sheetsync: Make the middleware interface explicit with a base class

The main purpose here is just documentation, so it's clear what the semantics of a middleware is.

sheetsync/sheetsync/middleware.py

@@ -0,0 +1,35 @@
+
+
+class Middleware:
+	"""A common interface for connecting sheetsync to a "sheet" source,
+	including specifics for a certain row type."""
+
+	def get_rows(self):
+		"""Fetch rows from the sheet, parsed into a list of dicts.
+		The returned dicts have the following guarenteed keys:
+			id: A unique identifier for the row
+			sheet_name: The worksheet associated with the row.
+				The concept of a worksheet is not common to all backends, but some identifying string
+				is still required.
+			_parse_errors: A list of error messages encountered when parsing, to be surfaced to the
+				user if possible.
+		In addition to the list of dicts, should return an "is_full" boolean which is True
+		if all rows were fetched or False if only some subset was fetched (eg. for quota management reasons).
+		Returns (is_full, rows).
+		"""
+		raise NotImplementedError
+
+	def write_value(self, row, key, value):
+		"""Write key=value to the given row. Takes the full row object so any identifying info
+		can be read from it as needed."""
+		raise NotImplementedError
+
+	def mark_modified(self, row):
+		"""Called if any sync action was performed due to this row.
+		Intended as a way to keep track of recently-changed rows for quota optimization."""
+		pass
+
+	def create_row(self, worksheet, id):
+		"""Create a new row with given id in the given worksheet and return it.
+		Only used for reverse sync."""
+		raise NotImplementedError

sheetsync/sheetsync/sheets.py

@@ -7,6 +7,8 @@ from monotonic import monotonic
 import common
 from common.googleapis import GoogleAPIClient
 
+from .middleware import Middleware
+
 
 class SheetsClient(object):
 	"""Manages Google Sheets API operations"""
@@ -70,7 +72,7 @@ class SheetsClient(object):
 		return ''.join(digits)
 
 
-class SheetsMiddleware():
+class SheetsMiddleware(Middleware):
 	# How many syncs of active sheets to do before checking inactive sheets.
 	# By checking inactive sheets less often, we stay within our API limits.
 	# For example, 4 syncs per inactive check * 5 seconds between syncs = 20s between inactive checks

sheetsync/sheetsync/streamlog.py

@@ -6,6 +6,9 @@ import requests
 
 from common.dateutil import parse_utc_only
 
+from .middleware import Middleware
+
+
 class StreamLogClient():
 	"""Client for Stream Log server"""
 
@@ -41,7 +44,7 @@ class StreamLogClient():
 		return None
 
 
-class StreamLogMiddleware:
+class StreamLogMiddleware(Middleware):
 	def __init__(self, client):
 		self.client = client
 		# Maps DB column names to streamlog fields.
@@ -121,9 +124,3 @@ class StreamLogMiddleware:
 		if key in self.column_encode:
 			value = self.column_encode[key](value)
 		self.client.write_value(row["id"], self.write_map[key], value)
-
-	def mark_modified(self, row):
-		pass # not a concept we have
-
-	def create_row(self, worksheet, id):
-		raise NotImplementedError