diff --git a/tests/backend/localmongodb/test_connection.py b/tests/backend/localmongodb/test_connection.py
index a76f5b85..77692abe 100644
--- a/tests/backend/localmongodb/test_connection.py
+++ b/tests/backend/localmongodb/test_connection.py
@@ -55,10 +55,8 @@ def test_get_connection_returns_the_correct_instance(db_host, db_port):
     assert conn.conn._topology_settings.replica_set_name == config['replicaset']
 
 
-@mock.patch('bigchaindb.backend.localmongodb.connection.initialize_replica_set')
 @mock.patch('pymongo.MongoClient.__init__')
-@mock.patch('time.sleep')
-def test_connection_error(mock_sleep, mock_client, mock_init_repl_set):
+def test_connection_error(mock_client):
     from bigchaindb.backend import connect
     from bigchaindb.backend.exceptions import ConnectionError
 
@@ -74,9 +72,7 @@ def test_connection_error(mock_sleep, mock_client, mock_init_repl_set):
     assert mock_client.call_count == 3
 
 
-@mock.patch('bigchaindb.backend.localmongodb.connection.initialize_replica_set')
-@mock.patch('pymongo.MongoClient')
-def test_connection_run_errors(mock_client, mock_init_repl_set):
+def test_connection_run_errors():
     from bigchaindb.backend import connect
     from bigchaindb.backend.exceptions import (DuplicateKeyError,
                                                OperationError,
diff --git a/tests/backend/localmongodb/test_queries.py b/tests/backend/localmongodb/test_queries.py
index 4d1d7bdf..363d02a1 100644
--- a/tests/backend/localmongodb/test_queries.py
+++ b/tests/backend/localmongodb/test_queries.py
@@ -7,7 +7,7 @@ from copy import deepcopy
 import pytest
 import pymongo
 
-pytestmark = [pytest.mark.tendermint, pytest.mark.localmongodb, pytest.mark.bdb]
+pytestmark = [pytest.mark.tendermint, pytest.mark.bdb]
 
 
 def test_get_txids_filtered(signed_create_tx, signed_transfer_tx):
diff --git a/tests/commands/conftest.py b/tests/commands/conftest.py
index 025d5708..ce09ff24 100644
--- a/tests/commands/conftest.py
+++ b/tests/commands/conftest.py
@@ -27,8 +27,8 @@ def mock_db_init_with_existing_db(monkeypatch):
 
 @pytest.fixture
 def mock_processes_start(monkeypatch):
-    from bigchaindb import processes
-    monkeypatch.setattr(processes, 'start', lambda *args: None)
+    from bigchaindb.utils import Process
+    monkeypatch.setattr(Process, 'run', lambda *args: None)
 
 
 @pytest.fixture
diff --git a/tests/commands/test_commands.py b/tests/commands/test_commands.py
index d834ae6b..a0c070d8 100644
--- a/tests/commands/test_commands.py
+++ b/tests/commands/test_commands.py
@@ -179,7 +179,7 @@ def test_run_configure_when_config_does_exist(monkeypatch,
                                               mock_bigchaindb_backup_config):
     value = {}
 
-    def mock_write_config(newconfig, filename=None):
+    def mock_write_config(newconfig):
         value['return'] = newconfig
 
     from bigchaindb.commands.bigchaindb import run_configure
@@ -244,11 +244,8 @@ def test_run_start_when_db_already_exists(mocker,
 
 
 @pytest.mark.tendermint
-@patch('argparse.ArgumentParser.parse_args')
-@patch('bigchaindb.commands.utils.base_parser')
 @patch('bigchaindb.commands.utils.start')
-def test_calling_main(start_mock, base_parser_mock, parse_args_mock,
-                      monkeypatch):
+def test_calling_main(start_mock, monkeypatch):
     from bigchaindb.commands.bigchaindb import main
 
     argparser_mock = Mock()
@@ -280,8 +277,7 @@ def test_calling_main(start_mock, base_parser_mock, parse_args_mock,
 @patch('bigchaindb.config_utils.autoconfigure')
 @patch('bigchaindb.commands.bigchaindb.run_recover')
 @patch('bigchaindb.start.start')
-def test_recover_db_on_start(mock_autoconfigure,
-                             mock_run_recover,
+def test_recover_db_on_start(mock_run_recover,
                              mock_start,
                              mocked_setup_logging):
     from bigchaindb.commands.bigchaindb import run_start
diff --git a/tests/conftest.py b/tests/conftest.py
index c5dabd2d..65d248ce 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -37,13 +37,6 @@ USER_PRIVATE_KEY = '8eJ8q9ZQpReWyQT5aFCiwtZ5wDZC4eDnCen88p3tQ6ie'
 USER_PUBLIC_KEY = 'JEAkEJqLbbgDRAtMm8YAjGp759Aq2qTn9eaEHUj2XePE'
 
