Sync: chunk user requests

The site can't handle huge syncs. Even a bulk patch of 10k users will
crash the service. Chunk the requests into groups of 1000 users and
await them sequentially. Testing showed that concurrent requests
are not scalable and would also crash the service.

1 files changed, 8 insertions, 2 deletions

diff --git a/bot/exts/backend/sync/_syncers.py b/bot/exts/backend/sync/_syncers.py
index 2eb9f9971..c9f2d2da8 100644
--- a/bot/exts/backend/sync/_syncers.py
+++ b/bot/exts/backend/sync/_syncers.py
@@ -5,12 +5,15 @@ from collections import namedtuple
 
 from discord import Guild
 from discord.ext.commands import Context
+from more_itertools import chunked
 
 import bot
 from bot.api import ResponseCodeError
 
 log = logging.getLogger(__name__)
 
+CHUNK_SIZE = 1000
+
 # These objects are declared as namedtuples because tuples are hashable,
 # something that we make use of when diffing site roles against guild roles.
 _Role = namedtuple('Role', ('id', 'name', 'colour', 'permissions', 'position'))
@@ -207,10 +210,13 @@ class UserSyncer(Syncer):
     @staticmethod
     async def _sync(diff: _Diff) -> None:
         """Synchronise the database with the user cache of `guild`."""
+        # Using asyncio.gather would still consume too many resources on the site.
         log.trace("Syncing created users...")
         if diff.created:
-            await bot.instance.api_client.post("bot/users", json=diff.created)
+            for chunk in chunked(diff.created, CHUNK_SIZE):
+                await bot.instance.api_client.post("bot/users", json=chunk)
 
         log.trace("Syncing updated users...")
         if diff.updated:
-            await bot.instance.api_client.patch("bot/users/bulk_patch", json=diff.updated)
+            for chunk in chunked(diff.updated, CHUNK_SIZE):
+                await bot.instance.api_client.patch("bot/users/bulk_patch", json=chunk)