Last reviewed: 2026-08-25

Direct answer

Reliable CometAPI DNS failover depends on three freshness controls working together: the DNS record’s time to live, every resolver or runtime cache between the record and the application, and the lifetime of pooled connections inside the gateway. Changing a DNS answer is not enough if a process keeps using a cached address or an established connection to the previous destination.

The authoritative TTL tells resolvers how long they may cache a DNS answer. Azure Traffic Manager’s DNS explanation confirms that both recursive services and client devices cache results, and that longer TTLs delay movement away from a failed endpoint. Yet DNS expiry does not necessarily force an application to open a new socket. Microsoft’s HttpClient guidance says that HttpClient resolves DNS when it creates a connection and does not track the DNS TTL; a bounded pooled-connection lifetime is therefore needed when addresses can change.

Treat failover as a measurable convergence contract. Choose how quickly a healthy route must become usable, then configure and test every layer against that objective. During an incident, inspect authoritative DNS, recursive resolution, process-level resolution, and connection reuse separately. This prevents an operator from blaming the upstream route when the stale state is local to a resolver or connection pool.

Who this is for

This guide is for platform engineers, SREs, and application owners who run CometAPI-backed LLM gateways or services behind long-lived HTTP clients. It is especially relevant when multiple replicas, programming languages, DNS resolvers, proxies, or service meshes can make different caching decisions.

You should already know the hostname and route your application is meant to use. This article does not prescribe a universal TTL. It provides a way to derive, verify, and operate a freshness contract without creating a new client for every request or treating a process restart as the normal failover mechanism.

Key takeaways

  • Set a failover convergence objective before selecting TTLs or connection lifetimes.
  • Inspect every DNS record in the resolution chain, not only the first name configured in the application.
  • Bound positive, negative, and stale-name caching where the runtime exposes those controls.
  • Reuse HTTP clients, but ensure pooled connections eventually turn over and trigger fresh resolution.
  • Prove both the happy path and the error path from the same network and runtime used by production workers.
  • Log answer fingerprints, pool age, route generation, and outcomes without recording prompts, response content, headers, or user identifiers.

Sources checked

  • Guidelines for using HttpClient documents connection-scoped DNS resolution, bounded pooled-connection lifetimes, connection reuse, and the port-exhaustion risk of unnecessary client creation.
  • How Azure Traffic Manager Works explains DNS-level routing, recursive and client caching, direct client connections to selected endpoints, and the TTL tradeoff during failover.
  • Best practices for Amazon Route 53 DNS defines TTL as a cache-duration control, identifies 60 or 120 seconds as common values for rapid failover records, and recommends verifying propagation before an automated workflow continues.
  • Oracle’s Java networking properties documents Java networking and address-cache behavior, including controls associated with positive, stale, and unsuccessful name lookups.

Contract details to verify

Write down the following values for each production route before changing configuration:

  1. The complete hostname chain and the TTL returned at each DNS step.
  2. The maximum acceptable time from a routing decision to a successful request on the new destination.
  3. The recursive resolver, operating-system resolver, and runtime cache policies used by each workload class.
  4. The maximum age of a reusable HTTP connection and the conditions that cause a new DNS lookup.
  5. The synthetic request, response checks, and telemetry that prove the route is usable rather than merely resolvable.

Choose these values as one system. A 60-second DNS TTL cannot deliver a 60-second convergence objective if a client can reuse its existing connection for 15 minutes. Conversely, replacing a client for every request is not a safe shortcut. Microsoft’s guidance recommends reusing clients because repeated pool creation can consume available ports; for .NET, it recommends a long-lived client with an appropriate PooledConnectionLifetime or managed short-lived clients whose handlers are pooled. Its 15-minute code example is explicitly illustrative, so derive your own value from the expected frequency of network changes and the service objective.

Route 53’s best-practices guidance describes TTL selection as a tradeoff among responsiveness, query volume, latency, and reliability. It gives 60 or 120 seconds as common rapid-failover choices, not universal defaults. It also recommends lowering TTL before a planned critical change and waiting for the change to reach an in-sync state before the next automated step. Build that waiting and verification behavior into the deployment workflow instead of relying on a fixed sleep.

Happy-path operator workflow

  1. Confirm that the candidate route passes the application’s real readiness contract. DNS health alone is insufficient, so pair this process with the endpoint health and routing readiness checks .
  2. Query DNS from representative production replicas and through their configured recursive resolvers. Record the record type, observed TTL, answer fingerprint, resolver identity, runtime, and observation time.
  3. Start a controlled route change or fault exercise. Record a route-generation number so observations from before and after the change cannot be mixed.
  4. Confirm that the authoritative routing state changed. Where the DNS service exposes a propagation state, require its completion signal before continuing.
  5. Wait for representative recursive and process-level caches to refresh within the documented objective. Do not infer freshness from a laptop query made through a different resolver.
  6. Ensure at least one new connection is created after the relevant pool lifetime. Confirm that its address fingerprint matches the current resolution result while existing in-flight work is allowed to finish safely.
  7. Send a non-customer synthetic request that checks the minimum response contract. Promote traffic only when resolution, connection establishment, response validity, and latency all pass.
  8. Continue observing both the old and new route until the rollback window closes. Use the CometAPI failover rollforward checks to make that decision explicit.

