private function depthData(string $symbol, array $json)
    {
        $bids = $asks = [];
        foreach ($json['bids'] as $obj) {
            $bids[$obj[0]] = $obj[1];
        }
        foreach ($json['asks'] as $obj) {
            $asks[$obj[0]] = $obj[1];
        }
        return $this->depthCache[$symbol] = [
            "bids" => $bids,
            "asks" => $asks,
        ];
    }