Inbound Channel Adapters: Controlling Remote File Fetching

配置入站通道适配器时，应考虑两个属性。与所有轮询器一样，max-messages-per-poll 可用于限制每次轮询发出的消息数（如果超出已配置值）。max-fetch-size（自版本 5.0 起）可以限制一次从远程服务器检索的文件数。

You should consider two properties when configuring inbound channel adapters. max-messages-per-poll, as with all pollers, can be used to limit the number of messages emitted on each poll (if more than the configured value are ready). max-fetch-size (since version 5.0) can limit the number of files retrieved from the remote server at a time.

以下场景假设起始状态是一个空本地目录：

The following scenarios assume the starting state is an empty local directory:

  • max-messages-per-poll=2 and max-fetch-size=1: The adapter fetches one file, emits it, fetches the next file, and emit it. Then it sleeps until the next poll.

  • max-messages-per-poll=2 and max-fetch-size=2): The adapter fetch both files and then emits each one.

  • max-messages-per-poll=2 and max-fetch-size=4: The adapter fetches up to 4 files (if available) and emits the first two (if there are at least two). The next two files will be emitted on the next poll.

  • max-messages-per-poll=2 and max-fetch-size not specified: The adapter fetches all remote files and emits the first two (if there are at least two). The subsequent files are emitted on subsequent polls (two at a time). When all are consumed, the remote fetch is attempted again, to pick up any new files.

在部署应用程序的多个实例时，我们建议设置一个小的 max-fetch-size，以避免一个实例 “grabbing” 所有文件，让其他实例挨饿。

When you deploy multiple instances of an application, we recommend setting a small max-fetch-size, to avoid one instance “grabbing” all the files and starving other instances.

另一个使用 max-fetch-size 的场景是当您希望停止获取远程文件，但继续处理已经获取的文件。通过编程方式、JMX 或 control bus 设置 MessageSource 上的 maxFetchSize 属性，可有效停止适配器获取更多文件，但能让轮询器继续为先前已获取的文件发出消息。如果在该属性发生改变时轮询器处于活动状态，那么此次改变将在下一次轮询时生效。

Another use for max-fetch-size is when you want to stop fetching remote files but continue to process files that have already been fetched. Setting the maxFetchSize property on the MessageSource (programmatically, via JMX, or via a control bus) effectively stops the adapter from fetching more files but lets the poller continue to emit messages for files that have previously been fetched. If the poller is active when the property is changed, the change takes effect on the next poll.

从版本 5.1 开始，可以为同步器提供 Comparator<?>。这在使用 maxFetchSize 限制获取的文件数时很有用。

Starting with version 5.1, the synchronizer can be provided with a Comparator<?>. This is useful when restricting the number of files fetched with maxFetchSize.