diff --git a/README.md b/README.md
index baea074..4cf0556 100644
--- a/README.md
+++ b/README.md
@@ -379,6 +379,16 @@ Accept: application/vnd.api+json
 Response is empty HTTP 204.
 
 
+### Delete a tag
+
+```
+DELETE /tags/1
+Accept: application/vnd.api+json
+```
+
+Response is empty HTTP 204.
+
+
 ### Delete a relationship between two papers
 
 ```
diff --git a/database.py b/database.py
index 8078c6c..cc4fc17 100644
--- a/database.py
+++ b/database.py
@@ -35,15 +35,12 @@ class RelationshipAssociation(Base):
                                         ondelete="CASCADE"))
     right_paper = sqlalchemy_relationship("Paper",
                                           foreign_keys=right_id,
-                                          back_populates="related_by",
-                                          passive_deletes=True)
-    relationship = sqlalchemy_relationship("Relationship",
-                                           passive_deletes=True)
+                                          back_populates="related_by")
+    relationship = sqlalchemy_relationship("Relationship")
 
     left_paper = sqlalchemy_relationship("Paper",
                                          foreign_keys=left_id,
-                                         back_populates="related_to",
-                                         passive_deletes=True)
+                                         back_populates="related_to")
 
 tag_association_table = Table(
     'tag_association', Base.metadata,
@@ -56,7 +53,7 @@ class Paper(Base):
     __tablename__ = "papers"
     id = Column(Integer, primary_key=True)
     doi = Column(String(), nullable=True, unique=True)
-    arxiv_id = Column(String(25), nullable=True, unique=True)
+    arxiv_id = Column(String(30), nullable=True, unique=True)
     # related_to are papers related to this paper (this_paper R …)
     related_to = sqlalchemy_relationship("RelationshipAssociation",
                                          foreign_keys="RelationshipAssociation.left_id",
@@ -70,7 +67,8 @@ class Paper(Base):
     # Tags relationship
     tags = sqlalchemy_relationship("Tag",
                                    secondary=tag_association_table,
-                                   backref="papers")
+                                   backref="papers",
+                                   passive_deletes=True)
 
     def __repr__(self):
         return "<Paper(id='%d', doi='%s', arxiv_id='%s')>" % (
@@ -142,3 +140,12 @@ class Tag(Base):
                 "self": "/tags/%d" % (self.id,)
             }
         }
+
+
+class CitationProcessingQueue(Base):
+    __tablename__ = "citationprocessingqueue"
+    id = Column(Integer, primary_key=True)
+    paper_id = Column(Integer,
+                      ForeignKey('papers.id', ondelete="CASCADE"),
+                      unique=True)
+    paper = sqlalchemy_relationship("Paper")
diff --git a/main.py b/main.py
index 7a8023c..a540f98 100755
--- a/main.py
+++ b/main.py
@@ -2,6 +2,7 @@
 import bottle
 from bottle.ext import sqlalchemy
 from sqlalchemy import create_engine
+from sqlalchemy.orm import sessionmaker
 
 import config
 import database
@@ -10,6 +11,8 @@ import tools
 
 # Initialize db and include the SQLAlchemy plugin in bottle
 engine = create_engine('sqlite:///%s' % (config.database,), echo=True)
+create_session = sessionmaker(bind=engine)
+database.Base.metadata.create_all(engine)
 
 app = bottle.Bottle()
 plugin = sqlalchemy.Plugin(
@@ -21,13 +24,15 @@ plugin = sqlalchemy.Plugin(
     keyword='db',
     # If it is true, execute `metadata.create_all(engine)` when plugin is
     # applied (default False).
-    create=True,
+    create=False,
     # If it is true, plugin commit changes after route is executed (default
     # True).
     commit=True,
     # If it is true and keyword is not defined, plugin uses **kwargs argument
     # to inject session database (default False).
-    use_kwargs=False
+    use_kwargs=False,
+    # Create session method
+    create_session=create_session
 )
 
 app.install(plugin)
@@ -53,6 +58,8 @@ app.route("/papers/<id:int>/relationships/<name>", method="DELETE",
 
 app.get("/tags", callback=routes.get.fetch_tags)
 app.get("/tags/<id:int>", callback=routes.get.fetch_tags_by_id)
+app.route("/tags/<id:int>", method="DELETE",
+          callback=routes.delete.delete_tag)
 
 
 app.post("/papers", callback=routes.post.create_paper)
@@ -66,4 +73,5 @@ app.route("/papers/<id:int>/relationships/<name>", method="PATCH",
 
 
 if __name__ == "__main__":
+    routes.post.fetch_citations_in_queue(create_session)
     app.run(host=config.host, port=config.port, debug=(not config.production))
diff --git a/routes/delete.py b/routes/delete.py
index d9b2c8e..3edff93 100644
--- a/routes/delete.py
+++ b/routes/delete.py
@@ -97,3 +97,28 @@ def delete_relationship(id, name, db):
             db.delete(relationship)
     # Return an empty 204 on success
     return tools.APIResponse(status=204, body="")
+
+
+def delete_tag(id, db):
+    """
+    Delete a given tag.
+
+    .. code-block:: bash
+
+        DELETE /tags/1
+        Accept: application/vnd.api+json
+
+
+    .. code-block:: bash
+
+        HTTP 204
+
+    :param id: The id of the requested tag to be deleted.
+    :param db: A database session, injected by the ``Bottle`` plugin.
+    :returns: An ``HTTPResponse``.
