Charges

Current Design

openmeter/billing/charges is the root facade for charge operations.

Charge-backed invoicing no longer relies on a charges-side InvoicePendingLines(...) wrapper. Billing owns invoice creation and dispatches gathering lines by billing.LineEngineType, while charge packages provide charge-specific line engines where needed.

Important layers:

• charges.Service
  • public entrypoint for Create(...), GetByID(...), GetByIDs(...), AdvanceCharges(...)
• charges/meta
  • shared charge metadata, charge type, short status, and common query surface
• charges/service
  • orchestration across charge types
• charges/usagebased
  • the deepest current lifecycle implementation

The generic rule is:

• the root charges package owns cross-type orchestration
• type-specific packages own type-specific lifecycle and persistence
• AdvanceCharges(...) is a facade method, not the state machine itself
• type-specific service subpackages may own reusable realization mechanics when the parent service becomes too broad; keep state-machine decisions in the lifecycle files and move only mechanical operations such as rating snapshots, realization persistence, credit allocation/correction, and realization lineage persistence into these helpers

Important types:

• charges.AdvanceChargesInput identifies the customer whose charges should advance
• meta.Charge and meta.ChargeID define the shared charge identity and type
• charges.Charge wraps concrete charge variants
• flatfee.Intent carries AmountBeforeProration, ProRating, SettlementMode, PaymentTerm, InvoiceAt — immutable inputs provided by the caller
• flatfee.ChargeBase stores the persisted flat-fee charge row: current Status plus durable State
• flatfee.State currently tracks:
  • AmountAfterProration
  • AdvanceAfter
  • FeatureID
• flatfee.Realizations stores expanded, non-base data loaded from child tables:
  • CreditRealizations
  • AccruedUsage
  • Payment
• flatfee.Intent.CalculateAmountAfterProration() computes the prorated amount from AmountBeforeProration, ServicePeriod/FullServicePeriod ratio, and ProRating config, with currency-precision rounding
• Charge-backed targets do not use invoice-style semantic proration or empty-period filtering; the charge stack materializes and prorates state itself, and the flat fee charge is responsible for omitting empty lines
• usagebased.Intent carries FeatureKey, Price, SettlementMode, InvoiceAt, and ServicePeriod
• usagebased.ChargeBase stores the current Status and State
• usagebased.State currently tracks:
  • CurrentRealizationRunID
  • AdvanceAfter
• usagebased.RealizationRunBase stores:
  • Type
  • AsOf
  • CollectionEnd
  • MeterValue
  • Totals

Billing Line Engines

Charge-backed gathering lines must carry the correct billing line engine when they are created.

Current engine values:

• billing.LineEngineTypeChargeFlatFee
• billing.LineEngineTypeChargeUsageBased
• billing.LineEngineTypeChargeCreditPurchase

Current implementations:

• flat fee line engine: openmeter/billing/charges/flatfee/lineengine
• credit purchase line engine: openmeter/billing/charges/creditpurchase/lineengine
• usage-based line engine: openmeter/billing/charges/usagebased/service/lineengine.go

Important rules:

• do not rely on billing to infer a charge-backed engine from ChargeID
• billing/service.CreatePendingInvoiceLines(...) rejects charge-backed gathering lines with empty Engine
• production wiring must register charge line engines through billing.Service.RegisterLineEngine(...)
• tests that temporarily add engines can remove them again through billing.Service.DeregisterLineEngine(...); use the public registry API instead of mutating billing internals from non-service packages
• charge test setups must also register those engines explicitly; keep this in openmeter/billing/charges/testutils
• if a charge create path stamps a new LineEngineType, app wiring and charge test wiring must register a matching implementation in the same change
• usage-based exposes its billing line engine from usagebased.Service.GetLineEngine(); register that returned engine instead of reusing the service type directly

Operational consequence:

• adding a new charge engine enum without a registered implementation causes invoice collection to fail when billing resolves the line engine
• a schema migration defaulting old persisted rows to invoicing is not enough for charge-backed lines; existing persisted gathering lines may need a backfill if they should route to a charge engine after rollout

Current shared contract details:

