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vcpkg/docs/users/assetcaching.md
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[docs] Improve assetcaching.md (#19625) 
* Improve asset caching readme

* Update assetcaching.md

Co-authored-by: Robert Schumacher <roschuma@microsoft.com>
2021-08-17 17:33:44 -07:00

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Asset Caching

The latest version of this documentation is available on GitHub.

Experimental feature: this may change or be removed at any time

Vcpkg can utilize mirrors to cache downloaded assets, ensuring continued operation even if the original source changes or disappears.

In-tool help is available via vcpkg help assetcaching.

Configuration

Asset caching can be configured by setting the environment variable X_VCPKG_ASSET_SOURCES to a semicolon-delimited list of source strings. Characters can be escaped using backtick (`).

Valid source strings

The <rw> optional parameter for certain strings controls how they will be accessed. It can be specified as read, write, or readwrite and defaults to read.

clear

Syntax: clear

Removes all previous sources

x-azurl

Syntax: x-azurl,<url>[,<sas>[,<rw>]]

Adds an Azure Blob Storage source, optionally using Shared Access Signature validation. URL should include the container path and be terminated with a trailing /. SAS, if defined, should be prefixed with a ?. Non-Azure servers will also work if they respond to GET and PUT requests of the form: <url><sha512><sas>. As an example, if you set X_VCPKG_ASSET_SOURCES to x-azurl,https://mydomain.com/vcpkg/,token=abc123,readwrite your server should respond to GET and PUT requests of the form https://mydomain.com/vcpkg/<sha512>?token=abc123.

You can also use the filesystem (e.g. a network drive) via file:// as asset cache. For example you then set X_VCPKG_ASSET_SOURCES to x-azurl,file:///Z:/vcpkg/assetcache/,,readwrite when you have a network folder mounted at Z:/.

The workflow of this asset source is:

  1. Attemp to read from the mirror
  2. (If step 1 failed) Read from the original url
  3. (If step 2 succeeded) Write back to the mirror

You can enable/disable steps 1 and 3 via the <rw> specifier and you can disable step 2 via x-block-origin below.

See also the binary caching documentation for Azure Blob Storage for more information on how to set up an x-azurl source.

x-block-origin

Syntax: x-block-origin

Disables use of the original URLs in case the mirror does not have the file available.