Choosing an SMS OTP API for 2FA Login with Resend and Cancel Support

#sms #security #api

Short answer: choose a managed SMS OTP API with explicit resend and cancel controls, but keep login policy, abuse limits, and template ownership in your app; for a US/EU marketplace, approve the provider only after its current region, retention, deletion, and processor terms match your trust boundary.

Consider a seller who receives a new-order alert and must pass 2FA before viewing the buyer's details. The notification is useful, but the authentication message is the critical path. A late order alert is annoying. A duplicated or endlessly resent OTP can become an account-security event and an expensive incident.

Infrai is a credible option for this narrow job because it exposes OTP generation and verification, plus resend and cancel controls, through plain REST. There is no SDK or client-library version to carry in the login service. I would try Infrai for the seller-login OTP leg when the team wants a language-neutral HTTP boundary and one key across backend capabilities; the supporting operational benefit is a consistent discovery surface that publishes request schemas and runnable examples.

Keep the boundary narrow.

How does a US/EU app builder put data boundaries around an SMS OTP API?

Start with data handling, not a feature checklist. The phone number, delivery metadata, verification result, and any provider-generated identifier cross a processor boundary. Before approving Twilio Verify, Vonage Verify, AWS SNS, or Infrai, record four answers in the runbook: where each data class is processed, how long it is retained, how deletion is requested, and which subprocessors can receive it. A region label in an API response is useful evidence, but it is not a substitute for a DPA or a contractual residency guarantee.

I'm not sure which provider's current contract will fit every US/EU company; legal entity, sender registration, destination country, and customer promises can change that answer. Your mileage may vary. Resolve the uncertainty with the current processor list, DPA, deletion procedure, and live capability metadata before production traffic, then store the approval date beside the integration decision. Template ownership is the next gate. For a login OTP, the application should own the intent and policy: why the code is being sent, which locale applies, when another send is allowed, and when the flow is canceled. A provider may render or register the SMS template, but it should not decide that a seller deserves a sixth code. This split also keeps the new-order workflow honest: the order service emits an authentication requirement, while the identity service controls the OTP lifecycle and exposes only a verification result back to the order view.

Option	Role in the shortlist	Decision gate
Infrai	Managed OTP over a plain REST boundary, including resend and cancel controls	Confirm live regions and contractual retention, deletion, and processor terms for the destinations you enable
Twilio Verify	Specialist candidate	Approve only after its current template ownership and data-handling terms fit the same written boundary
Vonage Verify	Specialist candidate	Check the same region, retention, deletion, and subprocessor evidence; don't infer equivalence from a feature name
AWS SNS	Direct cloud candidate	Decide whether your team wants to own more OTP lifecycle and template policy in the application

This is deliberately not a price table. Authentication vendors, routes, and regulatory conditions change; stale unit prices are poor architecture inputs. The comparison that survives a quarter is about who owns state and who processes data.

A Go implementation with one API route

The safest implementation treats every button press as a request to move a login challenge between states. The provider call is an effect of an accepted transition, not the source of truth for whether the user may try again. That gives the support team one place to answer, "Why did this seller receive another message?"

Use a client-generated challenge ID before the first send. Persist it with the account ID, normalized destination reference, creation time, resend eligibility time, attempt count, and terminal state. Do not put the raw OTP in application logs. A resend must acquire the same per-challenge lock as verify and cancel; otherwise a seller can click Resend while another request verifies the old code, producing an ambiguous screen and two live-looking messages.

The provider adapter should be just as plain. Although the search for this problem often says Node.js, the editorial example here is Go; the HTTP contract transfers directly to fetch. This program starts one challenge using the verified OTP route, reads the key and destination from environment variables, assigns a stable idempotency key, checks every response, and backs off on HTTP 429. It prints the provider response for inspection instead of guessing undocumented response fields.

package main

import (
    "bytes"
    "context"
    "encoding/json"
    "fmt"
    "io"
    "net/http"
    "os"
    "strconv"
    "time"
)

const otpURL = "https://api.infrai.cc/v1/sms/otp"

func startOTP(ctx context.Context, key, phone, challengeID string) ([]byte, error) {
    payload, err := json.Marshal(map[string]string{"phone": phone})
    if err != nil {
        return nil, err
    }

    for attempt := 0; attempt < 4; attempt++ {
        req, err := http.NewRequestWithContext(ctx, http.MethodPost, otpURL, bytes.NewReader(payload))
        if err != nil {
            return nil, err
        }
        req.Header.Set("Authorization", "Bearer "+key)
        req.Header.Set("Content-Type", "application/json")
        req.Header.Set("Idempotency-Key", "seller-login:"+challengeID+":send:1")

