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(resource-registration)=

Resource registration

As a Pulumi program is executed by the language host, it will make calls to the engine in order to declare resources and their desired state. Each contains:

The resource's type and name.
The resource's parent, if it has one.
A reference to the provider that manages the resource, if an explicit one has been specified. If no reference has been specified, Pulumi will use a default provider instance for the resource's package and version.
Values for the resource's input properties.
Any resource options that have been specified for the resource.

In order to determine what actions to take in order to arrive at the desired, the engine diffs the desired state of the resource against the current state as recorded in the state snapshot:

If there is no current state, the engine will attempt to create a new resource.
If there is a current state, the engine will diff the current state with the desired state to determine whether the resource is unchanged, requires updating, or must be replaced in some manner.

When the appropriate actions have been determined, the engine will invoke the relevant provider methods to carry them out. After the actions complete, the engine returns the new state of the resource to the program.

Although all of the above happens "in the engine", in practice these concerns are separated into a number of subsystems: the resource monitor, the step generator, and the step executor.

(resource-monitor)=

Resource monitor

The resource monitor (largely represented by resmon in the codebase; see gh-file:pulumi#pkg/resource/deploy/source_eval.go) implements the interface, which is the primary communication channel between language hosts and the engine. There is a single resource monitor per deployment. Aside from being a marshalling and unmarshalling layer between the engine and its gRPC boundary, the resource monitor is also responsible for resolving default providers and component providers, responding to s as follows:

The request is unmarshalled from the gRPC wire format into an engine-internal representation.
If the request lacks a provider reference, the resource monitor will resolve a default provider for the resource's package and version.
If the request registers a remote component, the resource monitor will dispatch an appropriate call to the component provider and await the result.
If the request does not register a remote component (a so-called custom resource, although this is in reality the default type of resource), the resource monitor will emit a RegisterResourceEvent and await a response.
When a result is received (either in response to a call or a RegisterResourceEvent), the resource monitor will marshal the result back into the gRPC wire format and return it to the language host.

(step-generation)=

Step generation

The step generator (gh-file:pulumi#pkg/resource/deploy/step_generator.go) is responsible for processing RegisterResourceEvents emitted by the resource monitor. When an event is received, the step generator proceeds as follows:

A URN is generated for the resource, using the type, name and parent fields from the event.
The URN is used to look up the resource's existing state, if it has any. If the event contains aliases, state under those aliases will also be looked up. It is an error if multiple pieces of existing state are found due to aliasing.
Input properties are pre-processed to implement the ignoreChanges resource option, by resetting the values of any properties which should be ignored to their previous values.
If the event indicates that the resource should be imported, the step generator will emit an ImportStep and return.
If the resource is not being imported, the step generator will continue by calling the provider's method with both the event's input properties and the resource's existing inputs.¹ Check will return a validated bag of input values that may be used in later calls to , , and .
At this point, the step generator will invoke any analyzers that have been configured in the stack to perform additional validation on the resource's input properties.
If the resource has no existing state, it must be created. Issue a CreateStep and return.
Diff the resource in order to determine whether it must be updated, replaced, or left as-is.
If there are no changes, issue a SameStep and return.
If the resource is not being replaced, issue an UpdateStep and return.
If the resource is being replaced, call the resource provider's method again, this time sending no existing inputs. This call ensures that the input properties used to create the replacement will not reuse generated defaults that should be unique to the existing resource.
If the replacement should be created before the original is deleted (a normal replacement, also "create-replace" or "create-before-replace"), issue an appropriate CreateStep and DeleteStep pair and return.
If the replacement should be created after the original has been deleted ("delete-replace", "delete-before-replace", "DBR"), calculate the list of resources that will have to be deleted in response to the deletion of the original (see the sections on deletions and dependent replacements later on for more details). Then, issue a DeleteStep and CreateStep pair for the replacement and return.

:::{note} Presently, step generation is a serial process (that is, steps are processed one at a time, in turn). This means that step generation is on the critical path for a deployment, so any significant blocking operations could slow down deployments considerably. In the case of an update, step generator latency is generally insignificant compared to the time spend performing provider operations (e.g. cloud updates), but in the case of a large preview operation, or an update where most resources are unchanged, the step generator could become a bottleneck. :::

Step generation is a fire-and-forget process. Once a step has been generated, the step generator immediately moves on to the next RegisterResourceEvent. It is the responsibility of the step executor to communicate the results of each step back to the resource monitor.

