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Inbound Channel Adapters: Controlling Remote File Fetching

There are two properties that you should consider when you configure 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 one time.

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, emits it, and then sleeps until the next poll.

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

  • max-messages-per-poll=2 and max-fetch-size=4: The adapter fetches up to four files (if available) and emits the first two (if there are at least two). The next two files are 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 files are consumed, the remote fetch is attempted again, to pick up any new files.

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

Another use for max-fetch-size is if 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, with JMX, or with 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.

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