public async Task SendAsync(SmsMessage sms, CancellationToken cancellation = default)
{
var error = SmsValidation.Validate(sms);
if (error != null)
{
throw new SmsInvalidException(error);
}
var serviceSid = !string.IsNullOrWhiteSpace(_options.ServiceSid) ? _options.ServiceSid : throw new InvalidOperationException("ServiceSid is missing");
// Extract the values from the argument
var to = new PhoneNumber(sms.ToPhoneNumber);
var message = sms.Message;
var messageId = sms.MessageId;
var tenantId = sms.TenantId;
// Calculate the callbackUri (if required)
Uri callbackUri = null;
if (messageId != 0)
{
string hostname = _options.CallbackHost?.WithoutTrailingSlash() ?? throw new InvalidOperationException("CallbackHost is missing");
string stringUri = $"{hostname}/api/sms-callback?{MessageIdParamName}={messageId}";
if (tenantId != 0)
{
stringUri += $"&{TenantIdParamName}={tenantId}";
}
callbackUri = new Uri(stringUri);
}
// Exponential backoff
const int maxAttempts = 5;
const int maxBackoff = 25000; // 25 Seconds
const int minBackoff = 1000; // 1 Second
const int deltaBackoff = 1000; // 1 Second
int attemptsSoFar = 0;
int backoff = minBackoff;
while (attemptsSoFar < maxAttempts && !cancellation.IsCancellationRequested)
{
attemptsSoFar++;
try
{
// Send using Twilio's Messaging Service
await MessageResource.CreateAsync(
body : message,
messagingServiceSid : serviceSid,
to : to,
statusCallback : callbackUri
);
break; // Success
}
catch (ApiException ex) when(ex.Status == (int)HttpStatusCode.TooManyRequests || ex.Status >= 500)
{
// Twilio imposes a maximum number of concurrent calls, and returns 429 when that maximum is reached
// Here we implement exponential backoff to retry the call few more times before giving up as
// recommended here https://bit.ly/2CWYrjQ
if (attemptsSoFar < maxAttempts)
{
var randomOffset = _rand.Next(0, deltaBackoff);
await Task.Delay(backoff + randomOffset, cancellation);
// Double the backoff for next attempt
backoff = Math.Min(backoff * 2, maxBackoff);
}
else
{
_logger.LogError($"Twilio: 429 Too Many Requests even after {attemptsSoFar} attempts with exponential backoff.");
throw; // Give up
}
}
}
}