 
-def pytest_runtest_setup(item):
-    if isinstance(item, item.Function):
-        backend = item.session.config.getoption('--database-backend')
-        if (item.get_marker('localmongodb') and backend != 'localmongodb'):
-            pytest.skip('Skip tendermint specific tests if not using localmongodb')
-
-
 def pytest_addoption(parser):
     from bigchaindb.backend.connection import BACKENDS
 
@@ -56,19 +49,6 @@ def pytest_addoption(parser):
     )
 
 
-def pytest_ignore_collect(path, config):
-    from bigchaindb.backend.connection import BACKENDS
-    path = str(path)
-
-    supported_backends = BACKENDS.keys()
-
-    if os.path.isdir(path):
-        dirname = os.path.split(path)[1]
-        if dirname in supported_backends and dirname != config.getoption('--database-backend'):
-            print('Ignoring unrequested backend test dir: ', path)
-            return True
-
-
 def pytest_configure(config):
     config.addinivalue_line(
         'markers',
@@ -282,12 +262,6 @@ def b():
     return BigchainDB()
 
 
-@pytest.fixture
-def tb():
-    from bigchaindb import BigchainDB
-    return BigchainDB()
-
-
 @pytest.fixture
 def create_tx(alice, user_pk):
     from bigchaindb.models import Transaction
@@ -349,24 +323,6 @@ def inputs(user_pk, b, alice):
         b.store_bulk_transactions(transactions)
 
 
-@pytest.fixture
-def inputs_shared(user_pk, user2_pk, alice):
-    from bigchaindb.models import Transaction
-
-    # create blocks with transactions for `USER` to spend
-    for block in range(4):
-        transactions = [
-            Transaction.create(
-                [alice.public_key],
-                [user_pk, user2_pk],
-                metadata={'msg': random.random()},
-            ).sign([alice.private_key]).to_dict()
-            for _ in range(10)
-        ]
-        block = Block(app_hash='', height=_get_height(b), transaction=transactions)
-        b.store_block(block._asdict())
-
-
 @pytest.fixture
 def dummy_db(request):
     from bigchaindb.backend import connect, schema
@@ -389,26 +345,6 @@ def dummy_db(request):
         pass
 
 
-@pytest.fixture
-def not_yet_created_db(request):
-    from bigchaindb.backend import connect, schema
-    from bigchaindb.common.exceptions import DatabaseDoesNotExist
-    conn = connect()
-    dbname = request.fixturename
-    xdist_suffix = getattr(request.config, 'slaveinput', {}).get('slaveid')
-    if xdist_suffix:
-        dbname = '{}_{}'.format(dbname, xdist_suffix)
-    try:
-        schema.drop_database(conn, dbname)
-    except DatabaseDoesNotExist:
-        pass
-    yield dbname
-    try:
-        schema.drop_database(conn, dbname)
-    except DatabaseDoesNotExist:
-        pass
-
-
 @pytest.fixture
 def db_config():
     from bigchaindb import config
@@ -464,29 +400,6 @@ def tendermint_ws_url(tendermint_host, tendermint_port):
     return 'ws://{}:{}/websocket'.format(tendermint_host, tendermint_port)
 
 
-@pytest.fixture
-def tendermint_context(tendermint_host, tendermint_port, tendermint_ws_url):
-    TendermintContext = namedtuple(
-        'TendermintContext', ('host', 'port', 'ws_url'))
-    return TendermintContext(
-        host=tendermint_host,
-        port=tendermint_port,
-        ws_url=tendermint_ws_url,
-    )
-
-
-@pytest.fixture
-def mocked_setup_pub_logger(mocker):
-    return mocker.patch(
-        'bigchaindb.log.setup.setup_pub_logger', autospec=True, spec_set=True)
-
-
-@pytest.fixture
-def mocked_setup_sub_logger(mocker):
-    return mocker.patch(
-        'bigchaindb.log.setup.setup_sub_logger', autospec=True, spec_set=True)
-
-
 @pytest.fixture(autouse=True)
 def _abci_http(request):
     if request.keywords.get('abci', None):
@@ -513,7 +426,7 @@ def abci_http(_setup_database, _configure_bigchaindb, abci_server,
 