• line-engine transport structs in openmeter/billing/lineengine.go are validated at the billing callsite before invoking the engine, and returned lines/results are validated after the call
• OnCollectionCompleted(...) takes billing.OnCollectionCompletedInput and returns updated billing.StandardLines
• collection-time engines must preserve the exact line ID set they were given; billing validates that returned line IDs match input line IDs exactly and then merges the returned lines back into the invoice
• CalculateLines(...) returns updated billing.StandardLines; billing treats this as a pure recalculation boundary, validates exact line ID preservation, and merges the returned lines back into the invoice instead of relying on in-place mutation
• CalculateLines(...) no longer takes context.Context; if a future charge engine needs context-aware recalculation, propagate that need deliberately through the contract instead of using context.Background() as a workaround
• SplitGatheringLine(...) takes a concrete billing.GatheringLine plus SplitAt and returns only the split line fragments; the billing caller owns fetching the current line from the gathering invoice and merging PreSplitAtLine / optional PostSplitAtLine back into the invoice aggregate
• optional split outputs use pointers; SplitGatheringLineResult.PostSplitAtLine is *billing.GatheringLine
• use billing.ValidateStandardLineIDsMatchExactly(...) when a charge-side test or helper needs to assert that returned standard-line identities are preserved across a line-engine boundary
• collection-time errors should be normalized through billing.NewLineEngineValidationError(...) instead of rebuilding the validation-issue wrapper at each billing callsite
• charge line engines are only responsible for invoice-backed charge flows such as credit_then_invoice; they are not the execution path for credit_only settlement mode
• because of that boundary, it is acceptable for a charge line engine to return an error when invoked with credit_only settlement mode; treat that as a lifecycle misuse rather than adding credit_only behavior to the engine

Timestamp Normalization

Charge persistence assumes timestamp precision is bounded by streaming aggregation precision.

Rules:

• persisted charge timestamps must be truncated to streaming.MinimumWindowSizeDuration
• meta.NormalizeTimestamp(...) is the shared primitive; it also converts to UTC
• meta.NormalizeClosedPeriod(...) and Intent.Normalized() helpers are the domain-level normalization entrypoints
• normalize intent timestamps before validation and before any derived calculation that depends on durations or boundaries
• flat-fee proration must use normalized periods, otherwise sub-second inputs can change AmountAfterProration
• for usage-based lifecycle timestamps (AdvanceAfter, AsOf, CollectionEnd, storedAtOffset), normalize the computed timestamp before persisting it or handing it to downstream persistence callbacks

Important timestamp surfaces:

• meta.Intent.ServicePeriod
• meta.Intent.FullServicePeriod
• meta.Intent.BillingPeriod
• flatfee.Intent.InvoiceAt
• usagebased.Intent.InvoiceAt
• flatfee.State.AdvanceAfter
• usagebased.State.AdvanceAfter
• usagebased.CreateRealizationRunInput.AsOf
• usagebased.CreateRealizationRunInput.CollectionEnd
• usagebased.UpdateRealizationRunInput.AsOf

Placement guidance:

• prefer domain-side normalization when constructing or mutating intents and state (Intent.Normalized(), state-machine transition logic, temporary patch remap)
• keep a persistence backstop in shared write helpers such as charges/models/chargemeta
• in adapters, normalize at the actual write setter (SetInvoiceAt(...), SetAsof(...), SetCollectionEnd(...), SetOrClearAdvanceAfter(...)) rather than rewriting the whole input object at the top of the adapter method
• do not add redundant .UTC() calls after meta.NormalizeTimestamp(...); the helper already returns UTC

Currency Normalization

Charge lifecycle code owns currency rounding.

Rules:

• all currency amounts that enter or leave the charge lifecycle must be rounded with the charge currency calculator
• normalize charge-domain totals and inputs in charge code where charges own the amount
• ledger-backed allocation realizations may be stored exactly as returned by ledger-owned handlers for credits-only flows
• correction request and correction creation outputs are normalized in the shared creditrealization.Realizations.Correct(...) / CorrectAll(...) path
• zero-valued corrections are allowed after rounding; treat them as a no-op instead of an error

Important money surfaces:

