Metadata Store

许多外部系统、服务或资源不是事务性的（Twitter、RSS、文件系统等），并且没有任何将数据标记为已读取的能力。此外，有时您可能需要在一些集成解决方案中实现 Enterprise Integration Pattern idempotent receiver。为了实现这一目标并存储端点与外部系统的下一次交互之前的某些先前状态或处理下一个消息，Spring Integration 提供元数据存储组件作为 org.springframework.integration.metadata.MetadataStore 接口的实现，并具有常规键值契约。

Many external systems, services, or resources are not transactional (Twitter, RSS, file systems, and so on), and there is not any ability to mark the data as read. Also, sometimes, you may need to implement the Enterprise Integration Pattern idempotent receiver in some integration solutions. To achieve this goal and store some previous state of the endpoint before the next interaction with external system or to deal with the next message, Spring Integration provides the metadata store component as an an implementation of the org.springframework.integration.metadata.MetadataStore interface with a general key-value contract.

元数据存储旨在存储各种类型的一般元数据（例如，已处理的最后一个提要条目的已发布日期），以帮助提要适配器之类的组件处理重复项。如果未直接向组件提供对 MetadataStore 的引用，则定位元数据存储的算法如下：首先，在应用程序上下文中查找具有 metadataStore ID 的 bean。如果找到一个，就使用它。否则，创建一个 SimpleMetadataStore 的新实例，这是一个内存实现，仅在当前运行的应用程序上下文的生命周期内保留元数据。这意味着，在重新启动时，你最终可能会得到重复的条目。

The metadata store is designed to store various types of generic metadata (for example, the published date of the last feed entry that has been processed) to help components such as the feed adapter deal with duplicates. If a component is not directly provided with a reference to a MetadataStore, the algorithm for locating a metadata store is as follows: First, look for a bean with a metadataStore ID in the application context. If one is found, use it. Otherwise, create a new instance of SimpleMetadataStore, which is an in-memory implementation that persists only metadata within the lifecycle of the currently running application context. This means that, upon restart, you may end up with duplicate entries.

如果你需要应用程序上下文重启之间坚持元数据，那么框架提供以下持久`MetadataStores`：

If you need to persist metadata between application context restarts, the framework provides the following persistent MetadataStores:

PropertiesPersistingMetadataStore 由属性文件和 PropertiesPersister 支持。

The PropertiesPersistingMetadataStore is backed by a properties file and a PropertiesPersister.

默认情况下，它在应用程序上下文中正常关闭时才坚持状态。它实现了`Flushable`，以便你可以通过调用 flush() 来坚持状态。以下示例演示如何使用 XML 配置一个‘PropertiesPersistingMetadataStore’：

By default, it persists only the state when the application context is closed normally. It implements Flushable so that you can persist the state at will, by invoking flush(). The following example shows how to configure a 'PropertiesPersistingMetadataStore' with XML:

<bean id="metadataStore"
    class="org.springframework.integration.metadata.PropertiesPersistingMetadataStore"/>

或者，你可以提供自己实现的`MetadataStore`接口（例如，JdbcMetadataStore）并在应用程序上下文中将它配置为 Bean。

Alternatively, you can provide your own implementation of the MetadataStore interface (for example, JdbcMetadataStore) and configure it as a bean in the application context.

从 4.0 版本开始，SimpleMetadataStore、PropertiesPersistingMetadataStore 和 RedisMetadataStore 实现了 ConcurrentMetadataStore。这些提供原子更新，可用于多个组件或应用程序实例。

Starting with version 4.0, SimpleMetadataStore, PropertiesPersistingMetadataStore, and RedisMetadataStore implement ConcurrentMetadataStore. These provide for atomic updates and can be used across multiple component or application instances.

Idempotent Receiver and Metadata Store

元数据存储在需要过滤传入消息（如果该消息已被处理，则可以将其丢弃或在丢弃时执行一些其他逻辑）时实现 EIP idempotent receiver 模式非常有用。以下配置显示了如何执行此操作的示例：

The metadata store is useful for implementing the EIP idempotent receiver pattern when there is need to filter an incoming message if it has already been processed and you can discard it or perform some other logic on discarding. The following configuration shows an example of how to do so:

<int:filter input-channel="serviceChannel"
			output-channel="idempotentServiceChannel"
			discard-channel="discardChannel"
			expression="@metadataStore.get(headers.businessKey) == null"/>

<int:publish-subscribe-channel id="idempotentServiceChannel"/>

<int:outbound-channel-adapter channel="idempotentServiceChannel"
                              expression="@metadataStore.put(headers.businessKey, '')"/>

<int:service-activator input-channel="idempotentServiceChannel" ref="service"/>

幂等条目的`value`可能是过期日期，超过该日期后该条目应由某个计划的整理程序从元数据存储中删除。

The value of the idempotent entry may be an expiration date, after which that entry should be removed from metadata store by some scheduled reaper.

另请参阅 Idempotent Receiver Enterprise Integration Pattern。

`MetadataStoreListener`

某些元数据存储（当前仅限于 zookeeper）支持注册一个侦听器，以便在项目更改时接收事件，如下例所示：

Some metadata stores (currently only zookeeper) support registering a listener to receive events when items change, as the following example shows:

public interface MetadataStoreListener {

	void onAdd(String key, String value);

	void onRemove(String key, String oldValue);

	void onUpdate(String key, String newValue);
}

有关更多信息，请参见 Javadoc。如果您仅对事件的子集感兴趣，则可以对 MetadataStoreListenerAdapter 进行子类化。

See the Javadoc for more information. The MetadataStoreListenerAdapter can be subclassed if you are interested only in a subset of events.