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conn = StrictRedis()

def online_size(sender, tree_name=None, edge=None):
    conn.incr('{0}.size'.format(tree_name))

def online_depth(sender, tree_name=None, edge=None):
    with conn.lock('{0}.depth'.format(tree_name)):
        current_depth = conn.get('{0}.depth'.format(tree_name)) or 0
        if int(current_depth) < edge.generation:
            conn.set('{0}.depth'.format(tree_name), edge.generation)