iptvnator/docs/architecture/host-connectivity-guard.md

Host Connectivity Guard

Per-host circuit breaker for portal requests, in both processes that make them: the Electron main process and the self-hosted web backend.

The problem

Every request to an unreachable portal costs its full axios timeout — 30 s for XTREAM_REQUEST, 15 s for STALKER_REQUEST (30 s for create_link), and the same budgets on the web backend's /xtream and /stalker routes. Browsing a dead portal's catalog issues dozens of those back to back, which shows up as 30-second spinners and a log full of identical failures. Once a host has refused to answer twice in a row there is nothing left to learn from waiting again.

The web backend had a worse version of the same problem first: its proxy routes passed no timeout at all, so a provider that accepted a connection and then went silent held the request until the OS gave up on the TCP connection. A breaker is only useful once not answering is bounded, which is why the timeouts and the breaker landed there together.

Where it lives

libs/shared/host-health (@iptvnator/shared/host-health, tagged scope:shared / domain:shared-runtime / type:util) — the breaker class, the failure classification and the redirect-attribution helpers, with no transport, logger or process singleton of its own. The owning app supplies the clock and decides how many guards exist.

Each runtime owns its instance:

• Electron — apps/electron-backend/src/app/util/host-connectivity-guard.ts holds one guard for the whole process, wired into both IPC handlers. The handlers are the choke point that sees all traffic to a host. The renderer's executeStalkerRequest is not: it has four documented bypasses (auth, endpoint discovery, account info, the row-less stream resolver), and that is exactly the traffic that hits dead hosts.
• PWA — apps/web-backend/src/app/host-guard.ts guards the /xtream and /stalker proxy routes. The guard instance is injected through WebBackendAppOptions.hostGuard, alongside now and guid, so specs drive it with a clock they own.

Request timeouts are the precondition

A breaker keyed on "the endpoint did not answer" is only useful once not answering is bounded. apps/web-backend originally passed no timeout at all, so a silent host hung on OS-level TCP timeouts. Both runtimes now use the same budgets: Xtream 30 s, Stalker 15 s (30 s for create_link, which mints a stream URL before answering), playlist and XMLTV downloads 30 s.

Those numbers are safe for large downloads. On axios' default (follow-redirects) transport timeout is not a wall-clock deadline for the whole response: it bounds the time to response headers and then continues as the socket's inactivity timeout for the body. A multi-megabyte XMLTV file that keeps delivering bytes is never cut off mid-transfer — only a stalled one is.

Scope: portal calls only

Both runtimes guard the portal API paths and nothing else. Playlist and XMLTV downloads (/parse, /parse-xml, and their Electron equivalents) get the timeouts but not the breaker, deliberately:

• The problem being solved is a catalog fan-out — dozens of requests to one endpoint back to back. A download is a single request.
• A download is usually the direct result of the user asking for it (the add-playlist dialog sends PLAYLIST_PARSE_BY_URL). Refusing an immediate retry is a regression, not a protection, and there is no natural reset site on that path the way portal Retry has one.
• A large XMLTV transfer can legitimately run for minutes — the timeout is idle-based, see above — which outlives the 45 s half-open trial expiry and would let a second trial in behind the first.

Rules

Being wrong here means refusing to talk to a portal that works, so every rule errs towards contacting the host:

Trip	2 consecutive host-level failures within an inclusive 120 s window
Open for	30 s (OPEN_DURATION_MS), matching the repo's other cooldowns
Half-open	exactly ONE trial request; the rest keep fast-failing until it settles
Reset	any HTTP response — 200, 404, even 502 — the host answered
Key	URL.origin — scheme, host and port (see below)
Kill switch	IPTVNATOR_DISABLE_CONNECTIVITY_GUARD=1 (read per call)

Host-level failure means an error with no HTTP response whose code is one of ETIMEDOUT, ECONNABORTED, ENOTFOUND, EAI_AGAIN, ECONNREFUSED, EHOSTUNREACH, ENETUNREACH. ECONNRESET is deliberately excluded: a reset mid-transfer happens on hosts that are very much alive. Cancelled requests (ERR_CANCELED) and SSRF-policy refusals are inconclusive — they say nothing about reachability and only release the half-open slot.

