WEB / WEB-21.02

Traffic routing, scaling, and availability

Trace name resolution, serving hops, desired/ready replicas, and user-flow results separately.

CORE · RED · BLUE · ARTIFACT · PRACTICE

Read every scenario without motion

Origin path

Start after miss
After the supplied DNS resolution and edge-cache miss, the edge has prepared a public origin read. The balancer has not received this request yet.
Client / resolver projection
Resolution supplied; waiting
Edge projection
Origin read prepared
Local balancer
Balancer request unknown
Application pool
Pool request unknown
Public note owner
Tea note authoritative
Balancer arrival
The edge prepared request arrives at the balancer; application-pool and client knowledge remain in their prior states.
Client / resolver projection
Resolution supplied; waiting
Edge projection
Origin read prepared
Local balancer
Public read received
Application pool
Pool request unknown
Public note owner
Tea note authoritative
Select target
The local literal-ready policy selects app-a and prepares the query; readiness has granted no private permission.
Client / resolver projection
Resolution supplied; waiting
Edge projection
Origin read prepared
Local balancer
app-a query prepared
Application pool
Pool request unknown
Public note owner
Tea note authoritative
Application arrival
The selected app-a request becomes known at pool arrival; reading the authoritative note remains separate local work.
Client / resolver projection
Resolution supplied; waiting
Edge projection
Origin read prepared
Local balancer
app-a query prepared
Application pool
app-a read received
Public note owner
Tea note authoritative
Local note read
The pool locally reads Tea title from its own note owner and prepares a response bound to serving identity app-a.
Client / resolver projection
Resolution supplied; waiting
Edge projection
Origin read prepared
Local balancer
app-a query prepared
Application pool
Tea response prepared
Public note owner
Tea note authoritative
Balancer return
The application response arrives at the balancer; the edge has neither received it nor delivered it to a viewer.
Client / resolver projection
Resolution supplied; waiting
Edge projection
Origin read prepared
Local balancer
app-a result received
Application pool
Tea response prepared
Public note owner
Tea note authoritative
Prepare for edge
The balancer locally prepares a response for the edge; this processing step is not receiver arrival.
Client / resolver projection
Resolution supplied; waiting
Edge projection
Origin read prepared
Local balancer
Edge response prepared
Application pool
Tea response prepared
Public note owner
Tea note authoritative
Edge arrival
The prepared result reaches the edge; the bounded scene ends here and the client remains waiting.
Client / resolver projection
Resolution supplied; waiting
Edge projection
Origin result received
Local balancer
Edge response prepared
Application pool
Tea response prepared
Public note owner
Tea note authoritative

One unready

Down snapshot
The declared readiness snapshot supplies a=false and b=true; this fixture is not a real replica kill.
Client / resolver projection
Client waiting
Edge projection
Edge waiting
Local balancer
Public read known
Application pool
a=false; b=true supplied
Public note owner
Tea note authoritative
Derive eligible
The local policy excludes observed-false a and admits observed-true b; failure domains are not measured.
Client / resolver projection
Client waiting
Edge projection
Edge waiting
Local balancer
Eligible b only
Application pool
a=false; b=true supplied
Public note owner
Tea note authoritative
Prepare for b
The balancer prepares the public request for selected b; the pool receiver has not received it yet.
Client / resolver projection
Client waiting
Edge projection
Edge waiting
Local balancer
app-b read prepared
Application pool
a=false; b=true supplied
Public note owner
Tea note authoritative
b arrival
The selected b request becomes known at pool arrival; no result has yet returned to the caller.
Client / resolver projection
Client waiting
Edge projection
Edge waiting
Local balancer
app-b read prepared
Application pool
app-b read received
Public note owner
Tea note authoritative
b result prepared
The own public-title read becomes b response; a prepared result does not establish a measured HA guarantee.
Client / resolver projection
Client waiting
Edge projection
Edge waiting
Local balancer
app-b read prepared
Application pool
app-b result prepared
Public note owner
Tea note authoritative
b result arrival
The response arrives at the balancer, showing that surviving b produced this finite request result in the source projection.
Client / resolver projection
Client waiting
Edge projection
Edge waiting
Local balancer
app-b result received
Application pool
app-b result prepared
Public note owner
Tea note authoritative
Scope boundary
The local routing result is not expanded into regional failover, parallel capacity, or an uptime SLA.
Client / resolver projection
Client waiting
Edge projection
Edge waiting
Local balancer
Limited route result
Application pool
app-b result prepared
Public note owner
Tea note authoritative

