Introducing the Prefetch Block

Prior to version 7.2, action scripts containing download commands were processed by scanning the action to deduce which downloads corresponded to which sha1’s and sizes. This was error prone because the download commands had to be correlated with continue if commands and the information was spread out throughout the action, comingled with other action logic. This made actions harder to understand than was necessary. The prefetch block resolves these issues.

The prefetch block must be the first entry in the action script (other than comments or blank lines). It contains all the download prefetch logic needed to prepare for subsequent action execution, making the action easier to understand. Some of the methods that can be used in a prefetch block include:

• Literal downloads. These are ordinary static downloads, which are still available.
• Conditional downloads. Only those commands inside TRUE condition pathways are performed.
• Variable Substitution. This includes downloads that use relevance substitution to determine which files to collect.
• Custom logic. This takes advantage of a plug-in to create download manifests.

Unlike the pre-parsing algorithm used in the traditional downloading actions, prefetch block downloads can be viewed as a top-down approach: the prefetch block comes first and must successfully complete before the rest of the action can continue. This provides greater control, flexibility and power.

Notes:

Only one prefetch block is allowed per action. When it is used, the begin prefetch block command must be the first executable in the script. Only blank lines and comments are allowed to precede it. An end prefetch block command is required for termination.