 
 @pytest.yield_fixture(scope='session')
-def event_loop(request):
+def event_loop():
     import asyncio
 
     loop = asyncio.get_event_loop_policy().new_event_loop()
diff --git a/tests/tendermint/conftest.py b/tests/tendermint/conftest.py
index 22dec41c..8efa8e76 100644
--- a/tests/tendermint/conftest.py
+++ b/tests/tendermint/conftest.py
@@ -8,12 +8,6 @@ import codecs
 import abci.types_pb2 as types
 
 
-@pytest.fixture
-def b():
-    from bigchaindb import BigchainDB
-    return BigchainDB()
-
-
 @pytest.fixture
 def validator_pub_key():
     return 'B0E42D2589A455EAD339A035D6CE1C8C3E25863F268120AA0162AD7D003A4014'
diff --git a/tests/tendermint/test_integration.py b/tests/tendermint/test_integration.py
index 1a8250bd..683404bc 100644
--- a/tests/tendermint/test_integration.py
+++ b/tests/tendermint/test_integration.py
@@ -16,13 +16,12 @@ from io import BytesIO
 
 @pytest.mark.tendermint
 @pytest.mark.bdb
-def test_app(tb, init_chain_request):
+def test_app(b, init_chain_request):
     from bigchaindb import App
     from bigchaindb.tendermint_utils import calculate_hash
     from bigchaindb.common.crypto import generate_key_pair
     from bigchaindb.models import Transaction
 
-    b = tb
     app = App(b)
     p = ProtocolHandler(app)
 
diff --git a/tests/tendermint/test_lib.py b/tests/tendermint/test_lib.py
index cc511b3a..d8223a25 100644
--- a/tests/tendermint/test_lib.py
+++ b/tests/tendermint/test_lib.py
@@ -57,10 +57,9 @@ def test_asset_is_separated_from_transaciton(b):
 
 
 @pytest.mark.bdb
-def test_get_latest_block(tb):
+def test_get_latest_block(b):
     from bigchaindb.lib import Block
 
-    b = tb
     for i in range(10):
         app_hash = os.urandom(16).hex()
         txn_id = os.urandom(16).hex()
@@ -75,8 +74,8 @@ def test_get_latest_block(tb):
 @pytest.mark.bdb
 @patch('bigchaindb.backend.query.get_block', return_value=None)
 @patch('bigchaindb.BigchainDB.get_latest_block', return_value={'height': 10})
-def test_get_empty_block(_0, _1, tb):
-    assert tb.get_block(5) == {'height': 5, 'transactions': []}
+def test_get_empty_block(_0, _1, b):
+    assert b.get_block(5) == {'height': 5, 'transactions': []}
 
 
 def test_validation_error(b):
@@ -172,15 +171,15 @@ def test_validator_updates(b, validator_pub_key):
 
 
 @pytest.mark.bdb
-def test_update_utxoset(tb, signed_create_tx, signed_transfer_tx, db_context):
+def test_update_utxoset(b, signed_create_tx, signed_transfer_tx, db_context):
     mongo_client = MongoClient(host=db_context.host, port=db_context.port)
-    tb.update_utxoset(signed_create_tx)
+    b.update_utxoset(signed_create_tx)
     utxoset = mongo_client[db_context.name]['utxos']
     assert utxoset.count() == 1
     utxo = utxoset.find_one()
     assert utxo['transaction_id'] == signed_create_tx.id
     assert utxo['output_index'] == 0
-    tb.update_utxoset(signed_transfer_tx)
+    b.update_utxoset(signed_transfer_tx)
     assert utxoset.count() == 1
     utxo = utxoset.find_one()
     assert utxo['transaction_id'] == signed_transfer_tx.id
@@ -188,14 +187,14 @@ def test_update_utxoset(tb, signed_create_tx, signed_transfer_tx, db_context):
 
