Can I emulate Wolfram's automatic package updating in my own packages?

Question

It seems that some Wolfram-provided packages are smart enough to automatically check with the Wolfram servers, notice when a newer version is available, and download it, so that the user always has the latest version. I'd like to implement this with packages that I've developed and distributed. Is this possible? If so, how?

Answer 1

You can home-brew something like this...

1. Host your package on a publicly-accessible server. You could use Amazon S3 or Google Cloud Storage (GCS) (they both have free or trial tiers), GitHub, or your own server. I've never tried it, but Wolfram's new cloud stuff could potentially work too.
2. For S3 or GCS, add a metadata tag, such as "version", to your hosted package that states the version (see links in note 2 below). For GitHub, tag your release.

3. Add a new file to your package that serves as a package loader/maintainer. This will be the file that users Get (e.g. `<<"MyPackage.wl"). The contents would be something like this:

   fileToLoad = FileNameJoin[{DirectoryName[System`Private`$InputFileName], "myActualPackage.wl"}];
   
   (* Read the first line of the package, which must be like 'version=1.0', with no semicolon. See note 1. *)
   Block[{f = OpenRead[fileToLoad]}, currentVersion = Read[f]; Close[f]];
   
   (* Use HEAD Object to get the latest version from the metadata. See note 2. *)
   Block[{headers},
       headers = URLFetch["url.for/your/package", "Headers", "Method"->"HEAD"];
       latestVersion = ToExpression[First@Cases[headers, {"x-amz-meta-version", v_} :> v]];
   ]
   
   (* Update if necessary. See note 3. *)
   If[latestVersion > currentVersion,
       (* Consider prompting the user if they want to update. *)
       PrintTemporary[Panel["Updating to version " <> ToString[latestVersion] <> "..."]];
       URLSave["url.for/your/package", fileToLoad];
   ]
   
   (* Finally load your actual package *)
   Get[fileToLoad]
   

Note 1: There are a few ways to store your current version. Perhaps a better way, especially if your package is already more than one file, is to use a new file that stores only the version.

Note 2: See the S3 docs for metadata, S3 docs for HEAD Object, Google Cloud Storage docs for metadata and GCS docs for HEAD Object. The metadata is encoded in the headers as "x-amz-meta-" for S3 and "x-goog-meta-" for Google, where * is your metadata key. Using HEAD fetches just the metadata without downloading the whole file. (You could also work out something a bit fancier using ETags...) If you decide to use GitHub, use the GitHub API to fetch the latest release.

Note 3: You can tweak this, e.g. if you want separate folders for new versions or if your package is multiple files distributed as a ZIP that needs to be unpacked. If the file is a ZIP, I would just use URLSave["url.for/your/package"] to save it to a temp directory, then use ExtractArchive, CopyDirectory and DeleteDirectory to move it into place.

Edit 11-Dec-2015: Changed NotebookDirectory[] on first line to DirectoryName[System`Private `$InputFileName].