+    """
+    resource = db.query(database.Tag).filter_by(id=id).first()
+    if resource:
+        db.delete(resource)
+        return tools.APIResponse(status=204, body="")
+    return bottle.HTTPError(404, "Not found")
diff --git a/routes/post.py b/routes/post.py
index 96f4988..420e60e 100644
--- a/routes/post.py
+++ b/routes/post.py
@@ -3,8 +3,10 @@ This file contains POST routes methods.
 """
 import bottle
 import json
+import threading
 from sqlalchemy.exc import IntegrityError
 
+import config
 import database
 import tools
 from reference_fetcher import arxiv
@@ -115,19 +117,19 @@ def create_by_doi(doi, db):
     return paper
 
 
-def create_by_arxiv(arxiv, db):
+def create_by_arxiv(arxiv_id, db):
     """
     Create a new resource identified by its arXiv eprint ID, if it does not
     exist.
 
-    :param arxiv: The arXiv eprint ID.
+    :param arxiv_id: The arXiv eprint ID.
     :param db: A database session.
     :returns: ``None`` if insertion failed, the ``Paper`` object otherwise.
     """
-    paper = database.Paper(arxiv_id=arxiv)
+    paper = database.Paper(arxiv_id=arxiv_id)
 
     # Try to fetch an arXiv id
-    doi = arxiv.get_doi(arxiv)
+    doi = arxiv.get_doi(arxiv_id)
     if doi:
         paper.doi = doi
 
@@ -156,20 +158,35 @@ def add_cite_relationship(paper, db):
     # If paper is on arXiv
     if paper.arxiv_id is not None:
         # Get the cited DOIs
-        cited_dois = arxiv.get_cited_dois(paper.arxiv_id)
+        cited_urls = arxiv.get_cited_dois(paper.arxiv_id)
         # Filter out the ones that were not matched
-        cited_dois = [cited_dois[k]
-                      for k in cited_dois if cited_dois[k] is not None]
-        for doi in cited_dois:
+        cited_urls = [cited_urls[k]
+                      for k in cited_urls if cited_urls[k] is not None]
+        for url in cited_urls:
+            type, identifier = tools.get_identifier_from_url(url)
+            if type is None:
+                # No identifier found
+                continue
             # Get the associated paper in the db
-            right_paper = (db.query(database.Paper).
-                           filter_by(doi=doi).first())
+            right_paper = (db.query(database.Paper)
+                           .filter(getattr(database.Paper, type) == identifier)
+                           .first())
             if right_paper is None:
-                # If paper does not exist in db, add it
-                right_paper = create_by_doi(doi, db)
-                # Update cite relationship for this paper, recursively
-                # TODO: Known bug: too many levels of recursion!
-                # add_cite_relationship(right_paper, db)
+                # If paper is not in db, add it
+                if type == "doi":
+                    right_paper = create_by_doi(identifier, db)
+                elif type == "arxiv_id":
+                    right_paper = create_by_arxiv(identifier, db)
+                else:
+                    continue
+                # Push this paper on the queue for update of cite relationships
+                queue = database.CitationProcessingQueue()
+                queue.paper = right_paper
+                try:
+                    db.add(queue)
+                except IntegrityError:
+                    # Unique constraint violation, relationship already exists
+                    db.rollback()
             # Update the relationships
             update_relationship_backend(paper.id, right_paper.id, "cite", db)
     # If paper is not on arXiv, nothing to do
@@ -177,6 +194,39 @@ def add_cite_relationship(paper, db):
         return
 
 
+def fetch_citations_in_queue(create_session):
+    """
+    Process the first item in the queue, waiting for citation processing.
+
+    i.. note::
+
+            Calls itself recursively after the time defined in ``config``, so
+            that queued articles are processed concurrently.
+
+    :param create_session: a ``SQLAlchemy`` ``sessionmaker``.
+    :returns: Nothing.
+    """
+    # Get a db Session
+    db = create_session()
+    queued = db.query(database.CitationProcessingQueue).first()
+    if queued:
+        print("Processing citation relationships for %s." % (queued.paper,))
+        # Process this paper
+        add_cite_relationship(queued.paper, db)
+        # Remove this paper from queue
+        db.delete(queued)
+        # Commit to the database
+        try:
+            db.commit()
+        except:
+            db.rollback()
+    # Call this function again after a while
+    threading.Timer(
+        config.queue_polling_interval,
+        lambda: fetch_citations_in_queue(create_session)
+    ).start()
+
+
 def update_relationships(id, name, db):
     """
     Update the relationships associated to a given paper.
diff --git a/tools.py b/tools.py
index 3221592..117834a 100644
--- a/tools.py
+++ b/tools.py
@@ -18,6 +18,27 @@ def pretty_json(data):
                       separators=(',', ': '))
 
 
+def get_identifier_from_url(url):
+    """
+    Get the identifier out of a DOI or arXiv URL.
+
+    :param url: An input URL.
+    :returns: A tuple ``(type, identifier)``. Returns ``(None, None)`` if \
+            could not match.
+    """
+    type = None
+    identifier = None
+
+    if "dx.doi.org" in url:
+        type = "doi"
+        identifier = url[url.find("dx.doi.org") + 11:]
+    elif "arxiv.org/abs" in url:
+        type = "arxiv_id"
+        identifier = url[url.find("arxiv.org/abs/") + 14:]
+
+    return (type, identifier)
+
+
 class APIResponse(bottle.HTTPResponse):
     """
     Extend bottle.HTTPResponse base class to add Content-Type header.