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During a stack update, some resources will need to be replaced (rather than updated in-place). In general, we can’t know in advance for which resources that will be the case, so we need to be able to create replacements on the fly.
When we detect that a resource needs to be replaced (i.e. Resource.update raises UpdateReplace), create a new resource with the same name in the same stack. Fill in a the replaces and replaced_by fields of the new and existing resources, respectively. Do not create a SyncPoint for the new resource.
Once the new Resource has been stored in the database, retrigger the current check with the same data except passing the key of the new resource. Then return immediately, without triggering any dependent nodes.
Note that no modification of the graph stored in the Stack is required. When we come to trigger the SyncPoints of nodes that are dependent on the replaced resource, the replacement should just use the old resource’s graph key to impersonate it. However the contents of the input data (not the keys) to the next resource will contain the resource ID of the replacement, so that dependent resources will update their dependency lists. The previous resource will be visited again in the clean-up phase of the graph, at which point it will be deleted.