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Useful to ensure\n+    only a few thing happen at a time on a given resource.\n+\n+    Example:\n+\n+        with (yield limiter.queue(\"test_key\")):\n+            # do some work.\n+\n+    \"\"\"\n+    def __init__(self, max_count):\n+        \"\"\"\n+        Args:\n+            max_count(int): The maximum number of concurrent access\n+        \"\"\"\n+        self.max_count = max_count\n+        self.key_to_defer = {}\n+\n+    @defer.inlineCallbacks\n+    def queue(self, key):\n+        entry = self.key_to_defer.setdefault(key, [0, []])\n+\n+        if entry[0] >= self.max_count:\n+            new_defer = defer.Deferred()\n+            entry[1].append(new_defer)\n+            with PreserveLoggingContext():\n+                yield new_defer\n+\n+        entry[0] += 1\n+\n+        @contextmanager\n+        def _ctx_manager():\n+            try:\n+                yield\n+            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