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