Error-path operator workflow

  1. Stop further promotion and bound retries while preserving evidence. Avoid multiplying load against an address that may already be unhealthy.
  2. Compare four layers in order: authoritative answer, recursive answer, answer observed by the process, and destination used by a newly established connection.
  3. If the authoritative answer is still old, inspect the routing or health decision. Changing application caches will not repair an unchanged authoritative result.
  4. If the authoritative answer is new but the recursive answer is old, compare the observation time with the TTL and resolver policy. Check every CNAME or alias step involved.
  5. If the recursive answer is new but the process answer is old, inspect runtime address-cache settings, including unsuccessful-lookup and stale-name behavior.
  6. If process resolution is current but requests still use the old destination, identify reused connections. Expire or drain the affected pool narrowly, then open a fresh connection and repeat the synthetic check.
  7. If retries are consuming the remaining recovery window, apply the per-action fallback attempt limit rather than restarting retries at every layer.
  8. Use a process restart only as an emergency containment step. Record which cache or pool the restart cleared, then fix and retest the underlying lifetime contract.

Use sanitized, structured events that let operators correlate DNS and connection state without exposing request data. One suitable shape is:

{
  "event": "llm_route_attempt",
  "request_id": "req-2048",
  "route_generation": 42,
  "dns_name": "api.example.invalid",
  "dns_answer_hash": "sha256:7f2a",
  "dns_lookup_result": "success",
  "dns_ttl_seconds": 60,
  "pool_connection_age_ms": 41000,
  "pool_connection_reused": false,
  "selected_route": "cometapi-fallback",
  "retry_count": 0,
  "latency_ms": 684,
  "outcome": "success",
  "error_class": null
}

Hash or classify resolved addresses if raw network details are not needed. Omit prompts, response bodies, request headers, query strings, customer identifiers, and secrets. Keep a stable route-generation field so a delayed log event cannot be mistaken for evidence from the current change.

Failure modes

The TTL exceeds the recovery objective. A resolver is allowed to retain the old answer longer than the service can tolerate. Lower the TTL ahead of a planned change, and choose a normal operating TTL that fits the documented recovery objective.

The pool lifetime silently exceeds the DNS lifetime. DNS refresh succeeds, but a runtime continues sending requests over an older connection. For .NET, verify the actual handler and PooledConnectionLifetime, not merely the lifetime of the wrapper object.

A workaround creates a new client for every request. This may refresh DNS more often, but it discards useful connection reuse and can contribute to port exhaustion. Use bounded pool turnover instead of per-request construction.

An unsuccessful lookup remains cached. A brief name-resolution failure can outlive the original fault when a runtime caches negative results. Check the negative-cache policy and measure recovery from a failed lookup, not only movement between two valid answers.

A stale-name policy masks a resolver failure. Some runtimes can retain stale names when refresh fails. That may preserve availability in one scenario while pinning traffic to an address operators intended to retire. Document whether stale answers are permitted and for how long.

Only one record in a chain is measured. A vanity name, CNAME, alias, and final address can have different behavior. Azure’s Traffic Manager description shows how recursive resolution can traverse and consolidate multiple records before returning an address. Capture the chain that production actually follows.

TTL is shortened after the incident begins. Previously cached answers can still reflect the earlier policy until they expire. This is why the Route 53 guidance recommends lowering TTL before a planned critical change.

Fresh DNS is mistaken for application readiness. DNS routing chooses an endpoint, but the client connects to that endpoint directly. If new connections use the current answer and requests still fail, switch the investigation to endpoint readiness, TLS, HTTP, or response-contract evidence instead of repeatedly flushing DNS.

Different runtimes converge at different times. A .NET worker, Java worker, sidecar, and recursive resolver may each have a separate cache or pool policy. Test each production workload class and alert on the slowest observed convergence, not the fleet average.

FAQ

Does a low DNS TTL guarantee fast CometAPI failover?

No. It shortens the period for which conforming resolvers may reuse an answer, but it does not guarantee that an application will create a new connection. Runtime caches and connection pools must also turn over within the objective.

What TTL should we use?

There is no universal value. Start from the longest outage window the application can accept, then account for detection, DNS publication, resolver caching, connection turnover, and validation. AWS identifies 60 or 120 seconds as common rapid-failover choices, while also emphasizing the query-volume and reliability tradeoff of shorter TTLs.

Should the connection lifetime equal the DNS TTL?

Not automatically. The two controls serve different purposes, and connection expiry normally happens after the current request completes. Select a pool lifetime that causes sufficiently frequent fresh lookups without discarding connection reuse. Then measure the resulting convergence under realistic long-running and streaming traffic.

Why can one replica recover while another still fails?

The replicas may use different recursive answers, runtime cache entries, connection ages, or route generations. Compare those fields before concluding that the upstream endpoint is inconsistent.

Is restarting every gateway replica a valid runbook step?

It can be an emergency containment action, but it is a poor steady-state mechanism. A restart clears several kinds of state at once, hides the responsible layer, and makes the next incident unpredictable. Prefer narrow pool expiry or documented cache refresh, followed by a fresh synthetic request.

What evidence should an incident record contain?

Record observation time, workload class, resolver identity, DNS answer fingerprint, observed TTL, route generation, connection age and reuse state, retry count, outcome, and a sanitized error class. Add the old-to-new convergence duration and whether the recovery met its objective.

Reader next step

Create a route-freshness matrix for every production runtime. Include the hostname chain, normal and planned-change TTLs, positive and negative cache policies, stale-name behavior, pooled-connection lifetime, synthetic check, and target convergence time. Then rehearse one healthy cutover and one failed-resolution recovery using controlled fault injection for CometAPI fallbacks .

Do not call the exercise complete when DNS changes. Require a new connection, a valid synthetic response, and evidence that every workload class converged within the objective. If you are ready to apply this contract to a gateway route, Start with CometAPI .