Defensive Programming x Fintech

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Defensive Programming X FinTech

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Defensive Programming X FinTech Tato Kutalia, eMoney/MyCoins Android Dev fb.com/tatocaster github.com/tatocaster soundcloud.com/debugger-podcast

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Defend against the impossible, because the impossible will happen.

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Multiple ways to plug Example of extremely bad design

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Only one way to plug Example of good design

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No content

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No content

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Unexpected bug

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Add validation

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Add error handling

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Add logging

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= code that should have been there in the ﬁrst place

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Suggestions

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Safe zones - Protect code from invalid data coming from “outside”

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Safe zones - Protect code from invalid data coming from “outside” - Everything outside of the boundary is dangerous

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Safe zones - Protect code from invalid data coming from “outside” - Everything outside of the boundary is dangerous - Double check for limits and bounds

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Safe zones - Protect code from invalid data coming from “outside” - Everything outside of the boundary is dangerous - Double check for limits and bounds - Do whitelists, not blacklists

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Handling - Defensive Programming is NOT about swallowing errors or hiding bugs

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Handling - Defensive Programming is NOT about swallowing errors or hiding bugs - Return an error and stop (fail fast)

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Handling - Defensive Programming is NOT about swallowing errors or hiding bugs - Return an error and stop (fail fast) - Return a neutral value

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Handling - Defensive Programming is NOT about swallowing errors or hiding bugs - Return an error and stop (fail fast) - Return a neutral value - Don’t be afraid to fail!!!

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Code doesn’t work as advertised - Don’t assume that a function call outside of code will work as advertised.

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Code doesn’t work as advertised - Don’t assume that a function call outside of code will work as advertised. - Always wrap 3rd party libraries with your own gateways.

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Code doesn’t work as advertised - Don’t assume that a function call outside of code will work as advertised. - Always wrap 3rd party libraries with your own gateways. - ALWAYS!

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Assertions - Use assertions to document assumptions and to highlight “impossible” conditions (dev env)

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fun register(facebookAccessToken : String) { val response = facebookSdkClient.getUserByToken(facebookAccessToken) if (response.email.isNullOrEmpty()) throw IllegalArgumentException("Invalid response from Facebook") // Register the user }

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Logging - Add logging and tracing to help explain what’s going on at run-time, especially if you run into a problem.

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Logging - Add logging and tracing to help explain what’s going on at run-time, especially if you run into a problem. - Use 3rd party systems to analyze logs and maintain the same level

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Standardize error handling - “Normal errors” vs “Expected errors”

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Standardize error handling - “Normal errors” vs “Expected errors” - Not all errors are errors

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authorizeUserWithUsernameAndPassword(user, pass) .subscribe({ when { it.errorCode != 0 -> { // API error } else -> { getUserInfo(it.accessToken) } } }, { // Any other error. Throwable })

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authorizeUserWithUsernameAndPassword(user, pass) .subscribe({ result: ApiResponse -> when { result.errorCode != 0 -> { // API error } else -> { getUserInfo(result.accessToken) } } }, { it: Throwable // Any other error. Throwable })

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Misc - Never ever wait forever on an external call, especially a remote call.

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Misc - Never ever wait forever on an external call, especially a remote call. - Forever can be a long time when something goes wrong. Use time-out/retry logic

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Misc - Never ever wait forever on an external call, especially a remote call. - Forever can be a long time when something goes wrong. Use time-out/retry logic. - When just checking null for an immutable variable, don’t use LET.

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// NOT RECOMMENDED fun process(str: String?) { str?.let { /*Do something*/ } } // Decompiled JAVA public final void process(@Nullable String str) { if (str != null) { boolean var4 = false; /*Do something*/ } }

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// RECOMMENDED fun process(str: String?) { if (str != null) // Do Something } // Decompiled JAVA public final void process(@Nullable String str) { if (str != null) /*Do something*/ }

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“A public method is like a child: once you've written it, you are going to maintain it for the rest of its life!” Stefan Priebsch

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Don’t go too far

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registerUsingEmail(EmailAddress("[email protected]"))

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- Defensive code is code – all code has bugs Misc

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- Defensive code is code – all code has bugs - Understanding what conditions to check for and how much defensive coding is needed takes experience Misc

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Don’t reinvent the wheel

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(!frameworks.active) ???

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- Well you like doing extra work for no reason, congratulations! ⭐ Frameworks

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- Well you like doing extra work for no reason, congratulations! ⭐ - Well tested, trusted by thousands of developers and stable Frameworks

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- Well you like doing extra work for no reason, congratulations! ⭐ - Well tested, trusted by thousands of developers and stable - That’s what is used to call intelligent code reuse Frameworks

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fun doSomething(count: Int){ if (count < 1 || count > 100) throw IllegalArgumentException("Invalid count") /* Do something */ } fun doSomethingElse(count: Int) { if (count < 1 || count > 100) throw IllegalArgumentException("Invalid count") /* Do something else */ }

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fun doSomething(count: Count) { /* Do something */ } fun doSomethingElse(count: Count) { /* Do something else */ } class Count(var value: Int) { init { if (value < 1 || value > 100) throw IllegalArgumentException("Invalid count") } }

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“just to be safe” aka overly defensive code

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- Passing empty objects instead of crash Overly Defensive Code

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- Passing empty objects instead of crash - Traverse nested object/array to check the key Overly Defensive Code

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*,body,body * { font-family: DejaVu Serif, serif !important; }

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(this.state.stuff && this.state.stuff.things && this.state.stuff.things.meta) || {}

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// DON'T! @Throws(Exception::class) fun doNotSpecifyException() { doSomething() } // DO THIS! @Throws(NumberFormatException::class, IllegalArgumentException::class) fun doSomething() { // do something }

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Money in Fintech

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double?

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val result = 0.1 + 0.2 - 0.3 println("0.1 + 0.2 - 0.3=$result") => 0.1 + 0.2 - 0.3 = 5.551115123125783E-17

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Roundings?

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- ROUND_CEILING - ROUND_DOWN - ROUND_FLOOR - ROUND_HALF_DOWN - ROUND_HALF_EVEN - ROUND_HALF_UP - ROUND_UP Roundings

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val result = 0.1 + 0.2 - 0.3 val resultRounded = BigDecimal(result).setScale(2, BigDecimal.ROUND_HALF_UP) println("0.1 + 0.2 - 0.3=$resultRounded") => 0.1 + 0.2 - 0.3 = 0.00

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- BigDecimal(String) constructor should always be preferred over BigDecimal(Double) because using BigDecimal(double)is unpredictable due to the inability of the double to represent 0.1 as exact 0.1. - If double must be used for initializing a BigDecimal, use BigDecimal.valueOf(double), which converts the Double value to String using Double.toString(double) method - Rounding mode should be provided while setting the scale - StripTrailingZeros chops off all the trailing zeros - toString() may use scientiﬁc notation but, toPlainString() will never return exponentiation in its result - BigDecimal

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Questions?