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Comment by decode | Hacker News Reader

A somewhat similar question: what do you do with frontend assets? If you have JS/CSS/image files with a cache-busting URL, and you have different versions on different servers, you can run into problems.

For example, if most of your web servers are running version 1 and you're in the middle of rolling out version 2. A page request goes to one of the web servers with version 2, which returns HTML containing links to assets with the version 2 cache-busting URLs. The browser requests those assets, perhaps through the CDN, but the load balancer sends the requests to a web server still running version 1, where the assets don't exist yet. This means the browser will get a 404 error.

What are the best options for dealing with this problem?

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Not sure that this is the best answer, but seems like a good use of feature flags. If the flag points to the old state, serve the old assets. If it points to the new state, serve the new assets. So make a deploy with both assets, deploy everywhere, then flip the flag.

A bit unsatisfying because it probably isn't always easy the include both assets.

Another idea (not sure if a good one) would be having the load balancers pin a certain session to a certain backend machine. Seems like this would make it better without fixing it, though: that session will still need to switch to a different set of assets when "their" server is deployed.

The underlying problem is the use of a naive cache buster which would cause the old server to reply one way and the new one another. For both the people who want the old asset and the new asset.

If instead of being something the web server throws away before replying, the version number actually caused different assets to be returned, then you'd not have this problem.

One pattern is to separate out assets into a different package that is deployed to a separate host group and have your clients request a different host name, or have your load balancers use a path match to use those servers for those requests.

Another is to push asset updates first to all hosts. All hosts, even without code update, will now be able to respond for the new assets.

Another is to use a local cache plus some backend service or database to serve the assets from the web servers - again, all hosts will now respond correctly for the old assets and the new.

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