diff --git a/bigchaindb/backend/admin.py b/bigchaindb/backend/admin.py
index 057b5995..f0ea62fd 100644
--- a/bigchaindb/backend/admin.py
+++ b/bigchaindb/backend/admin.py
@@ -20,3 +20,15 @@ def set_shards(connection, *, shards):
 @singledispatch
 def set_replicas(connection, *, replicas):
     raise NotImplementedError
+
+
+@singledispatch
+def add_replicas(connection, replicas):
+    raise NotImplementedError('This command is specific to the '
+                              'MongoDB backend.')
+
+
+@singledispatch
+def remove_replicas(connection, replicas):
+    raise NotImplementedError('This command is specific to the '
+                              'MongoDB backend.')
diff --git a/bigchaindb/backend/mongodb/__init__.py b/bigchaindb/backend/mongodb/__init__.py
index af5293ac..e3746fa3 100644
--- a/bigchaindb/backend/mongodb/__init__.py
+++ b/bigchaindb/backend/mongodb/__init__.py
@@ -16,7 +16,7 @@ generic backend interfaces to the implementations in this module.
 """
 
 # Register the single dispatched modules on import.
-from bigchaindb.backend.mongodb import schema, query, changefeed  # noqa
+from bigchaindb.backend.mongodb import admin, schema, query, changefeed  # noqa
 
 # MongoDBConnection should always be accessed via
 # ``bigchaindb.backend.connect()``.
diff --git a/bigchaindb/backend/mongodb/admin.py b/bigchaindb/backend/mongodb/admin.py
new file mode 100644
index 00000000..7d72c3a4
--- /dev/null
+++ b/bigchaindb/backend/mongodb/admin.py
@@ -0,0 +1,86 @@
+"""Database configuration functions."""
+import logging
+
+from pymongo.errors import OperationFailure
+
+from bigchaindb.backend import admin
+from bigchaindb.backend.utils import module_dispatch_registrar
+from bigchaindb.backend.exceptions import DatabaseOpFailedError
+from bigchaindb.backend.mongodb.connection import MongoDBConnection
+
+logger = logging.getLogger(__name__)
+
+register_admin = module_dispatch_registrar(admin)
+
+
+@register_admin(MongoDBConnection)
+def add_replicas(connection, replicas):
+    """Add a set of replicas to the replicaset
+
+    Args:
+        connection (:class:`~bigchaindb.backend.connection.Connection`):
+            A connection to the database.
+        replicas (:obj:`list` of :obj:`str`): replica addresses in the
+            form "hostname:port".
+
+    Raises:
+        DatabaseOpFailedError: If the reconfiguration fails due to a MongoDB
+            :exc:`OperationFailure`
+    """
+    # get current configuration
+    conf = connection.conn.admin.command('replSetGetConfig')
+
+    # MongoDB does not automatically add an id for the members so we need
+    # to choose one that does not exist yet. The safest way is to use
+    # incrementing ids, so we first check what is the highest id already in
+    # the set and continue from there.
+    cur_id = max([member['_id'] for member in conf['config']['members']])
+
+    # add the nodes to the members list of the replica set
+    for replica in replicas:
+        cur_id += 1
+        conf['config']['members'].append({'_id': cur_id, 'host': replica})
+
+    # increase the configuration version number
+    # when reconfiguring, mongodb expects a version number higher than the one
+    # it currently has
+    conf['config']['version'] += 1
+
+    # apply new configuration
+    try:
+        connection.conn.admin.command('replSetReconfig', conf['config'])
+    except OperationFailure as exc:
+        raise DatabaseOpFailedError(exc.details['errmsg'])
+
+
+@register_admin(MongoDBConnection)
+def remove_replicas(connection, replicas):
+    """Remove a set of replicas from the replicaset
+
+    Args:
+        connection (:class:`~bigchaindb.backend.connection.Connection`):
+            A connection to the database.
+        replicas (:obj:`list` of :obj:`str`): replica addresses in the
+            form "hostname:port".
+
+    Raises:
+        DatabaseOpFailedError: If the reconfiguration fails due to a MongoDB
+            :exc:`OperationFailure`
+    """
+    # get the current configuration
+    conf = connection.conn.admin.command('replSetGetConfig')
+
+    # remove the nodes from the members list in the replica set
+    conf['config']['members'] = list(
+        filter(lambda member: member['host'] not in replicas,
+               conf['config']['members'])
+    )
+
+    # increase the configuration version number
+    conf['config']['version'] += 1
+
+    # apply new configuration
+    try:
+        connection.conn.admin.command('replSetReconfig', conf['config'])
+    except OperationFailure as exc:
+        raise DatabaseOpFailedError(exc.details['errmsg'])
diff --git a/bigchaindb/backend/mongodb/query.py b/bigchaindb/backend/mongodb/query.py
index d7ee6afc..e3b71315 100644
--- a/bigchaindb/backend/mongodb/query.py
+++ b/bigchaindb/backend/mongodb/query.py
@@ -143,6 +143,10 @@ def get_asset_by_id(conn, asset_id):
 @register_query(MongoDBConnection)
 def get_spent(conn, transaction_id, output):
     cursor = conn.db['bigchain'].aggregate([
+        {'$match': {
+            'block.transactions.inputs.fulfills.txid': transaction_id,
+            'block.transactions.inputs.fulfills.output': output
+        }},
         {'$unwind': '$block.transactions'},
         {'$match': {
             'block.transactions.inputs.fulfills.txid': transaction_id,
@@ -157,12 +161,9 @@ def get_spent(conn, transaction_id, output):
 @register_query(MongoDBConnection)
 def get_owned_ids(conn, owner):
     cursor = conn.db['bigchain'].aggregate([
+        {'$match': {'block.transactions.outputs.public_keys': owner}},
         {'$unwind': '$block.transactions'},
-        {'$match': {
-            'block.transactions.outputs.public_keys': {
-                '$elemMatch': {'$eq': owner}
-            }
-        }}
+        {'$match': {'block.transactions.outputs.public_keys': owner}}
     ])
     # we need to access some nested fields before returning so lets use a
     # generator to avoid having to read all records on the cursor at this point
diff --git a/bigchaindb/backend/mongodb/schema.py b/bigchaindb/backend/mongodb/schema.py
index 2c526e7c..95c2d02a 100644
--- a/bigchaindb/backend/mongodb/schema.py
+++ b/bigchaindb/backend/mongodb/schema.py
@@ -63,6 +63,18 @@ def create_bigchain_secondary_index(conn, dbname):
         .create_index('block.transactions.transaction.asset.id',
                       name='asset_id')
 
