(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) converts RegisterResourceEvents and ReadResourceEvents from the Pulumi program into executable steps. Steps are the internal representation of everything that needs to happen to fulfill the goals of that program. The steps are passed to the step executor, which will do the actual work encoded in the steps.

This 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.

Process Overview

flowchart TD
    Start[RegisterResourceEvent] --> GenURN[Generate URN]
    GenURN --> LookupOld[Lookup existing state
Check aliases]
    LookupOld --> Import{Import?}

    Import -->|Yes| ImportStep[Generate ImportStep]
    Import -->|No| Check[Call Provider.Check]

    Check --> Analyzers[Run Analyzers]
    Analyzers --> HasOld{Existing
state?}

    HasOld -->|No| CreateStep[Generate CreateStep]
    HasOld -->|Yes| Diff[Compute Diff]

    Diff --> DiffResult{Changes?}
    DiffResult -->|None| SameStep[Generate SameStep]
    DiffResult -->|Update| UpdateStep[Generate UpdateStep]
    DiffResult -->|Replace| ReplaceFlow[Replacement Flow]

    ReplaceFlow --> ReplaceType{Replace
Type?}
    ReplaceType -->|Create-Before| CBR[CreateStep +
ReplaceStep +
DeleteStep]
    ReplaceType -->|Delete-Before| DBR[Calculate Dependents
DeleteStep +
CreateStep]

    CreateStep --> Submit[Submit to Executor]
    SameStep --> Submit
    UpdateStep --> Submit
    ImportStep --> Submit
    CBR --> Submit
    DBR --> Submit

Step Generation Algorithm

When a RegisterResourceEvent is received:

1. URN Generation: Create URN from type, name, and parent
2. State Lookup: Find existing state by URN or aliases (error if multiple matches)
3. Import Check: If importing, generate ImportStep and return
4. Check Inputs: Call provider's Check to validate and normalize inputs
5. Run Analyzers: Execute configured analyzers for validation
6. Decision Point:
   • No existing state: Generate CreateStep
   • Has existing state: Proceed to diff
7. Diff: Compute differences between old and new state (can happen async, see below)
8. Generate Steps:
   • No changes: SameStep
   • Update: UpdateStep
   • Replace: Replacement flow (see below)

:::{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.

There are some exceptions to this, e.g. with diffs (see below), but largely operations that happen in the step generator should be fast, to not slow the whole deployment down. :::

Read Resource Events

While RegisterResourceEvents represent resources managed by Pulumi, ReadResourceEvents fetch existing resources as external references. These resources are marked External = true, meaning Pulumi tracks them but doesn't own their lifecycle.

flowchart TD
    Start[ReadResourceEvent] --> GenURN[Generate URN]
    GenURN --> CreateState[Create new state
marked External]
    CreateState --> HasOld{Existing
resource?}

    HasOld -->|No| ReadStep[Generate ReadStep]
    HasOld -->|Yes| CheckExternal{Old is
External?}

    CheckExternal -->|Yes| ReadStep
    CheckExternal -->|No| CheckID{IDs
match?}

    CheckID -->|Yes| Relinquish[Relinquish:
Release from management]
    CheckID -->|No| ReadReplace[Read Replacement:
Delete old, read new]

    Relinquish --> ReadStep
    ReadReplace --> ReadReplaceStep[Generate ReadReplacementStep
+ ReplaceStep]

    ReadStep --> Submit[Submit to Executor]
    ReadReplaceStep --> Submit

1. Generate URN from type, name, and parent
2. Create state marked as External = true with provided ID
3. Check existing resource:
   • No existing: Simple ReadStep to fetch state
   • Existing external: Simple ReadStep (updating reference)
   • Existing managed, same ID: "Relinquish" - release from Pulumi management, generate ReadStep
   • Existing managed, different ID: "Read replacement" - generate ReadReplacementStep to delete old managed resource and ReplaceStep marker

(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:

1. 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.
2. 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.
3. 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.
4. 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.

Parallel Diff Optimization

To avoid blocking step generation, diffs can be computed in parallel using the step executor's worker pool (note that this is disabled by default as of the time of this writing, and can be enabled with the PULUMI_PARALLEL_DIFF=true env var):

sequenceDiagram
    participant SG as Step Generator
    participant SE as Step Executor
    participant P as Provider

    SG->>SE: Submit DiffStep
    Note over SG: Continues to next resource
    SE->>P: Call Diff
    P-->>SE: DiffResult
    SE->>SG: Fire ContinueResourceDiffEvent
    SG->>SG: Generate steps from diff

The DiffStep leverages parallel workers while the step generator continues processing other resources. When complete, a ContinueResourceDiffEvent re-enters step generation to produce the final steps.

