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<!doctype html><html lang="en"><head><title data-rh="true">Idempotency: The Key to Reliable Money Transfer APIs | by Agile Actors | Nov, 2024 | Medium</title><meta data-rh="true" charset="utf-8"/><meta data-rh="true" name="viewport" content="width=device-width,minimum-scale=1,initial-scale=1,maximum-scale=1"/><meta data-rh="true" name="theme-color" content="#000000"/><meta data-rh="true" name="twitter:app:name:iphone" content="Medium"/><meta data-rh="true" name="twitter:app:id:iphone" content="828256236"/><meta data-rh="true" property="al:ios:app_name" content="Medium"/><meta data-rh="true" property="al:ios:app_store_id" content="828256236"/><meta data-rh="true" property="al:android:package" content="com.medium.reader"/><meta data-rh="true" property="fb:app_id" content="542599432471018"/><meta data-rh="true" property="og:site_name" content="Medium"/><meta data-rh="true" property="og:type" content="article"/><meta data-rh="true" property="article:published_time" content="2024-11-22T15:08:29.756Z"/><meta data-rh="true" name="title" content="Idempotency: The Key to Reliable Money Transfer APIs | by Agile Actors | Nov, 2024 | Medium"/><meta data-rh="true" property="og:title" content="Idempotency: The Key to Reliable Money Transfer APIs"/><meta data-rh="true" property="al:android:url" content="medium://p/286c710fd3ce"/><meta data-rh="true" property="al:ios:url" content="medium://p/286c710fd3ce"/><meta data-rh="true" property="al:android:app_name" content="Medium"/><meta data-rh="true" name="description" content="In this article, we will explore how idempotency can enhance the reliability of a money transfer API. 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data-testid="storyTitle"><strong class="al">Idempotency: The Key to Reliable Money Transfer APIs</strong></h1><div><div class="speechify-ignore ab cp"><div class="speechify-ignore bh l"><div class="ik il im in io ab"><div><div class="ab ip"><div><div class="bm" aria-hidden="false"><a rel="noopener follow" href="/@agileactors?source=post_page---byline--286c710fd3ce--------------------------------"><div class="l iq ir by is it"><div class="l fj"><img alt="Agile Actors" class="l fd by dd de cx" src="https://miro.medium.com/v2/resize:fill:88:88/1*o2gqK5A6W3k5ztjF679Q7A.png" width="44" height="44" loading="lazy" data-testid="authorPhoto"/><div class="iu by l dd de fs n iv ft"></div></div></div></a></div></div></div></div><div class="bn bh l"><div class="ab"><div style="flex:1"><span class="bf b bg z bk"><div class="iw ab q"><div class="ab q ix"><div class="ab q"><div><div class="bm" aria-hidden="false"><p class="bf b iy iz bk"><a class="af ag ah ai aj ak al am an ao ap aq ar ja" 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du">Share</p></div></button></div></div></div></div></div></div></div></div></div><p id="921d" class="pw-post-body-paragraph mu mv hj mw b mx my mz na nb nc nd ne nf ng nh ni nj nk nl nm nn no np nq nr gn bk">By Roza Chatzigeorgiou, Java Software Engineer.</p><h1 id="a303" class="ns nt hj bf nu nv nw nx ny nz oa ob oc od oe of og oh oi oj ok ol om on oo op bk"><strong class="al">Introduction</strong></h1><p id="12bf" class="pw-post-body-paragraph mu mv hj mw b mx oq mz na nb or nd ne nf os nh ni nj ot nl nm nn ou np nq nr gn bk">In this article, we will explore how idempotency can enhance the reliability of a money transfer API.</p><p id="d8e1" class="pw-post-body-paragraph mu mv hj mw b mx my mz na nb nc nd ne nf ng nh ni nj nk nl nm nn no np nq nr gn bk">Let’s begin by defining what an idempotent request actually is. An idempotent request is like that trusty friend who never changes - no matter how many times you call, it’s always giving you the same response. <em class="ov">For APIs, an idempotent request will always give the same response to the client.</em> While some types of requests such as GET inherently have this behavior, others like POST do not guarantee it.</p><p id="263e" class="pw-post-body-paragraph mu mv hj mw b mx my mz na nb nc nd ne nf ng nh ni nj nk nl nm nn no np nq nr gn bk">But how important is idempotency in a money transfer application? 🤔 Let’s visualize a simple (happy path) example with a client timeout.