+    # secondary index on the public keys of outputs
+    conn.conn[dbname]['bigchain']\
+        .create_index('block.transactions.outputs.public_keys',
+                      name='outputs')
+
+    # secondary index on inputs/transaction links (txid, output)
+    conn.conn[dbname]['bigchain']\
+        .create_index([
+            ('block.transactions.inputs.fulfills.txid', ASCENDING),
+            ('block.transactions.inputs.fulfills.output', ASCENDING),
+        ], name='inputs')
+
 
 def create_backlog_secondary_index(conn, dbname):
     logger.info('Create `backlog` secondary index.')
diff --git a/bigchaindb/backend/rethinkdb/query.py b/bigchaindb/backend/rethinkdb/query.py
index aa7c3be6..99346984 100644
--- a/bigchaindb/backend/rethinkdb/query.py
+++ b/bigchaindb/backend/rethinkdb/query.py
@@ -111,21 +111,22 @@ def _get_asset_create_tx_query(asset_id):
 
 @register_query(RethinkDBConnection)
 def get_spent(connection, transaction_id, output):
-    # TODO: use index!
     return connection.run(
             r.table('bigchain', read_mode=READ_MODE)
-            .concat_map(lambda doc: doc['block']['transactions'])
-            .filter(lambda transaction: transaction['inputs'].contains(
-                lambda input: input['fulfills'] == {'txid': transaction_id, 'output': output})))
+             .get_all([transaction_id, output], index='inputs')
+             .concat_map(lambda doc: doc['block']['transactions'])
+             .filter(lambda transaction: transaction['inputs'].contains(
+                lambda input_: input_['fulfills'] == {'txid': transaction_id, 'output': output})))
 
