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/**
* The flow ledger: what the service observed of each flow (PRD R22, R23).
*
* Two questions, two answers, deliberately kept apart from `FlowShapes`:
*
* - **`FlowShapes`** accumulates the *template-ref sequence* of an execution,
* because that is what drift compares. It holds an execution only while it is
* in flight and hands the sequence over once, at termination.
* - **the ledger** retains what the *diagrams* need, and the two diagrams need
* different things: one execution's ordered calls; and, per flow **kind**, a
* running union of the calls ever observed under it — what was seen, not what
* was declared anywhere.
*
* Retaining per-execution state keyed by the caller's ULID is also what closes
* R9's *partial* verdict: records are now indexed by flow id, so one execution's
* chain is a lookup rather than a scan.
*
* ## A call is a declaration, not a deduction
*
* The caller names both ends, and states both: the leg id it mints is the method the call
* reaches its callee with (`gateway.bundle.find`, `db/gateway.bundle.find`), and the unit
* that made the call and the receiver it expects are the `legFrom` and `legTo` it declared
* with it. So an edge is known from **one** observation — the caller's — and a receiver that
* never answers (missing, failing, or wired to the wrong address) still appears in the
* observed shape. What its silence costs is the `observed` count on the edge, which is a
* fact about the deployment instead of a line that cannot be drawn.
*
* The caller used to be read out of the id, which began with it. It is a field now because
* the id is what a diagram draws on the arrow: repeating the caller there made every label
* say what its own arrow already said (`public.gateway.bundle.find` between `public` and
* `gateway`), and the longer it got the less of the diagram could be read. Nothing is derived
* from the id today — an observation whose identity carries no caller contributes no edge,
* because the source of an arrow is not something to infer from whichever process happened
* to write a record (D-210, D-249).
*
* Whether that edge was *answered* is a separate question, answered by the
* **declaration** rather than by the record that carries the receipt. A leg a caller
* declared toward one target, and then observed from the receiving side, was
* answered by that target — whatever name the process that wrote the record happens
* to have. One process hosts many namespaces, and in development a whole suite, so a
* credit that compared the writer's service to the callee would make an answered call
* a property of how the deployment is split rather than of what happened (D-210).
*
* ## Position, not time
*
* Calls are ordered by the **counter path** the caller assigned (`1`, `1.2`), not
* by any timestamp. Two records emitted inside one millisecond cannot be ordered by
* a millisecond-resolution clock; measured on the inter-scheme fixture, 7 of its 14
* legs tied, and a direction resting on arrival order is a coin flip that looks
* like a measurement. The counter is assigned without a coordinator — the receiver
* numbers its own calls as children of the position it was handed — so the paths
* form a depth-first order of the execution, which is what a sequence diagram is
* drawn in.
*
* The union is a running aggregate, like the drift history: it is fed as events
* arrive rather than when an execution ends, so a *stalled* flow still contributes
* the calls it was seen to make, and evicting an execution's detail does not
* un-observe it. Retention has two bounds, and neither loss is silent — an execution
* dropped by the cap is reported by {@link FlowLedger.evictions} and a call dropped
* by the per-execution cap by {@link FlowLedger.truncations}.