</p><figure class="ox oy oz pa pb ha gs gt paragraph-image"><div role="button" tabindex="0" class="hb hc fj hd bh he"><div class="gs gt ow"><picture><source srcSet="https://miro.medium.com/v2/resize:fit:640/format:webp/1*OU3MRqEmHuRoaV-bRj5AyQ.jpeg 640w, https://miro.medium.com/v2/resize:fit:720/format:webp/1*OU3MRqEmHuRoaV-bRj5AyQ.jpeg 720w, https://miro.medium.com/v2/resize:fit:750/format:webp/1*OU3MRqEmHuRoaV-bRj5AyQ.jpeg 750w, https://miro.medium.com/v2/resize:fit:786/format:webp/1*OU3MRqEmHuRoaV-bRj5AyQ.jpeg 786w, https://miro.medium.com/v2/resize:fit:828/format:webp/1*OU3MRqEmHuRoaV-bRj5AyQ.jpeg 828w, https://miro.medium.com/v2/resize:fit:1100/format:webp/1*OU3MRqEmHuRoaV-bRj5AyQ.jpeg 1100w, https://miro.medium.com/v2/resize:fit:1400/format:webp/1*OU3MRqEmHuRoaV-bRj5AyQ.jpeg 1400w" sizes="(min-resolution: 4dppx) and (max-width: 700px) 50vw, (-webkit-min-device-pixel-ratio: 4) and (max-width: 700px) 50vw, (min-resolution: 3dppx) and (max-width: 700px) 67vw, (-webkit-min-device-pixel-ratio: 3) and (max-width: 700px) 65vw, (min-resolution: 2.5dppx) and (max-width: 700px) 80vw, (-webkit-min-device-pixel-ratio: 2.5) and (max-width: 700px) 80vw, (min-resolution: 2dppx) and (max-width: 700px) 100vw, (-webkit-min-device-pixel-ratio: 2) and (max-width: 700px) 100vw, 700px" type="image/webp"/><source data-testid="og" srcSet="https://miro.medium.com/v2/resize:fit:640/1*OU3MRqEmHuRoaV-bRj5AyQ.jpeg 640w, https://miro.medium.com/v2/resize:fit:720/1*OU3MRqEmHuRoaV-bRj5AyQ.jpeg 720w, https://miro.medium.com/v2/resize:fit:750/1*OU3MRqEmHuRoaV-bRj5AyQ.jpeg 750w, https://miro.medium.com/v2/resize:fit:786/1*OU3MRqEmHuRoaV-bRj5AyQ.jpeg 786w, https://miro.medium.com/v2/resize:fit:828/1*OU3MRqEmHuRoaV-bRj5AyQ.jpeg 828w, https://miro.medium.com/v2/resize:fit:1100/1*OU3MRqEmHuRoaV-bRj5AyQ.jpeg 1100w, https://miro.medium.com/v2/resize:fit:1400/1*OU3MRqEmHuRoaV-bRj5AyQ.jpeg 1400w" sizes="(min-resolution: 4dppx) and (max-width: 700px) 50vw, (-webkit-min-device-pixel-ratio: 4) and (max-width: 700px) 50vw, (min-resolution: 3dppx) and (max-width: 700px) 67vw, (-webkit-min-device-pixel-ratio: 3) and (max-width: 700px) 65vw, (min-resolution: 2.5dppx) and (max-width: 700px) 80vw, (-webkit-min-device-pixel-ratio: 2.5) and (max-width: 700px) 80vw, (min-resolution: 2dppx) and (max-width: 700px) 100vw, (-webkit-min-device-pixel-ratio: 2) and (max-width: 700px) 100vw, 700px"/><img alt="A happy path example with a timeout." class="bh hf hg c" width="700" height="445" loading="lazy"/></picture></div></div><figcaption class="pc ff pd gs gt pe pf bf b bg z du">A happy path example with a timeout.</figcaption></figure><p id="8944" class="pw-post-body-paragraph mu mv hj mw b mx my mz na nb nc nd ne nf ng nh ni nj nk nl nm nn no np nq nr gn bk">A client requests a transfer of 10$ from account A to account B.</p><p id="9ca1" class="pw-post-body-paragraph mu mv hj mw b mx my mz na nb nc nd ne nf ng nh ni nj nk nl nm nn no np nq nr gn bk">The server receives the request, performs the transfer and sends a response to the client. However, due to a timeout, the client fails to receive the response.</p><p id="f2fd" class="pw-post-body-paragraph mu mv hj mw b mx my mz na nb nc nd ne nf ng nh ni nj nk nl nm nn no np nq nr gn bk">The client, assuming their request experienced a non-functional failure, decides to submit it again.</p><p id="3633" class="pw-post-body-paragraph mu mv hj mw b mx my mz na nb nc nd ne nf ng nh ni nj nk nl nm nn no np nq nr gn bk">But the server has already processed the first request and proceeds to double-charging Account A 😐. This is a critical error for a money transfer app! How can we handle such tricky situations? 🤔</p><p id="dddf" class="pw-post-body-paragraph mu mv hj mw b mx my mz na nb nc nd ne nf ng nh ni nj nk nl nm nn no np nq nr gn bk">This is where idempotent behavior comes into play!</p><p id="4ed9" class="pw-post-body-paragraph mu mv hj mw b mx my mz na nb nc nd ne nf ng nh ni nj nk nl nm nn no np nq nr gn bk">It ensures that even if a request is submitted multiple times, the server handles it only once; the client will always receive the same response.</p><h1 id="d9fd" class="ns nt hj bf nu nv nw nx ny nz oa ob oc od oe of og oh oi oj ok ol om on oo op bk"><strong class="al">Data Model</strong></h1><p id="7469" class="pw-post-body-paragraph mu mv hj mw b mx oq mz na nb or nd ne nf os nh ni nj ot nl nm nn ou np nq nr gn bk">Let us begin by defining the data model of an idempotent money transfer API. The Account entity represents a bank account, while the Transfer entity a money transfer between two bank accounts.</p><figure class="ox oy oz pa pb ha"><div class="pg ph l fj"><div class="pi pj l"></div></div><figcaption class="pc ff pd gs gt pe pf bf b bg z du">The Account entity represents a bank account.</figcaption></figure><figure class="ox oy oz pa pb ha"><div class="pg ph l fj"><div class="pi pj l"></div></div><figcaption class="pc ff pd gs gt pe pf bf b bg z du">The Transfer entity represents a money transfer between two bank accounts.