandTest.php
new file mode 100644
index 0000000..4397dd6
--- /dev/null
+++ b/Tests/Unit/Command/IndexCommandTest.php
@@ -0,0 +1,258 @@
+<?php
+
+namespace Slub\LisztBibliography\Tests\Unit\Controller;
+
+use Elastic\Elasticsearch\ClientInterface;
+use Elastic\Elasticsearch\Endpoints\Indices;
+use GuzzleHttp\Psr7\Response;
+use Hedii\ZoteroApi\ZoteroApi;
+use Illuminate\Support\Str;
+use Psr\Log\LoggerInterface;
+use Slub\LisztBibliography\Command\IndexCommand;
+use Slub\LisztBibliography\Exception\TooManyRequestsException;
+use Slub\LisztBibliography\Processing\BibEntryConfig;
+use Slub\LisztBibliography\Processing\BibEntryProcessor;
+use Slub\LisztBibliography\Tests\Fixtures\ClientMock;
+use Slub\LisztCommon\Common\Collection;
+use Slub\LisztCommon\Common\ElasticClientBuilder;
+use Symfony\Component\Console\Input\InputInterface;
+use Symfony\Component\Console\Output\OutputInterface;
+use TYPO3\CMS\Core\Configuration\ExtensionConfiguration;
+use TYPO3\CMS\Core\Site\SiteFinder;
+use TYPO3\CMS\Core\Utility\GeneralUtility;
+use TYPO3\TestingFramework\Core\Unit\UnitTestCase;
+
+/**
+ * @covers Slub\LisztBibliography\Controller\BibliographyController
+ */
+final class IndexCommandTest extends UnitTestCase
+{
+    private ?IndexCommand $subject = null;
+    private string $exampleEntry = '';
+
+    private string $authorFirstName = 'ex_author_first';
+    private string $authorLastName = 'ex_author_last';
+    private string $editorFirstName = 'ex_editor_first';
+    private string $editorLastName = 'ex_editor_last';
+    private string $translatorFirstName = 'ex_translator_first';
+    private string $translatorLastName = 'ex_translator_last';
+    private string $title = 'ex_title';
+    private string $bookTitle = 'ex_book_title';
+    private string $place = 'ex_place';
+    private string $date = 'ex_date';
+    private string $pages = 'ex_pages';
+    private string $volume = 'ex_volume';
+    private string $numberOfVolumes = 'ex_number_of_volumes';
+    private string $issue = 'ex_issue';
+
+    private string $exampleBook = '';
+    private string $exampleBookWithoutAuthor = '';
+    private string $exampleBookWithAnonymousAuthor = '';
+    private string $exampleArticle = '';
+    private string $exampleBookSection = '';
+
+    protected function setUp(): void
+    {
+        parent::setUp();
+
+        $this->exampleEntry =
+            <<<JSON
+            {
+                "key": "key",
+                "itemType": "book",
+                "title": "$this->title",
+                "creators": [
+                    {
+                        "creatorType": "author",
+                        "firstName": "$this->authorFirstName",
+                        "lastName": "$this->authorLastName"
+                    },
+                    {
+                        "creatorType": "editor",
+                        "firstName": "$this->editorFirstName",
+                        "lastName": "$this->editorLastName"
+                    },
+                    {
+                        "creatorType": "translator",
+                        "firstName": "$this->translatorFirstName",
+                        "lastName": "$this->translatorLastName"
+                    }
+                ],
+                "place": "$this->place",
+                "volume": "$this->volume",
+                "numberOfVolumes": "$this->numberOfVolumes",
+                "date": "$this->date"
+            }
+            JSON;
+
+        $siteFinderMock = $this->createMock(SiteFinder::class);
+        $loggerMock = $this->createMock(LoggerInterface::class);
+        $this->subject = GeneralUtility::makeInstance(IndexCommand::class, $siteFinderMock, $loggerMock);
+    }
+
+    public function tearDown(): void
+    {
+        parent::tearDown();
+    }
+
+    /**
+     * @test
+     */
+    public function commandBreaksOn429(): void
+    {
+        $response = $this->createStub(Response::class);
+        $client = $this->createStub(ZoteroApi::class);
+        $response->method('getStatusCode')->
+            willReturn(429);
+        $client->method('send')->
+            willReturn($response);
+/*
+        $response = $this->getAccessibleMock(Response::class, ['getStatusCode'], [], '', false);
+        $response->method('getStatusCode')->
+            willReturn(429);
+        $selfReturningMethods = [ 'group', 'items', 'top', 'start', 'limit', 'setSince' ];
+        $client = $this->getAccessibleMock(ZoteroApi::class,
+            ['send', ...$selfReturningMethods ],
+            [],
+            '',
+            false
+        );
+        $client->method('send')->
+            willReturn($response);
+        foreach ($selfReturningMethods as $method) {
+            $client->
+                method($method)->
+                willReturn($client);
+        }
+*/
+
+/*
+        $indices = $this->createStub(Indices::class);
+        $indices->method('exists')->
+            willReturn(true);
+*/