*/
import {isUlid} from '../ulid.ts';
import {legOf, refFromFingerprint, type IngestEvent} from './registry.ts';
/** Executions whose detail is retained for the instance view. */
const DEFAULT_EXECUTION_LIMIT = 1000;
/** Calls one execution retains in detail, each end of a call counted apart. */
const DEFAULT_STEP_LIMIT = 256;
/** One observed traversal of one call, as one event reported it. */
export interface LegObservation {
/** The method the call reached its callee with — what a diagram labels the arrow. */
leg: string;
/** The process that emitted the record — information, never the caller. */
service: string;
/** The logical unit that declared the call; absent when the identity named none. */
from?: string;
/** The receiver the **caller** declared; absent on a receiver's own records. */
to?: string;
/** The call's position in the execution, as the caller assigned it. */
seq?: string;
/** The step the record sat in, when the emitter reported one. */
step?: string;
/** The flow position the emitter reported, when it reported one. */
index?: number;
time: number;
/** The record the observation came from, so a diagram can name its evidence. */
ref: string;
}
/** One declared call, and how much of it was seen. */
export interface LegEnd {
caller: string;
callee: string;
/** Declarations observed for this pair — one per execution, however many records it logged. */
count: number;
/** Of those, the attempts whose receiver was also observed. */
observed: number;
}
/** A call as the union knows it. */
export interface ObservedLeg {
leg: string;
/** Declarations observed for it, over every caller. */
count: number;
first: number;
last: number;
/** The step it was observed in, most often — the phase it belongs to. */
step?: string;
/** The position its declarations carried, first seen. Orders the union. */
seq?: string;
/** Services seen logging it, in first-observation order. */
services: string[];
/** Ends observed, most frequently declared first. */
ends: LegEnd[];
}
/** What was observed of one flow kind. */
export interface FlowUnion {
kind: string;
/** Executions observed under this kind. */
executions: number;
/** Every service seen in one of them, in first-observation order. */
services: string[];
/** Calls observed, in the order their positions give them. */
legs: ObservedLeg[];
}
/** What was observed of one execution, without its per-record detail. */
export interface FlowSummary {
id: string;
kind?: string;
/** Every service seen emitting a record of it, in first-observation order. */
services: string[];
/** Template refs traversed, in observation order — the shape drift compares. */
refs: string[];
/** Calls traversed, ordered by their positions. */
legs: string[];
/** Has the emitter reported a terminal status for it? */
closed: boolean;
}
/** What was observed of one execution. */
export interface FlowExecution extends FlowSummary {
observations: LegObservation[];
}
/** One declaration this execution made, and what has been counted for it. */
interface Declaration {
caller: string;
callee: string;
/** Has it been counted in the aggregate? */
counted: boolean;
/** Has a receipt been credited to it? */
credited: boolean;
}
/** What one execution has declared and seen answered for one call. */
interface LegSeen {
/** Declarations made for the call, by pair. */
declared: Map<string, Declaration>;
/**
* Services observed carrying the call without declaring it — the receiving side.
* Who wrote a receipt is **information**, for the reader of a diagram: the credit
* below rests on the declaration, never on these names.
*/
answered: Set<string>;
}
/**
* Whether a receipt seen for a leg can be attributed to a declaration.
*
* A record with no declared target says *that* the leg was answered, and not which target
* answered it. That is enough when the leg was declared toward one callee in this
* execution — and not when it was declared toward two, which is one call id reused toward
* two receivers, i.e. the caller misuse the id exists to make impossible. Contradictory
* data gets the conservative reading: neither edge is credited, so both are drawn
* unanswered, instead of a coin flip deciding which of them claims to have been answered.
*
* Exported so the diagram and the ledger reach the same verdict about an arrow
* from this rule, and a second copy of it is how the two views drifted once already.
*/
export function attributable(declaredTargets: number, receipts: number): boolean {
return declaredTargets === 1 && receipts > 0;
}
/**
* Per-execution state.
*/
interface ExecutionState {
kind?: string;
/**
* The kind's aggregate, once the kind is known. Held here rather than looked up
* per observation because a kindless execution has none — which is the one
* reachable case the aggregation has to pass over, and it cannot be expressed as
* a lookup that is allowed to fail.
*/
union?: KindState;
/** Every service seen emitting a record of this execution — a record needs no leg to count. */
services: Set<string>;
observations: LegObservation[];
/** Event ids already observed, so a redelivery is not a second attempt. */
seen: Set<string>;
/**
* The observations this execution already retains, one per call and end.