• creditpurchase.Intent.CreditAmount
• flatfee.Intent.AmountBeforeProration
• flatfee.State.AmountAfterProration
• usage-based realization Totals
• flatfee.OnAssignedToInvoiceInput.PreTaxTotalAmount
• flatfee.OnCreditsOnlyUsageAccruedInput.AmountToAllocate
• usagebased.CreditsOnlyUsageAccruedInput.AmountToAllocate
• creditrealization.CreateAllocationInputs
• creditrealization.CreateCorrectionInputs

Placement guidance:

• normalize durable charge inputs in domain helpers such as Intent.Normalized()
• for flat fees, make sure AmountAfterProration is already rounded when calculated; adapters should persist it as-is
• when calling charge handlers that feed ledger-backed allocations/corrections, decide explicitly whether charges or ledger owns the returned amount; for current credits-only allocation flows, store ledger-returned allocation amounts as-is
• prefer shared normalization in creditrealization helpers for correction flows instead of repeating callback-local normalization at each callsite
• do not mutate whole intents inside adapters just to normalize currency; if an adapter needs a persistence backstop, keep it local to the Set* write
• when ledger logic derives monetary values from balances, entries, or its own calculations, round those values in ledger code as well; do not rely only on upstream callers

Zero Invoice Accrual

Invoice accrual uses a non-negative, no-op-aware contract.

Rules:

• negative invoice-accrual amounts are invalid
• zero invoice-accrual amounts are valid no-ops
• positive invoice-accrual amounts must produce a non-empty ledger transaction group reference
• charges-side services should short-circuit zero before calling persistence that expects a real ledger transaction
• do not persist invoicedusage.AccruedUsage rows with an empty LedgerTransaction.TransactionGroupID

Current expected behavior:

• usagebased.OnInvoiceUsageAccruedInput.Validate() allows zero and rejects only negatives
• usage-based and flat-fee service/orchestration layers should skip invoice-accrual persistence when the invoice line total is zero
• ledger handlers may still defensively tolerate zero and return ledgertransaction.GroupReference{}
• when a service proceeds with non-zero invoice accrual, it must require a non-empty transaction group reference before storing accrued usage

HTTP/API Conversion

Credit-purchase charges have an API/domain enum mismatch for promotional grants.

Rules:

• in the billing domain, promotional credit grants are represented as creditpurchase.SettlementTypePromotional
• in the v3 customer credits API, the same case is represented as funding_method=none
• v3 API responses for promotional grants must omit the purchase block entirely
• conversion code in api/v3/handlers/customers/credits must map this case explicitly instead of treating promotional as an unsupported settlement type

Important files:

• api/v3/handlers/customers/credits/convert.go
• openmeter/billing/charges/creditpurchase/settlement.go
• openmeter/billing/creditgrant/service/service.go
• api/spec/packages/aip/src/customers/credits/grant.tsp

Realization Helper Subpackages

Use small type-specific realization helper subpackages to keep charge services and state machines from becoming kitchen-sink orchestration layers.

The purpose of these subpackages is to separate reusable realization mechanics from lifecycle decisions:

• state-machine files decide which trigger/status/action applies for a settlement mode
• realization helper packages execute reusable mechanics once that decision has already been made
• helper packages must not decide which trigger to fire, which status to enter, or whether a charge lifecycle event should advance

Naming should describe the charge-domain unit being manipulated rather than the current ledger operation. Prefer realizations for flat-fee helpers because flat fees have credit realizations today and will also support invoiced/payment realization flows. For usage-based charges, a run helper is appropriate when the helper owns realization-run mechanics such as rated run creation, run persistence, credit allocation/correction, and run credit-realization lineage.

Keep these helpers type-specific instead of forcing a generic cross-charge state machine. Flat-fee and usage-based lifecycles share some mechanics, but their durable state and lifecycle semantics differ: usage-based has realization runs, collection cutoffs, and CurrentRealizationRunID; flat-fee has charge-level realizations, proration, invoice hooks, and payment hooks.