+        $elasticClient = $this->createStub(ClientInterface::class);
+        //$elasticClient->method('search')->willReturn(true);
+/*
+        $elasticClient->method('indices')->
+            willReturn($indices);
+        $elasticClient->method('exists')->
+            willReturn(true);
+*/
+        $elasticClientBuilder = $this->createStub(ElasticClientBuilder::class);
+        $elasticClientBuilder->method('getClient')->
+            willReturn($elasticClient);
+        GeneralUtility::addInstance(ElasticClientBuilder::class, $elasticClientBuilder);
+
+        $this->expectException(TooManyRequestsException::class);
+        GeneralUtility::addInstance(ZoteroApi::class, $client);
+
+        $extConfMap = [
+            [
+                'liszt_bibliography',
+                '',
+                [
+                    'zoteroApiKey' => 'abc',
+                    'zoteroGroupId' => 'abc',
+                    'zoteroBulkSize' => 10,
+                    'elasticIndexName' => 'zotero'
+                ]
+            ],
+            [
+                'liszt_common',
+                '',
+                [
+                    'elasticHostName' => 'https://elasticsearch',
+                    'elasticCaFilePath' => '',
+                    'elasticPwdFileName' => '',
+                    'elasticCredentialsFilePath' => '',
+                    'zoteroBulkSize' => 10
+                ]
+            ]
+        ];
+
+        $extConf = $this->createStub(ExtensionConfiguration::class);
+        $extConf->method('get')->willReturnMap($extConfMap);
+        GeneralUtility::addInstance(ExtensionConfiguration::class, $extConf);
+        GeneralUtility::addInstance(ExtensionConfiguration::class, $extConf);
+
+        $inputMock = $this->createStub(InputInterface::class);
+        $outputMock = $this->createStub(OutputInterface::class);
+
+        $this->subject->run($inputMock, $outputMock);
+
+    }
+
+    /**
+     * @test
+     */
+    public function commandBreaksAfterThreeTimes500(): void
+    {
+        $book = $this->subject->process($this->exampleBookArray, new Collection(), new Collection());
+        $bookSection = $this->subject->process($this->exampleBookSectionArray, new Collection(), new Collection());
+        $article = $this->subject->process($this->exampleArticleArray, new Collection(), new Collection());
+
+        self::assertEquals(Str::of($this->title), $book['tx_lisztcommon_body']);
+        self::assertEquals(Str::of($this->title), $bookSection['tx_lisztcommon_body']);
+        self::assertEquals(Str::of($this->title), $article['tx_lisztcommon_body']);
+    }
+
+    /**
+     * @test
+     */
+    public function unsetGroupIdLeadsToException(): void
+    {
+        $book = $this->subject->process($this->exampleBookArray, new Collection(), new Collection());
+        $expected = Str::of(
+            'hg. von ' . $this->editorFirstName . ' ' . $this->editorLastName . ', ' .
+            'übers. von ' . $this->translatorFirstName . ' ' . $this->translatorLastName . ', ' .
+            $this->numberOfVolumes . 'Bde., ' .
+            'Bd. ' . $this->volume . ', ' .
+            $this->place . ' ' .
+            $this->date
+        );
+
+        self::assertEquals($expected, $book['tx_lisztcommon_footer']);
+    }
+
+    /**
+     * @test
+     */
+    public function unsetApiKeyLeadsToException(): void
+    {
+        $bookSection = $this->subject->process($this->exampleBookSectionArray, new Collection(), new Collection());
+        $expected = Str::of(
+            'In ' . $this->bookTitle . ', ' .
+            'hg. von ' . $this->editorFirstName . ' ' . $this->editorLastName . ', ' .
+            'übers. von ' . $this->translatorFirstName . ' ' . $this->translatorLastName . ', ' .
+            $this->numberOfVolumes . 'Bde., ' .
+            'Bd. ' . $this->volume . ', ' .
+            $this->place . ' ' .
+            $this->date . ', ' .
+            $this->pages
+        );
+
+        self::assertEquals($expected, $bookSection['tx_lisztcommon_footer']);
+    }
+
+    /**
+     * @test
+     */
+    public function exampleEntryIsIndexedCorrectly(): void
+    {
+        $article = $this->subject->process($this->exampleArticleArray, new Collection(), new Collection());
+        $expected = Str::of(
+            $this->bookTitle . ' ' .
+            $this->volume .
+            ' (' . $this->date . '), Nr. ' .
+            $this->issue . ', ' .
+            $this->pages
+        );
+
+        self::assertEquals($expected, $article['tx_lisztcommon_footer']);
+    }
+
+
+}
+
+class SomeClass { public function doSomething($a) { return 'a';}}
diff --git a/composer.json b/composer.json
index 72c4aee..6bb3931 100644
--- a/composer.json
+++ b/composer.json
@@ -21,7 +21,8 @@
     },
     "autoload": {
         "psr-4": {
-            "Slub\\LisztBibliography\\": "Classes/"
+            "Slub\\LisztBibliography\\": "Classes/",
+            "Slub\\LisztBibliography\\Tests\\": "Tests/"
         }
     },
     "extra": {