 
 @register_query(RethinkDBConnection)
 def get_owned_ids(connection, owner):
-    # TODO: use index!
     return connection.run(
             r.table('bigchain', read_mode=READ_MODE)
-            .concat_map(lambda doc: doc['block']['transactions'])
-            .filter(lambda tx: tx['outputs'].contains(
+             .get_all(owner, index='outputs')
+             .distinct()
+             .concat_map(lambda doc: doc['block']['transactions'])
+             .filter(lambda tx: tx['outputs'].contains(
                 lambda c: c['public_keys'].contains(owner))))
 
 
diff --git a/bigchaindb/backend/rethinkdb/schema.py b/bigchaindb/backend/rethinkdb/schema.py
index 4a76a06b..997ec5fc 100644
--- a/bigchaindb/backend/rethinkdb/schema.py
+++ b/bigchaindb/backend/rethinkdb/schema.py
@@ -66,6 +66,31 @@ def create_bigchain_secondary_index(connection, dbname):
         .table('bigchain')
         .index_create('asset_id', r.row['block']['transactions']['asset']['id'], multi=True))
 
+    # secondary index on the public keys of outputs
+    # the last reduce operation is to return a flatten list of public_keys
+    # without it we would need to match exactly the public_keys list.
+    # For instance querying for `pk1` would not match documents with
+    # `public_keys: [pk1, pk2, pk3]`
+    connection.run(
+        r.db(dbname)
+         .table('bigchain')
+         .index_create('outputs',
+                       r.row['block']['transactions']
+                        .concat_map(lambda tx: tx['outputs']['public_keys'])
+                        .reduce(lambda l, r: l + r), multi=True))
+
+    # secondary index on inputs/transaction links (txid, output)
+    connection.run(
+        r.db(dbname)
+         .table('bigchain')
+         .index_create('inputs',
+                       r.row['block']['transactions']
+                        .concat_map(lambda tx: tx['inputs']['fulfills'])
+                        .with_fields('txid', 'output')
+                        .map(lambda fulfills: [fulfills['txid'],
+                                               fulfills['output']]),
+                       multi=True))
+
     # wait for rethinkdb to finish creating secondary indexes
     connection.run(
         r.db(dbname)
diff --git a/bigchaindb/commands/bigchain.py b/bigchaindb/commands/bigchain.py
index 272f8107..4e8de28b 100644
--- a/bigchaindb/commands/bigchain.py
+++ b/bigchaindb/commands/bigchain.py
@@ -22,7 +22,8 @@ from bigchaindb.models import Transaction
 from bigchaindb.utils import ProcessGroup
 from bigchaindb import backend
 from bigchaindb.backend import schema
-from bigchaindb.backend.admin import set_replicas, set_shards
+from bigchaindb.backend.admin import (set_replicas, set_shards, add_replicas,
+                                      remove_replicas)
 from bigchaindb.backend.exceptions import DatabaseOpFailedError
 from bigchaindb.commands import utils
 from bigchaindb import processes
@@ -264,6 +265,32 @@ def run_set_replicas(args):
         logger.warn(e)
 
 
+def run_add_replicas(args):
+    # Note: This command is specific to MongoDB
+    bigchaindb.config_utils.autoconfigure(filename=args.config, force=True)
+    conn = backend.connect()
+
+    try:
+        add_replicas(conn, args.replicas)
+    except (DatabaseOpFailedError, NotImplementedError) as e:
+        logger.warn(e)
+    else:
+        logger.info('Added {} to the replicaset.'.format(args.replicas))
+
+
+def run_remove_replicas(args):
+    # Note: This command is specific to MongoDB
+    bigchaindb.config_utils.autoconfigure(filename=args.config, force=True)
+    conn = backend.connect()
+
+    try:
+        remove_replicas(conn, args.replicas)
+    except (DatabaseOpFailedError, NotImplementedError) as e:
+        logger.warn(e)
+    else:
+        logger.info('Removed {} from the replicaset.'.format(args.replicas))
+
+
 def create_parser():
     parser = argparse.ArgumentParser(
         description='Control your BigchainDB node.',
@@ -329,6 +356,32 @@ def create_parser():
                                  type=int, default=1,
                                  help='Number of replicas (i.e. the replication factor)')
 