</figcaption></figure><p id="2e7d" class="pw-post-body-paragraph mu mv hj mw b mx my mz na nb nc nd ne nf ng nh ni nj nk nl nm nn no np nq nr gn bk">To keep track of incoming transfer requests, we need a reliable way of storing them. For this purpose, let’s define a TransferRequest entity.</p><figure class="ox oy oz pa pb ha"><div class="pg ph l fj"><div class="pi pj l"></div></div><figcaption class="pc ff pd gs gt pe pf bf b bg z du">The TransferRequest entity represents a request for a money transfer between two bank accounts.</figcaption></figure><p id="75fc" class="pw-post-body-paragraph mu mv hj mw b mx my mz na nb nc nd ne nf ng nh ni nj nk nl nm nn no np nq nr gn bk">The status of a TransferRequest (TransferRequestStatus) is either in-progress or completed.</p><p id="3370" class="pw-post-body-paragraph mu mv hj mw b mx my mz na nb nc nd ne nf ng nh ni nj nk nl nm nn no np nq nr gn bk">A completed TransferRequest associated with a Transfer entity signifies a successful money transfer.</p><p id="2dae" class="pw-post-body-paragraph mu mv hj mw b mx my mz na nb nc nd ne nf ng nh ni nj nk nl nm nn no np nq nr gn bk">On the other hand, a completed TransferRequest with a null Transfer field indicates a failed money transfer. It also contains the infoMessage and httpStatus fields, which represent a failure message and a corresponding HTTP status.</p><p id="e726" class="pw-post-body-paragraph mu mv hj mw b mx my mz na nb nc nd ne nf ng nh ni nj nk nl nm nn no np nq nr gn bk">A TransferRequest in progress only contains the transferRequestId, amount, sourceAccountId, and targetAccountId fields.</p><h1 id="c2d4" class="ns nt hj bf nu nv nw nx ny nz oa ob oc od oe of og oh oi oj ok ol om on oo op bk">Idempotent Aspect</h1><p id="ce31" class="pw-post-body-paragraph mu mv hj mw b mx oq mz na nb or nd ne nf os nh ni nj ot nl nm nn ou np nq nr gn bk">How can we implement the functionality for idempotent transfer requests? The initial thought might be to create an IdempotentTransferService.</p><p id="07e9" class="pw-post-body-paragraph mu mv hj mw b mx my mz na nb nc nd ne nf ng nh ni nj nk nl nm nn no np nq nr gn bk">Sounds reasonable, right?</p><p id="95b2" class="pw-post-body-paragraph mu mv hj mw b mx my mz na nb nc nd ne nf ng nh ni nj nk nl nm nn no np nq nr gn bk">Imagine we’d like to re-use idempotent behavior throughout the application. Defining such service would make the idempotency feature tightly coupled to the business logic; that’s a recipe for poor code reusability and maintainability 😵.</p><p id="9e8c" class="pw-post-body-paragraph mu mv hj mw b mx my mz na nb nc nd ne nf ng nh ni nj nk nl nm nn no np nq nr gn bk">No worries, <a class="af pk" href="https://docs.spring.io/spring-framework/reference/core/aop.html" rel="noopener ugc nofollow" target="_blank">Aspect Oriented Programming</a> (AOP) comes to the rescue! AOP is a programming technique that allows us to handle a feature such as idempotency as a cross-cutting concern that is defined, managed and maintained in one place 😉.</p><blockquote class="pl pm pn"><p id="e570" class="mu mv ov mw b mx my mz na nb nc nd ne nf ng nh ni nj nk nl nm nn no np nq nr gn bk">Let’s define an IdempotentTransferAspect to implement the idempotency logic. Using the <a class="af pk" href="http://twitter.com/Around" rel="noopener ugc nofollow" target="_blank">@Around</a> annotation, the aspect can fully control the execution of the target method.</p></blockquote><figure class="ox oy oz pa pb ha"><div class="pg ph l fj"><div class="pi pj l"></div></div><figcaption class="pc ff pd gs gt pe pf bf b bg z du">The method that handles a new TransferRequest in IdempotencyTransferAspect.</figcaption></figure><h2 id="fd9f" class="po nt hj bf nu pp pq dy ny pr ps ea oc nf pt pu pv nj pw px py nn pz qa qb qc bk">Transaction Management</h2><p id="1afc" class="pw-post-body-paragraph mu mv hj mw b mx oq mz na nb or nd ne nf os nh ni nj ot nl nm nn ou np nq nr gn bk">By default, the @Around aspect inherits the transactional boundaries of the target method. This behavior is unwanted; we would like us to strictly define the transactional boundaries inside the aspect. The transactional boundaries of the target method (if any) will be ignored and its transaction will be automatically committed by Spring during transaction clean-up for @Around aspect.