How to handle Prometheus metrics in class extending netty's ChannelInboundHandlerAdapter in exceptionCaught() method?

We have a metered method legaced from ChannelInboundHandlerAdapter

public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
...
//where we do our metering part here:
            meterRegistry.timer(
                    "processing.handler.command.duration",
                    "command_name", commandName,
                    "success", Boolean.toString(true)
            ).record(duration, java.util.concurrent.TimeUnit.MILLISECONDS);

...

}

however, if there is an exception before this metering has happened, the

    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
...
}

is thrown and we can't pass the same values we have initialized in channelRead to put metrics on the same page with attributes channel and duration, but just mark them with homegrown attribute success=false.

How to pass the values between channelRead and exceptionCaught?

Solution

It is possible to pass the attributes to the channel, String or Integer, they can be used for that.

Static initialization of the class:

private static final AttributeKey<Long> START_TIME_KEY = AttributeKey.valueOf("startTime");
private static final AttributeKey<String> COMMAND_NAME_KEY = AttributeKey.valueOf("commandName");

In channelRead():

    ctx.channel().attr(START_TIME_KEY).set(System.currentTimeMillis());
    String commandName = "unknown";
    ctx.channel().attr(COMMAND_NAME_KEY).set(commandName);

In exceptionCaught():

    long duration = System.currentTimeMillis() -  ctx.channel().attr(START_TIME_KEY).get();
    meterRegistry.timer(
            "processing.handler.command.duration",
            "command_name",ctx.channel().attr(COMMAND_NAME_KEY).get(),
            "success", Boolean.toString(false)
    ).record(duration, java.util.concurrent.TimeUnit.MILLISECONDS);