private Deque<PooledChannel> getOrCreatePool(SessionProtocol protocol, PoolKey key) {
        return pool[protocol.ordinal()].computeIfAbsent(key, k -> new ArrayDeque<>());
    }
Returns an array whose index signifies {@link SessionProtocol#ordinal()}. Similar to {@link EnumMap}.
core/src/main/java/com/linecorp/armeria/client/HttpChannelPool.java