</p><blockquote class="pl pm pn"><p id="6aba" class="mu mv ov mw b mx my mz na nb nc nd ne nf ng nh ni nj nk nl nm nn no np nq nr gn bk">Considering the proxy nature of aspect oriented programming, <a class="af pk" href="https://docs.spring.io/spring-framework/reference/data-access/transaction/declarative.html" rel="noopener ugc nofollow" target="_blank">declarative transaction management</a> will not allow us to fully control the transactional boundaries inside the aspect, so we resort to <a class="af pk" href="https://docs.spring.io/spring-framework/reference/data-access/transaction/programmatic.html#transaction-programmatic-tm" rel="noopener ugc nofollow" target="_blank">programmatic transaction management</a> instead.</p></blockquote><p id="b87b" class="pw-post-body-paragraph mu mv hj mw b mx my mz na nb nc nd ne nf ng nh ni nj nk nl nm nn no np nq nr gn bk">For programmatic transaction management, <a class="af pk" href="https://docs.spring.io/spring-framework/docs/current/javadoc-api/org/springframework/transaction/PlatformTransactionManager.html" rel="noopener ugc nofollow" target="_blank">PlatformTransactionManager </a>will be used to create a new transaction (defined by a DefaultTransactionDefinition object) for a given transaction name, isolation and propagation level. The associated TransactionStatus will be then used to commit (and rollback if necessary) the transaction.</p><figure class="ox oy oz pa pb ha"><div class="pg ph l fj"><div class="pi pj l"></div></div><figcaption class="pc ff pd gs gt pe pf bf b bg z du">Programmatic creation of a new Transaction in Idempotent aspect.</figcaption></figure><p id="c24b" class="pw-post-body-paragraph mu mv hj mw b mx my mz na nb nc nd ne nf ng nh ni nj nk nl nm nn no np nq nr gn bk">A <a class="af pk" href="https://docs.oracle.com/javase/8/docs/api/java/lang/ThreadLocal.html" rel="noopener ugc nofollow" target="_blank">ThreadLocal</a> variable for the current TransactionStatus allows us to access it in a thread-safe manner; the associated entry will be removed after the aspect completes processing.</p><h2 id="06ad" class="po nt hj bf nu pp pq dy ny pr ps ea oc nf pt pu pv nj pw px py nn pz qa qb qc bk">Identifying TransferRequests</h2><p id="5d0a" class="pw-post-body-paragraph mu mv hj mw b mx oq mz na nb or nd ne nf os nh ni nj ot nl nm nn ou np nq nr gn bk">Every new transfer request requires <em class="ov">at least one</em> <em class="ov">transaction </em>with default isolation to create or retrieve the associated TransferRequest, as shown in the getAndCompareTransferRequest method.</p><figure class="ox oy oz pa pb ha"><div class="pg ph l fj"><div class="pi pj l"></div></div><figcaption class="pc ff pd gs gt pe pf bf b bg z du">Idempotent aspect gets the TransferRequest and processes it, given its TransactionRequestStatus.</figcaption></figure><figure class="ox oy oz pa pb ha"><div class="pg ph l fj"><div class="pi pj l"></div></div><figcaption class="pc ff pd gs gt pe pf bf b bg z du">NewTransferDto is a data transfer object representing the payload of a new transfer request.</figcaption></figure><figure class="ox oy oz pa pb ha"><div class="pg ph l fj"><div class="pi pj l"></div></div><figcaption class="pc ff pd gs gt pe pf bf b bg z du">TransferRequest is retrieved and then compared to the payload or created in IdempotencyTransferAspect.</figcaption></figure><p id="6b06" class="pw-post-body-paragraph mu mv hj mw b mx my mz na nb nc nd ne nf ng nh ni nj nk nl nm nn no np nq nr gn bk">A retrieved transfer request should be compared with the payload of the new transfer request to ensure they are identical. The payload of a transfer request contains important information such as account ids, amount which should also be taken into account; any alteration to these fields would signify a different transfer request.</p><figure class="ox oy oz pa pb ha"><div class="pg ph l fj"><div class="pi pj l"></div></div><figcaption class="pc ff pd gs gt pe pf bf b bg z du">Comparison of new and retrieved request in IdempotentTransferAspect.</figcaption></figure><h2 id="52bb" class="po nt hj bf nu pp pq dy ny pr ps ea oc nf pt pu pv nj pw px py nn pz qa qb qc bk"><strong class="al">Caching TransferRequests</strong></h2><p id="47bd" class="pw-post-body-paragraph mu mv hj mw b mx oq mz na nb or nd ne nf os nh ni nj ot nl nm nn ou np nq nr gn bk">To cut down on frequent database interactions, transaction requests are being cached. This way, we can avoid fetching resources repeatedly, resulting in a more efficient system. After all, time is money; every second saved contributes to greater efficiency and profitability 😉.