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Using production images when developing locally (IIS)

I develop locally on an app that has millions of “library” files that have been uploaded by users over the years. The image library is approaching 1TB in size, so obviously, I cannot be expected to sync or copy down that data for use locally. When any files are not local, then you do not get the true experience your users will.

I don’t know why I never though about this solution before but over on Lullabot, Sean Lange explained how he solved this issue using rewrite rules in Apache.

I use IIS so I needed something similar. IIS uses a web.config file so all I had to do was convert his logic into something IIS could use. The rewrite rule will inspect all outbound HTML and rewrite the src attribute on any img tag with a path of ‘/photos/’ for me. Obviously, I don’t want this rule to fire on the production server and since I do check-in my web.config file into SVN, I needed a precondition check so production does not process this rule too:

Under the <rules> block in web.config:

<outboundRules>
    <rule name="dev - rewrite SRC of library files to come from production" preCondition="IsHTML" stopProcessing="true">
        <match filterByTags="Img" pattern="^/photos/(.*)" />
        <action type="Rewrite" value="http://production.domain.com{R:0}" />
    </rule>
    <preConditions>
	<preCondition name="IsHTML">
	    <add input="{RESPONSE_CONTENT_TYPE}" pattern="^text/html" />
	    <add input="{HTTP_HOST}" pattern="devdomain|localhost" />
	</preCondition>
    </preConditions>
</outboundRules>

</rule>

</preCondition>

</preConditions>

</outboundRules>

Line 3: only match on IMG files in the path that begins with /photos/

Line 4: prepend the match ({R:0}) with the production domain

Line 8: only inspect HTML as it is leaving the server – no need to check images or JS

Line 9: only perform this if the webserver is serving for the ‘devdomain’ or ‘localhost’ domains. Since my local dev environment will answer for either devdomain or localhost, I check the {HTTP_HOST} variable to detect if this a locally running version of the app – if not, then the app is on production.

Setting the rule’s preCondition="IsHTML" ensures it will only alter image files on production.

One caveat: you IIS can’t process outbound rules if compression (gzip) is active. I turn that off for development anyway.

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