No ready

Read known
The public read is assumed received; false readiness for a and missing readiness for another instance are distinct knowledge.
Client / resolver projection
Client waiting
Edge projection
Edge waiting
Local balancer
Public read known
Application pool
a=false; unknown missing
Public note owner
Tea note authoritative
No eligible
No literal true is found, so eligibility is empty; unknown health is not renamed as a confirmed failure.
Client / resolver projection
Client waiting
Edge projection
Edge waiting
Local balancer
Eligible set empty
Application pool
a=false; unknown missing
Public note owner
Tea note authoritative
Unavailable prepared
The selected unavailable policy prepares 503, keeps forwarded=false, and invents no application request.
Client / resolver projection
Client waiting
Edge projection
Edge waiting
Local balancer
503 response prepared
Application pool
a=false; unknown missing
Public note owner
Tea note authoritative
Edge arrival
The balancer prepared unavailable response arrives at the edge; the pool retains its previous no-request state.
Client / resolver projection
Client waiting
Edge projection
503 response received
Local balancer
503 response prepared
Application pool
a=false; unknown missing
Public note owner
Tea note authoritative
Separate category
The edge classifies the result locally; it gains no new evidence about missing-health causes or viewer delivery.
Client / resolver projection
Client waiting
Edge projection
Unavailable classified
Local balancer
503 response prepared
Application pool
a=false; unknown missing
Public note owner
Tea note authoritative

Desired / ready

Prior pool
Initially desired2 and ready2 match; this coincidence does not equate their lifecycle identities.
Client / resolver projection
Public flow selected
Edge projection
Edge unchanged
Local balancer
Desired2; ready2
Application pool
a/b ready
Public note owner
Tea note authoritative
Intent grows
The supplied control intent becomes desired3; no new ready instance or measured capacity has been added.
Client / resolver projection
Public flow selected
Edge projection
Edge unchanged
Local balancer
Desired3; ready2
Application pool
a/b ready
Public note owner
Tea note authoritative
Create separate
The own fixture shows c exists but ready=false; created count is3 while ready count remains2.
Client / resolver projection
Public flow selected
Edge projection
Edge unchanged
Local balancer
Desired3; ready2
Application pool
a/b ready; c created
Public note owner
Tea note authoritative
Traffic gate
The local routing list does not select initializing c; desired count grants neither request permission nor usable capacity.
Client / resolver projection
Public flow selected
Edge projection
Edge unchanged
Local balancer
Eligible a/b only
Application pool
a/b ready; c created
Public note owner
Tea note authoritative
Ready supplied
Declared completed initialization supplies ready=true for c; no actual HPA/probe runtime or timing is measured.
Client / resolver projection
Public flow selected
Edge projection
Edge unchanged
Local balancer
Eligible a/b only
Application pool
a/b/c ready supplied
Public note owner
Tea note authoritative
Derive eligible3
A ready set of3 is now derived; this finite count establishes no throughput, regional redundancy, or HA percentage.
Client / resolver projection
Public flow selected
Edge projection
Edge unchanged
Local balancer
Desired3; ready3
Application pool
a/b/c ready supplied
Public note owner
Tea note authoritative

ACTORS · STATE · DATA FLOW

A DNS answer is not an HTTP response

DNS helps resolve names to resource records. It is not a forwarding hop carrying an HTTP request body to the application. After a resolver obtains an address or related record data, the client makes a separate connection to the selected endpoint. DNS answers concern name authority and resolver-cache lifetime; they do not establish that the application can read a note now or authorize the caller. public.example.invalid and edge-demo are supplied aliases, with no queries or record changes.