When extracting helpers:

• inject only the dependencies the helper needs, usually adapter, handler, lineage, and any rating helper
• expose struct-returning methods for multi-value outcomes
• keep credit allocation/correction exactness explicit at the call site, because credit_only and credit_then_invoice can differ
• keep lineage persistence next to realization creation so allocation and correction paths do not duplicate lineage bookkeeping
• keep the parent service responsible for transactions, locking, and building settlement-mode-specific state machines

Supported Behavior

• charges.AdvanceCharges(...) advances both usage-based and flat-fee credit-only charges
• usagebased.Service.AdvanceCharge(...) routes to the settlement-mode-specific state machine: CreditOnly uses the credits-only state machine and CreditThenInvoice uses NewCreditThenInvoiceStateMachine(...)
• flatfee.Service.AdvanceCharge(...) currently only supports CreditOnly; it does not have a CreditThenInvoice state machine path
• Both AdvanceCharge(...) methods return *Charge (nil means noop, non-nil means at least one transition)
• For invoice-settled flows, invoice creation and collection completion are still billing-triggered downstream events (invoice_created, collection_completed), not generic advance-loop transitions

Create + Auto-Advance Flow

charges.Create(...) runs in two phases:

1. Transaction phase — creates all charge records (flat-fee, usage-based, credit-purchase) and any gathering invoice lines
2. Post-create auto-advance — autoAdvanceCreatedCharges(...) runs outside the transaction so that creation is persisted even if advancing fails (a worker can retry later)

autoAdvanceCreatedCharges(...) (charges/service/create.go):

• Iterates over the created charges using a type switch (ChargeTypeUsageBased, ChargeTypeFlatFee)
• Collects unique customer IDs that have credit-only charges of either type
• Calls s.AdvanceCharges(...) (the facade) once per unique customer
• Merges advanced charges back into the result by charge ID

This means a newly created credit-only charge (usage-based or flat fee) that is eligible for immediate activation will be returned as active (or final) from Create(...) itself.

For invoice-settled charges:

• flat fee and credit purchase creation now stamp the gathering line engine in their type-specific Create(...) flows
• usage-based charge creation also stamps LineEngineTypeChargeUsageBased; do not introduce this discriminator unless a corresponding billing engine exists or the path is intentionally blocked
• usage-based IsLineBillableAsOf(...) is currently billable only once asOf >= resolved service period end; keep the existing progressive-billing TODO in place when touching that logic
• for usage-based credit_then_invoice, BuildStandardInvoiceLines(...) is allowed to drive charge lifecycle transitions needed to create the invoice-backed realization run
• OnCollectionCompleted(...) is the single collection-time line-engine hook; do not reintroduce a generic shared SnapshotLines() abstraction for charge engines
• prefer explicit usage-based lifecycle triggers such as invoice_created and collection_completed over generic line-snapshot callbacks
• collection-time charge-engine failures must surface as invoice validation issues through the billing line-engine validation flow, not as raw invoice-state-machine failures

Root Charges Advance Flow

The root-facade advance flow is:

1. charges.AdvanceCharges(...) lists non-final charge metas for the customer
2. It partitions charges by type using chargesByType(...)
3. Early return if no usage-based and no flat-fee charges
4. For flat-fee credit-only charges: calls flatfee.Service.AdvanceCharge(...) per charge (no customer override or feature meters needed)
5. For usage-based charges: resolves merged customer profile, feature meters, then calls usagebased.Service.AdvanceCharge(...) per charge

Key package responsibilities:

• charges/service/advance.go
  • customer-scoped orchestration
  • preloads customer/profile + feature context for usage-based only
  • flat-fee credit-only charges are self-contained (fixed amount, no meters)
• charges/meta/adapter
  • lists non-final charges for a customer
• charges/lock
  • provides NewChargeKey(...) for charge-scoped locking

Usage-Based Advance Flow

Usage-based advance currently does:

1. Validates:
   • ChargeID
   • expanded customer in CustomerOverrideWithDetails
   • valid merged profile
   • resolved feature meter
2. Takes a charge-scoped lock using:
   • charges/lock.NewChargeKey(...)
   • *lockr.Locker
3. Reloads the charge with realizations expanded
4. Routes by settlement mode
5. Builds the settlement-mode-specific state machine (NewCreditsOnlyStateMachine(...) or NewCreditThenInvoiceStateMachine(...))
6. Calls AdvanceUntilStateStable(...)

This is the main place where charge lifecycle logic exists today.

State-machine organization rule:

• keep state-machine-specific methods and helpers in the state-machine file that owns them
• do not spread one charge state machine across multiple files unless the split is the standard service/ or adapter/ package boundary
• line-engine files may call into a state machine, but they should not define that state machine's transition handlers

Credits-Only State Machine

The credits-only lifecycle is implemented in usagebased/service/creditsonly.go and usagebased/service/statemachine.go.

