diff --git a/swh/indexer/metadata.py b/swh/indexer/metadata.py
--- a/swh/indexer/metadata.py
+++ b/swh/indexer/metadata.py
@@ -43,7 +43,7 @@
             } for sha1 in ids
         ))
 
-    def index(self, id, data):
+    def index(self, id, data, log_suffix='unknown revision'):
         """Index sha1s' content and store result.
 
         Args:
@@ -63,8 +63,9 @@
         }
         try:
             mapping_name = self.tool['tool_configuration']['context']
-            result['translated_metadata'] = MAPPINGS[mapping_name] \
-                .translate(data)
+            log_suffix += ', content_id=%s' % hashutil.hash_to_hex(id)
+            result['translated_metadata'] = \
+                MAPPINGS[mapping_name](log_suffix).translate(data)
         except Exception:
             self.log.exception(
                 "Problem during metadata translation "
@@ -111,7 +112,7 @@
             'version': '0.0.2',
             'configuration': {
                 'type': 'local',
-                'context': ['NpmMapping', 'CodemetaMapping']
+                'context': list(MAPPINGS),
             },
         }),
     }
@@ -158,7 +159,9 @@
             files = [entry for entry in dir_ls if entry['type'] == 'file']
             detected_files = detect_metadata(files)
             result['translated_metadata'] = self.translate_revision_metadata(
-                    detected_files)
+                detected_files,
+                log_suffix='revision=%s' % hashutil.hash_to_hex(rev['id'])
+            )
         except Exception as e:
             self.log.exception(
                 'Problem when indexing rev: %r', e)
@@ -181,7 +184,7 @@
         self.idx_storage.revision_metadata_add(
             results, conflict_update=(policy_update == 'update-dups'))
 
-    def translate_revision_metadata(self, detected_files):
+    def translate_revision_metadata(self, detected_files, log_suffix):
         """
         Determine plan of action to translate metadata when containing
         one or multiple detected files:
@@ -236,7 +239,8 @@
                 # content indexing
                 try:
                     c_metadata_indexer.run(sha1s_filtered,
-                                           policy_update='ignore-dups')
+                                           policy_update='ignore-dups',
+                                           log_suffix=log_suffix)
                     # on the fly possibility:
                     for result in c_metadata_indexer.results:
                         local_metadata = result['translated_metadata']
diff --git a/swh/indexer/metadata_dictionary.py b/swh/indexer/metadata_dictionary.py
--- a/swh/indexer/metadata_dictionary.py
+++ b/swh/indexer/metadata_dictionary.py
@@ -23,7 +23,7 @@
 
 
 def register_mapping(cls):
-    MAPPINGS[cls.__name__] = cls()
+    MAPPINGS[cls.__name__] = cls
     return cls
 
 
@@ -69,13 +69,15 @@
     - inherit this class
     - override translate function
     """
-    def __init__(self):
+    def __init__(self, log_suffix=''):
+        self.log_suffix = log_suffix
         self.log = logging.getLogger('%s.%s' % (
             self.__class__.__module__,
             self.__class__.__name__))
 
+    @classmethod
     @abc.abstractmethod
-    def detect_metadata_files(self, files):
+    def detect_metadata_files(cls, files):
         """
         Detects files potentially containing metadata
 
@@ -104,9 +106,10 @@
         """The .json file to extract metadata from."""
         pass
 
-    def detect_metadata_files(self, file_entries):
+    @classmethod
+    def detect_metadata_files(cls, file_entries):
         for entry in file_entries:
-            if entry['name'] == self.filename:
+            if entry['name'] == cls.filename:
                 return [entry['sha1']]
         return []
 
@@ -184,12 +187,12 @@
         try:
             raw_content = raw_content.decode()
         except UnicodeDecodeError:
-            self.log.warning('Error unidecoding %r', raw_content)
+            self.log.warning('Error unidecoding from %s', self.log_suffix)
             return
         try:
             content_dict = json.loads(raw_content)
         except json.JSONDecodeError:
-            self.log.warning('Error unjsoning %r' % raw_content)
+            self.log.warning('Error unjsoning from %s', self.log_suffix)
             return
         return self.translate_dict(content_dict)
 