+    # parser for adding nodes to the replica set
+    add_replicas_parser = subparsers.add_parser('add-replicas',
+                                                help='Add a set of nodes to the '
+                                                     'replica set. This command '
+                                                     'is specific to the MongoDB'
+                                                     ' backend.')
+
+    add_replicas_parser.add_argument('replicas', nargs='+',
+                                     type=utils.mongodb_host,
+                                     help='A list of space separated hosts to '
+                                          'add to the replicaset. Each host '
+                                          'should be in the form `host:port`.')
+
+    # parser for removing nodes from the replica set
+    rm_replicas_parser = subparsers.add_parser('remove-replicas',
+                                               help='Remove a set of nodes from the '
+                                                    'replica set. This command '
+                                                    'is specific to the MongoDB'
+                                                    ' backend.')
+
+    rm_replicas_parser.add_argument('replicas', nargs='+',
+                                    type=utils.mongodb_host,
+                                    help='A list of space separated hosts to '
+                                         'remove from the replicaset. Each host '
+                                         'should be in the form `host:port`.')
+
     load_parser = subparsers.add_parser('load',
                                         help='Write transactions to the backlog')
 
diff --git a/bigchaindb/commands/utils.py b/bigchaindb/commands/utils.py
index 510eb2f6..80ee7a6b 100644
--- a/bigchaindb/commands/utils.py
+++ b/bigchaindb/commands/utils.py
@@ -3,14 +3,15 @@ for ``argparse.ArgumentParser``.
 """
 
 import argparse
-from bigchaindb.common.exceptions import StartupError
 import multiprocessing as mp
 import subprocess
 
 import rethinkdb as r
+from pymongo import uri_parser
 
 import bigchaindb
 from bigchaindb import backend
+from bigchaindb.common.exceptions import StartupError
 from bigchaindb.version import __version__
 
 
@@ -95,6 +96,34 @@ def start(parser, argv, scope):
     return func(args)
 
 
+def mongodb_host(host):
+    """Utility function that works as a type for mongodb ``host`` args.
+
+    This function validates the ``host`` args provided by to the
+    ``add-replicas`` and ``remove-replicas`` commands and checks if each arg
+    is in the form "host:port"
+
+    Args:
+        host (str): A string containing hostname and port (e.g. "host:port")
+
+    Raises:
+        ArgumentTypeError: if it fails to parse the argument
+    """
+    # check if mongodb can parse the host
+    try:
+        hostname, port = uri_parser.parse_host(host, default_port=None)
+    except ValueError as exc:
+        raise argparse.ArgumentTypeError(exc.args[0])
+
+    # we do require the port to be provided.
+    if port is None or hostname == '':
+        raise argparse.ArgumentTypeError('expected host in the form '
+                                         '`host:port`. Got `{}` instead.'
+                                         .format(host))
+
+    return host
+
+
 base_parser = argparse.ArgumentParser(add_help=False, prog='bigchaindb')
 
