If you're sending email via Exchange Web Services, cache the service URL for a while!

In my recent series of posts about sending emails through Office 365 from an Azure Function, triggered by a blob being uploaded to Azure storage, I noted that:

because of the amount of time Office 365 autodiscover takes, it's not a particularly quick process

There's a few references that a quick search returned that back this up:

1. EWS Best Practices: Use Autodiscover…  - "It’s good practice to refresh the cached URL for a given mailbox every 24 hours or so"
2. How to: Set the EWS service URL by using the EWS Managed API - "you might want to cache the URL value you get from Autodiscover and manually set the EWS service URL with this cached value"
3. Stack Overflow: How to cache Exchange web service API autodiscoverurl? - "please make AutoDiscover request at least once every 24 hours" (in an answer from a Program Manager @ Microsoft for Mail, calendar, & contact APIs for Office 365, Exchange, Outlook.com, Outlook Outlook/Exchange connectivity)

By instrumenting the console app I put together in my post Creating an Azure function that triggers when a blob is uploaded - Sending an email via Office 365  (very quickly and lazily!) to find out how long each email takes to send when run in a loop over 10 iterations:

var durations = new List();
for (int i = 0; i < 10; i++)
{
    Stopwatch watch = new Stopwatch();
    watch.Start();
    SendEmail(credentials, username, "<RECIPIENT_EMAIL_ADDRESS>", "Subject Line", $"<h1>Content of email generated at {DateTime.Now}</h1>");
    watch.Stop();
    durations.Add(watch.ElapsedMilliseconds);
}

var average = durations.Average();

I got an average duration of 12,246.2 milliseconds, or just over 12 seconds per email. By tweaking the code inside the SendEmail method so that it caches the URL, by adding a property to store it in and tweaking the code that calls the AutodiscoverUrl method:

if (EWSUrl == null)
{
    service.AutodiscoverUrl(fromAddress, RedirectionUrlValidationCallback);
    EWSUrl = service.Url;
}
else
{
    service.Url = EWSUrl;
}

All of a sudden, the average time taken drops down to 1,578.9 milliseconds. Being very lazy (i.e. not going back and checking the values), I can extrapolate that the ten requests took a total of 15,789 milliseconds, so deducting the average from the previous test (time taken when calling Autodiscover) of 12,246 milliseconds gives a remainder of 3,543 milliseconds.

Finally (and stretching the numbers even further into potential inaccuracy) dividing by 9 gives a result of 393 milliseconds. So, by caching the service url, the average time taken to send an email will reduce from c. 12 seconds to c. 1/3 of a second (when the time taken to autodiscover is stripped out from the latter). That's a pretty significant win.

Next up: How do I cache the service URL in an Azure function?