(step-generation-diff)=

Diffing

While in most cases diffing boils down to calling a provider's method, there are a number of cases where this might not happen. The full algorithm that the engine currently implements is as follows:

If the resource has been marked for replacement out-of-band (e.g. by the use of the --target-replace command-line option), the resource must be replaced.
If the resource's provider has changed, the resource must be replaced. Default providers are allowed to change without triggering a replacement if and only if the provider's configuration allows the new default provider to continue to manage existing resources. This is intended to allow default providers to be upgraded without causing all resources they manage to be replaced.
If the engine is configured to use legacy (pre-1.0) diffs, the engine will compare old and new inputs itself (without consulting the provider). If these differ, the resource must be updated.
In all other cases, the engine will call the provider's method to determine whether the resource must be updated, replaced, or left as-is.

(step-generation-deletions)=

Deletions

Once the Pulumi program has exited, the step generator determines the set of resources that must be deleted by computing the difference between the set of registered resources and the set of resources that were present in the previous state snapshot. This set is sorted topologically by reverse dependencies (that is, resources that depend on other resources are deleted first). This sorted list is then decomposed into a list of lists where the sublists must be processed serially but the contents of each sublist can be processed in parallel.

(step-generation-dependent-replacements)=

Dependent replacements

By default, Pulumi will replace resources by first creating the replacement and then deleting the original (create-before-replace). There are cases however where Pulumi will delete the original first (a delete-before-replace):

If the resource's provider specifies that the resource must be deleted before it can be replaced as using a 's deleteBeforeReplace field.
If the program specifies the deleteBeforeReplace resource option for the resource.

In such cases, it may be necessary to first delete resources that depend on that being replaced, since there will be a moment between the delete and create steps where no version of the resource exists (and thus dependent resources will have broken dependencies). The step generator does this as follows:

Compute the full set of resources that transitively depend on the resource being replaced.
Remove from this set any resources that would not themselves be replaced by changes to their dependencies. This is determined by substituting unknown values for any inputs that stem from a dependency and calling the provider's method.
Process the replacements in reverse topological order.

To better illustrate this, consider the following example (written in pseudo-TypeScript):

const a = new Resource("a", {})
const b = new Resource("b", {}, { dependsOn: a })
const c = new Resource("c", { input: a.output })
const d = new Resource("d", { input: b.output })

The dependency graph for this program is as follows:

flowchart TD
    b --> a
    c --> a
    d --> b

We see that the transitive set of resources that depend on a is {b, c, d}. In the event that a is subject to a delete-before-replace, then each of b, c, and d must also be considered. Since b's relationship is only due to dependsOn, its inputs will not be affected by the deletion of a, so it does not need to be replaced. c's inputs are affected by the deletion of a, so we must call Diff to see whether it needs to be replaced or not. d's dependency on a is through b, which we have established does not need to be replaced, so d does not need to be replaced either.

(step-execution)=

Step execution

The step executor is responsible for executing steps yielded by the step generator. Steps are processed in sequences called chains. While the steps within a chain must be processed serially, chains may be processed in parallel. The step executor uses a pool of workers to execute steps. Once a step completes, the executor communicates its results to the resource monitor. If a step fails, the executor notes its failure and cancels the deployment. Once the Pulumi program has exited and the step generator has issued all required deletions, the step executor waits for all outstanding steps to complete and then returns.

(resource-registration-examples)=

Examples

The following subsections give some example sequence diagrams for the processes described in this document. Use your mouse to zoom in/out and move around as necessary.

Creating a resource

:zoom:

sequenceDiagram
    participant LH as Language host
    box Engine
        participant RM as Resource monitor
        participant SG as Step generator
        participant SE as Step executor
    end
    participant P as Provider

    LH->>+RM: RegisterResourceRequest(type, name, inputs, options)
    RM->>+SG: RegisterResourceEvent(type, name, inputs, options)
    SG->>+P: CheckRequest(type, inputs)
    P->>-SG: CheckResponse(inputs', failures)
    SG->>+SE: CreateStep(inputs', options)
    SE->>+P: CreateRequest(type, inputs')
    P->>-SE: CreateResponse(new state)
    SE->>-RM: Done(new state)
    RM->>-LH: RegisterResourceResponse(URN, ID, new state)

Updating a resource

:zoom:

sequenceDiagram
    participant LH as Language host
    box Engine
        participant RM as Resource monitor
        participant SG as Step generator
        participant SE as Step executor
    end
    participant P as Provider