*
* A call site that makes the same call again in one execution — a loop over a
* list, a retry — is still the one call a diagram draws, and {@link FlowLedger.aggregate}
* already counts it once. Without this, every record of every repetition was
* retained, and the per-execution budget was spent on one call: measured on the
* gateway's test flow, 296 records for four leg ids truncated 150 steps, and the
* diagram showed only the first step of the group.
*/
retained: Set<string>;
/**
* What this execution declared and saw answered, per call.
*
* Kept per execution because both counters are **per execution**: a receiver that
* logs three records about one call answered it once, and credit has to be given
* when the two halves arrive in either order — a receipt can reach the service
* before the declaration that aimed at it, since two processes flush their sinks
* independently.
*/
legs: Map<string, LegSeen>;
refs: string[];
closed: boolean;
/** Monotonic recency, for eviction. */
touched: number;
}
interface LegAggregate {
count: number;
first: number;
last: number;
/** Step name to the declarations observed in it. */
steps: Map<string, number>;
/** Services seen logging this call, in first-observation order. */
services: Set<string>;
seq?: string;
ends: Map<string, LegEnd>;
}
interface KindState {
executions: number;
services: Set<string>;
legs: Map<string, LegAggregate>;
touched: number;
}
/** The key one declared pair is aggregated under. */
function pairKey(caller: string, callee: string): string {
return `${caller}\u0000${callee}`;
}
/**
* A path of counters, zero-padded per component so a plain string compare orders
* paths numerically: `1` < `1.1` < `2`. Six digits is more siblings than an
* execution can have records.
*/
function seqKey(seq: string): string {
return seq
.split('.')
.map(part => part.padStart(6, '0'))
.join('.');
}
/** Order two calls by the position they were given, a tie by id. */
export function comparePosition(
a: {id: string; seq?: string},
b: {id: string; seq?: string},
): number {
if (a.seq !== undefined && b.seq !== undefined) {
return seqKey(a.seq).localeCompare(seqKey(b.seq)) || a.id.localeCompare(b.id);
}
// A call observed without a position (an emitter that sent none) sorts last, and
// two of those are ordered by id: the order stays total and deterministic.
if (a.seq === b.seq) {
return a.id.localeCompare(b.id);
}
return a.seq === undefined ? 1 : -1;
}
/** The most frequent key of a count map, ties settled by first sight. */
function mostFrequent(counts: Map<string, number>): string | undefined {
let best: string | undefined;
let bestCount = 0;
for (const [key, count] of counts) {
if (count > bestCount) {
best = key;
bestCount = count;
}
}
return best;
}
/**
* The flow ledger. Fed from the ingest's accepted-event seam, beside the lineage
* index, so both views are built from one stream of events in one order.
*/
export class FlowLedger {
private readonly executionLimit: number;
private readonly stepLimit: number;
private readonly byExecution = new Map<string, ExecutionState>();
private readonly byKind = new Map<string, KindState>();
private sequence = 0;
private evictionCount = 0;
private truncationCount = 0;
constructor(
executionLimit: number = DEFAULT_EXECUTION_LIMIT,
stepLimit: number = DEFAULT_STEP_LIMIT,
) {
this.executionLimit = executionLimit;
this.stepLimit = stepLimit;
}
/**
* Record one accepted event.
*
* An event with no flow, a flow id that is not a ULID, or no call is not an
* error. The first two are already reported by `FlowShapes` (which counts a flow
* id it cannot place as a straggler) and are declined here rather than counted
* twice; the third simply means the record belongs to no observed call. Nothing
* on this path throws: it runs on the ingest, where a peer's malformed value must
* not break a batch (D3).
*
* A record naming an execution whose terminal status was already reported is
* **still observed**, and that is deliberate: between two processes arrival order is
* not emission order. Measured on the single-scheme fixture, the payer's sink is
* flushed first, so its `transfer complete` reached the service before the hub's
* last four records — and refusing them lost the hub's own declaration of
* `hub.transfer.deliver`, i.e. a whole edge of the flow, plus three receipts. The
* execution's end is a fact about the emitter's report (`closed`), not a claim about
* what other participants have yet to deliver.