Relevant statuses:

• created
• active
• active.final_realization.started
• active.final_realization.waiting_for_collection
• active.final_realization.processing
• active.final_realization.completed
• final

High-level transitions:

1. created -> active
   • guarded by IsInsideServicePeriod()
   • sets AdvanceAfter to service-period start while waiting
2. active -> active.final_realization.started
   • guarded by IsAfterServicePeriod()
   • sets AdvanceAfter to service-period end while waiting
3. active.final_realization.started -> active.final_realization.waiting_for_collection
   • StartFinalRealizationRun(...) creates the realization run
4. active.final_realization.waiting_for_collection -> active.final_realization.processing
   • guarded by IsAfterCollectionPeriod(...)
5. active.final_realization.processing -> active.final_realization.completed
   • FinalizeRealizationRun(...) re-rates usage, computes delta vs initial run totals, then:
     • positive delta → allocates additional credits via allocateCredits
     • negative delta → corrects existing allocations via Realizations.Correct() with handler callback OnCreditsOnlyUsageAccruedCorrection
     • zero delta → no-op
6. active.final_realization.completed -> final
   • clears AdvanceAfter

AdvanceUntilStateStable(...) loops until the machine can no longer fire TriggerNext.

Flat Fee Credits-Only State Machine

The flat fee credits-only lifecycle is implemented in flatfee/service/creditsonly.go and flatfee/service/triggers.go. Types are in flatfee/statemachine.go.

Statuses (much simpler than usage-based — no collection period):

• created
• active
• final

Transitions:

1. created -> active
   • guarded by IsAfterInvoiceAt() (clock.Now() >= charge.Intent.InvoiceAt)
   • sets AdvanceAfter to InvoiceAt while waiting
2. active -> final
   • unconditional (fires immediately after active)
   • AllocateCredits(...) calls handler.OnCreditsOnlyUsageAccrued(...) with State.AmountAfterProration
   • validates credit allocations sum equals amount
   • persists credit realizations via adapter.CreateCreditAllocations(...)
   • clears AdvanceAfter on entering final

Key differences from usage-based credits-only:

• No collection period, no two-phase realization
• Amount is computed at creation from Intent.AmountBeforeProration and stored in State.AmountAfterProration, no meter snapshot or rating
• No FeatureMeter or CustomerOverride needed
• Uses flatfee.Status with only top-level states (not sub-statuses like active.final_realization.*)
• Persists only flatfee.ChargeBase; credit allocations / payment / accrued usage live in flatfee.Realizations

Service construction requires a *lockr.Locker (same as usage-based).

Handler interface: OnCreditsOnlyUsageAccrued(ctx, OnCreditsOnlyUsageAccruedInput) returns creditrealization.CreateAllocationInputs. The production implementation in ledger/chargeadapter/flatfee.go is stubbed as not-implemented; the test handler is in charges/service/handlers_test.go.

Flat fee credit-only charges start with InitialStatus: flatfee.StatusCreated (not Active). The invoiced path still starts as flatfee.StatusActive.

Collection Period Semantics

The collection-period logic is central to this package.

Rules:

• usagebased.InternalCollectionPeriod is 1 minute
• StartFinalRealizationRun(...) computes storedAtOffset = clock.Now() - InternalCollectionPeriod
• the realization run persists CollectionEnd
• waiting logic must use the persisted run CollectionEnd, not a recomputed value
• AdvanceAfterCollectionPeriodEnd(...) sets AdvanceAfter = CollectionEnd + InternalCollectionPeriod
• IsAfterCollectionPeriod(...) checks clock.Now() >= CollectionEnd + InternalCollectionPeriod

GetCollectionPeriodEnd(...) currently uses:

• CustomerOverride.MergedProfile.WorkflowConfig.Collection.Interval
• added to Charge.Intent.ServicePeriod.To

Do not depend on a concrete customer-override record being present. The merged profile is the important input.

Rating and Event Snapshot Semantics

Usage-based quantity is derived through snapshotQuantity(...).

