Described are techniques used in connection with subscribe and alert services as may be embodied in a subscription system. The subscription system uses a preprocessor for preprocessing subscription and notification messages prior to submission to the subscription/publication server. The preprocessor outputs a preprocessed message including one or more identifiers categorizing an input subscription or notification message. The preprocessor uses a content graph to produce the preprocessed message. The subscription system uses a content based routing system with a tree-like topology to process preprocessed subscriptions and notifications using content graphs maintained at each node in the system. Also described is a loss detection technique using sequence numbers.