*/
observe(event: IngestEvent): void {
const flow = event.flow;
if (flow === undefined || !isUlid(flow.id)) {
return;
}
const state = this.byExecution.get(flow.id) ?? this.open(flow.id);
if (state.kind === undefined && typeof flow.kind === 'string' && flow.kind.length > 0) {
state.kind = flow.kind;
state.union = this.countKind(flow.kind);
}
state.touched = ++this.sequence;
// A record needs no call to prove its service took part: the entry records that
// sit between hops belong to the flow, and a diagram that omitted their service
// would be missing a participant the reader can see in the records.
state.services.add(event.service);
const leg = legOf(event);
if (leg !== undefined && !state.seen.has(event.id)) {
state.seen.add(event.id);
const observation: LegObservation = {
leg: leg.id,
service: event.service,
from: leg.from,
to: leg.to,
seq: leg.seq,
step: flow.step,
index: flow.index,
time: event.time,
ref: event.id,
};
// The counters are kept for every record — they are per execution and
// idempotent, and a repetition's own timestamps are the honest ones — while
// the *detail* is retained once per call and end: a leg is one call however
// many records were logged about it, which is the unit the cap is meant to
// bound. The key carries the caller as well as the method, because two units
// may call the same method in one execution — two calls, two arrows, and the
// second is not a repeat of the first.
this.aggregate(state, observation);
const call = `${leg.id}\u0000${leg.from ?? ''}\u0000${leg.to === undefined ? 'receipt' : 'declaration'}`;
if (!state.retained.has(call)) {
state.retained.add(call);
if (state.observations.length < this.stepLimit) {
state.observations.push(observation);
} else {
this.truncationCount++;
}
}
}
if (state.refs.length < this.stepLimit) {
state.refs.push(refFromFingerprint(event.fingerprint));
}
if (flow.status === 'completed' || flow.status === 'failed') {
state.closed = true;
}
this.enforceLimit();
}
/** The retained detail of one execution, or `undefined` when it is not held. */
executionOf(flowId: string): FlowExecution | undefined {
const state = this.byExecution.get(flowId);
if (state === undefined) {
return undefined;
}
return {
...this.summaryOf(flowId, state),
observations: state.observations.map(o => ({...o})),
};
}
/**
* Every retained execution, most recently observed first, without its per-record
* detail. Bounded by the retention cap, like the detail itself.
*
* The recency counter is bumped by every accepted event and never reset, so two
* retained executions cannot hold the same one and the order needs no tie-break.
*/
executions(): FlowSummary[] {
return [...this.byExecution.entries()]
.sort((a, b) => b[1].touched - a[1].touched)
.map(([flowId, state]) => this.summaryOf(flowId, state));
}
/** The summary view of one execution's state. */
private summaryOf(flowId: string, state: ExecutionState): FlowSummary {
return {
id: flowId,
kind: state.kind,
services: [...state.services],
refs: [...state.refs],
legs: callsOf(state.observations),
closed: state.closed,
};
}
/** The union observed for one kind, or `undefined` when it has none. */
unionOf(kind: string): FlowUnion | undefined {
const state = this.byKind.get(kind);
if (state === undefined) {
return undefined;
}
return {
kind,
executions: state.executions,
services: [...state.services],
legs: [...state.legs.entries()]
.map(([leg, aggregate]) => ({
leg,
count: aggregate.count,
first: aggregate.first,
last: aggregate.last,
step: mostFrequent(aggregate.steps),
seq: aggregate.seq,
services: [...aggregate.services],
ends: [...aggregate.ends.values()]
.map(end => ({...end}))
.sort(
(a, b) =>
b.count - a.count ||
a.caller.localeCompare(b.caller) ||
a.callee.localeCompare(b.callee),
),
}))
.sort((a, b) => comparePosition({id: a.leg, seq: a.seq}, {id: b.leg, seq: b.seq})),
};
}
/** The kinds observed, most recently touched first. */
kinds(): string[] {
return [...this.byKind.entries()]
.sort((a, b) => b[1].touched - a[1].touched || a[0].localeCompare(b[0]))
.map(([kind]) => kind);
}
/** How many executions are retained in detail. */
size(): number {
return this.byExecution.size;
}
/** Executions dropped by the retention cap. */
evictions(): number {
return this.evictionCount;
}
/** Calls dropped by the per-execution cap. */
truncations(): number {
return this.truncationCount;
}
/** Every union, for the snapshot. */
unions(): FlowUnion[] {
return this.kinds().map(kind => this.unionOf(kind) as FlowUnion);
}
/**
* Replace the unions with those read from a snapshot.