Important behavior:

• query window uses the charge service period
• stored-at filtering uses stored_at < cutoff
• the cutoff is the current storedAtOffset
• the service-period end is expected to behave as exclusive in lifecycle tests

This means late-arriving events can become eligible in later advances if their stored_at was previously too new but later falls before the next cutoff.

Realization Runs

Realization runs are the persisted checkpoint for collection progress.

Important rules:

• the first final-realization advance creates a run
• CollectionEnd must be persisted on the run and mapped back into the domain model
• CurrentRealizationRunID points at the active run while waiting/finalizing
• finalization must clear CurrentRealizationRunID

Persistence gotcha:

• in usagebased/adapter/charge.go, use SetOrClearCurrentRealizationRunID(...)
• do not hand-roll separate Set... and Clear... branches unless there is a specific reason

Status Persistence

Charge status persistence is split across:

• the shared meta charge row
• the charge-type-specific row

For all three charge types (usage-based, flat-fee, credit-purchase):

When status changes:

• update the meta charge status to the short/meta status
• update the type-specific charge status_detailed to the full type-specific status

usagebased.Status.ToMetaChargeStatus(), flatfee.Status.ToMetaChargeStatus(), and creditpurchase.Status.ToMetaChargeStatus() are the bridges between the full state-machine status and the root charge meta status.

Testing Guidance

Key tests:

• openmeter/billing/charges/service/advance_test.go
• openmeter/billing/charges/service/invoicable_test.go

Use these conventions for lifecycle tests:

• validate root-facade behavior through AdvanceCharges(...) when testing orchestration
• validate persisted state through mustGetChargeByID(...)
• only use the direct AdvanceCharges(...) return as a secondary assertion
• if a returned charge is non-nil, at minimum match its status to the DB-loaded charge
• install usage-based handler callbacks only in the subtests that expect them (handler is reset in TearDownTest)
• use streaming/testutils.WithStoredAt(...) to simulate late events
• prefer clock.FreezeTime(...) for exact AsOf / AllocateAt assertions
• rely on the default billing profile unless the test explicitly needs customer-specific override behavior
• for credit-only charges (usage-based or flat fee), Create(...) itself may return an already-advanced charge — assert the returned charge's status, do not assume it will be created
• for flat fee credit-only tests, use mustAdvanceFlatFeeCharges(...) helper — it filters the advance result to flat fee charges only
• flat fee credit-only handler callbacks (onCreditsOnlyUsageAccrued) must return credit allocations that sum to the input AmountToAllocate
• when testing timestamp truncation, use sub-second fixtures and assert the persisted charge/run fields are second-aligned after create/advance
• cover the temporary shrink/extend remap path as well; it synthesizes new intents and must normalize the replacement period ends before re-create

Test suite teardown:

• BaseSuite.TearDownTest() (capital D — testify calls this automatically between tests) resets FlatFeeTestHandler, CreditPurchaseTestHandler, UsageBasedTestHandler, and MockStreamingConnector
• Use TearDownTest (capital D) in all sub-suites; TeardownTest (lowercase d) is not called by testify
• MockStreamingConnector events are shared across all tests in the suite — always rely on TearDownTest to reset them rather than defer

Billing-profile test gotcha:

• ProvisionBillingProfile(...) supports multiple edit options
• when asserting collection timing, verify the created profile is default and, if needed, compare it to GetDefaultProfile(...)

Running Tests

For direct package runs, use the repo env and Postgres. Prefer direct command execution; do not wrap these in sh -lc, bash -lc, or similar helper shells when a direct invocation works.

POSTGRES_HOST=127.0.0.1 direnv exec . go test -run TestInvoicableCharges/TestUsageBasedCreditOnlyLifecycle -v ./openmeter/billing/charges/service
POSTGRES_HOST=127.0.0.1 direnv exec . go test ./openmeter/billing/charges/...

Editing Checklist

When changing charges:

• decide whether the change belongs in:
  • the root facade
  • meta queries
  • charge locking
  • a type-specific package
• preserve the current root rule that AdvanceCharges(...) only advances supported types (usage-based and flat-fee credit-only)
• keep meta status and type-specific status in sync

When changing usage-based charges:

• confirm whether the change belongs in the facade, usage-based service, state machine, or adapter

• preserve the nil means noop contract for AdvanceCharge(...)

