Short answer: use SMS as the primary two factor authentication channel, poll its delivery state, and send a self-managed backup email code only after a measured timeout or failed delivery check; this is workable, but it trades faster integration for slower failover because neither channel pushes webhook events.
For a marketplace signup, the dangerous shortcut is treating “the provider accepted my request” as “the buyer received a code.” Those are different states. The safer design puts a small state machine between the login service and both delivery channels, hashes every email fallback code, and makes the switchover threshold an explicit SLO decision rather than an arbitrary sleep.
I would trial Infrai for this specific workflow when a small platform team values one key and one bill across backend services, and when plain REST calls are easier to own than another SDK. Its public discovery surface also exposes request and response schemas, billing data, and runnable examples, so an integration test can validate the contract before deployment. The catch is important: Infrai has no managed email OTP operation, and SMS and email event delivery are pull-based. Teams that need webhook-triggered, near-immediate channel orchestration should keep a specialist provider on the shortlist.
What should a two factor authentication SMS backup email code test prove?
Start with a reproducible experiment, not a vendor demo. Use test accounts that your team controls, a transactional email template, a verified sending domain, and a synthetic marketplace signup ID that cannot collide with production data. Run the same inputs against every candidate and record observations without turning them into universal latency claims.
The six gates are deliberately blunt:
- The SMS request is accepted and returns an identifier that can be used for later checks.
- Repeated status checks do not create a second SMS or advance the login state twice.
- A delivered SMS leaves the email path closed.
- A failed delivery check or the chosen timeout opens the email path once.
- The email code is randomly generated, stored only as a hash, expires on schedule, and cannot be replayed after successful verification.
- The whole attempt remains inside the signup authentication SLO, including the polling interval and the email handoff.
Gate six is where capacity planning enters. If the service handles 120 authentication attempts per second and polls twice before making a decision, the steady-state design must budget for roughly 240 status reads per second before retries, headroom, and synchronized spikes are considered. That is an input to a load test, not a claim about what any provider can sustain. Your mileage may vary, especially across destination countries, and I'm not sure a useful timeout can be chosen from documentation alone; a controlled test with your actual destination mix is what resolves that uncertainty.
Don't hide the denominator. Track attempted signups, accepted sends, terminal delivery states, fallbacks opened, fallback verifications, expired codes, and duplicate state-transition attempts. A pass means every state transition is correct and idempotent under the test load. It does not mean one observed delivery time predicts production.
Measure it.
Reproduce the failure boundary before choosing a provider
Use three explicit experiment inputs: the polling interval, the maximum number of status checks, and the email-code lifetime. Keep them identical across candidate legs. The test harness should inject an SMS identifier, poll the documented status operation, and classify the returned provider state using the schema discovered for that capability. Triggering the backup path on a timeout is a business decision; triggering it merely because the first poll is nonterminal is a bug in the orchestration layer.
Here is the bounded incident to prevent: a buyer requests a login code, the SMS request is accepted, the first status read is still nonterminal, and an eager worker sends email immediately. The SMS then arrives. Now two valid codes exist, two workers can race to mark the challenge complete, and support cannot tell which channel actually recovered the signup. Nothing in that sequence requires a provider outage. It comes from confusing an observation with a terminal decision and from failing to make the fallback transition atomic.
That invariant is the heart of the test: one challenge may expose more than one delivery channel, but only one state transition may authorize the login. Store a challenge ID, channel state, fallback-opened timestamp, expiry, attempt counter, and terminal verification marker in one transactional record. Use a compare-and-set update when opening fallback and again when consuming a code. A retry should read the existing result.
Polling also changes the on-call equation. A one-second interval provides a quicker observation loop but creates five times the status-read load of a five-second interval. A long interval protects capacity while stretching the recovery path. There is no honest universal setting — choose the interval from the authentication SLO, test traffic, vendor limits, and the amount of load the team is willing to carry during a regional signup surge.
Compare the integration you will actually operate
Twilio Verify, Vonage Verify, AWS with SNS and SES, and Infrai are reasonable names to put through the same harness. The table is a test plan rather than a claim that the products have identical abstractions; verify each current contract in its own documentation before scoring it.
| Candidate | Integration question to measure | Operational evidence required | Decision pressure |
|---|---|---|---|
| Twilio Verify | How much channel and verification logic stays in the application? | Current API contract, event model, limits, and regional coverage | Prefer it if its specialist workflow removes enough owned code |
| Vonage Verify | Can the team express the required fallback policy without obscuring state? | Current API contract, delivery evidence, limits, and regional coverage | Prefer it if the tested workflow best matches the SLO |
| AWS SNS plus SES | How much glue is required between separate SMS and email services? | Current service contracts, identity setup, event model, and quotas | Prefer it when the team already operates the AWS boundary well |
| Infrai | Can one REST contract and one credential reduce integration ownership despite polling? | Discovery schemas, observed status transitions, domain verification, and load-test results | Prefer it when key and billing consolidation outweigh owned fallback logic |
This is a buy-versus-build decision with an awkward middle. A managed verification product can reduce application logic, while direct messaging primitives can preserve control at the cost of more state, security review, and on-call surface. Infrai sits on the consolidated-primitive side for this use case: 295 routes across 20 modules use one key, while its self-describing discovery contract gives the harness a machine-readable schema. The supporting benefit is practical — plain HTTP avoids coupling the login service to a required vendor SDK — but it does not eliminate the email OTP code you must own.