 base_parser.add_argument('-c', '--config',
diff --git a/bigchaindb/common/transaction.py b/bigchaindb/common/transaction.py
index 65b12eed..e4bd642f 100644
--- a/bigchaindb/common/transaction.py
+++ b/bigchaindb/common/transaction.py
@@ -482,8 +482,8 @@ class Transaction(object):
             Args:
                 tx_signers (:obj:`list` of :obj:`str`): A list of keys that
                     represent the signers of the CREATE Transaction.
-                recipients (:obj:`list` of :obj:`str`): A list of keys that
-                    represent the recipients of the outputs of this
+                recipients (:obj:`list` of :obj:`tuple`): A list of
+                    ([keys],amount) that represent the recipients of this
                     Transaction.
                 metadata (dict): The metadata to be stored along with the
                     Transaction.
@@ -549,7 +549,7 @@ class Transaction(object):
                 inputs (:obj:`list` of :class:`~bigchaindb.common.transaction.
                     Input`): Converted `Output`s, intended to
                     be used as inputs in the transfer to generate.
-                recipients (:obj:`list` of :obj:`str`): A list of
+                recipients (:obj:`list` of :obj:`tuple`): A list of
                     ([keys],amount) that represent the recipients of this
                     Transaction.
                 asset_id (str): The asset ID of the asset to be transferred in
diff --git a/bigchaindb/models.py b/bigchaindb/models.py
index c6e81956..c3683a03 100644
--- a/bigchaindb/models.py
+++ b/bigchaindb/models.py
@@ -202,11 +202,6 @@ class Block(object):
             OperationError: If a non-federation node signed the Block.
             InvalidSignature: If a Block's signature is invalid.
         """
-
-        # First, make sure this node hasn't already voted on this block
-        if bigchain.has_previous_vote(self.id, self.voters):
-            return self
-
         # Check if the block was created by a federation node
         possible_voters = (bigchain.nodes_except_me + [bigchain.me])
         if self.node_pubkey not in possible_voters:
diff --git a/tests/backend/mongodb/test_admin.py b/tests/backend/mongodb/test_admin.py
new file mode 100644
index 00000000..a7784369
--- /dev/null
+++ b/tests/backend/mongodb/test_admin.py
@@ -0,0 +1,108 @@
+"""Tests for the :mod:`bigchaindb.backend.mongodb.admin` module."""
+import copy
+from unittest import mock
+
+import pytest
+from pymongo.database import Database
+from pymongo.errors import OperationFailure
+
+
+@pytest.fixture
+def mock_replicaset_config():
+    return {
+        'config': {
+            '_id': 'bigchain-rs',
+            'members': [
+                {
+                    '_id': 0,
+                    'arbiterOnly': False,
+                    'buildIndexes': True,
+                    'hidden': False,
+                    'host': 'localhost:27017',
+                    'priority': 1.0,
+                    'slaveDelay': 0,
+                    'tags': {},
+                    'votes': 1
+                }
+            ],
+            'version': 1
+        }
+    }
+
+
+@pytest.fixture
+def connection():
+    from bigchaindb.backend import connect
+    connection = connect()
+    # connection is a lazy object. It only actually creates a connection to
+    # the database when its first used.
+    # During the setup of a MongoDBConnection some `Database.command` are
+    # executed to make sure that the replica set is correctly initialized.
+    # Here we force the the connection setup so that all required
+    # `Database.command` are executed before we mock them it in the tests.
+    connection._connect()
+    return connection
+
+
+def test_add_replicas(mock_replicaset_config, connection):
+    from bigchaindb.backend.admin import add_replicas
+
+    expected_config = copy.deepcopy(mock_replicaset_config)
+    expected_config['config']['members'] += [
+        {'_id': 1, 'host': 'localhost:27018'},
+        {'_id': 2, 'host': 'localhost:27019'}
+    ]
+    expected_config['config']['version'] += 1
+
+    with mock.patch.object(Database, 'command') as mock_command:
+        mock_command.return_value = mock_replicaset_config
+        add_replicas(connection, ['localhost:27018', 'localhost:27019'])
+
+    mock_command.assert_called_with('replSetReconfig',
+                                    expected_config['config'])
+
+
+def test_add_replicas_raises(mock_replicaset_config, connection):
+    from bigchaindb.backend.admin import add_replicas
+    from bigchaindb.backend.exceptions import DatabaseOpFailedError
+
+    with mock.patch.object(Database, 'command') as mock_command:
+        mock_command.side_effect = [
+            mock_replicaset_config,
+            OperationFailure(error=1, details={'errmsg': ''})
+        ]
+        with pytest.raises(DatabaseOpFailedError):
+            add_replicas(connection, ['localhost:27018'])
+
+
+def test_remove_replicas(mock_replicaset_config, connection):
+    from bigchaindb.backend.admin import remove_replicas
+
+    expected_config = copy.deepcopy(mock_replicaset_config)
+    expected_config['config']['version'] += 1
+
+    # add some hosts to the configuration to remove
+    mock_replicaset_config['config']['members'] += [
+        {'_id': 1, 'host': 'localhost:27018'},
+        {'_id': 2, 'host': 'localhost:27019'}
+    ]
+
+    with mock.patch.object(Database, 'command') as mock_command:
+        mock_command.return_value = mock_replicaset_config
+        remove_replicas(connection, ['localhost:27018', 'localhost:27019'])
+
+    mock_command.assert_called_with('replSetReconfig',
+                                    expected_config['config'])
+
+
+def test_remove_replicas_raises(mock_replicaset_config, connection):
+    from bigchaindb.backend.admin import remove_replicas
+    from bigchaindb.backend.exceptions import DatabaseOpFailedError
+
+    with mock.patch.object(Database, 'command') as mock_command:
+        mock_command.side_effect = [
+            mock_replicaset_config,
+            OperationFailure(error=1, details={'errmsg': ''})
+        ]
+        with pytest.raises(DatabaseOpFailedError):
+            remove_replicas(connection, ['localhost:27018'])
diff --git a/tests/backend/mongodb/test_schema.py b/tests/backend/mongodb/test_schema.py
index 34b6edf9..71eac7ff 100644
--- a/tests/backend/mongodb/test_schema.py
+++ b/tests/backend/mongodb/test_schema.py
@@ -21,8 +21,8 @@ def test_init_creates_db_tables_and_indexes():
     assert sorted(collection_names) == ['backlog', 'bigchain', 'votes']
 