@@ -355,7 +358,7 @@
         try:
             d = xmltodict.parse(content).get('project') or {}
         except xml.parsers.expat.ExpatError:
-            self.log.warning('Error parsing XML of %r', content)
+            self.log.warning('Error parsing XML from %s', self.log_suffix)
             return None
         metadata = self.translate_dict(d, normalize=False)
         metadata[SCHEMA_URI+'codeRepository'] = self.parse_repositories(d)
@@ -512,7 +515,8 @@
 
     mapping = CROSSWALK_TABLE['Ruby Gem']
 
-    def detect_metadata_files(self, file_entries):
+    @classmethod
+    def detect_metadata_files(cls, file_entries):
         for entry in file_entries:
             if entry['name'].endswith(b'.gemspec'):
                 return [entry['sha1']]
@@ -522,7 +526,7 @@
         try:
             raw_content = raw_content.decode()
         except UnicodeDecodeError:
-            self.log.warning('Error unidecoding %r', raw_content)
+            self.log.warning('Error unidecoding from %s', self.log_suffix)
             return
 
         # Skip lines before 'Gem::Specification.new'
@@ -533,8 +537,8 @@
         try:
             next(lines)  # Consume 'Gem::Specification.new'
         except StopIteration:
-            self.log.warning('Could not find Gem::Specification in %r',
-                             raw_content)
+            self.log.warning('Could not find Gem::Specification in %s',
+                             self.log_suffix)
             return
 
         content_dict = {}
@@ -607,19 +611,3 @@
         if isinstance(authors, list):
             return {"@list": [author for author in authors
                               if isinstance(author, str)]}
-
-
-def main():
-    raw_content = """{"name": "test_name", "unknown_term": "ut"}"""
-    raw_content1 = b"""{"name": "test_name",
-                        "unknown_term": "ut",
-                        "prerequisites" :"packageXYZ"}"""
-    result = MAPPINGS["NpmMapping"].translate(raw_content)
-    result1 = MAPPINGS["MavenMapping"].translate(raw_content1)
-
-    print(result)
-    print(result1)
-
-
-if __name__ == "__main__":
-    main()
diff --git a/swh/indexer/tests/test_metadata.py b/swh/indexer/tests/test_metadata.py
--- a/swh/indexer/tests/test_metadata.py
+++ b/swh/indexer/tests/test_metadata.py
@@ -137,7 +137,7 @@
         # None if no metadata was found or an error occurred
         declared_metadata = None
         # when
-        result = MAPPINGS["NpmMapping"].translate(content)
+        result = MAPPINGS["NpmMapping"]().translate(content)
         # then
         self.assertEqual(declared_metadata, result)
 
@@ -177,7 +177,7 @@
         }
 
         # when
-        result = MAPPINGS["NpmMapping"].translate(content)
+        result = MAPPINGS["NpmMapping"]().translate(content)
         # then
         self.assertEqual(declared_metadata, result)
 