    LH->>+RM: RegisterResourceRequest(type, name, inputs, options)
    RM->>+SG: RegisterResourceEvent(type, name, inputs, options)
    SG->>+P: CheckRequest(type, inputs, old inputs)
    P->>-SG: CheckResponse(inputs', failures)
    SG->>+P: DiffRequest(type, inputs', old state, options)
    P->>-SG: DiffResponse(diff)
    SG->>+SE: UpdateStep(inputs', old state, options)
    SE->>+P: UpdateRequest(type, inputs', old state)
    P->>-SE: UpdateResponse(new state)
    SE->>-RM: Done(new state)
    RM->>-LH: RegisterResourceResponse(URN, ID, new state)

Replacing a resource (create-before-replace)

:zoom:

sequenceDiagram
    participant LH as Language host
    box Engine
        participant RM as Resource monitor
        participant SG as Step generator
        participant SE as Step executor
    end
    participant P as Provider

    LH->>+RM: RegisterResourceRequest(type, name, inputs, options)
    RM->>+SG: RegisterResourceEvent(type, name, inputs, options)
    SG->>+P: CheckRequest(type, inputs, old inputs)
    P->>-SG: CheckResponse(inputs', failures)
    SG->>+P: DiffRequest(type, inputs', old state, options)
    P->>-SG: DiffResponse(diff)
    SG->>+SE: CreateStep(inputs', old state, options)
    SE->>+P: CreateRequest(type, inputs', old state)
    P->>-SE: CreateResponse(new state)
    SE->>-RM: Done(new state)
    RM->>-LH: RegisterResourceResponse(URN, ID, new state)

    Note over SG: Pulumi program exits

    SG->>SG: Generate delete steps
    SG->>+SE: DeleteStep(old state)
    SE->>+P: DeleteRequest(type, old state)
    P->>-SE: DeleteResponse()

Replacing a resource (delete-before-replace)

:zoom:

sequenceDiagram
    participant LH as Language host
    box Engine
        participant RM as Resource monitor
        participant SG as Step generator
        participant SE as Step executor
    end
    participant P as Provider

    LH->>+RM: RegisterResourceRequest(type, name, inputs, options)
    RM->>+SG: RegisterResourceEvent(type, name, inputs, options)
    SG->>+P: CheckRequest(type, inputs, old inputs)
    P->>-SG: CheckResponse(inputs', failures)
    SG->>+P: DiffRequest(type, inputs', old state, options)
    P->>-SG: DiffResponse(diff)
    SG->>+SE: DeleteStep(old state), CreateStep(inputs', options)
    SE->>+P: DeleteRequest(type, old state)
    P->>-SE: DeleteResponse()
    SE->>+P: CreateRequest(type, inputs', old state)
    P->>-SE: CreateResponse(new state)
    SE->>-RM: Done(new state)
    RM->>-LH: RegisterResourceResponse(URN, ID, new state)

Importing a resource

:zoom:

sequenceDiagram
    participant LH as Language host
    box Engine
        participant RM as Resource monitor
        participant SG as Step generator
        participant SE as Step executor
    end
    participant P as Provider

    LH->>+RM: RegisterResourceRequest(type, name, inputs, options)
    RM->>+SG: RegisterResourceEvent(type, name, inputs, options)
    SG->>+SE: ImportStep(inputs, options)
    SE->>+P: ReadRequest(type, id)
    P->>-SE: ReadResponse(current inputs, current state)
    SE->>+P: CheckRequest(type, inputs, current inputs)
    P->>-SE: CheckResponse(inputs', failures)
    SE->>+P: DiffRequest(type, inputs', current state, options)
    P->>-SE: DiffResponse(diff)
    SE->>-RM: Done(current state)
    RM->>-LH: RegisterResourceResponse(URN, ID, current state)

Leaving a resource unchanged

:zoom:

sequenceDiagram
    participant LH as Language host
    box Engine
        participant RM as Resource monitor
        participant SG as Step generator
        participant SE as Step executor
    end
    participant P as Provider

    LH->>+RM: RegisterResourceRequest(type, name, inputs, options)
    RM->>+SG: RegisterResourceEvent(type, name, inputs, options)
    SG->>+P: CheckRequest(type, inputs, old inputs)
    P->>-SG: CheckResponse(inputs', failures)
    SG->>+P: DiffRequest(type, inputs', old state, options)
    P->>-SG: DiffResponse(diff)
    SG->>+SE: SameStep(inputs', old state, options)
    SE->>-RM: Done(old state)
    RM->>-LH: RegisterResourceResponse(URN, ID, old state)

Existing inputs inputs may be used to repopulate default values for input properties that are automatically generated when the resource is created but that are not changed on subsequent updates (e.g. automatically generated names). ↩︎

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