 
 @pytest.mark.bdb
-def test_store_transaction(mocker, tb, signed_create_tx,
+def test_store_transaction(mocker, b, signed_create_tx,
                            signed_transfer_tx, db_context):
     mocked_store_asset = mocker.patch('bigchaindb.backend.query.store_assets')
     mocked_store_metadata = mocker.patch(
         'bigchaindb.backend.query.store_metadatas')
     mocked_store_transaction = mocker.patch(
         'bigchaindb.backend.query.store_transactions')
-    tb.store_bulk_transactions([signed_create_tx])
+    b.store_bulk_transactions([signed_create_tx])
     # mongo_client = MongoClient(host=db_context.host, port=db_context.port)
     # utxoset = mongo_client[db_context.name]['utxos']
     # assert utxoset.count() == 1
@@ -204,40 +203,40 @@ def test_store_transaction(mocker, tb, signed_create_tx,
     # assert utxo['output_index'] == 0
 
     mocked_store_asset.assert_called_once_with(
-        tb.connection,
+        b.connection,
         [{'id': signed_create_tx.id, 'data': signed_create_tx.asset['data']}],
     )
     mocked_store_metadata.assert_called_once_with(
-        tb.connection,
+        b.connection,
         [{'id': signed_create_tx.id, 'metadata': signed_create_tx.metadata}],
     )
     mocked_store_transaction.assert_called_once_with(
-        tb.connection,
+        b.connection,
         [{k: v for k, v in signed_create_tx.to_dict().items()
          if k not in ('asset', 'metadata')}],
     )
     mocked_store_asset.reset_mock()
     mocked_store_metadata.reset_mock()
     mocked_store_transaction.reset_mock()
-    tb.store_bulk_transactions([signed_transfer_tx])
+    b.store_bulk_transactions([signed_transfer_tx])
     # assert utxoset.count() == 1
     # utxo = utxoset.find_one()
     # assert utxo['transaction_id'] == signed_transfer_tx.id
     # assert utxo['output_index'] == 0
     assert not mocked_store_asset.called
     mocked_store_metadata.asser_called_once_with(
-        tb.connection,
+        b.connection,
         [{'id': signed_transfer_tx.id, 'metadata': signed_transfer_tx.metadata}],
     )
     mocked_store_transaction.assert_called_once_with(
-        tb.connection,
+        b.connection,
         [{k: v for k, v in signed_transfer_tx.to_dict().items()
          if k != 'metadata'}],
     )
 
 
 @pytest.mark.bdb
-def test_store_bulk_transaction(mocker, tb, signed_create_tx,
+def test_store_bulk_transaction(mocker, b, signed_create_tx,
                                 signed_transfer_tx, db_context):
     mocked_store_assets = mocker.patch(
         'bigchaindb.backend.query.store_assets')
@@ -245,7 +244,7 @@ def test_store_bulk_transaction(mocker, tb, signed_create_tx,
         'bigchaindb.backend.query.store_metadatas')
     mocked_store_transactions = mocker.patch(
         'bigchaindb.backend.query.store_transactions')
-    tb.store_bulk_transactions((signed_create_tx,))
+    b.store_bulk_transactions((signed_create_tx,))
     # mongo_client = MongoClient(host=db_context.host, port=db_context.port)
     # utxoset = mongo_client[db_context.name]['utxos']
     # assert utxoset.count() == 1
@@ -253,34 +252,34 @@ def test_store_bulk_transaction(mocker, tb, signed_create_tx,
     # assert utxo['transaction_id'] == signed_create_tx.id
     # assert utxo['output_index'] == 0
     mocked_store_assets.assert_called_once_with(
-        tb.connection,
+        b.connection,
         [{'id': signed_create_tx.id, 'data': signed_create_tx.asset['data']}],
     )
     mocked_store_metadata.assert_called_once_with(
-        tb.connection,
+        b.connection,
         [{'id': signed_create_tx.id, 'metadata': signed_create_tx.metadata}],
     )
     mocked_store_transactions.assert_called_once_with(
-        tb.connection,
+        b.connection,
         [{k: v for k, v in signed_create_tx.to_dict().items()
          if k not in ('asset', 'metadata')}],
     )
     mocked_store_assets.reset_mock()
     mocked_store_metadata.reset_mock()
     mocked_store_transactions.reset_mock()
-    tb.store_bulk_transactions((signed_transfer_tx,))
+    b.store_bulk_transactions((signed_transfer_tx,))
     # assert utxoset.count() == 1
     # utxo = utxoset.find_one()
     # assert utxo['transaction_id'] == signed_transfer_tx.id
     # assert utxo['output_index'] == 0
     assert not mocked_store_assets.called
     mocked_store_metadata.asser_called_once_with(
-        tb.connection,
+        b.connection,
         [{'id': signed_transfer_tx.id,
           'metadata': signed_transfer_tx.metadata}],
     )
     mocked_store_transactions.assert_called_once_with(
-        tb.connection,
+        b.connection,
         [{k: v for k, v in signed_transfer_tx.to_dict().items()
           if k != 'metadata'}],
     )
diff --git a/tests/test_config_utils.py b/tests/test_config_utils.py
index 05a5ad83..35f0e486 100644
--- a/tests/test_config_utils.py
+++ b/tests/test_config_utils.py
@@ -24,7 +24,7 @@ def clean_config(monkeypatch, request):
     monkeypatch.setattr('bigchaindb.config', original_config)
 