@@ -302,7 +302,7 @@
                 "email": "foo@example.com"
             }
         }"""
-        result = MAPPINGS["NpmMapping"].translate(package_json)
+        result = MAPPINGS["NpmMapping"]().translate(package_json)
         self.assertEqual(result, {
             '@context': 'https://doi.org/10.5063/schema/codemeta-2.0',
             'name': 'foo',
@@ -317,7 +317,7 @@
                 "email": "foo@example.com"
             }
         }"""
-        result = MAPPINGS["NpmMapping"].translate(package_json)
+        result = MAPPINGS["NpmMapping"]().translate(package_json)
         self.assertEqual(result, {
             '@context': 'https://doi.org/10.5063/schema/codemeta-2.0',
             'name': 'foo',
@@ -329,7 +329,7 @@
             "name": "foo",
             "bugs": "https://github.com/owner/project/issues"
         }"""
-        result = MAPPINGS["NpmMapping"].translate(package_json)
+        result = MAPPINGS["NpmMapping"]().translate(package_json)
         self.assertEqual(result, {
             '@context': 'https://doi.org/10.5063/schema/codemeta-2.0',
             'name': 'foo',
@@ -346,7 +346,7 @@
                 "url" : "https://github.com/npm/cli.git"
             }
         }"""
-        result = MAPPINGS["NpmMapping"].translate(package_json)
+        result = MAPPINGS["NpmMapping"]().translate(package_json)
         self.assertEqual(result, {
             '@context': 'https://doi.org/10.5063/schema/codemeta-2.0',
             'name': 'foo',
@@ -361,7 +361,7 @@
                 "type" : "git"
             }
         }"""
-        result = MAPPINGS["NpmMapping"].translate(package_json)
+        result = MAPPINGS["NpmMapping"]().translate(package_json)
         self.assertEqual(result, {
             '@context': 'https://doi.org/10.5063/schema/codemeta-2.0',
             'name': 'foo',
@@ -373,7 +373,7 @@
             "name": "foo",
             "repository": "github:npm/cli"
         }"""
-        result = MAPPINGS["NpmMapping"].translate(package_json)
+        result = MAPPINGS["NpmMapping"]().translate(package_json)
         expected_result = {
             '@context': 'https://doi.org/10.5063/schema/codemeta-2.0',
             'name': 'foo',
@@ -387,7 +387,7 @@
             "name": "foo",
             "repository": "npm/cli"
         }"""
-        result = MAPPINGS["NpmMapping"].translate(package_json)
+        result = MAPPINGS["NpmMapping"]().translate(package_json)
         self.assertEqual(result, expected_result)
 
         # gitlab shortcut
@@ -395,7 +395,7 @@
             "name": "foo",
             "repository": "gitlab:user/repo"
         }"""
-        result = MAPPINGS["NpmMapping"].translate(package_json)
+        result = MAPPINGS["NpmMapping"]().translate(package_json)
         self.assertEqual(result, {
             '@context': 'https://doi.org/10.5063/schema/codemeta-2.0',
             'name': 'foo',
@@ -549,7 +549,7 @@
             "datePublished": "2017-06-05",
             "programmingLanguage": "JSON-LD"
           }
-        result = MAPPINGS["CodemetaMapping"].translate(raw_content)
+        result = MAPPINGS["CodemetaMapping"]().translate(raw_content)
         self.assertEqual(result, expected_result)
 
     def test_compute_metadata_maven(self):
@@ -580,7 +580,7 @@
             </license>
           </licenses>
         </project>"""
-        result = MAPPINGS["MavenMapping"].translate(raw_content)
+        result = MAPPINGS["MavenMapping"]().translate(raw_content)
         self.assertEqual(result, {
             '@context': 'https://doi.org/10.5063/schema/codemeta-2.0',
             'type': 'SoftwareSourceCode',
@@ -596,7 +596,7 @@
         raw_content = b"""
         <project>
         </project>"""
-        result = MAPPINGS["MavenMapping"].translate(raw_content)
+        result = MAPPINGS["MavenMapping"]().translate(raw_content)
         self.assertEqual(result, {
             '@context': 'https://doi.org/10.5063/schema/codemeta-2.0',
             'type': 'SoftwareSourceCode',
@@ -607,20 +607,29 @@
         <project>
           <foo/>
         </project>"""
-        result = MAPPINGS["MavenMapping"].translate(raw_content)
+        result = MAPPINGS["MavenMapping"]().translate(raw_content)
         self.assertEqual(result, {
             '@context': 'https://doi.org/10.5063/schema/codemeta-2.0',
             'type': 'SoftwareSourceCode',
         })
 
     def test_compute_metadata_maven_invalid_xml(self):
+        expected_warning = (
+            'WARNING:swh.indexer.metadata_dictionary.MavenMapping:'
+            'Error parsing XML from foo')
         raw_content = b"""
         <project>"""
-        result = MAPPINGS["MavenMapping"].translate(raw_content)
+        with self.assertLogs('swh.indexer.metadata_dictionary',
+                             level='WARNING') as cm:
+            result = MAPPINGS["MavenMapping"]('foo').translate(raw_content)
+            self.assertEqual(cm.output, [expected_warning])
         self.assertEqual(result, None)
         raw_content = b"""
         """
-        result = MAPPINGS["MavenMapping"].translate(raw_content)
+        with self.assertLogs('swh.indexer.metadata_dictionary',
+                             level='WARNING') as cm:
+            result = MAPPINGS["MavenMapping"]('foo').translate(raw_content)
+            self.assertEqual(cm.output, [expected_warning])
         self.assertEqual(result, None)
 