DNS → CDN → load balancer → application is a common responsibility chain, not a mandatory sequence for every website. A CDN can serve public representations at the edge and contact an origin on a selected miss. A balancer selects instances within a target pool. One product can combine roles and another application may have no CDN. Each HTTP boundary produces separate request/response knowledge; a cached DNS result is neither a CDN hit nor an origin response.

The normal scene starts after supplied resolution and an edge-miss decision: the edge has prepared the backend request, which the balancer has not seen. The balancer receives it and selects ready application-a. The application pool produces an own Tea note response. The bounded scene ends when that response returns through the balancer to the edge. Client/resolver remains waiting because final viewer delivery is outside this slice. Native adapters observe an in-process relationship without DNS, CDN, TLS, or network delivery.

Scale-out, ready capacity, and high availability

Horizontal scaling shares work across more instances or replicas, whereas vertical scaling adds resources to existing instances. Desired replica count is controller intent; created and ready counts may not reach it simultaneously. Kubernetes HPA is a named example of a periodic metric-driven control loop updating a workload scale target, while startup, dependency initialization, and readiness progress separately. desired3/ready2 are explicit finite fixture values, not production thresholds or HPA-formula output.

High availability aims to keep a selected user flow usable under failures. Multiple replicas do not automatically solve a shared-database, edge-configuration, or fault-domain failure. Remaining replicas need sufficient capacity, correct routing, and access to required state. The fixture routes a public read to b when a has observed ready=false. That limited result is not regional failover or an uptime/SLA measurement.

Health checks report a selected predicate within an observation scope. Liveness can support recovery decisions, readiness supports traffic eligibility, and an end-to-end synthetic check asks a different user-flow question. A missing readiness field is unknown, not an observed false health fact. A policy can exclude both unknown and false instances while preserving different diagnostic reasons. Probe responses are not evidence of authentication or note-level authorization.

Read every scenario without motion

Desired vs ready

Scaler intent
desiredBefore is 2, desiredAfter and createdAfter 3. These indicate provisioning intent and existence; readyDuringInit remains 2 and new app-c is not ready.
Pool and scale ledger
Desired3 / created3 / ready2
Traffic eligibility gate
Not yet evaluated
Public-read result
Not yet evaluated
Calculate eligible pool
Traffic selection requires literal ready true. Although app-c exists, it is ineligible during initialization; copying desired count into target count is incorrect.
Pool and scale ledger
Desired3 / created3 / ready2
Traffic eligibility gate
Init eligible app-a/app-b only
Public-read result
Not yet evaluated
Bound capacity claims
readyAfter 3 adds app-c to the pool, but capacityMeasured is false. Three ready instances do not establish three independent failure domains or an SLA.
Pool and scale ledger
Desired3 / created3 / ready2
Traffic eligibility gate
Init eligible app-a/app-b only
Public-read result
Ready3 / capacity unmeasured
One target down
oneDown marks app-a unready and app-b ready; the deterministic policy selects app-b. Correct servedBy app-b association answers the routing question.
Pool and scale ledger
app-a false / app-b selected
Traffic eligibility gate
Init eligible app-a/app-b only
Public-read result
Ready3 / capacity unmeasured
Authority belongs elsewhere
Health or readiness grants no private-operation authority. This is public-note reading; privateAuthorizationVerified is false, and business policy remains with its application owner.
Pool and scale ledger
app-a false / app-b selected
Traffic eligibility gate
Readiness ≠ private authorization
Public-read result
Ready3 / capacity unmeasured
Retain the delivery boundary
The normal result stops at edge received response. Client delivery is unmeasured; reaching the edge is not completed viewer flow or measured regional failover.
Pool and scale ledger
app-a false / app-b selected
Traffic eligibility gate
Readiness ≠ private authorization
Public-read result
Edge response known / client delivery outside scope