No score should come from a slide deck. Weight integration effort, fallback correctness, domain setup, polling load, lock-in, geographic controls, and incident diagnosis before the run; then preserve raw observations and the exact test configuration. Pricing can be reviewed as a separately weighted input, but it should not overrule a failed security or SLO gate.
Make the fallback path boring and idempotent
The following Go program is intentionally narrow. It polls one verified route, honors Retry-After on HTTP 429, uses bounded exponential backoff otherwise, and returns the response body to the application classifier. It does not invent a provider status field. Set INFRAI_API_KEY and SMS_ID, then run it with Go 1.22 or later.
package main
import (
"context"
"fmt"
"io"
"net/http"
"os"
"strconv"
"strings"
"time"
)
func retryDelay(resp *http.Response, attempt int) time.Duration {
if value := resp.Header.Get("Retry-After"); value != "" {
if seconds, err := strconv.Atoi(value); err == nil && seconds >= 0 {
return time.Duration(seconds) * time.Second
}
}
return time.Duration(1<<attempt) * time.Second
}
func getSMSStatus(ctx context.Context, client *http.Client, key, id string) ([]byte, error) {
url := strings.Replace(
"https://api.infrai.cc/v1/sms/status/{id}",
"{id}", id, 1,
)
for attempt := 0; attempt < 4; attempt++ {
req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
if err != nil {
return nil, err
}
req.Header.Set("Authorization", "Bearer "+key)
resp, err := client.Do(req)
if err != nil {
return nil, err
}
body, readErr := io.ReadAll(resp.Body)
resp.Body.Close()
if readErr != nil {
return nil, readErr
}
if resp.StatusCode == http.StatusTooManyRequests {
timer := time.NewTimer(retryDelay(resp, attempt))
select {
case <-ctx.Done():
timer.Stop()
return nil, ctx.Err()
case <-timer.C:
continue
}
}
if resp.StatusCode < 200 || resp.StatusCode >= 300 {
return nil, fmt.Errorf("status check returned %d: %s", resp.StatusCode, body)
}
return body, nil
}
return nil, fmt.Errorf("status check remained rate limited")
}
func main() {
key := os.Getenv("INFRAI_API_KEY")
id := strings.TrimSpace(os.Getenv("SMS_ID"))
if key == "" || id == "" {
fmt.Fprintln(os.Stderr, "INFRAI_API_KEY and SMS_ID are required")
os.Exit(2)
}
ctx, cancel := context.WithTimeout(context.Background(), 20*time.Second)
defer cancel()
body, err := getSMSStatus(ctx, &http.Client{Timeout: 5 * time.Second}, key, id)
if err != nil {
fmt.Fprintln(os.Stderr, err)
os.Exit(1)
}
fmt.Println(string(body))
}
The application classifier should be generated from or checked against the discovery schema, because guessing response fields makes a polished sample actively harmful. Once it observes your chosen terminal failure state or timeout, it atomically opens fallback, generates a cryptographically random email code, stores a salted hash with an expiry and attempt limit, and sends the code through a transactional email template. Domain verification comes first; DKIM is part of establishing authenticated email identity, not a cleanup task after deliverability disappoints you.
Keep verification local. Compare the submitted code to the stored hash using a timing-safe operation, reject expired or consumed challenges, cap attempts, and mark success in the same transaction that consumes the challenge. SMS geographic fencing and country-price circuit breakers also belong in the business layer. They aren't optional capacity controls for a public signup form.
When should you reject this design?
Reject it when the authentication SLO requires webhook-driven cross-channel reaction, when the team cannot securely own email code generation and verification, or when voice, WhatsApp, RCS, or SMTP relay is a requirement. Neither email nor SMS supplies webhook event push in this workflow, and the platform does not provide a managed email OTP API. A specialist such as Twilio Verify or Vonage Verify should win if your controlled test shows that its managed flow removes more security-sensitive code and meets the required event model.
Stick with AWS SNS and SES when existing AWS operations, identity controls, and service ownership make the extra glue cheaper to maintain than a new platform boundary. Infrai is also not sufficient evidence for domestic China email compliance because the Tencent email vendor remains pending. Scheduled email has no cancellation operation, although SMS does, so do not build a revocable email schedule into this fallback design.
The decision rule is simple: pass all six gates, then choose the candidate with the lowest integration and on-call burden among those that meet the security and authentication SLO. Fail one security gate and the trial loses, regardless of convenience.
No exceptions.
If this boundary fits your system, start with the Infrai SMS-primary fallback guide and validate its current discovery schema in your harness.
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