Node-REDのsplitノードとjoinノード(split-join)は簡単に非同期処理を実行できて便利です。しかし、msgオブジェクトに大量データを載せているとmsg.payloadの配列要素数だけmsgオブジェクトのクローン（ディープコピー）が作成されるので、リソースを大量に消費してしまいます。
split-joinを使わずないで順次処理にする方法もありますが、それでは処理効率が落ちてしまいますので、多段でsplit-joinを使う方法を紹介します。
やりたいこと（順次処理の例）
以下のような順次処理を考えてみましょう。
set dataでは以下のような処理をします。
msg.original = {a:1, bigData:[1,2,3]}
msg.payload = msg.original.bigData;
return msg;
元データをmsg.originalに残しておいて、msg.payloadのデータを処理します。しかしこれはうまく動きません。
ディープコピーされていないので、msg.originalも更新されてしまいました。
非同期処理の例
順次処理でもmsg.original.bigDataをディープコピーすれば良いのですが、ここではsplit-joinを使います。
これはうまく動作します。
splitノードではpayloadの配列を分解して複数のmsgオブジェクトを作ります。その際にmsgオブジェクトをディープコピーしてクローンを作るからです。
この例のようなデータ量であれば問題にならないのですが、処理すべきデータが何千何万と大量の場合は、msg.originalにあるデータが何千倍何万倍に増えますから、メモリを大量消費するのはもちろん、オブジェクト生成やガベージコレクションでCPUも消費します。この例の場合はmsg.originalをあきらめればよいのですが、後続処理で使う場合はそうもいきません。
対策
そこでmsg.originalを含むmsgのクローンを別経路で渡す方法を考えました。
clone & deleteでは受け取ったメッセージのクローンを作成した後、各データの処理に不要でリソースを消費してしまうmsg.originalを消します。
そして元のmsg.payloadと作成したクローンを配列にしてmsg.payloadにセットします。
let tmp = RED.util.cloneMessage(msg);
msg.payload = [msg.payload, tmp]
delete msg.original;
return msg;
splitではmsgオブジェクトがクローンされますが、すでにmsg.originalはdeleteされているのでリソースを消費しません。
処理の後はmake answerでmsg.originalのあるオブジェクトを元に処理結果を統合してmsgオブジェクトとします。
let newMsg = msg.payload.pop();
newMsg.payload = msg.payload[0];
return newMsg;
いかがでしょう？
split-joinのリソース消費対策としてsplit-joinを使うというお話でした。
今回のソース
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