Unknown pool

Missing readiness
app-unknown has readinessObserved false and health unknown. An absent field proves neither health nor failure; both require different evidence.
Pool and scale ledger
Unknown health / missing readiness
Traffic eligibility gate
Not yet evaluated
Public-read result
Not yet evaluated
Bounded eligible set
The selected control requires literal true. With app-a false and app-unknown missing, eligibility is empty; defaulting unknown to true violates this policy.
Pool and scale ledger
Unknown health / missing readiness
Traffic eligibility gate
False + missing → eligible empty
Public-read result
Not yet evaluated
503 without forwarding
noReady has selected null, forwarded false and status 503. There is no application receipt or note read; unavailability does not diagnose a particular process crash.
Pool and scale ledger
Unknown health / missing readiness
Traffic eligibility gate
False + missing → eligible empty
Public-read result
No target / no forwarding / 503
DNS is another mechanism
dnsAnswerAlias edge-demo is supplied resolution knowledge, with dnsQueryExecuted false. DNS is not an HTTP forwarder; resolving a name does not guarantee a ready application.
Pool and scale ledger
DNS answer ≠ ready target
Traffic eligibility gate
False + missing → eligible empty
Public-read result
No target / no forwarding / 503
Failure-domain question
App-a and app-b can share a dependency. Ready count or one finite rerouting case does not establish measured availability, capacity or independent failure domains.
Pool and scale ledger
DNS answer ≠ ready target
Traffic eligibility gate
Replica count ≠ independent availability
Public-read result
No target / no forwarding / 503
Retain the unknown reason
Not routing is a known policy result while missing-readiness cause remains unknown. Even health-check 200 or process-alive evidence would require separate caller-authority checks.
Pool and scale ledger
DNS answer ≠ ready target
Traffic eligibility gate
Replica count ≠ independent availability
Public-read result
No-ready decision known / cause unknown

ASSUMPTION · BOUNDARY · CONTROL

Mistaking resolution and box counts for availability

A successful DNS lookup does not establish completion of the selected public request across the whole chain. An edge miss, an empty eligible target pool, and an application dependency error are different boundaries. Review actual request/response knowledge instead of assigning every final status to an origin failure. Ending at edge arrival prevents counting an undelivered representation as a completed viewer flow.

Reading desired3 as capacity3 overstates capability during ordinary provisioning or startup. Three instances sharing one dependency do not create three independent availability paths. Record scaling intent separately from traffic eligibility. The fixture supplies ready=false for new c, leaving only a/b eligible; supplied ready=true adds c later. No real parallel capacity or latency was measured.

Treating missing observations as healthy or authorized

Defaulting missing readiness to true can route traffic to an unknown target. Defaulting it to false and reporting confirmed failure invents different knowledge. The selected control admits only literal true and retains unknown as the missing reason. An empty eligible pool produces a selected unavailable result without sending a request to the application. That does not replace diagnostic evidence about why each instance is unready.

A health200 or a live process confers no caller authority for a private operation. Authentication, authorization, and business-result verification remain with their owners. This fixture reads a public note only and does not test private-object permissions. Production review also considers observation age and failure-domain dependencies. The boolean adapter is not a universal vendor fail-open or fail-closed policy.