     indexes = conn.conn[dbname]['bigchain'].index_information().keys()
-    assert sorted(indexes) == ['_id_', 'asset_id', 'block_timestamp',
-                               'transaction_id']
+    assert sorted(indexes) == ['_id_', 'asset_id', 'block_timestamp', 'inputs',
+                               'outputs', 'transaction_id']
 
     indexes = conn.conn[dbname]['backlog'].index_information().keys()
     assert sorted(indexes) == ['_id_', 'assignee__transaction_timestamp',
@@ -81,8 +81,8 @@ def test_create_secondary_indexes():
 
     # Bigchain table
     indexes = conn.conn[dbname]['bigchain'].index_information().keys()
-    assert sorted(indexes) == ['_id_', 'asset_id', 'block_timestamp',
-                               'transaction_id']
+    assert sorted(indexes) == ['_id_', 'asset_id', 'block_timestamp', 'inputs',
+                               'outputs', 'transaction_id']
 
     # Backlog table
     indexes = conn.conn[dbname]['backlog'].index_information().keys()
diff --git a/tests/backend/rethinkdb/test_schema.py b/tests/backend/rethinkdb/test_schema.py
index 1447e80f..e19dfdc2 100644
--- a/tests/backend/rethinkdb/test_schema.py
+++ b/tests/backend/rethinkdb/test_schema.py
@@ -85,6 +85,10 @@ def test_create_secondary_indexes():
         'transaction_id')) is True
     assert conn.run(r.db(dbname).table('bigchain').index_list().contains(
         'asset_id')) is True
+    assert conn.run(r.db(dbname).table('bigchain').index_list().contains(
+        'inputs')) is True
+    assert conn.run(r.db(dbname).table('bigchain').index_list().contains(
+        'outputs')) is True
 
     # Backlog table
     assert conn.run(r.db(dbname).table('backlog').index_list().contains(
diff --git a/tests/backend/test_generics.py b/tests/backend/test_generics.py
index daf50c10..2f86d417 100644
--- a/tests/backend/test_generics.py
+++ b/tests/backend/test_generics.py
@@ -71,6 +71,8 @@ def test_changefeed_class(changefeed_class_func_name, args_qty):
     ('reconfigure', {'table': None, 'shards': None, 'replicas': None}),
     ('set_shards', {'shards': None}),
     ('set_replicas', {'replicas': None}),
+    ('add_replicas', {'replicas': None}),
+    ('remove_replicas', {'replicas': None}),
 ))
 def test_admin(admin_func_name, kwargs):
     from bigchaindb.backend import admin
diff --git a/tests/commands/test_commands.py b/tests/commands/test_commands.py
index 1a1291e3..95bb0db7 100644
--- a/tests/commands/test_commands.py
+++ b/tests/commands/test_commands.py
@@ -1,6 +1,6 @@
 import json
 from unittest.mock import Mock, patch
-from argparse import Namespace
+from argparse import Namespace, ArgumentTypeError
 import copy
 
