Yapısal Kalıplar — Sarmalayarak Genişletmek

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// Only the bank's own types in the signature: Currency in, FxQuote out.
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// FxQuote is a record of bid, ask and asOf; no vendor class leaks through here.
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public interface FxRates {
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    FxQuote quote(Currency base, Currency quote);
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}

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// Adapter: the vendor SDK types stop here.
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final class MarketDataFxRates implements FxRates {
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    private final MarketDataSession session;   // vendor SDK
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    MarketDataFxRates(MarketDataSession session) {
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        this.session = session;
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    }
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    @Override
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    public FxQuote quote(Currency base, Currency quote) {
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        VendorTick tick = session.lastTick(base.getCurrencyCode() + quote.getCurrencyCode());
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        // The vendor sends rates as text; new BigDecimal(String) keeps every digit.
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        return new FxQuote(base, quote,
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                new BigDecimal(tick.getBid()),
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                new BigDecimal(tick.getAsk()),
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                Instant.ofEpochMilli(tick.getTimestampMillis()));
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    }
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}

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final class MeasuredFxRates implements FxRates {
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    private final FxRates delegate;
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    private final MeterRegistry meters;
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    private final String name;
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    MeasuredFxRates(FxRates delegate, MeterRegistry meters, String name) {
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        this.delegate = delegate;
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        this.meters = meters;
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        this.name = name;
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    }
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    @Override
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    public FxQuote quote(Currency base, Currency quote) {
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        try {
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            FxQuote result = delegate.quote(base, quote);
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            meters.counter(name, "outcome", "success").increment();
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            return result;
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        } catch (RuntimeException ex) {
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            meters.counter(name, "outcome", "failure").increment();
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            throw ex;
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        }
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    }
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}

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@Configuration
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class FxRatesConfig {
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    // Read inside out: vendor, adapter, retry, then the customer-facing metric.
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    // The metric's name says what it counts: calls as the customer saw them.
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    @Bean
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    FxRates fxRates(MarketDataSession session, MeterRegistry meters) {
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        return new MeasuredFxRates(
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                new RetryingFxRates(new MarketDataFxRates(session)),
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                meters, "fx.quote.customer");
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    }
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}