Read every scenario without motion

Pool controls

Normal ready baseline
The normal pool marks app-a/app-b true and deterministic first-ready selects app-a. Result servedBy is app-a with Tea note; instance association is checked separately.
Readiness records
app-a selected / servedBy app-a
Routing review
Not yet evaluated
Outcome evidence
Not yet evaluated
Down-branch comparison
When app-a becomes false, app-b remains true and is selected. The fallback identifies the serving target without measuring a real balancer or cloud failover.
Readiness records
app-a selected / servedBy app-a
Routing review
app-b selected / servedBy app-b
Outcome evidence
Not yet evaluated
Unknown versus false
noReady contains false and missing readiness. Neither is eligible, but the missing reason is unknown and is not merged with confirmed unreadiness.
Readiness records
app-a selected / servedBy app-a
Routing review
app-b selected / servedBy app-b
Outcome evidence
Both ineligible / evidence reasons distinct
Scale-time context
Desired, created and ready counts are retained together. Initialization has 2 ready instances and completion 3; these authored stages are not latency or autoscaler benchmarks.
Readiness records
Intent/existence/readiness distinct
Routing review
app-b selected / servedBy app-b
Outcome evidence
Both ineligible / evidence reasons distinct
Outcome recipient
Edge response receipt is not client delivery. User-flow success requires correct title, usable status and actual recipient stage; the final owner narrows the scope.
Readiness records
Intent/existence/readiness distinct
Routing review
Edge receipt / client completion unmeasured
Outcome evidence
Both ineligible / evidence reasons distinct
Finite routing evidence
The input list is independently filtered and target compared with servedBy. This establishes the route decision without adding DNS, Kubernetes, capacity or SLA results.
Readiness records
Intent/existence/readiness distinct
Routing review
Edge receipt / client completion unmeasured
Outcome evidence
Finite selection verified / availability broader

Health gap

Visible target count
The review sees only created count 3 without readiness or selected-target records. Listing three processes does not identify the recipient of a public request.
Readiness records
Created3 / readiness missing
Routing review
Not yet evaluated
Outcome evidence
Not yet evaluated
Do not copy the expected pool
The scale fixture expects readyDuringInit 2 and readyAfter 3. Copying completion state into an incomplete event could present initialization as finished.
Readiness records
Created3 / readiness missing
Routing review
Expected ready set ≠ observation
Outcome evidence
Not yet evaluated
User-flow uncertainty
Current eligibility, selected target and edge/client delivery remain UNKNOWN. Created count is existence evidence only, not health, usable capacity or request success.
Readiness records
Created3 / readiness missing
Routing review
Expected ready set ≠ observation
Outcome evidence
Eligibility/selection/delivery UNKNOWN
Required route correlation
Readiness observation time, route purpose, selected instance and response source are needed. Pool and application outcome must be associated with the same public request without private bodies.
Readiness records
Needed: ready time/target/result recipient
Routing review
Expected ready set ≠ observation
Outcome evidence
Eligibility/selection/delivery UNKNOWN
Signal health is separate
Missing readiness could reflect target or telemetry failure. Without observation-pipeline coverage, an empty health table does not prove the entire pool down.
Readiness records
Needed: ready time/target/result recipient
Routing review
Target health ≠ observation pipeline health
Outcome evidence
Eligibility/selection/delivery UNKNOWN
Purpose of new evidence
New evidence must answer routing and delivery-recipient questions separately. This event retains only creation count without adding SLA or server-permission success.
Readiness records
Needed: ready time/target/result recipient
Routing review
Target health ≠ observation pipeline health
Outcome evidence
Collect scoped routing/delivery evidence

IMPLEMENTATION · SECURITY CONTROL · VERIFICATION

Separate intent, observation, and selected targets

Keep desired count, observed instances, and the ready set together but as separate fields. For the public read, expose route purpose, the balancer-selected instance, and response source. Artifact/version equality across ready instances requires configuration evidence, not a count. Deterministic first-ready selection is only fixture policy. The native checker independently filters explicit lists and checks distinctions among unknown, false, and true fields.

Trace edge-to-balancer request arrival, balancer-to-application arrival, and the response relationship as distinct snapshots. Only the receiver gains knowledge on arrival. The authoritative note read inside the pool is a local adapter, not a fabricated database packet. Independent JSON parsing checks the finite public-note result, selected instance, and title. No DNS, balancer, or cloud API call occurs.

Bind availability claims to the user flow

Define a successful result for the selected operation: correct public title, usable status, and the endpoint where delivery occurred. A result from b in the replica-down case differs from absence of forwarding in the no-ready case. An HA claim needs measured user-flow availability, remaining capacity, and failure-domain evidence; these finite cases produce no percentage or SLA. DNS TTL and CDN lifetime do not imply globally instantaneous propagation.