</p><p id="2611" class="pw-post-body-paragraph mu mv hj mw b mx my mz na nb nc nd ne nf ng nh ni nj nk nl nm nn no np nq nr gn bk">For caching declaration, Spring’s caching abstraction provides a set of Java annotations such as @Cachable and @CachePut. Based on a key value, these annotations indicate that the result of a method will be cached (@Cachable) or that the cache contents will be updated (@CachePut).</p><figure class="ox oy oz pa pb ha"><div class="pg ph l fj"><div class="pi pj l"></div></div><figcaption class="pc ff pd gs gt pe pf bf b bg z du">Get (retrieve) or create a new request in TransferRequestServiceImpl and update/save new entry in cache.</figcaption></figure><h2 id="5688" class="po nt hj bf nu pp pq dy ny pr ps ea oc nf pt pu pv nj pw px py nn pz qa qb qc bk"><strong class="al">Processing TransferRequests</strong></h2><p id="d68c" class="pw-post-body-paragraph mu mv hj mw b mx oq mz na nb or nd ne nf os nh ni nj ot nl nm nn ou np nq nr gn bk">A TransferRequest is processed depending on its status i.e., IN_PROGRESS or COMPLETED.</p><blockquote class="pl pm pn"><p id="b41c" class="mu mv ov mw b mx my mz na nb nc nd ne nf ng nh ni nj nk nl nm nn no np nq nr gn bk">For a COMPLETED TransferRequest, further processing becomes unnecessary. The retrieved TransferRequest entity already contains the information for creating the client response.</p></blockquote><figure class="ox oy oz pa pb ha"><div class="pg ph l fj"><div class="pi pj l"></div></div><figcaption class="pc ff pd gs gt pe pf bf b bg z du">IdempotentTransferAspect returns a Transfer or an exception message and HTTP status for a completed TransferRequest.</figcaption></figure><blockquote class="pl pm pn"><p id="2612" class="mu mv ov mw b mx my mz na nb nc nd ne nf ng nh ni nj nk nl nm nn no np nq nr gn bk">On the other hand, an IN_PROGRESS idempotent money transfer request requires<em class="hj"> two more</em> transactions:</p><p id="8ac5" class="mu mv ov mw b mx my mz na nb nc nd ne nf ng nh ni nj nk nl nm nn no np nq nr gn bk">1. Execution of money transfer target method (with serializable isolation)</p><p id="2055" class="mu mv ov mw b mx my mz na nb nc nd ne nf ng nh ni nj nk nl nm nn no np nq nr gn bk">2. Completion of TransferRequest (with default isolation).</p></blockquote><figure class="ox oy oz pa pb ha"><div class="pg ph l fj"><div class="pi pj l"></div></div><figcaption class="pc ff pd gs gt pe pf bf b bg z du">Execution of transfer target method, request completion and definition of their transactional boundaries.</figcaption></figure><p id="1dd9" class="pw-post-body-paragraph mu mv hj mw b mx my mz na nb nc nd ne nf ng nh ni nj nk nl nm nn no np nq nr gn bk">A successfully completed TransferRequest indicates is always associated with a Transfer entity.</p><p id="89bb" class="pw-post-body-paragraph mu mv hj mw b mx my mz na nb nc nd ne nf ng nh ni nj nk nl nm nn no np nq nr gn bk">For a failed money transfer, the TransferRequest is completed using the exception message and corresponding HTTP status.</p><p id="6023" class="pw-post-body-paragraph mu mv hj mw b mx my mz na nb nc nd ne nf ng nh ni nj nk nl nm nn no np nq nr gn bk">We use Spring’s @Retry annotation to handle retry logic for request completion in cases of temporary loss of database communication. The default number of retries before failing is three.</p><figure class="ox oy oz pa pb ha"><div class="pg ph l fj"><div class="pi pj l"></div></div><figcaption class="pc ff pd gs gt pe pf bf b bg z du">Request completion using Spring’s @Retryable in TransferRequestServiceImpl.</figcaption></figure><p id="d060" class="pw-post-body-paragraph mu mv hj mw b mx my mz na nb nc nd ne nf ng nh ni nj nk nl nm nn no np nq nr gn bk">Adding the idempotency behavior feature to different parts of the application is now a piece of cake; it can be achieved by simply annotating a transfer method with @IdempotentTransferRequest!</p><figure class="ox oy oz pa pb ha"><div class="pg ph l fj"><div class="pi pj l"></div></div><figcaption class="pc ff pd gs gt pe pf bf b bg z du">Annotating a transfer method as an idempotent transfer request.</figcaption></figure><h1 id="1ef9" class="ns nt hj bf nu nv nw nx ny nz oa ob oc od oe of og oh oi oj ok ol om on oo op bk">Testing</h1><p id="ed7e" class="pw-post-body-paragraph mu mv hj mw b mx oq mz na nb or nd ne nf os nh ni nj ot nl nm nn ou np nq nr gn bk">It’s time to test the idempotent feature! We write a unit test including tests for the following scenarios:</p><ol class=""><li id="edee" class="mu mv hj mw b mx my mz na nb nc nd ne nf ng nh ni nj nk nl nm nn no np nq nr qd qe qf bk">Happy path (aspect always returns the same Transfer).