rdkafka 保存offset到本地文件

为了支持断点续传功能，需要将offset保存在一个地方，下次从这个offset开始。librdkafka提供了本地文件保存的方式。

下面的代码演示了

1. 要用topic config对象设置 offset.store.path和offset.store.method

2. start函数接受参数OFFSET_STORED

		std::unique_ptr<RdKafka::Consumer> consumer(RdKafka::Consumer::create(global_conf_.get(), err_));
		if (!consumer) {
			throw KafkaError("Failed to create consumer");
		}

		stringstream stream;
		stream << topic_name_ << "-" << partition_idx << ".txt";
		string file_path = stream.str();
		string lasterr;
		topic_conf_->set("offset.store.path", file_path, lasterr);
		topic_conf_->set("offset.store.method", "file", lasterr);
		std::unique_ptr<RdKafka::Topic> topic(RdKafka::Topic::create(consumer.get(), topic_name_, topic_conf_.get(), err_));

		//		RdKafka::ErrorCode resp = consumer->start(topic.get(), partition_idx, RdKafka::Topic::OFFSET_BEGINNING);
		RdKafka::ErrorCode resp = consumer->start(topic.get(), partition_idx, RdKafka::Topic::OFFSET_STORED);
		if (resp != RdKafka::ERR_NO_ERROR) {
			throw KafkaError(RdKafka::err2str(resp));
		}

注意：

1. 当前进程目录下会出现$topic-$partiton-idx.txt文件

2. 运行后正常读取数据后需要过一会儿才会写入数值

这个写入时间可以设置:offset.store.sync.interval.ms

3. 程序启动时，可以先手动将offset写入文件，然后再启动

详细配置参考github

https://github.com/edenhill/librdkafka/blob/master/CONFIGURATION.md