• preserve merged-profile based collection-period resolution

• keep CollectionEnd persisted on realization runs

• keep the stored_at < cutoff behavior explicit in tests

• update lifecycle tests if late-event visibility changes When changing flat-fee charges:

• the invoiced path (CreditThenInvoice/InvoiceOnly) starts as Active and is driven by invoice lifecycle hooks

• the credit-only path starts as Created and is driven by the state machine — do not mix the two

• AmountAfterProration lives on flatfee.State, not flatfee.Intent — it is computed at creation via Intent.CalculateAmountAfterProration() and persisted on the base charge row. Callers must not provide it; they set AmountBeforeProration, ServicePeriod, FullServicePeriod, and ProRating on the Intent

• IntentWithInitialStatus carries AmountAfterProration alongside InitialStatus to pass the computed value from the service to the adapter at creation time

• flatfee.State.AdvanceAfter must be passed through chargemeta.UpdateInput.AdvanceAfter on every UpdateCharge(...) call

• flatfee.Adapter.UpdateCharge(...) takes flatfee.ChargeBase and persists only base-row fields; do not call it just because flatfee.Realizations changed

• CreatePayment(...), UpdatePayment(...), CreateInvoicedUsage(...), and CreateCreditAllocations(...) already persist the realization-side rows; a follow-up UpdateCharge(...) is redundant unless base-row fields changed too

• flatfee.Charge.Realizations is expand-only data loaded from child tables; tests and service code should read payment/accrued-usage/credit-allocation state there, not from flatfee.State

• charge_flat_fees.status_detailed mirrors status today; schema changes or migrations that introduce new flat-fee statuses must keep both columns consistent through

Usage-based handler interface (usagebased.Handler):

• OnCreditsOnlyUsageAccrued(ctx, CreditsOnlyUsageAccruedInput) → creditrealization.CreateAllocationInputs — allocate credits for a realization run
• OnCreditsOnlyUsageAccruedCorrection(ctx, CreditsOnlyUsageAccruedCorrectionInput) → creditrealization.CreateCorrectionInputs — correct (partially revert) existing credit allocations when finalization discovers usage decreased

Credit purchase handler interface (creditpurchase.Handler):

• OnPromotionalCreditPurchase(ctx, Charge) → ledgertransaction.GroupReference
• OnCreditPurchaseInitiated(ctx, Charge) → ledgertransaction.GroupReference
• OnCreditPurchasePaymentAuthorized(ctx, Charge) → ledgertransaction.GroupReference
• OnCreditPurchasePaymentSettled(ctx, Charge) → ledgertransaction.GroupReference

flatfee/service/service.go Config requires a *lockr.Locker — when constructing in tests, create the locker before the flat fee service

When changing credit purchase charges:

• creditpurchase.ChargeBase stores base-row data: ManagedResource, Intent, Status (own creditpurchase.Status type); State exists but is an empty struct
• creditpurchase.Charge embeds ChargeBase + Realizations — all lifecycle outcomes live in Realizations, not State
• creditpurchase.Realizations holds CreditGrantRealization, ExternalPaymentSettlement, and InvoiceSettlement (all loaded from edge tables)
• CreditGrantRealization is stored in its own charge_credit_purchase_credit_grants table, not on the base row
• creditpurchase.Status mirrors the flatfee pattern: StatusCreated, StatusActive, StatusFinal, StatusDeleted with ToMetaChargeStatus() bridge
• charge_credit_purchases.status_detailed column mirrors status and is set via SetStatusDetailed(...) on create/update
• UpdateCharge(ctx, ChargeBase) (ChargeBase, error) only updates base-row fields — do not call it just because realization edges changed
• Realization edges are created/updated through dedicated adapter methods: CreateCreditGrant, CreateExternalPayment, UpdateExternalPayment, CreateInvoicedPayment, UpdateInvoicedPayment
• Adapter interface is composite: ChargeAdapter + CreditGrantAdapter + ExternalPaymentAdapter + InvoicedPaymentAdapter
• The withExpands helper in creditpurchase/adapter/charge.go adds .WithCreditGrant().WithExternalPayment().WithInvoicedPayment() to queries when ExpandRealizations is requested — use this helper instead of repeating the expand chain
• Service methods that only change edge data (e.g., HandleExternalPaymentAuthorized) update charge.Realizations in memory and return the full Charge without calling UpdateCharge
• Service methods that change status (e.g., HandleExternalPaymentSettled, onPromotionalCreditPurchase) call UpdateCharge(ctx, charge.ChargeBase) and merge the result back: charge.ChargeBase = updatedBase

Credit Realization Model

The creditrealization package (openmeter/billing/charges/models/creditrealization/) defines the domain model for credit allocations and corrections (partial/full reverts).

