假如DataWriter先起来，并且已经写了一些数据，之后有新的DataReader起来，那么新起来的DataReader能不能接收到它启动之前，DataWriter发布的数据呢。DurabilityQosPolicy用来做这种控制。

VOLATILE_DURABILITY_QOS：易失的，新上线的DataReader只能读取上线之后，DataWriter发布的数据。

TRANSIENT_LOCAL_DURABILITY_QOS：会在内存中保存数据，新上线的DataReader也能读取上线之前的数据，前提是DataWriter还在，如果这个时候DataWriter已经退出了，那么DataReader就无法获取到。

/*** Enum DurabilityQosPolicyKind_t, different kinds of durability for DurabilityQosPolicy.*/
typedef enum DurabilityQosPolicyKind : fastdds::rtps::octet
{/*** The Service does not need to keep any samples of data-instances on behalf of any DataReader that is not* known by the DataWriter at the time the instance is written. In other words the Service will only attempt* to provide the data to existing subscribers*/VOLATILE_DURABILITY_QOS,/*** For TRANSIENT_LOCAL, the service is only required to keep the data in the memory of the DataWriter that* wrote the data and the data is not required to survive the DataWriter.*/TRANSIENT_LOCAL_DURABILITY_QOS,/*** For TRANSIENT, the service is only required to keep the data in memory and not in permanent storage; but* the data is not tied to the lifecycle of the DataWriter and will, in general, survive it.*/TRANSIENT_DURABILITY_QOS,/*** Data is kept on permanent storage, so that they can outlive a system session.** @warning Not Supported*/PERSISTENT_DURABILITY_QOS
} DurabilityQosPolicyKind_t;

1hello world

仍然以fastdds自带的hello world做为例子进行测试。

默认情况下，qos类型为VOLATILE_DURABILITY_QOS，即易失的。如果直接像下边这样进行修改，不生效的，因为DurabilityQosPolicy的使用要同时结合HistoryQosPolicy和ResourceLimitsQosPolicy以及ReliabilityQosPolicy进行统一配置。

• ReliabilityQosPolicyKind需要配置可靠类型
• HistoryQosPolicy配置KEEP_LAST_HISTORY_QOS，则depth需要大于0;或者配置为KEEP_ALL_HISTORY_QOS
• ResourceLimitsQosPolicy中ResourceLimitsQosPolicy不可小于HistoryQosPolicy中的depth

    DataWriterQos writer_qos = DATAWRITER_QOS_DEFAULT;
publisher_->get_default_datawriter_qos(writer_qos);

    auto max_samples = 500;
writer_qos.reliability().kind = ReliabilityQosPolicyKind::RELIABLE_RELIABILITY_QOS;
writer_qos.durability().kind = DurabilityQosPolicyKind::TRANSIENT_LOCAL_DURABILITY_QOS;
writer_qos.history().kind = HistoryQosPolicyKind::KEEP_LAST_HISTORY_QOS;
writer_qos.history().depth = max_samples;
writer_qos.resource_limits().ResourceLimitsQosPolicy= max_samples;
writer_qos.resource_limits().max_samples = writer_qos.resource_limits().max_instances * max_samples;
writer_ = publisher_->create_datawriter(topic_, writer_qos, this, StatusMask::all());