BFF — Backend for Frontend

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// Shaped for this one screen. The app shows a quiet placeholder for each unavailable section.
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public record HomeScreenView(
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        List<AccountTile> accounts,
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        List<CardTile> cards,
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        List<RateRow> rates,
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        Set<Section> unavailable) {
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    public enum Section { CARDS, RATES }
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    public record AccountTile(String name, String maskedIban, String balance) {}   // "12.450,75 TL"
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    public record CardTile(String last4, String periodDebt) {}
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    public record RateRow(String pair, String buy, String sell) {}
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    public HomeScreenView {
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        unavailable = Set.copyOf(unavailable);
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    }
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    // Formatting helpers (Ibans, Money) are left out.
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    static HomeScreenView of(List<AccountDto> accounts, List<CardSummary> cards,
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                             List<FxQuote> rates, Set<Section> unavailable) {
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        return new HomeScreenView(
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                accounts.stream().map(a -> new AccountTile(a.name(), Ibans.mask(a.iban()), Money.format(a.available()))).toList(),
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                cards.stream().map(c -> new CardTile(c.last4(), Money.format(c.periodDebt()))).toList(),
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                rates.stream().map(q -> new RateRow(q.pair(), q.buy().toPlainString(), q.sell().toPlainString())).toList(),
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                unavailable);
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    }
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}

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@RestController
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@RequestMapping("/mobile/home")
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class HomeScreenController {
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    // The app gives up after 3 s. One second per call and no retry keeps us inside that.
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    private static final long CALL_BUDGET_MS = 1_000;
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    private final AccountsClient accounts;
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    private final CardsClient cards;
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    private final FxClient fx;
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    private final Executor io;                  // a bounded pool for the blocking HTTP calls
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    HomeScreenController(AccountsClient accounts, CardsClient cards, FxClient fx, Executor io) {
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        this.accounts = accounts;
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        this.cards = cards;
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        this.fx = fx;
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        this.io = io;
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    }
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    @GetMapping
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    HomeScreenView home(@AuthenticationPrincipal Jwt token) {
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        String customerNo = token.getSubject();
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        // All three start at once: the screen waits for the slowest call, not the sum.
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        var accountsCall = call(() -> accounts.listFor(customerNo));
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        var cardsCall = optional(call(() -> cards.summariesFor(customerNo)));
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        var ratesCall = optional(call(fx::homeScreenRates));
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        // Required: without accounts there is no home screen, so this join may fail the request.
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        List<AccountDto> accountList = accountsCall.join();
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        Optional<List<CardSummary>> cardList = cardsCall.join();
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        Optional<List<FxQuote>> rateList = ratesCall.join();
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        Set<Section> unavailable = EnumSet.noneOf(Section.class);
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        if (cardList.isEmpty()) unavailable.add(Section.CARDS);
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        if (rateList.isEmpty()) unavailable.add(Section.RATES);
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        return HomeScreenView.of(accountList, cardList.orElse(List.of()), rateList.orElse(List.of()), unavailable);
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    }
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    private <T> CompletableFuture<T> call(Supplier<T> remote) {
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        return CompletableFuture.supplyAsync(remote, io).orTimeout(CALL_BUDGET_MS, TimeUnit.MILLISECONDS);
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    }
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    // Optional sections: a failure or a timeout becomes "not available", not an error page.
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    private static <T> CompletableFuture<Optional<T>> optional(CompletableFuture<T> call) {
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        return call.thenApply(Optional::of).exceptionally(ex -> Optional.empty());
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    }
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}

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// Another way to write it: shorter, and it works on a good day.
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@RestController
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@RequestMapping("/mobile/home")
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class HomeScreenControllerAllRequired {
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    private final AccountsClient accounts;      // each client: 2 s timeout and 1 retry
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    private final CardsClient cards;
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    private final FxClient fx;
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    HomeScreenControllerAllRequired(AccountsClient accounts, CardsClient cards, FxClient fx) {
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        this.accounts = accounts;
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        this.cards = cards;
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        this.fx = fx;
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    }
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    @GetMapping
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    HomeScreenView home(@AuthenticationPrincipal Jwt token) {
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        String customerNo = token.getSubject();
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        // One after another: the waits add up, and one slow call alone can take 4 s.
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        var accountList = accounts.listFor(customerNo);
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        var cardList = cards.summariesFor(customerNo);
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        var rateList = fx.homeScreenRates();    // if the FX service is down, the whole home screen fails
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        return HomeScreenView.of(accountList, cardList, rateList, Set.of());
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    }
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}