*
* Only the unions are restored: the per-execution detail is a process-lifetime
* surface, like the lineage index, the incident store and the drift history. What
* should outlive a restart is what a restart cannot re-derive — the calls observed
* under a kind — while one execution's steps are of interest only while it is
* recent. The loader validates the shape before this runs (`persistence.ts`), so a
* malformed union is reported there rather than half-restored here.
*/
restore(unions: readonly FlowUnion[]): void {
this.byKind.clear();
for (const union of unions) {
const legs = new Map<string, LegAggregate>();
for (const leg of union.legs) {
legs.set(leg.leg, {
count: leg.count,
first: leg.first,
last: leg.last,
steps: new Map(leg.step === undefined ? [] : [[leg.step, leg.count]]),
services: new Set(leg.services),
seq: leg.seq,
ends: new Map(leg.ends.map(end => [pairKey(end.caller, end.callee), {...end}])),
});
}
this.byKind.set(union.kind, {
executions: union.executions,
services: new Set(union.services),
legs,
touched: ++this.sequence,
});
}
}
/** Open a fresh execution. The kind is adopted (and counted) by the observer. */
private open(flowId: string): ExecutionState {
const state: ExecutionState = {
services: new Set<string>(),
observations: [],
seen: new Set<string>(),
retained: new Set<string>(),
legs: new Map<string, LegSeen>(),
refs: [],
closed: false,
touched: 0,
};
this.byExecution.set(flowId, state);
return state;
}
/** Count one execution toward a kind, creating the kind's aggregate if needed. */
private countKind(kind: string): KindState {
const state = this.byKind.get(kind) ?? {
executions: 0,
services: new Set<string>(),
legs: new Map<string, LegAggregate>(),
touched: 0,
};
state.executions++;
this.byKind.set(kind, state);
return state;
}
/**
* Add one observation to its kind's aggregate.
*
* Both counters are **per execution, not per record**: a receiver that logs three
* records about one call answered it once, and one call declared once cannot be
* answered twice. A record that **declares** a receiver states the edge; a record
* that merely carries the call — the receiver's own — records who answered it.
*
* The declaration's bookkeeping is therefore *re-applied* after either half
* arrives, and the flags make that idempotent: a receipt can reach the service
* before the declaration that aimed at it, since two processes flush their sinks
* independently, and crediting on arrival order would have made the answer depend
* on which of them happened to flush first.
*
* A call seen only from the receiver's side appears with **no end at all**, which is
* the honest shape of it: nobody said who called, and the union reports that rather
* than inventing a caller from whichever service it happened to see first.
*
* A receipt is credited to the declaration, and attributed through it (see
* {@link attributable}): the receiving side's own name is on the observation and in
* the union's `services`, but it is never the condition.