 import pytest
@@ -22,6 +22,8 @@ def test_make_sure_we_dont_remove_any_command():
     assert parser.parse_args(['set-shards', '1']).command
     assert parser.parse_args(['set-replicas', '1']).command
     assert parser.parse_args(['load']).command
+    assert parser.parse_args(['add-replicas', 'localhost:27017']).command
+    assert parser.parse_args(['remove-replicas', 'localhost:27017']).command
 
 
 def test_start_raises_if_command_not_implemented():
@@ -376,3 +378,73 @@ def test_calling_main(start_mock, base_parser_mock, parse_args_mock,
                                                'distributed equally to all '
                                                'the processes')
     assert start_mock.called is True
+
+
+@pytest.mark.usefixtures('ignore_local_config_file')
+@patch('bigchaindb.commands.bigchain.add_replicas')
+def test_run_add_replicas(mock_add_replicas):
+    from bigchaindb.commands.bigchain import run_add_replicas
+    from bigchaindb.backend.exceptions import DatabaseOpFailedError
+
+    args = Namespace(config=None, replicas=['localhost:27017'])
+
+    # test add_replicas no raises
+    mock_add_replicas.return_value = None
+    assert run_add_replicas(args) is None
+    assert mock_add_replicas.call_count == 1
+    mock_add_replicas.reset_mock()
+
+    # test add_replicas with `DatabaseOpFailedError`
+    mock_add_replicas.side_effect = DatabaseOpFailedError()
+    assert run_add_replicas(args) is None
+    assert mock_add_replicas.call_count == 1
+    mock_add_replicas.reset_mock()
+
+    # test add_replicas with `NotImplementedError`
+    mock_add_replicas.side_effect = NotImplementedError()
+    assert run_add_replicas(args) is None
+    assert mock_add_replicas.call_count == 1
+    mock_add_replicas.reset_mock()
+
+
+@pytest.mark.usefixtures('ignore_local_config_file')
+@patch('bigchaindb.commands.bigchain.remove_replicas')
+def test_run_remove_replicas(mock_remove_replicas):
+    from bigchaindb.commands.bigchain import run_remove_replicas
+    from bigchaindb.backend.exceptions import DatabaseOpFailedError
+
+    args = Namespace(config=None, replicas=['localhost:27017'])
+
+    # test add_replicas no raises
+    mock_remove_replicas.return_value = None
+    assert run_remove_replicas(args) is None
+    assert mock_remove_replicas.call_count == 1
+    mock_remove_replicas.reset_mock()
+
+    # test add_replicas with `DatabaseOpFailedError`
+    mock_remove_replicas.side_effect = DatabaseOpFailedError()
+    assert run_remove_replicas(args) is None
+    assert mock_remove_replicas.call_count == 1
+    mock_remove_replicas.reset_mock()
+
+    # test add_replicas with `NotImplementedError`
+    mock_remove_replicas.side_effect = NotImplementedError()
+    assert run_remove_replicas(args) is None
+    assert mock_remove_replicas.call_count == 1
+    mock_remove_replicas.reset_mock()
+
+
+def test_mongodb_host_type():
+    from bigchaindb.commands.utils import mongodb_host
+
+    # bad port provided
+    with pytest.raises(ArgumentTypeError):
+        mongodb_host('localhost:11111111111')
+
+    # no port information provided
+    with pytest.raises(ArgumentTypeError):
+        mongodb_host('localhost')
+
+    # bad host provided
+    with pytest.raises(ArgumentTypeError):
+        mongodb_host(':27017')
diff --git a/tests/test_models.py b/tests/test_models.py
index 7ab97e9e..58aa64fd 100644
--- a/tests/test_models.py
+++ b/tests/test_models.py
@@ -163,16 +163,3 @@ class TestBlockModel(object):
 
         public_key = PublicKey(b.me)
         assert public_key.verify(expected_block_serialized, block.signature)
-
-    def test_validate_already_voted_on_block(self, b, monkeypatch):
-        from unittest.mock import Mock
-        from bigchaindb.models import Transaction
-
-        tx = Transaction.create([b.me], [([b.me], 1)])
-        block = b.create_block([tx])
-
-        has_previous_vote = Mock()
-        has_previous_vote.return_value = True
-        monkeypatch.setattr(b, 'has_previous_vote', has_previous_vote)
-        assert block == block.validate(b)
-        assert has_previous_vote.called is True