Type hierarchy

• CreateAllocationInput — positive-amount allocation input (has LineID). Collection type: CreateAllocationInputs.
• CreateCorrectionInput — positive-amount correction request (has CorrectsRealizationID). Collection type: CreateCorrectionInputs.
• CreateInput — unified DB write input (used by both allocations and corrections). Has Type field (TypeAllocation or TypeCorrection). Collection type: CreateInputs.
• Realization — full model read from DB, embeds CreateInput + NamespacedModel + ManagedModel + SortHint.
• Realizations — slice of Realization with query/aggregation methods.

Sign convention

• Allocations have positive Amount in CreateInput and DB.
• Corrections have negative Amount in CreateInput and DB (negated by AsCreateInputs).
• CreateCorrectionInput.Amount is always positive (the amount to correct). It gets negated when converting to CreateInput via CreateCorrectionInputs.AsCreateInputs().
• Realizations.Sum() returns the net total (allocations minus corrections).
• allocationsWithCorrections() computes remaining amounts by calling .Sub(corrections.Sum()) on each allocation. Since corrections have negative amounts in the DB, corrections.Sum() is negative, and .Sub(negative) correctly adds back — resulting in remaining = allocation + |corrections|. This is wrong — it makes remaining larger than the allocation. This is a known sign-convention risk: the Sub works correctly only if corrections are stored with positive amounts (old convention) or if the code is updated to use .Add(corrections.Sum()) with the new negative convention.

Correction flow

The full correction orchestration is Realizations.Correct(amount, currency, callback):

1. CreateCorrectionRequest(amount, currency) — builds CorrectionRequest items in reverse creation order (latest allocation first)
2. CorrectionRequest.ValidateWith(currency) — validates the request items
3. Calls callback(req) — caller (ledger handler) maps request items to CreateCorrectionInputs with ledger transaction references
4. CreateCorrectionInputs.ValidateWith(realizations, totalAmount, currency) — validates corrections don't exceed remaining per-allocation amounts
5. CreateCorrectionInputs.AsCreateInputs(realizations) — maps to []CreateInput with negated amounts, copies ServicePeriod from the corrected allocation

Unit test patterns for creditrealization

Tests are in correction_test.go (same package, not _test). Reusable helpers:

• allocationBuilder — builds allocation Realization entries with auto-incrementing SortHint and configurable CreatedAt
• correctionFor(allocation, amount) — builds a correction Realization targeting a given allocation
• correctionCallback(txGroupID) — returns a func(CorrectionRequest) (CreateCorrectionInputs, error) for use with Correct()
• correctionRequestAmounts(cr) / correctionRequestAllocationIDs(cr) — extract slices for assertions
• correctionInputsSum(inputs) — sums CreateCorrectionInputs amounts
• testCurrency(t) — returns a USD currencyx.Calculator

Test structure follows the rate_test pattern: declarative test cases with t.Run subtests, shared helpers, no DB required.

Adapter Gotchas

• resolveFeatureMeters(ctx, namespace, charges) takes an explicit namespace argument — do not access charges[0].Namespace directly (panics on empty slice)
• GetByMetas re-orders output to match input order; use lo.KeyBy (not lo.GroupBy) when building an intermediate lookup map — GroupBy produces map[K][]V and requires [0] indexing, KeyBy gives map[K]V directly
• refetchCharge in the state machine is a known interim pattern — the preferred direction is in-memory charge updates after adapter writes; avoid adding new refetchCharge calls without discussion
• buildCreateUsageBasedCharge is a builder chain — do not call the same setter twice (Ent builder chains accept duplicate .SetX calls silently, the last one wins)
• currencyx.Calculator.IsRoundedToPrecision(amount) is the preferred way to check if an amount is rounded to currency precision — use it instead of manual RoundToPrecision(x).Equal(x) patterns