add backlog count (#806)

bigchaindb/db/backends/rethinkdb.py

@@ -293,6 +293,17 @@ class RethinkDBBackend:
                 r.table('bigchain', read_mode=self.read_mode)
                 .count())
 
+    def count_backlog(self):
+        """Count the number of transactions in the backlog table.
+
+        Returns:
+            The number of transactions in the backlog.
+        """
+
+        return self.connection.run(
+                r.table('backlog', read_mode=self.read_mode)
+                .count())
+
     def write_vote(self, vote):
         """Write a vote to the votes table.
 

tests/db/test_bigchain_api.py

@@ -592,6 +592,15 @@ class TestBigchainApi(object):
         with pytest.raises(TransactionDoesNotExist) as excinfo:
             tx.validate(Bigchain())
 
+    def test_count_backlog(self, b, user_vk):
+        from bigchaindb.models import Transaction
+
+        for _ in range(4):
+            tx = Transaction.create([b.me], [user_vk]).sign([b.me_private])
+            b.write_transaction(tx)
+
+        assert b.backend.count_backlog() == 4
+
 
 class TestTransactionValidation(object):
     def test_create_operation_with_inputs(self, b, user_vk, create_tx):