A failure is only charged to the endpoint that produced it. Reaching any later hop proves the guarded endpoint answered — the first hop is always the URL we asked for, and only a redirect status advances the chain — so a failure there CLEARS the guarded endpoint's record, exactly like any other response. Merely declining to count it would leave an earlier direct failure standing, and a single later timeout would then fast-fail an endpoint that answered in between. Every caller passes the URL it asked for as the baseline.

Where the failed hop is found depends on the transport, and both are in play.

	Electron	Web backend
Redirects	followed hop by hop (maxRedirects: 0), each its own request	followed inside one request by follow-redirects
Failed hop is in	error.config.url	error.request._currentUrl

failedRequestUrlOf reads _currentUrl first and falls back to config.url, which is correct for both: a per-hop request exposes no _currentUrl, and on the following transport config is built once and keeps the URL we asked for — so reading config.url there would compare a URL with itself, find no redirect, and charge a dead destination to the provider that answered. Anything added here must work on both, because the same helper serves both.

The comparison is origin + path, not the whole URL. A same-origin redirect (/player_api.php → /slow/player_api.php) proves the endpoint answered just as much as a cross-origin one, and charging it would fast-fail every OTHER call to a portal that answers — so the path has to be part of it. The query must NOT be: the web backend passes Xtream credentials through axios' params, so the sent URL always carries a query the baseline does not, and comparing whole URLs made every ordinary failure look like a redirect and stopped the breaker from ever opening. What that gives up is a redirect that changes nothing but the query, which is then counted as an ordinary failure — the safe direction.

It requires positive evidence — anything unparseable or unknown counts the failure as usual, because guessing "redirect" here would stop the guard from ever tripping — and a failure that names no URL at all is still counted.

Known gap: the failing hop is not guarded either (it has no token of its own), so a permanently broken redirect chain keeps costing a full timeout.

The key is the origin, not the host. URL.host omits a default port, so http://panel.example and https://panel.example would share one record — two genuinely different endpoints, and a panel whose TLS listener is broken while plain HTTP works is a routine IPTV setup. Sharing state there would let the dead one fast-fail the working one without ever contacting it. URL.origin also leaves out any user:pass@ userinfo, so no credential reaches the key or the log line.

Two more rules exist because of specific failure modes:

• Siblings are not a streak. Catalog initialization fans out three category requests at once; one network hiccup failing all three is one piece of evidence, not a trip. A failure counts only if its request started at or after the moment the previous failure was recorded. Timestamps are millisecond coarse, so this only separates siblings once a request actually took time — which is precisely the expensive case worth protecting. Only a failure that was actually counted may trip the threshold: a sibling settling after the open window elapsed would otherwise start a fresh one off the existing count and push the half-open trial past the intended cooldown.
• A reset invalidates reports already in flight. reset() bumps a per-host epoch instead of deleting the record, and a failure reported under an older epoch is discarded. Without that, the 30-second stragglers a user was waiting behind settle right after they press Retry and re-open the breaker underneath the very retry that cleared it.

A half-open trial that never reports back expires after 45 s, so a leaked token cannot leave the breaker open forever. That expiry is why the slot has an identity: a trial can genuinely outlive its window — requestWithValidatedRedirects gives each of up to five redirect hops its own 30 s budget — and once a replacement has been admitted, the abandoned request's late report must not free the replacement's slot and let a third request through. trial: true alone cannot tell the two apart, so the token carries the slot id it owns; an abandoned trial's failure is still counted as ordinary evidence.

The fast-fail error is a renderer contract

HostConnectivityGuardError is a real Error: Electron serializes a rejected plain object to [object Object], which would destroy the renderer's classification. It carries no status property, because getStalkerRequestErrorStatus reads that field first.

