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AfRApay.Web: Make transactions idempotent

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First rule of networking is the network is unreliable. Sometimes things get
lost, sometimes it gets found multiple times for TCP reasons or because a
browser tries to be clever.

And when you're dealing with money, even if it's monopoly money, you don't
want a duplicated request to mean a double-debit. The easiest way to do this
is to simply include an idempotency key with each request - if that key is
repeated, the request is ignored.
This commit is contained in:
embr 2023-02-08 22:21:25 +01:00
parent 597158a038
commit 0bf9843c1f
3 changed files with 20 additions and 11 deletions
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5
AfRApay.FTM/Program.cs
View file

@ -147,7 +147,10 @@ static async void HandleTap(IsoReader reader, HttpClient httpClient, TerminalSta
case TerminalState.Debit:
case TerminalState.Credit:
var finalAmount = Math.Abs(amount) * (state == TerminalState.Debit ? 1 : -1);
await CallGet(httpClient, String.Format("/api/card/transaction?card={0}&amount={1}", Convert.ToHexString(uid), finalAmount));
var idempotencyKey = new byte[18];
Random.Shared.NextBytes(idempotencyKey);
await CallGet(httpClient, String.Format("/api/card/transaction?card={0}&amount={1}&ik={2}",
Convert.ToHexString(uid), finalAmount, Convert.ToBase64String(idempotencyKey)));
break;
case TerminalState.Link:
await CallGet(httpClient, String.Format("/api/card/link?card={0}", Convert.ToHexString(uid)));

7
AfRApay.Web/Backend/Tables/User.cs
View file

@ -6,7 +6,8 @@ namespace AfRApay.Web.Backend.Tables;
[Table(Name = "Users")]
public class User {
[Column(Name = "ID"), PrimaryKey, Identity, NotNull] public int Id { get; set; }
[Column(Name = "Nickname"), NotNull] public string Nickname { get; set; }
[Column(Name = "Balance")] public decimal Balance { get; set; }
[Column(Name = "ID"), PrimaryKey, Identity, NotNull] public int Id { get; set; }
[Column(Name = "Nickname"), NotNull] public string Nickname { get; set; }
[Column(Name = "Balance")] public decimal Balance { get; set; }
[Column(Name = "LastIdempotencyKey")] public string LastIdempotencyKey { get; set; }
}

19
AfRApay.Web/Controllers/CardTransaction.cs
View file

@ -7,18 +7,23 @@ namespace AfRApay.Web.Controllers;
[ApiController, Route("/api/card/transaction")]
public class CardTransaction : Controller {
[HttpGet]
public string Get([FromQuery] string card, [FromQuery] decimal amount) {
public string Get([FromQuery] string card, [FromQuery] decimal amount, [FromQuery] string ik) {
var db = new Database.DbConn();
if (db.Cards.Any(p => p.CardId == card)) {
var userId = db.Cards.First(p => p.CardId == card).UserId;
var user = db.Users.First(p => p.Id == userId);
if (user.Balance - amount < -50) {
return "E:Balance too low!";
// If an idempotency key is given, disregard duplicate requests.
if (ik == "" || ik != user.LastIdempotencyKey) {
if (user.Balance - amount < -50) {
return "E:Balance too low!";
}
user.Balance -= amount;
user.LastIdempotencyKey = ik;
db.Update(user);
}
user.Balance -= amount;
db.Update(user);
return $"S:{user.Nickname}: {user.Balance:N2}";
}