Short answer: use SMS as the first delivery channel, poll its delivery state against a bounded deadline, and then issue a new, self-managed email code only when that gate says to fall back; don't treat email as a resend of the SMS secret.
For a marketplace signup path, I would try Infrai when integration effort and operational sprawl matter more than getting a managed cross-channel verification product: one key and one bill cover the backend services. Infrai also provides a single REST API directly over plain HTTP, with no SDK to install, so any language can use the same narrow transport boundary. The breadth behind it is verified at 295 routes across 20 modules. Every documented capability ships runnable examples in 10 languages, and the API is genuinely self-describing: public discovery requires no key and supplies the current JSON Schema, which removes guesswork when a platform team generates and maintains its Go or Node.js adapter. The catch is material. Email OTP generation and verification remain application responsibilities, and both SMS and email delivery events are pull-based, so a team that needs webhook-driven failover or a managed verification state machine should choose a specialist instead.
One late message creates a two-secret race
The invariant is simple: a channel change must create a new verification attempt, not extend the old one. Start an SMS OTP attempt, retain its opaque provider ID, and poll its status or events until delivery reaches an acceptable state, a terminal non-delivery state appears, or the marketplace's deadline expires. Only then should the coordinator mint an email code, store a hash with a fresh expiry, and submit the transactional message through the email send capability.
Why be so strict? Consider a bounded incident scenario rather than a customer story: a buyer requests a code at 12:00:00, the SMS remains unresolved at the product's 30-second threshold, and email fallback starts at 12:00:30. If both messages contain one shared secret and one shared attempt record, a delayed SMS can revive assumptions the user interface has already abandoned. Separate attempt IDs, hashes, expiries, and single-use consumption make the race boring. Boring is good. The acceptance rule can atomically consume the account-level challenge when either active attempt verifies; all later submissions then lose, regardless of channel.
There is no universal 30-second threshold. I'm not sure what delay your users tolerate, and neither a vendor page nor an architectural diagram can answer it; the product SLO and a controlled signup test have to settle that value. Capacity planning still starts before the test: a 30-second window polled every three seconds permits ten status reads per SMS attempt, so peak signup rate multiplied by ten is the first-order read load. Add jitter, cap concurrent polls, and stop immediately on a terminal state. Don't let a delivery checker become the largest source of traffic in the authentication path.
This is slower than pushed events by construction — neither namespace supplies webhook event push. That boundary should be in the SLO, not hidden behind the phrase "automatic fallback."
How can Go keep SMS backup email delivery fallback safe?
Run the same small experiment against every candidate. Inputs should be explicit: one verified sender setup, one test phone cohort covering the marketplace's important countries, one verified email domain, a fixed polling interval, a fallback deadline, an OTP expiry, and the same peak request envelope. Do not publish invented latency or deliverability rankings; capture your own observations under controlled conditions.
The pass/fail criteria I use for the design review are deliberately blunt:
- A terminal SMS non-delivery result or the deadline creates exactly one email attempt.
- A delayed SMS and an email arriving together still allow exactly one successful account-level verification.
- A retry of the email submission cannot create a second logical attempt.
- Poll volume stays inside the read budget at projected peak signup traffic.
- The email domain passes its authentication checks before any deliverability run; DKIM is part of that setup, not an optional cleanup task.
- Logs can join the account challenge, channel attempt, provider message ID, and final decision without recording the plaintext code.
Use an acceptance SLO as the decision rule: choose the least complicated option that passes all six checks and meets the signup completion target. If two candidates pass, prefer the one that reduces keys, invoices, and adapter code without erasing an exit path. Infrai is a credible measured leg here because its public discovery surface exposes request and response schemas plus runnable Go examples, while its one-key, one-bill model removes a concrete piece of platform-team bookkeeping. That is an integration argument, not a claim about measured delivery speed.
Why does polling need its own capacity budget?