     def test_compute_metadata_maven_minimal(self):
@@ -632,7 +641,7 @@
           <artifactId>my-app</artifactId>
           <version>1.2.3</version>
         </project>"""
-        result = MAPPINGS["MavenMapping"].translate(raw_content)
+        result = MAPPINGS["MavenMapping"]().translate(raw_content)
         self.assertEqual(result, {
             '@context': 'https://doi.org/10.5063/schema/codemeta-2.0',
             'type': 'SoftwareSourceCode',
@@ -682,7 +691,7 @@
             </license>
           </licenses>
         </project>"""
-        result = MAPPINGS["MavenMapping"].translate(raw_content)
+        result = MAPPINGS["MavenMapping"]().translate(raw_content)
         self.assertEqual(result, {
             '@context': 'https://doi.org/10.5063/schema/codemeta-2.0',
             'type': 'SoftwareSourceCode',
@@ -730,7 +739,7 @@
 Description-Content-Type: text/markdown
 Provides-Extra: testing
 """) # noqa
-        result = MAPPINGS["PythonPkginfoMapping"].translate(raw_content)
+        result = MAPPINGS["PythonPkginfoMapping"]().translate(raw_content)
         self.assertCountEqual(result['description'], [
             'Software Heritage core utilities',  # note the comma here
             'swh-core\n'
@@ -763,7 +772,7 @@
 Name: foo
 License: MIT
 """) # noqa
-        result = MAPPINGS["PythonPkginfoMapping"].translate(raw_content)
+        result = MAPPINGS["PythonPkginfoMapping"]().translate(raw_content)
         self.assertEqual(result, {
             '@context': 'https://doi.org/10.5063/schema/codemeta-2.0',
             'type': 'SoftwareSourceCode',
@@ -785,7 +794,7 @@
   s.homepage    = 'https://rubygems.org/gems/example'
   s.metadata    = { "source_code_uri" => "https://github.com/example/example" }
 end"""
-        result = MAPPINGS['GemspecMapping'].translate(raw_content)
+        result = MAPPINGS['GemspecMapping']().translate(raw_content)
         self.assertCountEqual(result.pop('description'), [
             "This is an example!",
             "Much longer explanation of the example!"
@@ -807,7 +816,7 @@
   s.authors     = ["Ruby Coder1"]
   s.author      = "Ruby Coder2"
 end"""
-        result = MAPPINGS['GemspecMapping'].translate(raw_content)
+        result = MAPPINGS['GemspecMapping']().translate(raw_content)
         self.assertCountEqual(result.pop('author'), [
             'Ruby Coder1', 'Ruby Coder2'])
         self.assertEqual(result, {
@@ -820,7 +829,7 @@
 Gem::Specification.new do |s|
   s.author      = ["Ruby Coder"]
 end"""
-        result = MAPPINGS['GemspecMapping'].translate(raw_content)
+        result = MAPPINGS['GemspecMapping']().translate(raw_content)
         self.assertEqual(result, {
             '@context': 'https://doi.org/10.5063/schema/codemeta-2.0',
             'type': 'SoftwareSourceCode',
@@ -829,7 +838,7 @@
 Gem::Specification.new do |s|
   s.author      = "Ruby Coder1",
 end"""
-        result = MAPPINGS['GemspecMapping'].translate(raw_content)
+        result = MAPPINGS['GemspecMapping']().translate(raw_content)
         self.assertEqual(result, {
             '@context': 'https://doi.org/10.5063/schema/codemeta-2.0',
             'type': 'SoftwareSourceCode',
@@ -838,7 +847,7 @@
 Gem::Specification.new do |s|
   s.authors     = ["Ruby Coder1", ["Ruby Coder2"]]
 end"""
-        result = MAPPINGS['GemspecMapping'].translate(raw_content)
+        result = MAPPINGS['GemspecMapping']().translate(raw_content)
         self.assertEqual(result, {
             '@context': 'https://doi.org/10.5063/schema/codemeta-2.0',
             'type': 'SoftwareSourceCode',