The message comes from buildHostConnectivityFastFailMessage() in libs/shared/interfaces/src/lib/host-connectivity.util.ts. It names the full endpoint, scheme included, so a user who imported the same panel over both HTTP and HTTPS can tell which one was skipped. Its wording is load-bearing — the Stalker renderer classifies transport failures purely from message text:

Must not contain	Otherwise
HTTP Error <code>	reads as "endpoint absent, probe the next candidate"; 404/401/403 also fire lazy portal repair against a host we just declared dead
timed out, timeout of Nms, ETIMEDOUT	discovery walks every candidate instead of aborting early
authorization, unauthorized, access denied, invalid token, auth failed, handshake failed	fires lazy portal repair (a bare authorization matches)

What remains is the "connection-level failure" slot the renderer already has for ECONNREFUSED/ENOTFOUND: discovery stops probing and reports the host unreachable, shouldAttemptRepair returns false, and the message reaches error snackbars verbatim — which is why it reads like a sentence and names the endpoint.

The endpoint in that sentence is user data, so the marker outranks the heuristics. A portal at https://authorization.example would otherwise make its own fast-fail message match the broad auth phrase set and send an unreachable host into lazy portal repair. isStalkerAuthFailureMessage and isStalkerProbeTimeout therefore both return false for a message isHostConnectivityFastFailMessage recognises, before their phrase matching runs. (getStalkerRequestErrorStatus needs no such guard: HTTP Error <code> contains a space, which a hostname cannot.) stalker-portal-discovery.utils.spec.ts pins all three properties for the bare and the IPC-wrapped form.

Exemption: endpoint discovery

STALKER_REQUEST accepts skipConnectionGuard, set only by StalkerPortalDiscoveryService.probeContent. Semantics: bypass the check, never count failures, but still report successes.

Discovery walks several candidate paths on one host and expects most of them to fail; counting that would let it declare a slow-but-alive portal unreachable. Reporting successes is equally load-bearing: confirmFullPortal runs the full non-exempt authentication flow against auth-gated candidates, so without it two hung handshakes could open the breaker mid-discovery and the next authenticate would fast-fail into the "host unreachable" slot — abandoning a portal that works. "Success" here means any observed response, including one attached to a rejection: validateStatus lets 4xx through but rejects 5xx with error.response set, and a 5xx proves the origin answered just as well as a body does. That is why the exempt path reports through reportGuardedHostFailure(token, error, { countFailures: false }) rather than skipping the report.

The flag has to survive the PWA transport too, or discovery is exempt on the desktop and policed on the web. PwaService.forwardStalkerRequest forwards it as a /stalker control param and the route applies the same semantics — and, like macAddress/token/serialNumber, it is stripped from the query forwarded to the portal, because it is our control flag and not protocol content.

Explicit reset

CONNECTIVITY_GUARD_RESET ({ url }) is handled by apps/electron-backend/src/app/events/connectivity-guard.events.ts. One key derivation is enough: both normalizeXtreamServerUrl and buildStalkerRequestUrl rebuild their request URL from URL.origin, so the origin a request ends up using is always the origin of the URL stored on the playlist.

The rule: every user-driven retry or refresh that issues portal requests must reset the guard before its first request. The failures that opened the breaker are usually the very ones the user is retrying, so a reset placed after the request — or missing — makes the affordance do nothing until the window expires. Automatic and first-load paths deliberately do NOT reset: only a user action means "contact this host now", and clearing evidence the guard just collected would defeat it.

