double check that your "keywords" field is defined as multiValued in the schema. "Indexing Failed" generally tells you the reason, and gives a full stack trace as well.
also this following implementation might work better for what you are trying to do. Notice I removed your "group concat" query completely and added it as an entity below. Double check the joins and field names, I've only assumed connections based on your naming. This way you are not doing the additional concatenate - split action for each and every row
<dataSource name="db2" type="JdbcDataSource"
driver="com.mysql.jdbc.Driver"
url="jdbc:mysql://localhost/website"
user="webname"
password="123456"/>
<document name="content">
<entity name="web"
transformer="RegexTransformer"
query="SELECT
Article.id, Article.title, Article.sub_title, Article.category_id,
Article.section_title, Article.section_body, Category.id as cat_id,
Category.name as cat_name, Recipe.id as recipe_id, Recipe.recipe_title, ArticleMultiBody.id as multi_id,
ArticleMultiBody.body_title_1, ArticleMultiBody.body_text_1 FROM zahra.articles AS Article
LEFT JOIN zahra.categories AS Category ON (Article.category_id = Category.id)
LEFT JOIN zahra.recipes AS Recipe ON (Recipe.article_id = Article.id)
LEFT JOIN zahra.article_multi_bodies AS ArticleMultiBody ON (ArticleMultiBody.article_id = Article.id) ORDER BY Article.title"
deltaQuery="SELECT article.id, article.title, article.sub_title, article.category_id, article.section_title, article.section_body, category.id AS cat_id, category.name AS cat_name, recipe.id AS recipe_id, recipe.recipe_title, articlemultibody.id AS multi_id, articlemultibody.body_title_1, articlemultibody.body_text_1 FROM zahra.articles AS Article LEFT JOIN zahra.categories AS CATEGORY ON (Article.category_id = CATEGORY.id) LEFT JOIN zahra.recipes AS Recipe ON (Recipe.article_id = Article.id) LEFT JOIN zahra.article_multi_bodies AS ArticleMultiBody ON (ArticleMultiBody.article_id = Article.id) ORDER BY Article.title">
<field column="ID" name="id"/>
<field column="TITLE" name="title"/>
<field column="SUB_TITLE" name="sub_title"/>
<field column="SECTION_TITLE" name="section_title"/>
<field column="SECTION_BODY" name="section_body"/>
<field column="CATEGORY_ID" name="category_id"/>
<field column="CREATED" name="created" dateTimeFormat="Y-m-d h:i:s"/>
<field column="KEYWORDS" name="keywords" splitBy="," sourceColName="tagnames"/>
<entity name="keywords"
pk="ARTICLE_ID"
query="SELECT name FROM tags JOIN article_tags ON tags.id = article_tags.tag_id WHERE article_tags.article_id = '${web.ID}'">
<field column="KEYWORDS" name="keywords"/>
</entity>
</entity>
</document>