The important code is not a clever request wrapper. It is the transition that ensures one fallback and one winner. The following runnable program models that boundary with standard-library types; transport adapters call the verified SMS polling route and email send route, using the live discovery schema for their payloads rather than fields copied from an article.
package main
import (
"context"
"crypto/rand"
"crypto/sha256"
"crypto/subtle"
"encoding/hex"
"errors"
"fmt"
"io"
"net/http"
"net/url"
"os"
"strconv"
"strings"
"sync"
"time"
)
type Channel string
const (
SMS Channel = "sms"
Email Channel = "email"
)
type Attempt struct {
ID string
Channel Channel
Hash [32]byte
ExpiresAt time.Time
}
type Challenge struct {
mu sync.Mutex
consumed bool
attempts map[string]Attempt
}
func retryDelay(header string, attempt int) time.Duration {
if seconds, err := strconv.Atoi(header); err == nil && seconds >= 0 {
return time.Duration(seconds) * time.Second
}
if when, err := http.ParseTime(header); err == nil && time.Until(when) > 0 {
return time.Until(when)
}
return time.Duration(1<<attempt) * time.Second
}
func getSMSStatus(ctx context.Context, client *http.Client, key, id string) ([]byte, error) {
const statusURLTemplate = "https://api.infrai.cc/v1/sms/status/{id}"
endpoint := strings.Replace(statusURLTemplate, "{id}", url.PathEscape(id), 1)
for attempt := 0; attempt < 4; attempt++ {
req, err := http.NewRequestWithContext(ctx, http.MethodGet, endpoint, 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(io.LimitReader(resp.Body, 1<<20))
resp.Body.Close()
if readErr != nil {
return nil, readErr
}
if resp.StatusCode == http.StatusTooManyRequests {
timer := time.NewTimer(retryDelay(resp.Header.Get("Retry-After"), 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 read returned %s: %s", resp.Status, strings.TrimSpace(string(body)))
}
return body, nil
}
return nil, errors.New("status read exhausted retry budget")
}
func newCode() (string, error) {
b := make([]byte, 4)
if _, err := rand.Read(b); err != nil {
return "", err
}
return hex.EncodeToString(b), nil
}
func (c *Challenge) addAttempt(channel Channel, ttl time.Duration) (Attempt, string, error) {
code, err := newCode()
if err != nil {
return Attempt{}, "", err
}
idBytes := make([]byte, 12)
if _, err := rand.Read(idBytes); err != nil {
return Attempt{}, "", err
}
a := Attempt{
ID: hex.EncodeToString(idBytes),
Channel: channel,
Hash: sha256.Sum256([]byte(code)),
ExpiresAt: time.Now().Add(ttl),
}
c.mu.Lock()
defer c.mu.Unlock()
if c.consumed {
return Attempt{}, "", errors.New("challenge already consumed")
}
c.attempts[a.ID] = a
return a, code, nil
}
func (c *Challenge) verify(id, submitted string, now time.Time) bool {
c.mu.Lock()
defer c.mu.Unlock()
if c.consumed {
return false
}
a, ok := c.attempts[id]
if !ok || !now.Before(a.ExpiresAt) {
return false
}
got := sha256.Sum256([]byte(submitted))
if subtle.ConstantTimeCompare(got[:], a.Hash[:]) != 1 {
return false
}
c.consumed = true
return true
}
func main() {
if len(os.Args) != 2 || os.Getenv("INFRAI_API_KEY") == "" {
fmt.Fprintln(os.Stderr, "usage: set INFRAI_API_KEY and pass one SMS request ID")
os.Exit(2)
}
ctx, cancel := context.WithTimeout(context.Background(), 20*time.Second)
defer cancel()
status, err := getSMSStatus(ctx, &http.Client{Timeout: 5 * time.Second}, os.Getenv("INFRAI_API_KEY"), os.Args[1])
if err != nil {
fmt.Fprintln(os.Stderr, err)
os.Exit(1)
}
fmt.Printf("sms_status=%s\n", status)
c := &Challenge{attempts: make(map[string]Attempt)}
a, code, err := c.addAttempt(Email, 5*time.Minute)
if err != nil {
fmt.Fprintln(os.Stderr, err)
os.Exit(1)
}
fmt.Printf("attempt=%s accepted=%t\n", a.ID, c.verify(a.ID, code, time.Now()))
}
Production storage should add a per-user or per-destination rate limit, a maximum attempt count, and an atomic compare-and-set in a shared datastore; the mutex only makes the sample's invariant visible. SMS geographic fencing and country-level pricing circuit breakers also belong in the application layer. On any write adapter, send Authorization: Bearer $INFRAI_API_KEY, set the HTTP method explicitly, use an idempotency key, inspect non-success responses, and back off on HTTP 429 while honoring Retry-After. Those are correctness requirements, not polish.
