helmfile sync vs helmfile apply

Question

sync      sync all resources from state file (repos, releases and chart deps)
apply     apply all resources from state file only when there are changes

sync

The helmfile sync sub-command sync your cluster state as described in your helmfile ... Under 
the covers, Helmfile executes helm upgrade --install for each release declared in the 
manifest, by optionally decrypting secrets to be consumed as helm chart values. It also 
updates specified chart repositories and updates the dependencies of any referenced local 
charts.

For Helm 2.9+ you can use a username and password to authenticate to a remote repository.

apply

The helmfile apply sub-command begins by executing diff. If diff finds that there is any changes
sync is executed. Adding --interactive instructs Helmfile to request your confirmation before sync.
An expected use-case of apply is to schedule it to run periodically, so that you can auto-fix skews
between the desired and the current state of your apps running on Kubernetes clusters.

I went through the Helmfile repo Readme to figure out the difference between helmfile sync and helmfile apply. It seems like unlike the apply command, the sync command doesn't do a diff and helm upgrades the hell out of all releases 😃. But from the word sync, you'd expect the command to apply those releases that have been changed. There is also mention of the potential application of helmfile apply to periodically syncing of releases. Why not use helmfile sync for this purpose? Overall, the difference didn't become crystal clear, and I though there could probably be more to it. So, I'm asking.

Answer 1

Consider the use case where you have a Jenkins job that gets triggered every 5 minutes and in that job you want to upgrade your helm chart, but only if there are changes.

If you use helmfile sync which calls helm upgrade --install every five minutes, you will end up incrementing Chart revision every five minutes.

$ helm upgrade --install httpd bitnami/apache > /dev/null
$ helm list
NAME    REVISION        UPDATED                         STATUS          CHART           APP VERSION     NAMESPACE
httpd   1               Thu Feb 13 11:27:14 2020        DEPLOYED        apache-7.3.5    2.4.41          default
$ helm upgrade --install httpd bitnami/apache > /dev/null
$ helm list
NAME    REVISION        UPDATED                         STATUS          CHART           APP VERSION     NAMESPACE
httpd   2               Thu Feb 13 11:28:39 2020        DEPLOYED        apache-7.3.5    2.4.41          default

So, each helmfile sync will result new revision. Now if you were to run helmfile apply, which will first check for diffs and only then (if found) will call helmfile sync which will in turn call helm upgrade --install this will not happen.

Answer 2

Everything in the other answers is correct. However, there is one additional important thing to understand with sync vs apply:

• sync will call helm upgrade on all releases. Thus, a new helm secret will be created for each release, and releases' revisions will all be incremented by one. Helm 3 uses three-way strategic merge algorithm to compute patches, meaning it will revert manual changes made to resources in the live cluster handled by Helm;
• apply will first use the helm-diff plugin to determine which releases changed, and only run helm upgrade on the changed ones. However, helm-diff only compares the previous Helm state to the new one, it doesn't take the state of the live cluster into account. This means if you modified something manually, helmfile apply (or more precisely the helm-diff plugin) may not detect it, and leave it as-is.

Thus, if you always modify the live cluster through helm, helmfile apply is the way to go. However, if you can have manual changes and want to ensure the live state is coherent with what is defined in Helm/helmfile, then helmfile sync is necessary.

---

If you want more details, checkout my blog post helmfile: difference between sync and apply (Helm 3)