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This feature is available when the rendering of the macro body indicates a temporary error condition has occurred during the rendering. This requires underlying macros to be enabled to report this type of error so that the cache or future macro can behave differently. For instance, the SQL macros (with release 7.0) have been enable enabled to do this for certain temporary errors.
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The Future Macro does not show the error data and. Instead, instead, it retries rendering the body after waiting a certain amount of time specified by the retryInterval parameter on the macro. The retryInterval parameter defaults to Default which recognizes a setting of an adminstrator controlled value that sets the number of seconds to wait. This means that the loading message will continue to show until good data is retrieved.
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A standard, recommended approach for improving the user experience and performance of access accessing external data is to use the Cache Macro inside of a Future Macro. This approach works extremely well when both future and cache are enabled for the retry feature! The expected (error free) data is cached when it is available and automatically show shown on the page at that time.