(step-generation-deletions)=

Deletions

After the program exits, resources not registered in the new state are deleted:

flowchart LR
    A[Old State Resources] --> B[Subtract Registered
Resources]
    B --> C[Resources to Delete]
    C --> D[Topological Sort
Reverse Dependencies]
    D --> E[Decompose into
Parallel Groups]
    E --> F[Submit Delete Steps]

    style C fill:#f99

To determine which deletions can safely run in parallel, the step generator treats the resource dependency graph as a partially ordered set and decomposes it into antichains — subsets of resources that do not depend on one another and can therefore be deleted concurrently. The algorithm is:

1. While condemned resources remain: a. Identify all resources with no outgoing dependency edges (the current "maximal" elements of the remaining poset). b. Remove them from the graph. They form one antichain (a parallel batch of deletes). c. Repeat.

Because dependent resources must be deleted before the resources they depend on, the resulting list of batches is reversed before being submitted to the step executor. Each batch is executed to completion before the next begins.

Replacement Decision Flow

Pulumi supports two different type of replaces, either creating the new resource before deleting the old one (default), or deleting the old resource first, and then replacing it.

Delete-before-replace (DBR) is triggered when:

• Provider indicates via deleteBeforeReplace in
• User specifies deleteBeforeReplace resource option

flowchart TD
    Replace[Resource Requires
Replacement] --> CheckType{Delete
Before?}

    CheckType -->|No| CBR[Create-Before-Replace]
    CheckType -->|Yes| DBR[Delete-Before-Replace]

    CBR --> CBRCheck[Check inputs
without old defaults]
    CBRCheck --> CBRCreate[Generate CreateStep
pendingDelete=true]
    CBRCreate --> CBRReplace[Generate ReplaceStep]
    CBRReplace --> CBRDelete[Generate DeleteStep]

    DBR --> DBRCalc[Calculate dependent
replacements]
    DBRCalc --> DBRDelete[Generate DeleteStep]
    DBRDelete --> DBRCheck[Check inputs
without old defaults]
    DBRCheck --> DBRCreate[Generate CreateStep]

    CBRDelete --> End[Return Steps]
    DBRCreate --> End

For DBR replacements, a second Check call is made without old inputs to ensure new auto-generated properties (like names) don't reuse old values.¹

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:

1. Compute the full set of resources that transitively depend on the resource being replaced.
2. 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.
3. 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.

Dependent Replacement Algorithm

flowchart TD
    Start[Resource A needs DBR] --> Find[Find all transitive
dependents]
    Find --> Filter[Filter: Keep only
property dependents]
    Filter --> TestDiff[For each dependent:
Substitute unknowns
for A's outputs]
    TestDiff --> CallDiff[Call Provider.Diff]
    CallDiff --> NeedsReplace{Needs
replace?}
    NeedsReplace -->|Yes| AddToList[Add to replacement list]
    NeedsReplace -->|No| Skip[Skip this dependent]
    AddToList --> Next{More
dependents?}
    Skip --> Next
    Next -->|Yes| TestDiff
    Next -->|No| Sort[Sort by reverse
topological order]
    Sort --> Execute[Execute replacements]

Process:

1. Find all resources transitively depending on the DBR resource
2. Filter to only those with property dependencies (not just dependsOn)
3. For each dependent, test if it would be replaced:
   • Substitute unknowns for the DBR resource's outputs
   • Call provider's Diff to determine if replacement needed
4. Sort remaining dependents in reverse topological order
5. Execute cascading replacements

(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)

Reading an external 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: ReadResourceRequest(type, name, id, parent)
    RM->>+SG: ReadResourceEvent(type, name, id, parent)
    SG->>SG: Generate URN
    SG->>SG: Create state marked External=true
    SG->>+SE: ReadStep(id, provider)
    SE->>+P: ReadRequest(type, id)
    P->>-SE: ReadResponse(current state)
    SE->>-RM: Done(external state)
    RM->>-LH: ReadResourceResponse(URN, ID, state)