 
-def test_bigchain_instance_is_initialized_when_conf_provided(request):
+def test_bigchain_instance_is_initialized_when_conf_provided():
     import bigchaindb
     from bigchaindb import config_utils
     assert 'CONFIGURED' not in bigchaindb.config
diff --git a/tests/test_core.py b/tests/test_core.py
index 0e3a2f12..03701267 100644
--- a/tests/test_core.py
+++ b/tests/test_core.py
@@ -66,7 +66,6 @@ def test_bigchain_class_initialization_with_parameters():
     assert bigchain.consensus == BaseConsensusRules
 
 
-@pytest.mark.genesis
 def test_get_spent_issue_1271(b, alice, bob, carol):
     from bigchaindb.models import Transaction
 
diff --git a/tests/web/test_assets.py b/tests/web/test_assets.py
index 800537ba..2c658cdc 100644
--- a/tests/web/test_assets.py
+++ b/tests/web/test_assets.py
@@ -23,8 +23,7 @@ def test_get_assets_with_missing_text_search(client):
 
 @pytest.mark.bdb
 @pytest.mark.tendermint
-@pytest.mark.localmongodb
-def test_get_assets_tendermint(client, tb, alice):
+def test_get_assets_tendermint(client, b, alice):
     from bigchaindb.models import Transaction
 
     # test returns empty list when no assets are found
@@ -37,7 +36,7 @@ def test_get_assets_tendermint(client, tb, alice):
     tx = Transaction.create([alice.public_key], [([alice.public_key], 1)],
                             asset=asset).sign([alice.private_key])
 
-    tb.store_bulk_transactions([tx])
+    b.store_bulk_transactions([tx])
 
     # test that asset is returned
     res = client.get(ASSETS_ENDPOINT + '?search=abc')
@@ -51,11 +50,9 @@ def test_get_assets_tendermint(client, tb, alice):
 
 @pytest.mark.bdb
 @pytest.mark.tendermint
-@pytest.mark.localmongodb
-def test_get_assets_limit_tendermint(client, tb, alice):
+def test_get_assets_limit_tendermint(client, b, alice):
     from bigchaindb.models import Transaction
 
-    b = tb
     # create two assets
     asset1 = {'msg': 'abc 1'}
     asset2 = {'msg': 'abc 2'}
diff --git a/tests/web/test_block_tendermint.py b/tests/web/test_block_tendermint.py
index a726a117..a92822c4 100644
--- a/tests/web/test_block_tendermint.py
+++ b/tests/web/test_block_tendermint.py
@@ -14,9 +14,8 @@ pytestmark = pytest.mark.tendermint
 
 @pytest.mark.bdb
 @pytest.mark.usefixtures('inputs')
-def test_get_block_endpoint(tb, client, alice):
+def test_get_block_endpoint(b, client, alice):
     import copy
-    b = tb
     tx = Transaction.create([alice.public_key], [([alice.public_key], 1)], asset={'cycle': 'hero'})
     tx = tx.sign([alice.private_key])
 
@@ -49,8 +48,7 @@ def test_get_block_returns_404_if_not_found(client):
 