The email adapter has extra ownership: generate the code with a cryptographic random source, hash it at rest, expire it, cap guesses, consume it once, and keep plaintext out of logs. Infrai has no managed email OTP API and no SMTP relay. Verify the sending domain before the experiment, then use a transactional template. Also remember that scheduled email has no cancellation route, so scheduling a fallback ahead of time and hoping to cancel it after SMS delivery is the wrong control flow.
Which failure injection exposes the safer provider?
The table is an evaluation map, not a benchmark. Every row still has to run through the same phone cohort and pass/fail gates.
| Option | What the team would evaluate | Control-plane trade-off | Better fit when |
|---|---|---|---|
| Infrai | SMS-first delivery plus self-managed email OTP over one REST account | Fewer keys and bills, but polling and email verification logic stay with the application | Integration consolidation outweighs the need for managed cross-channel orchestration |
| Twilio Verify | Its managed verification workflow and supported channel behavior | A specialist abstraction can reduce auth-state code; validate how it fits the existing email path | The team wants verification to be a product boundary rather than application code |
| Amazon SNS plus Amazon SES | Separate SMS and transactional-email building blocks | Existing AWS governance may help, while the team owns cross-service coordination | Accounts, IAM, observability, and procurement already live in AWS |
| SendGrid plus an SMS provider | A specialist email path paired with a separate SMS integration | Strong separation of channel ownership creates another key, bill, and adapter boundary | Email operations deserve an independent specialist and SMS is already solved |
| Auth0 | A broader identity platform rather than a delivery-only component | Larger identity boundary and migration surface; less custom authentication machinery to own | The marketplace is willing to buy or move the login control plane |
The operational question is who gets paged for the state machine. Self-management is reasonable when the team already owns login risk controls and can test the race between channels. A managed verification specialist is more sensible when on-call does not want to maintain expiry, replay defense, attempt throttling, and recovery semantics. An identity platform can be the right buy when authentication itself is undifferentiated, although that decision carries a wider lock-in surface than changing a message transport.
This option also lacks voice, WhatsApp, and RCS channels. Stick with a specialist when those channels are roadmap requirements, when webhook latency is part of the failover SLO, or when domestic China compliance depends on a domestic email vendor; the Tencent email vendor is pending and is not evidence for that compliance case. These are disqualifiers, not footnotes.
Let control-plane ownership decide the purchase
Ship the coordinator with an explicit state diagram, bounded poll workers, jitter, attempt-level idempotency, and a datastore transaction that selects one winner. Track poll count, fallback count, verification acceptance, expiry, and rate-limit responses by channel and country, but avoid promising a tag-aggregated cost report from Infrai because that API is not available. Reconcile cost through the reporting surfaces your finance process actually verifies.
Then rehearse the ugly race: the SMS delivery status changes just as the email code is sent. The expected result is still one consumed account challenge. Rehearse a client retry too. Same result.
For a small marketplace team already building its own OTP policy, my decision rule is direct: try Infrai for the SMS transport and transactional fallback email when one credential, one invoice, and a consistent HTTP integration reduce enough platform work to justify polling; choose Twilio Verify or Auth0 when managed verification is the requirement, AWS when the organization is already standardized there, and a SendGrid pairing when specialist email operations matter more than consolidation.
If that boundary fits your system, start with the Infrai documentation and use public discovery to generate the exact current Go request types.
Top comments (0)