DDS QoS - HISTORY

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18. HISTORY

 

OpenDDS Developer's Guide（OpenDDS Version 3.14）

The HISTORY policy determines how samples are held in the data writer and data reader for a particular instance. For data writers these values are held until the publisher retrieves them and successfully sends them to all connected subscribers. For data readers these values are held until “taken” by the application. This policy applies to the topic, data reader, and data writer entities via the history member of their respective QoS structures. Below is the IDL related to the history QoS policy:

历史记录策略确定特定实例在数据写入器和数据读取器中如何保存样本。 对于数据编写者，这些值将一直保留到发布者检索到它们并将它们成功发送给所有连接的订户为止。 对于数据读取器，这些值将保留到应用程序“获取”为止。 此策略通过各自的QoS结构的历史记录成员适用于主题、数据读取器和数据写入器实体。 以下是与历史QoS策略相关的IDL：

enum HistoryQosPolicyKind {

KEEP_LAST_HISTORY_QOS,

KEEP_ALL_HISTORY_QOS

};

struct HistoryQosPolicy {

HistoryQosPolicyKind kind;

long depth;

};

The “keep all” value ( KEEP_ALL_HISTORY_QOS ) specifies that all possible samples for that instance should be kept. When “keep all” is specified and the number of unread samples is equal to the “resource limits” field of max_samples_per_instance then any incoming samples are rejected.

“全部保留”值（KEEP_ALL_HISTORY_QOS）指定应保留该实例的所有可能样本。 当指定“全部保留”并且未读样本的数量等于max_samples_per_instance的“资源限制”字段时，则拒绝任何传入的样本。

The “keep last” value ( KEEP_LAST_HISTORY_QOS ) specifies that only the last depth values should be kept. When a data writer contains depth samples of a given instance, a write of new samples for that instance are queued for delivery and the oldest unsent samples are discarded. When a data reader contains depth samples of a given instance, any incoming samples for that instance are kept and the oldest samples are discarded.

“保留最后一个”值（KEEP_LAST_HISTORY_QOS）指定仅保留最后一个深度值。 当数据写入器包含给定实例的深度样本时，该实例的新样本写入将排队等待交付，而最旧的未发送样本将被丢弃。 当数据读取器包含给定实例的深度样本时，该实例的所有传入样本都会保留，最旧的样本将被丢弃。

This policy defaults to a “keep last” with a depth of one.

此策略默认为深度为1的“保留最后”。

 

Data Distribution Service Version 1.4（ formal/2015-04-10）

 

1) This policy controls the behavior of the Service when the value of an instance changes before it is finally communicated to some of its existing DataReader entities.

1）当实例的值在最终与它的某些现有DataReader实体通信之前发生更改时，此策略控制Service的行为。

2) If the kind is set to KEEP_LAST, then the Service will only attempt to keep the latest values of the instance and discard the older ones. In this case, the value of depth regulates the maximum number of values (up to and including the most current one) the Service will maintain and deliver. The default (and most common setting) for depth is one, indicating that only the most recent value should be delivered.

2）如果将种类设置为KEEP_LAST，则服务将仅尝试保留实例的最新值并丢弃较旧的值。 在这种情况下，深度值可调节服务将维持和提供的最大值数（包括最新值）。 深度的默认值（也是最常用的设置）是1，表示仅应传递最新的值。

3) If the kind is set to KEEP_ALL, then the Service will attempt to maintain and deliver all the values of the instance to existing subscribers. The resources that the Service can use to keep this history are limited by the settings of the RESOURCE_LIMITS QoS. If the limit is reached, then the behavior of the Service will depend on the RELIABILITY QoS. If the reliability kind is BEST_EFFORT, then the old values will be discarded. If reliability is RELIABLE, then the Service will block the DataWriter until it can deliver the necessary old values to all subscribers.

3）如果将种类设置为KEEP_ALL，则服务将尝试维护实例的所有值并将其传递给现有订户。 服务可用于保留此历史记录的资源受RESOURCE_LIMITS QoS的设置限制。 如果达到限制，则服务的行为将取决于可靠性QoS。 如果可靠性类型为BEST_EFFORT，则旧值将被丢弃。 如果可靠性是可靠的，则服务将阻止DataWriter，直到它可以将必要的旧值传递给所有订户。

The setting of HISTORY depth must be consistent with the RESOURCE_LIMITS max_samples_per_instance . For these two QoS to be consistent, they must verify that depth <= max_samples_per_instance .

历史记录深度的设置必须与RESOURCE_LIMITS max_samples_per_instance相一致。 为了使这两个QoS保持一致，它们必须验证 depth<= max_samples_per_instance。