</li><li id="6d71" class="mu mv hj mw b mx qg mz na nb qh nd ne nf qi nh ni nj qj nl nm nn qk np nq nr qd qe qf bk">Unhappy path (aspect always returns the same exception message and HTTP status).</li><li id="8172" class="mu mv hj mw b mx qg mz na nb qh nd ne nf qi nh ni nj qj nl nm nn qk np nq nr qd qe qf bk">Same TransferRequest with different payload (should throw a RequestConflictException).</li></ol><figure class="ox oy oz pa pb ha"><div class="pg ph l fj"><div class="pi pj l"></div></div><figcaption class="pc ff pd gs gt pe pf bf b bg z du">Unit test for IdempotentTransferAspect</figcaption></figure><p id="2eee" class="pw-post-body-paragraph mu mv hj mw b mx my mz na nb nc nd ne nf ng nh ni nj nk nl nm nn no np nq nr gn bk">Thanks for reading! :)</p><p id="2f5c" class="pw-post-body-paragraph mu mv hj mw b mx my mz na nb nc nd ne nf ng nh ni nj nk nl nm nn no np nq nr gn bk">You can find the source code here: <a class="af pk" href="https://github.com/rozachatz/CashFlowz" rel="noopener ugc nofollow" target="_blank">https://github.com/rozachatz/CashFlowz</a></p></div></div></div></div></section></div></div></article></div><div class="ab cb"><div class="ci bh fz ga gb gc"><div class="ql qm ab jk"><div class="qn ab"><a class="qo ay am ao" rel="noopener follow" href="/tag/technology?source=post_page-----286c710fd3ce--------------------------------"><div class="qp fj cx qq ge qr qs bf b bg z bk qt">Technology</div></a></div><div class="qn ab"><a class="qo ay am ao" rel="noopener follow" href="/tag/software-development?source=post_page-----286c710fd3ce--------------------------------"><div class="qp fj cx qq ge qr qs bf b bg z bk qt">Software Development</div></a></div><div class="qn ab"><a class="qo ay am ao" rel="noopener 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An idempotent request is like that trusty friend who never changes - no matter how many times you call, it’s always giving you the same response. For APIs, an idempotent request will always give the same response to the client. While some types of requests such as GET inherently have this behavior, others like POST do not guarantee it.","hasDropCap":null,"dropCapImage":null,"markups":[{"__typename":"Markup","type":"EM","start":210,"end":291,"href":null,"anchorType":null,"userId":null,"linkMetadata":null}],"codeBlockMetadata":null,"iframe":null,"mixtapeMetadata":null},"Paragraph:eddc97fef7c4_6":{"__typename":"Paragraph","id":"eddc97fef7c4_6","name":"263e","type":"P","href":null,"layout":null,"metadata":null,"text":"But how important is idempotency in a money transfer application? 🤔 Let’s visualize a simple (happy path) example with a client timeout.","hasDropCap":null,"dropCapImage":null,"markups":[],"codeBlockMetadata":null,"iframe":null,"mixtapeMetadata":null},"ImageMetadata:1*OU3MRqEmHuRoaV-bRj5AyQ.jpeg":{"__typename":"ImageMetadata","id":"1*OU3MRqEmHuRoaV-bRj5AyQ.jpeg","originalHeight":959,"originalWidth":1510,"focusPercentX":null,"focusPercentY":null,"alt":"A happy path example with a timeout."},"Paragraph:eddc97fef7c4_7":{"__typename":"Paragraph","id":"eddc97fef7c4_7","name":"d229","type":"IMG","href":null,"layout":"INSET_CENTER","metadata":{"__ref":"ImageMetadata:1*OU3MRqEmHuRoaV-bRj5AyQ.jpeg"},"text":"A happy path example with a timeout.","hasDropCap":null,"dropCapImage":null,"markups":[],"codeBlockMetadata":null,"iframe":null,"mixtapeMetadata":null},"Paragraph:eddc97fef7c4_8":{"__typename":"Paragraph","id":"eddc97fef7c4_8","name":"8944","type":"P","href":null,"layout":null,"metadata":null,"text":"A client requests a transfer of 10$ from account A to account B.","hasDropCap":null,"dropCapImage":null,"markups":[],"codeBlockMetadata":null,"iframe":null,"mixtapeMetadata":null},"Paragraph:eddc97fef7c4_9":{"__typename":"Paragraph","id":"eddc97fef7c4_9","name":"9ca1","type":"P","href":null,"layout":null,"metadata":null,"text":"The server receives the request, performs the transfer and sends a response to the client. However, due to a timeout, the client fails to receive the response.","hasDropCap":null,"dropCapImage":null,"markups":[],"codeBlockMetadata":null,"iframe":null,"mixtapeMetadata":null},"Paragraph:eddc97fef7c4_10":{"__typename":"Paragraph","id":"eddc97fef7c4_10","name":"f2fd","type":"P","href":null,"layout":null,"metadata":null,"text":"The client, assuming their request experienced a non-functional failure, decides to submit it again.","hasDropCap":null,"dropCapImage":null,"markups":[],"codeBlockMetadata":null,"iframe":null,"mixtapeMetadata":null},"Paragraph:eddc97fef7c4_11":{"__typename":"Paragraph","id":"eddc97fef7c4_11","name":"3633","type":"P","href":null,"layout":null,"metadata":null,"text":"But the server has already processed the first request and proceeds to double-charging Account A 😐. This is a critical error for a money transfer app! How can we handle such tricky situations? 