Call sites:

• retryContentInitialization (with-content.feature.ts) — the Xtream content-gate Retry button. The reset is the first awaited statement, before the portal status check: a tripped guard fast-fails that check, its unavailable verdict returns early, and a reset placed any later would never run.
• StalkerPortalDiscoveryService.discover() — one site covering import, the Edit dialog and lazy repair, which also guarantees a freshly edited address never inherits a refusal recorded for the previous one.
• retryContentPage (with-stalker-content.feature.ts) — the Stalker grid tail's append retry.
• StalkerSearchComponent's search-page retry — the search results have their own append error and retry, separate from the catalog's.
• StalkerItvCacheService.refresh() — the Live TV refresh button, on the same path that already clears the cache's own error cooldown. This also covers refreshChannels() in the live layout.
• Both account-info dialogs' Retry buttons (AccountInfoComponent.reload() for Xtream, StalkerAccountInfoComponent.reload() for Stalker). Their automatic load on open goes through a private load() that does not reset.
• The destructive Xtream refresh — before anything is deleted. It removes the cached catalog and then bootstraps a re-import whose status request an open guard would fast-fail, leaving the user with no catalog at all until the cooldown expires. The reset lives in XtreamRefreshFlowService.runRefresh() (libs/playlist/shared/ui), which owns the whole flow for both of its entry points: PlaylistRefreshActionService.refreshXtream() and RecentPlaylistsComponent.refreshXtreamPlaylist() (the Workspace sources page). Those two used to be independent near-duplicates, which is exactly why the second one was missed first time round; they now differ only in the XtreamRefreshProgressReporter they hand over, and a reporter cannot reach the reset. Keep it that way — a third entry point should pass a reporter, not copy the sequence.
• PortalStatusService.checkPortalStatusDetails when skipCache is set — the user-initiated "Test Connection".

Two retry paths deliberately have no reset: Xtream's retryAppend() is a no-op because in-memory appends cannot fail, and the guard's own half-open trial is not a user action.

Where a retry clears a UI error flag, that flag is cleared synchronously before awaiting the reset — otherwise the retry branch stays re-enterable and the next nearEnd event fires a second retry.

They all go through resetHostConnectivityGuard() (libs/services/src/lib/host-connectivity-reset.ts), which holds the one rule they share: the reset is best effort, because the guard only ever delays a request and a failed reset must not block the action that asked for it. Both runtimes honour it, so neither keeps fast-failing an endpoint the user just asked to retry — in the PWA PwaService forwards it to the web backend's POST /connectivity-guard/reset, since the breaker lives in the backend process and a local reset would clear nothing. That route takes the raw provider URL rather than a registered targetId, because callers reset precisely when the address may have changed, which is before any target exists for it; it fetches nothing, reads only the origin, and never logs the URL.

Interaction with VOD multi-source

No exemption is needed. Multi-source resolution reaches get_vod_info over XTREAM_REQUEST, but VodSourceResolverService.loadVodDetails already catches that failure and falls back to its learned container cache or null, and probeSource maps null to the verdict unknown — which VodSourceProbeCacheService deliberately does not cache. A guard fast-fail therefore degrades to a retryable "could not check", exactly matching the module's contract that unreachable ≠ contacted-and-refused. The STREAM_PROBE_URL reachability half never rejects and is untouched.

Tests

• libs/shared/host-health/src/lib/host-connectivity-guard.spec.ts — the state machine, with an injected clock.
• apps/electron-backend/src/app/util/host-connectivity-guard.spec.ts — the main-process singleton and redirect attribution through it.
• apps/web-backend/src/app/web-backend-app.host-guard.spec.ts — the proxy routes: the HTTP 200 refusal shape, that the outbound request really is skipped, that an open endpoint is refused before a DNS lookup is spent on it, redirect attribution, the discovery exemption, half-open, the reset endpoint, and that playlist/EPG downloads are never fast-failed. The per-route timeouts are asserted in web-backend-app.spec.ts, which pins the whole outbound request shape.
• apps/web/src/app/services/pwa.service.spec.ts — that a fast-fail reaches the renderer with no numeric status and no HTTP Error <code> in its message, that the discovery bypass is forwarded, and that a reset calls the backend.
• apps/electron-backend/src/app/events/stalker.events.spec.ts and xtream.events.spec.ts — trip, fast-fail without contacting axios, reset, exemption, and the absence of per-request log spam.
• libs/portal/stalker/data-access/src/lib/stalker-portal-discovery.utils.spec.ts and stalker-portal-repair.service.spec.ts — the message contract.