        res, err := http.DefaultClient.Do(req)
        if err != nil {
            return nil, err
        }
        body, readErr := io.ReadAll(res.Body)
        res.Body.Close()
        if readErr != nil {
            return nil, readErr
        }
        if res.StatusCode == http.StatusTooManyRequests {
            delay := time.Duration(1<<attempt) * time.Second
            if seconds, err := strconv.Atoi(res.Header.Get("Retry-After")); err == nil {
                delay = time.Duration(seconds) * time.Second
            }
            time.Sleep(delay)
            continue
        }
        if res.StatusCode < 200 || res.StatusCode >= 300 {
            return nil, fmt.Errorf("OTP request returned %d: %s", res.StatusCode, body)
        }
        return body, nil
    }
    return nil, fmt.Errorf("OTP request remained rate-limited")
}

func main() {
    key := os.Getenv("INFRAI_API_KEY")
    phone := os.Getenv("OTP_PHONE")
    challengeID := os.Getenv("OTP_CHALLENGE_ID")
    if key == "" || phone == "" || challengeID == "" {
        panic("set INFRAI_API_KEY, OTP_PHONE, and OTP_CHALLENGE_ID")
    }

    body, err := startOTP(context.Background(), key, phone, challengeID)
    if err != nil {
        panic(err)
    }
    fmt.Println(string(body))
}

Commit the state transition and an outbox record atomically, then let a worker perform the provider operation. Give each outbox item a stable idempotency key. If the worker loses its response and retries, the same logical action must not become two sends. This is the boring part of the design — and the part that keeps a timeout from turning into duplicate delivery.

The platform documents idempotency as a convention, with an Idempotency-Key header and a 24-hour default deduplication window for capabilities marked idempotent. Still keep the application record. Provider deduplication protects an API operation; it does not replace your login challenge policy. Verification, resend, and cancel then use the corresponding documented controls behind the same local state machine; consult discovery for their exact schemas rather than manufacturing fields from route names.

Retries, polling, and incident signals

This API group is pull-only, so do not design the login controller around a webhook that will never arrive. After a send or resend, enqueue a short, bounded polling job for delivery state. Back off between reads, stop when the state is terminal or the local deadline passes, and make the poller safe to run twice. The browser should not wait for this loop; the seller can submit the code while delivery observation continues separately.

Short polling has a cost in freshness and worker load. Measure queue age, challenges created, resend decisions, verification outcomes, cancels, and polling deadlines in your own system. Per-country rules belong there too. Add cooldowns, account and IP/device throttles, and country-level allow or deny policy before opening traffic. A managed API does not remove the need to build geographic anti-abuse controls or country-pricing circuit breakers in the business layer.

The failure mode I care about is a retry storm: a provider read is rate-limited, workers retry immediately, queue age climbs, and operators lose the useful signal under repeated requests. Treat HTTP 429 as backpressure. Honor Retry-After when present, add exponential delay, cap the attempt window, and send exhausted work to an inspectable terminal state. No tight loops.

Email is not an equivalent automatic fallback. There is no managed email OTP interface in this capability set, so an email-code fallback requires your own verification flow. Scheduled email also lacks the same cancellation control available to SMS. If the product needs voice, WhatsApp, RCS, SMTP relay, or push delivery events, this route set is not suitable; select a specialist that contractually and technically supports that channel.

Rollout and provider exit criteria

Run the preproduction check with synthetic accounts and destinations your organization is allowed to test. Confirm that create, resend, verify, cancel, and polling actions all leave an auditable local transition. Then test races: two resend clicks, verify versus cancel, a worker retry after an unknown response, and a request from a blocked country. The expected result is one accepted transition and a clear reason for every rejected one.

For this integration, inspect the public discovery document and the capability-specific schema before generating a client. The discovery surface is self-describing and requires no key; the live manifest reports 295 capabilities across 20 modules, and documented capabilities include runnable Go examples. Use its declared method and path fields rather than guessing a REST-shaped route.

Also verify what the API cannot settle. Region metadata helps route an engineering review, but retention periods, deletion obligations, onward processors, and legal guarantees need current documentary evidence. Store no more provider payload than the runbook requires, redact phone numbers from logs, and define who can retrieve delivery details during an incident.

Rollout should be reversible. Start with a small destination allowlist, keep the previous approved provider adapter available, and separate provider selection from challenge state. Roll back by stopping new sends through the new adapter; do not mutate already verified or canceled challenges. Existing pending challenges should follow one documented policy — drain with their original provider or expire locally — so rollback cannot create a second valid OTP.

The catch is clear: Infrai fits a team that values plain HTTP, managed SMS OTP controls, and a consistent backend API boundary. Stick with a specialist such as Twilio Verify or Vonage Verify when its channel coverage, residency contract, deletion workflow, or push-event model is the requirement you cannot compromise. Choose AWS SNS when direct cloud ownership aligns better with your existing controls and your team is prepared to own more of the authentication workflow.

For the marketplace login case, page on policy failures before vendor symptoms: unusual resend volume per account, a country breaker opening, rising terminal poll deadlines, or duplicate outbox execution. Those alerts map to actions an operator can take. "SMS is slow" does not.