🤔","hasDropCap":null,"dropCapImage":null,"markups":[],"codeBlockMetadata":null,"iframe":null,"mixtapeMetadata":null},"Paragraph:eddc97fef7c4_12":{"__typename":"Paragraph","id":"eddc97fef7c4_12","name":"dddf","type":"P","href":null,"layout":null,"metadata":null,"text":"This is where idempotent behavior comes into play!","hasDropCap":null,"dropCapImage":null,"markups":[],"codeBlockMetadata":null,"iframe":null,"mixtapeMetadata":null},"Paragraph:eddc97fef7c4_13":{"__typename":"Paragraph","id":"eddc97fef7c4_13","name":"4ed9","type":"P","href":null,"layout":null,"metadata":null,"text":"It ensures that even if a request is submitted multiple times, the server handles it only once; the client will always receive the same response.","hasDropCap":null,"dropCapImage":null,"markups":[],"codeBlockMetadata":null,"iframe":null,"mixtapeMetadata":null},"Paragraph:eddc97fef7c4_14":{"__typename":"Paragraph","id":"eddc97fef7c4_14","name":"d9fd","type":"H3","href":null,"layout":null,"metadata":null,"text":"Data Model","hasDropCap":null,"dropCapImage":null,"markups":[{"__typename":"Markup","type":"STRONG","start":0,"end":10,"href":null,"anchorType":null,"userId":null,"linkMetadata":null}],"codeBlockMetadata":null,"iframe":null,"mixtapeMetadata":null},"Paragraph:eddc97fef7c4_15":{"__typename":"Paragraph","id":"eddc97fef7c4_15","name":"7469","type":"P","href":null,"layout":null,"metadata":null,"text":"Let us begin by defining the data model of an idempotent money transfer API. The Account entity represents a bank account, while the Transfer entity a money transfer between two bank accounts.","hasDropCap":null,"dropCapImage":null,"markups":[],"codeBlockMetadata":null,"iframe":null,"mixtapeMetadata":null},"MediaResource:045fd86a95b420acb888f4faf6ba026b":{"__typename":"MediaResource","id":"045fd86a95b420acb888f4faf6ba026b","iframeSrc":"","iframeHeight":0,"iframeWidth":0,"title":"Account.java"},"Paragraph:eddc97fef7c4_16":{"__typename":"Paragraph","id":"eddc97fef7c4_16","name":"1d54","type":"IFRAME","href":null,"layout":"INSET_CENTER","metadata":null,"text":"The Account entity represents a bank account.","hasDropCap":null,"dropCapImage":null,"markups":[],"codeBlockMetadata":null,"iframe":{"__typename":"Iframe","mediaResource":{"__ref":"MediaResource:045fd86a95b420acb888f4faf6ba026b"}},"mixtapeMetadata":null},"MediaResource:a21bbf522a53516ba0f6efc6248e3156":{"__typename":"MediaResource","id":"a21bbf522a53516ba0f6efc6248e3156","iframeSrc":"","iframeHeight":0,"iframeWidth":0,"title":"Transfer.java"},"Paragraph:eddc97fef7c4_17":{"__typename":"Paragraph","id":"eddc97fef7c4_17","name":"243f","type":"IFRAME","href":null,"layout":"INSET_CENTER","metadata":null,"text":"The Transfer entity represents a money transfer between two bank accounts.","hasDropCap":null,"dropCapImage":null,"markups":[],"codeBlockMetadata":null,"iframe":{"__typename":"Iframe","mediaResource":{"__ref":"MediaResource:a21bbf522a53516ba0f6efc6248e3156"}},"mixtapeMetadata":null},"Paragraph:eddc97fef7c4_18":{"__typename":"Paragraph","id":"eddc97fef7c4_18","name":"2e7d","type":"P","href":null,"layout":null,"metadata":null,"text":"To keep track of incoming transfer requests, we need a reliable way of storing them. For this purpose, let’s define a TransferRequest entity.","hasDropCap":null,"dropCapImage":null,"markups":[],"codeBlockMetadata":null,"iframe":null,"mixtapeMetadata":null},"MediaResource:8810832d6655c4ddb679e37a0fd24085":{"__typename":"MediaResource","id":"8810832d6655c4ddb679e37a0fd24085","iframeSrc":"","iframeHeight":0,"iframeWidth":0,"title":"TransferRequest.java"},"Paragraph:eddc97fef7c4_19":{"__typename":"Paragraph","id":"eddc97fef7c4_19","name":"b4e6","type":"IFRAME","href":null,"layout":"INSET_CENTER","metadata":null,"text":"The TransferRequest entity represents a request for a money transfer between two bank accounts.","hasDropCap":null,"dropCapImage":null,"markups":[],"codeBlockMetadata":null,"iframe":{"__typename":"Iframe","mediaResource":{"__ref":"MediaResource:8810832d6655c4ddb679e37a0fd24085"}},"mixtapeMetadata":null},"Paragraph:eddc97fef7c4_20":{"__typename":"Paragraph","id":"eddc97fef7c4_20","name":"75fc","type":"P","href":null,"layout":null,"metadata":null,"text":"The status of a TransferRequest (TransferRequestStatus) is either in-progress or