Preboot | Heroku Dev Center

Preboot changes the standard dyno start behavior for web dynos. Instead of stopping the existing set of web dynos before starting the new ones, preboot ensures that the new web dynos are started (and receive traffic) before the existing ones are terminated. This can contribute to zero downtime deployments. Preboot is only available on the Common Runtime to apps that are using professional dyno typ

[YassLab]

"Preboot ensures that the new web dynos are started (and receive traffic) before existing ones are terminated. / Only available on the Common Runtime to apps that are using professional dyno types (standard and performance). Applications using free or hobby dynos do not have access to preboot."

[willnet]

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