*/
private aggregate(state: ExecutionState, observation: LegObservation): void {
const kindState = state.union;
if (kindState === undefined) {
// A flow with no stable name has no union to be observed under. Its calls
// are still retained for the instance view, and the loss is the same one
// `FlowShapes` counts as kindless — counted there, not twice.
return;
}
kindState.touched = this.sequence;
kindState.services.add(observation.service);
const aggregate = kindState.legs.get(observation.leg) ?? {
count: 0,
first: observation.time,
last: observation.time,
steps: new Map<string, number>(),
services: new Set<string>(),
seq: observation.seq,
ends: new Map<string, LegEnd>(),
};
kindState.legs.set(observation.leg, aggregate);
aggregate.first = Math.min(aggregate.first, observation.time);
aggregate.last = Math.max(aggregate.last, observation.time);
aggregate.services.add(observation.service);
if (observation.step !== undefined) {
aggregate.steps.set(observation.step, (aggregate.steps.get(observation.step) ?? 0) + 1);
}
const seen = state.legs.get(observation.leg) ?? {
declared: new Map<string, Declaration>(),
answered: new Set<string>(),
};
state.legs.set(observation.leg, seen);
if (observation.to === undefined) {
// A record with no declared target is the receiving side of this leg. Which
// service wrote it is information, not a condition: one service can host many
// realms, adapters and orchestrators, so requiring its name to equal the callee
// would make an answered call a property of how the deployment is split rather
// than of what happened.
seen.answered.add(observation.service);
} else {
// The caller is the unit the identity declares — the logical unit that made the
// call, which in blong is the namespace that served it — so an arrow starts at a
// namespace and reads the same in a monolith and in microservices. It is *not*
// derived from the id any more, and the process that wrote the record is never
// consulted: one process hosts many namespaces, so an identity taken from the
// writer collapses every participant of a monolith into one, which is a property
// of how the deployment is split rather than of what happened (D-210). An
// identity that named no caller contributes no end at all: the source of an arrow
// is not something to invent, and the counters below already treat silence as
// silence.
const caller = observation.from;
if (caller !== undefined) {
// Created **once** per pair: a call site that logs several records about the
// one call it made (a request and its answer, a preparation and its commit)
// is one attempt, and re-creating the entry would reset the flags below and
// count the same attempt again — which is how the payer's discovery, logged
// twice inside one leg, came to be reported as two calls.
const key = pairKey(caller, observation.to);
if (!seen.declared.has(key)) {
seen.declared.set(key, {
caller,
callee: observation.to,
counted: false,
credited: false,
});
}
}
}
for (const [key, declared] of seen.declared) {
const end = aggregate.ends.get(key) ?? {
caller: declared.caller,
callee: declared.callee,
count: 0,
observed: 0,
};
aggregate.ends.set(key, end);
if (!declared.counted) {
declared.counted = true;
aggregate.count++;
end.count++;
}
if (!declared.credited && attributable(seen.declared.size, seen.answered.size)) {
declared.credited = true;
end.observed++;
}
}
}
/** Evict the least-recently-observed execution when the cap is exceeded. */
private enforceLimit(): void {
if (this.byExecution.size <= this.executionLimit) {
return;
}
// The same shape `FlowShapes` uses, down to the sentinel: the map is never
// empty here (its size exceeds the cap), so the loop always finds a victim,
// and a guard for "no victim" would be a branch nothing can take.
let victim = '';
let oldest = Number.POSITIVE_INFINITY;
for (const [flowId, state] of this.byExecution) {
if (state.touched < oldest) {
oldest = state.touched;
victim = flowId;
}
}
this.byExecution.delete(victim);
this.evictionCount++;
}
}
/** The calls one execution made or answered, deduplicated, in position order. */
function callsOf(observations: readonly LegObservation[]): string[] {
const seen = new Map<string, string | undefined>();
for (const observation of observations) {
if (!seen.has(observation.leg)) {
seen.set(observation.leg, observation.seq);
}
}
return [...seen.entries()]
.map(([leg, seq]) => ({id: leg, seq}))
.sort(comparePosition)
.map(leg => leg.id);
}
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