diff --git a/requirements-test.txt b/requirements-test.txt
--- a/requirements-test.txt
+++ b/requirements-test.txt
@@ -1,6 +1,7 @@
 Click
-pytest < 4
+pytest
 pytest-postgresql
 requests-mock
 hypothesis >= 3.11.0
 pre-commit
+pytz
diff --git a/swh/core/tests/test_logger.py b/swh/core/tests/test_logger.py
new file mode 100644
--- /dev/null
+++ b/swh/core/tests/test_logger.py
@@ -0,0 +1,120 @@
+# Copyright (C) 2019  The Software Heritage developers
+# See the AUTHORS file at the top-level directory of this distribution
+# License: GNU General Public License version 3, or any later version
+# See top-level LICENSE file for more information
+
+from datetime import datetime
+import logging
+import pytz
+import inspect
+
+from unittest.mock import patch
+
+from swh.core import logger
+
+
+def lineno():
+    """Returns the current line number in our program."""
+    return inspect.currentframe().f_back.f_lineno
+
+
+def test_db_level():
+    assert logger.db_level_of_py_level(10) == 'debug'
+    assert logger.db_level_of_py_level(20) == 'info'
+    assert logger.db_level_of_py_level(30) == 'warning'
+    assert logger.db_level_of_py_level(40) == 'error'
+    assert logger.db_level_of_py_level(50) == 'critical'
+
+
+def test_flatten_scalar():
+    assert list(logger.flatten('')) == [('', '')]
+    assert list(logger.flatten('toto')) == [('', 'toto')]
+
+    assert list(logger.flatten(10)) == [('', 10)]
+    assert list(logger.flatten(10.5)) == [('', 10.5)]
+
+
+def test_flatten_list():
+    assert list(logger.flatten([])) == []
+    assert list(logger.flatten([1])) == [('0', 1)]
+
+    assert list(logger.flatten([1, 2, ['a', 'b']])) == [
+        ('0', 1),
+        ('1', 2),
+        ('2_0', 'a'),
+        ('2_1', 'b'),
+    ]
+
+    assert list(logger.flatten([1, 2, ['a', ('x', 1)]])) == [
+        ('0', 1),
+        ('1', 2),
+        ('2_0', 'a'),
+        ('2_1_0', 'x'),
+        ('2_1_1', 1),
+    ]
+
+
+def test_flatten_dict():
+    assert list(logger.flatten({})) == []
+    assert list(logger.flatten({'a': 1})) == [('a', 1)]
+
+    assert sorted(logger.flatten({'a': 1,
+                                  'b': (2, 3,),
+                                  'c': {'d': 4, 'e': 'f'}})) == [
+        ('a', 1),
+        ('b_0', 2),
+        ('b_1', 3),
+        ('c_d', 4),
+        ('c_e', 'f'),
+    ]
+
+
+def test_stringify():
+    assert logger.stringify(None) == 'None'
+    assert logger.stringify(123) == '123'
+    assert logger.stringify('abc') == 'abc'
+
+    date = datetime(2019, 9, 1, 16, 32)
+    assert logger.stringify(date) == '2019-09-01T16:32:00'
+
+    tzdate = datetime(2019, 9, 1, 16, 32, tzinfo=pytz.utc)
+    assert logger.stringify(tzdate) == '2019-09-01T16:32:00+00:00'
+
+
+@patch('swh.core.logger.send')
+def test_journal_handler(send):
+    log = logging.getLogger('test_logger')
+    log.addHandler(logger.JournalHandler())
+    log.setLevel(logging.DEBUG)
+
+    _, ln = log.info('hello world'), lineno()
+
+    send.assert_called_with(
+        'hello world',
+        CODE_FILE=__file__,
+        CODE_FUNC='test_journal_handler',
+        CODE_LINE=ln,
+        LOGGER='test_logger',
+        PRIORITY='6',
+        THREAD_NAME='MainThread')
+
+
+@patch('swh.core.logger.send')
+def test_journal_handler_w_data(send):
+    log = logging.getLogger('test_logger')
+    log.addHandler(logger.JournalHandler())
+    log.setLevel(logging.DEBUG)
+
+    _, ln = log.debug('something cool %s', ['with', {'extra': 'data'}]), lineno()  # noqa
+
+    send.assert_called_with(
+        "something cool ['with', {'extra': 'data'}]",
+        CODE_FILE=__file__,
+        CODE_FUNC='test_journal_handler_w_data',
+        CODE_LINE=ln,
+        LOGGER='test_logger',
+        PRIORITY='7',
+        THREAD_NAME='MainThread',
+        SWH_LOGGING_ARGS_0_0='with',
+        SWH_LOGGING_ARGS_0_1_EXTRA='data'
+    )
diff --git a/swh/core/tests/test_tarball.py b/swh/core/tests/test_tarball.py
new file mode 100644
--- /dev/null
+++ b/swh/core/tests/test_tarball.py
@@ -0,0 +1,67 @@
+# Copyright (C) 2019  The Software Heritage developers
+# See the AUTHORS file at the top-level directory of this distribution
+# License: GNU General Public License version 3, or any later version
+# See top-level LICENSE file for more information
+
+from zipfile import ZipFile
+
+from swh.core import tarball
+
+
+def test_is_tarball(tmp_path):
+
+    nozip = tmp_path / 'nozip.zip'
+    nozip.write_text('Im no zip')
+
+    assert tarball.is_tarball(str(nozip)) is False
+
+    notar = tmp_path / 'notar.tar'
+    notar.write_text('Im no tar')
+
+    assert tarball.is_tarball(str(notar)) is False
+
+    zipfile = tmp_path / 'truezip.zip'
+    with ZipFile(str(zipfile), 'w') as myzip:
+        myzip.writestr('file1.txt', 'some content')
+
+    assert tarball.is_tarball(str(zipfile)) is True
+
+
+def test_compress_uncompress_zip(tmp_path):
+    tocompress = tmp_path / 'compressme'
+    tocompress.mkdir()
+
+    for i in range(10):
+        fpath = tocompress / ('file%s.txt' % i)
+        fpath.write_text('content of file %s' % i)
+
+    zipfile = tmp_path / 'archive.zip'
+    tarball.compress(str(zipfile), 'zip', str(tocompress))
+
+    assert tarball.is_tarball(str(zipfile))
+
+    destdir = tmp_path / 'destdir'
+    tarball.uncompress(str(zipfile), str(destdir))
+
+    lsdir = sorted(x.name for x in destdir.iterdir())
+    assert ['file%s.txt' % i for i in range(10)] == lsdir
+
+
+def test_compress_uncompress_tar(tmp_path):
+    tocompress = tmp_path / 'compressme'
+    tocompress.mkdir()
+
+    for i in range(10):
+        fpath = tocompress / ('file%s.txt' % i)
+        fpath.write_text('content of file %s' % i)
+
+    tarfile = tmp_path / 'archive.tar'
+    tarball.compress(str(tarfile), 'tar', str(tocompress))
+
+    assert tarball.is_tarball(str(tarfile))
+
+    destdir = tmp_path / 'destdir'
+    tarball.uncompress(str(tarfile), str(destdir))
+
+    lsdir = sorted(x.name for x in destdir.iterdir())
+    assert ['file%s.txt' % i for i in range(10)] == lsdir