Assess application health separately from telemetry-pipeline health, developed in the next lesson. A missing-observation case must not equate no event with no failure. The XML selector detects unknown readiness with selected=true; zero matching events alone do not prove safe routing. Native, authored-model, and browser/UI scopes stay distinct. Model motion creates neither deployment status nor a permission grant.

NOW APPLY IT TO THE CASE

Is desired replica count traffic eligibility or availability?

A normal public read gets Tea note from app-a. When app-a is unready, app-b is selected; creating app-c does not grow the ready pool until it becomes ready. app-unknown is not treated as healthy, and no ready targets yields 503 without forwarding.

Evaluate the case data journey together with environment, dependency and telemetry context. Does an operational-success label establish every application outcome?

Open the related case file ↗

READ THE EXAMPLE TOGETHER

Traffic / readiness / replica ledger

Select a field and examine what it supports and where the evidence ends.

Read the raw artifact and field explanations

{
  "topology": {
    "logicalChain": [
      "DNS resolution",
      "CDN role",
      "balancer role",
      "application role"
    ],
    "mandatoryEverySite": false,
    "dnsAnswerAlias": "edge-demo",
    "dnsNameAlias": "public.example.invalid",
    "dnsQueryExecuted": false,
    "dnsIsHttpForwarder": false
  },
  "normal": {
    "edgeDecision": "supplied miss",
    "ready": [
      {
        "id": "app-a",
        "ready": true
      },
      {
        "id": "app-b",
        "ready": true
      }
    ],
    "selected": "app-a",
    "response": {
      "id": "tea",
      "title": "Tea note",
      "servedBy": "app-a"
    },
    "stopsAt": "edge received response; client delivery outside scene"
  },
  "oneDown": {
    "ready": [
      {
        "id": "app-a",
        "ready": false
      },
      {
        "id": "app-b",
        "ready": true
      }
    ],
    "selected": "app-b",
    "response": {
      "id": "tea",
      "title": "Tea note",
      "servedBy": "app-b"
    },
    "regionalFailoverMeasured": false
  },
  "scale": {
    "desiredBefore": 2,
    "desiredAfter": 3,
    "createdAfter": 3,
    "readyBefore": 2,
    "readyDuringInit": 2,
    "readyAfter": 3,
    "newInstance": "app-c",
    "newReadyDuring": false,
    "newReadyAfter": true,
    "hpaAlgorithmExecuted": false,
    "capacityMeasured": false
  },
  "unknownHealth": {
    "instance": "app-unknown",
    "readinessObserved": false,
    "health": "unknown",
    "selected": false,
    "reason": "missing-readiness"
  },
  "noReady": {
    "instances": [
      {
        "id": "app-a",
        "ready": false
      },
      {
        "id": "app-unknown"
      }
    ],
    "eligible": [],
    "selected": null,
    "forwarded": false,
    "status": 503
  },
  "limits": {
    "realDnsExecuted": false,
    "realCdnExecuted": false,
    "realLoadBalancerExecuted": false,
    "kubernetesExecuted": false,
    "realNetworkExecuted": false,
    "slaMeasured": false,
    "privateAuthorizationVerified": false
  },
  "selector": {
    "language": "XPath",
    "xml": "<audit><health observation=\"unknown\" selected=\"false\"/></audit>",
    "xpath": "count(/audit/health[@observation='unknown' and @selected='true'])",
    "expected": 0,
    "positiveFixtureXml": "<audit><health observation=\"unknown\" selected=\"true\"/></audit>",
    "positiveExpected": 1,
    "scope": "Own inert decision observations, not vendor log fields.",
    "missingFieldBehavior": "Missing observation is insufficient evidence; zero matches alone proves no healthy runtime/control."
  }
}

topology/normal: Supplied resolution and bounded origin path carry different knowledge.
scale: Desired/created/ready counts differ; no HPA or capacity measurement.
unknownHealth/noReady: Missing observation is unknown; literal-true policy selects traffic.
selector/limits: Inert health decision; no actual DNS, cloud, HA, or private permission.