completed.","hasDropCap":null,"dropCapImage":null,"markups":[],"codeBlockMetadata":null,"iframe":null,"mixtapeMetadata":null},"Paragraph:eddc97fef7c4_21":{"__typename":"Paragraph","id":"eddc97fef7c4_21","name":"3370","type":"P","href":null,"layout":null,"metadata":null,"text":"A completed TransferRequest associated with a Transfer entity signifies a successful money transfer.","hasDropCap":null,"dropCapImage":null,"markups":[],"codeBlockMetadata":null,"iframe":null,"mixtapeMetadata":null},"Paragraph:eddc97fef7c4_22":{"__typename":"Paragraph","id":"eddc97fef7c4_22","name":"2dae","type":"P","href":null,"layout":null,"metadata":null,"text":"On the other hand, a completed TransferRequest with a null Transfer field indicates a failed money transfer. It also contains the infoMessage and httpStatus fields, which represent a failure message and a corresponding HTTP status.","hasDropCap":null,"dropCapImage":null,"markups":[],"codeBlockMetadata":null,"iframe":null,"mixtapeMetadata":null},"Paragraph:eddc97fef7c4_23":{"__typename":"Paragraph","id":"eddc97fef7c4_23","name":"e726","type":"P","href":null,"layout":null,"metadata":null,"text":"A TransferRequest in progress only contains the transferRequestId, amount, sourceAccountId, and targetAccountId fields.","hasDropCap":null,"dropCapImage":null,"markups":[],"codeBlockMetadata":null,"iframe":null,"mixtapeMetadata":null},"Paragraph:eddc97fef7c4_24":{"__typename":"Paragraph","id":"eddc97fef7c4_24","name":"c2d4","type":"H3","href":null,"layout":null,"metadata":null,"text":"Idempotent Aspect","hasDropCap":null,"dropCapImage":null,"markups":[],"codeBlockMetadata":null,"iframe":null,"mixtapeMetadata":null},"Paragraph:eddc97fef7c4_25":{"__typename":"Paragraph","id":"eddc97fef7c4_25","name":"ce31","type":"P","href":null,"layout":null,"metadata":null,"text":"How can we implement the functionality for idempotent transfer requests? The initial thought might be to create an IdempotentTransferService.","hasDropCap":null,"dropCapImage":null,"markups":[],"codeBlockMetadata":null,"iframe":null,"mixtapeMetadata":null},"Paragraph:eddc97fef7c4_26":{"__typename":"Paragraph","id":"eddc97fef7c4_26","name":"07e9","type":"P","href":null,"layout":null,"metadata":null,"text":"Sounds reasonable, right?","hasDropCap":null,"dropCapImage":null,"markups":[],"codeBlockMetadata":null,"iframe":null,"mixtapeMetadata":null},"Paragraph:eddc97fef7c4_27":{"__typename":"Paragraph","id":"eddc97fef7c4_27","name":"95b2","type":"P","href":null,"layout":null,"metadata":null,"text":"Imagine we’d like to re-use idempotent behavior throughout the application. Defining such service would make the idempotency feature tightly coupled to the business logic; that’s a recipe for poor code reusability and maintainability 😵.","hasDropCap":null,"dropCapImage":null,"markups":[],"codeBlockMetadata":null,"iframe":null,"mixtapeMetadata":null},"Paragraph:eddc97fef7c4_28":{"__typename":"Paragraph","id":"eddc97fef7c4_28","name":"9e8c","type":"P","href":null,"layout":null,"metadata":null,"text":"No worries, Aspect Oriented Programming (AOP) comes to the rescue! AOP is a programming technique that allows us to handle a feature such as idempotency as a cross-cutting concern that is defined, managed and maintained in one place 😉.","hasDropCap":null,"dropCapImage":null,"markups":[{"__typename":"Markup","type":"A","start":12,"end":39,"href":"https:\u002F\u002Fdocs.spring.io\u002Fspring-framework\u002Freference\u002Fcore\u002Faop.html","anchorType":"LINK","userId":null,"linkMetadata":null}],"codeBlockMetadata":null,"iframe":null,"mixtapeMetadata":null},"Paragraph:eddc97fef7c4_29":{"__typename":"Paragraph","id":"eddc97fef7c4_29","name":"e570","type":"BQ","href":null,"layout":null,"metadata":null,"text":"Let’s define an IdempotentTransferAspect to implement the idempotency logic. 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We write a unit test including tests for the following scenarios:","hasDropCap":null,"dropCapImage":null,"markups":[],"codeBlockMetadata":null,"iframe":null,"mixtapeMetadata":null},"Paragraph:eddc97fef7c4_64":{"__typename":"Paragraph","id":"eddc97fef7c4_64","name":"edee","type":"OLI","href":null,"layout":null,"metadata":null,"text":"Happy path (aspect always returns the same Transfer).","hasDropCap":null,"dropCapImage":null,"markups":[],"codeBlockMetadata":null,"iframe":null,"mixtapeMetadata":null},"Paragraph:eddc97fef7c4_65":{"__typename":"Paragraph","id":"eddc97fef7c4_65","name":"6d71","type":"OLI","href":null,"layout":null,"metadata":null,"text":"Unhappy path (aspect always returns the same exception message and HTTP 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