 
 @pytest.mark.bdb
-def test_get_block_containing_transaction(tb, client, alice):
-    b = tb
+def test_get_block_containing_transaction(b, client, alice):
     tx = Transaction.create([alice.public_key], [([alice.public_key], 1)], asset={'cycle': 'hero'})
     tx = tx.sign([alice.private_key])
     b.store_bulk_transactions([tx])
diff --git a/tests/web/test_metadata.py b/tests/web/test_metadata.py
index aa0a2382..248ef946 100644
--- a/tests/web/test_metadata.py
+++ b/tests/web/test_metadata.py
@@ -23,11 +23,9 @@ def test_get_metadata_with_missing_text_search(client):
 
 @pytest.mark.bdb
 @pytest.mark.tendermint
-def test_get_metadata_tendermint(client, tb, alice):
+def test_get_metadata_tendermint(client, b, alice):
     from bigchaindb.models import Transaction
 
-    b = tb
-
     # test returns empty list when no assets are found
     res = client.get(METADATA_ENDPOINT + '?search=abc')
     assert res.json == []
@@ -53,11 +51,9 @@ def test_get_metadata_tendermint(client, tb, alice):
 
 @pytest.mark.bdb
 @pytest.mark.tendermint
-def test_get_metadata_limit_tendermint(client, tb, alice):
+def test_get_metadata_limit_tendermint(client, b, alice):
     from bigchaindb.models import Transaction
 
-    b = tb
-
     # create two assets
     asset1 = {'msg': 'abc 1'}
     meta1 = {'key': 'meta 1'}
diff --git a/tests/web/test_parameters.py b/tests/web/test_parameters.py
index ccf3119b..3eb8ab2a 100644
--- a/tests/web/test_parameters.py
+++ b/tests/web/test_parameters.py
@@ -4,6 +4,8 @@
 
 import pytest
 
+pytestmark = pytest.mark.tendermint
+
 
 def test_valid_txid():
     from bigchaindb.web.views.parameters import valid_txid
diff --git a/tests/web/test_server.py b/tests/web/test_server.py
index 215aa980..5fce3629 100644
--- a/tests/web/test_server.py
+++ b/tests/web/test_server.py
@@ -2,6 +2,10 @@
 # SPDX-License-Identifier: (Apache-2.0 AND CC-BY-4.0)
 # Code is Apache-2.0 and docs are CC-BY-4.0
 
+import pytest
+
+pytestmark = pytest.mark.tendermint
+
 
 def test_settings():
     import bigchaindb
diff --git a/tests/web/test_validators.py b/tests/web/test_validators.py
index d26d8cf1..714ceee4 100644
--- a/tests/web/test_validators.py
+++ b/tests/web/test_validators.py
@@ -9,7 +9,7 @@ pytestmark = pytest.mark.tendermint
 VALIDATORS_ENDPOINT = '/api/v1/validators/'
 
 
-def test_get_validators_endpoint(b, client, monkeypatch):
+def test_get_validators_endpoint(b, client):
     validator_set = [{'address': 'F5426F0980E36E03044F74DD414248D29ABCBDB2',
                       'pub_key': {'data': '4E2685D9016126864733225BE00F005515200727FBAB1312FC78C8B76831255A',
                                   'type': 'ed25519'},
diff --git a/tests/web/test_websocket_server.py b/tests/web/test_websocket_server.py
index 64309137..1a2b55af 100644
--- a/tests/web/test_websocket_server.py
+++ b/tests/web/test_websocket_server.py
@@ -136,7 +136,6 @@ def test_websocket_block_event(b, test_client, loop):
 
 
 @pytest.mark.skip('Processes are not stopping properly, and the whole test suite would hang')
-@pytest.mark.genesis
 def test_integration_from_webapi_to_websocket(monkeypatch, client, loop):
     # XXX: I think that the `pytest-aiohttp` plugin is sparkling too much
     # magic in the `asyncio` module: running this test without monkey-patching
@@ -152,6 +151,8 @@ def test_integration_from_webapi_to_websocket(monkeypatch, client, loop):
     import aiohttp
 
     from bigchaindb.common import crypto
+    # TODO processes does not exist anymore, when reactivating this test it
+    # will fail because of this
     from bigchaindb import processes
     from bigchaindb.models import Transaction