Add spanner stale data sample (#1107)

Author: Jon Wayne Parrott

spanner/cloud-client/snippets.py

@@ -119,6 +119,27 @@ def read_data(instance_id, database_id):
             print(u'SingerId: {}, AlbumId: {}, AlbumTitle: {}'.format(*row))
 
 
+def read_stale_data(instance_id, database_id):
+    """Reads sample data from the database. The data is exactly 10 seconds
+    stale."""
+    import datetime
+
+    spanner_client = spanner.Client()
+    instance = spanner_client.instance(instance_id)
+    database = instance.database(database_id)
+    staleness = datetime.timedelta(seconds=10)
+
+    with database.snapshot(exact_staleness=staleness) as snapshot:
+        keyset = spanner.KeySet(all_=True)
+        results = snapshot.read(
+            table='Albums',
+            columns=('SingerId', 'AlbumId', 'AlbumTitle',),
+            keyset=keyset)
+
+        for row in results:
+            print(u'SingerId: {}, AlbumId: {}, AlbumTitle: {}'.format(*row))
+
+
 def query_data_with_new_column(instance_id, database_id):
     """Queries sample data from the database using SQL.
 
@@ -424,6 +445,7 @@ def read_only_transaction(instance_id, database_id):
     subparsers.add_parser('insert_data', help=insert_data.__doc__)
     subparsers.add_parser('query_data', help=query_data.__doc__)
     subparsers.add_parser('read_data', help=read_data.__doc__)
+    subparsers.add_parser('read_stale_data', help=read_stale_data.__doc__)
     subparsers.add_parser('add_column', help=add_column.__doc__)
     subparsers.add_parser('update_data', help=update_data.__doc__)
     subparsers.add_parser(
@@ -454,6 +476,8 @@ def read_only_transaction(instance_id, database_id):
         query_data(args.instance_id, args.database_id)
     elif args.command == 'read_data':
         read_data(args.instance_id, args.database_id)
+    elif args.command == 'read_stale_data':
+        read_stale_data(args.instance_id, args.database_id)
     elif args.command == 'add_column':
         add_column(args.instance_id, args.database_id)
     elif args.command == 'update_data':

spanner/cloud-client/snippets_test.py

@@ -73,6 +73,17 @@ def test_read_data(temporary_database, capsys):
     assert 'Total Junk' in out
 
 
+def test_read_stale_data(temporary_database, capsys):
+    snippets.read_stale_data(SPANNER_INSTANCE, temporary_database.database_id)
+
+    out, _ = capsys.readouterr()
+
+    # It shouldn't be in the output because it was *just* inserted by the
+    # temporary database fixture and this sample reads 10 seconds into the
+    # past.
+    assert 'Total Junk' not in out
+
+
 @pytest.fixture(scope='module')
 def temporary_database_with_column(temporary_database):
     snippets.add_column(SPANNER_INSTANCE, temporary_database.database_id)