Actual finite own in-process readiness and route adapters only; remote scene messages are conceptual. No DNS query, CDN/balancer/cloud deployment, SLA, or capacity measurement.

Two reality checks

DNS success or desired3 means three healthy instances and a completed user flow.

Resolution, desired count, ready eligibility, and response delivery require separate observations.

A missing health event proves no failure or permission.

Missing observation remains unknown; health predicates are neither authorization nor complete-flow verification.

YOUR TURN

Work through this story.

Predict the outcome, find the evidence boundary, and explain it in your own words.

Desired instances increased from 2 to 3 and three were created. New app-c is not ready yet. Are three instances immediately eligible?

No. readyDuringInit=2; app-c must wait.
Yes. Increasing the desired count proves measured capacity growth.

Read the explained answer

No. readyDuringInit=2; app-c must wait. — An instance existing does not prove it can accept requests. readyAfter reaches 3 only when app-c becomes ready.

app-a is not ready, and app-unknown has no readiness field. Which node receives the request?

Selected lesson artifact fields; teaching example

noReady.instances: [{"id":"app-a","ready":false},{"id":"app-unknown"}]
noReady.eligible: []
noReady.forwarded: false
noReady.status: 503

app-unknown is selected; no visible error proves it is healthy.
None. The eligible list is empty; no forwarding occurs and status is 503.

Read the explained answer

None. The eligible list is empty; no forwarding occurs and status is 503. — Missing readiness is not assumed healthy. Known false and unknown differ, but this policy dispatches to neither.

The normal flow ends when the edge receives the response, while oneDown selects app-b. Why are these not client-delivery or regional-failover measurements?

Read the explained answer

The edge received the response; client delivery is outside the scene. oneDown selects app-b from the supplied readiness list, with regionalFailoverMeasured=false. DNS is a resolution role here, not an HTTP forwarder. This logical topology is not mandatory for every site, and no real DNS/CDN/load-balancer or SLA measurement is performed.

□ I separated edge receipt from client delivery.

□ I did not treat instance selection as regional failover or SLA evidence.

Open the application console · request / response / browser

Also open request, URL, cookie and DOM examples

Sources and scope

RFC 1034 — Domain Names: Concepts and Facilities2.4. Elements of the DNS
Amazon CloudFront — How CloudFront delivers contentHow CloudFront delivers content to your users · How CloudFront works with regional edge caches
Kubernetes — Horizontal Pod AutoscalingOpening paragraphs: horizontal versus vertical scaling · How does a HorizontalPodAutoscaler work? · Pod readiness and autoscaling metrics
Kubernetes — Pod LifecyclePod phase · Readiness probe · Liveness probe
Microsoft Learn — Monitoring workload reliabilityMonitor workload functionality · Track availability targets · Make alerts actionable with a health model · Monitor load distribution across redundant instances

YOUR NOTES / THIS BROWSER

Traffic routing, scaling, and availability

1. Core · Mechanism

Origin path

One unready

No ready

Desired / ready

A DNS answer is not an HTTP response

Scale-out, ready capacity, and high availability

2. Red · Trust & Impact

Desired vs ready

Unknown pool

Mistaking resolution and box counts for availability

Treating missing observations as healthy or authorized

3. Blue · Secure development & defense

Pool controls

Health gap

Separate intent, observation, and selected targets

Bind availability claims to the user flow

Is desired replica count traffic eligibility or availability?

Two reality checks

Desired instances increased from 2 to 3 and three were created. New app-c is not ready yet. Are three instances immediately eligible?

app-a is not ready, and app-unknown has no readiness field. Which node receives the request?

The normal flow ends when the edge receives the response, while oneDown selects app-b. Why are these not client-delivery or regional-failover measurements?

Sources and scope