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      "body": "Congratulations @goam! You received a personal award!\n\n<table><tr><td>https://steemitimages.com/70x70/http://steemitboard.com/@goam/birthday1.png</td><td>Happy Birthday! - You are on the Steem blockchain for 1 year!</td></tr></table>\n\n<sub>_You can view [your badges on your Steem Board](https://steemitboard.com/@goam) and compare to others on the [Steem Ranking](https://steemitboard.com/ranking/index.php?name=goam)_</sub>\n\n\n###### [Vote for @Steemitboard as a witness](https://v2.steemconnect.com/sign/account-witness-vote?witness=steemitboard&approve=1) to get one more award and increased upvotes!",
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      "body": "<center><a href='https://d.tube/#!/v/goam/v4d6uk5t'><img src='https://ipfs.io/ipfs/QmYGarBXc812xbJx6CNNwvcCY7g2gQsksy9qKesYXCQjKx'></a></center><hr>\n\nWhat are Ethereum Improvement Proposals (or EIPs)?\n\nEthereum is an open source project with no one company or organization that controls the direction of the project. There is an open governance model where everyone is free to propose and discuss changes to the system. The EIP system is the forum to do this. You can view the EIP website here: https://eips.ethereum.org/\n\nSome notable features of the EIP system are the submissions are open to everyone with a github account, which is very easy to get. The incorporation of a proposal into the platform is dictated by the quality of the idea rather than the reputation of the proposer. \n\nThere are several different stages that an EIP can be in. Draft EIPs are works in progress, are open for consideration and discussed on github. Accepted EIPs can be expected to be included in the next hard fork. Final EIPs are proposals that have already been adopted and deferred EIPs are not being considered for immediate adoption, but may be considered again in the future.\n\nThe structure of an EIP is important for clear and effective communication among so many project participants. An EIP includes the following sections:\n\n* Preamble (metadata)\n* Simple summary\n* Abstract - A short description of the issue\n* Motivation - Why is the existing protocol inadequate\n* Specification - Describes the syntax and semantics of the new feature\n* Rationale - Why did you make these design decisions?\n* Backwards compatibility - Explain any backward incompatibilities and how they will be addressed\n* Test Cases - These are mandatory for EIPs proposing consensus changes\n* Implementation - What does an implementation of the EIP look like? This section must be completed before the EIP is given the status “Final”.\n\nIn practice, this system creates a set of community standards for changing the project. \n\n# EIP-20, a Standard Token Interface: https://eips.ethereum.org/EIPS/eip-20\n\nLet’s look at one of the most popular EIPs, EIP-20 which specifies a standardized token interface. The implementation specification enables easy interoperability between Ethereum tokens and allows applications with complex functionality to reliably handle Ethereum tokens. \n\nThe development of tokens on Ethereum is under active development. The following EIPs are in the draft phase. Explore these EIPs to follow the development of token standards.\n\nhttps://eips.ethereum.org/EIPS/eip-721\nhttps://eips.ethereum.org/EIPS/eip-918\nhttps://eips.ethereum.org/EIPS/eip-777\nhttps://eips.ethereum.org/EIPS/eip-1046\nhttps://eips.ethereum.org/EIPS/eip-1080\nhttps://github.com/ethereum/EIPs\nhttps://medium.com/@codetractio/walkthrough-of-an-ethereum-improvement-proposal-eip-6fda3966d171\nhttps://github.com/ethereum/EIPs/blob/master/EIPS/eip-20.md\nhttps://theethereum.wiki/w/index.php/ERC20_Token_Standard\nhttps://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md\n\n<hr><a href='https://d.tube/#!/v/goam/v4d6uk5t'> ▶️ DTube</a><br /><a href='https://ipfs.io/ipfs/QmSJeLm5GGRy9ubZ5MKzAaS1BfnqdSYStcdVMEUKSYKe4J'> ▶️ IPFS</a>",
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      "body": "<center><a href='https://d.tube/#!/v/goam/539ewdz0'><img src='https://ipfs.io/ipfs/QmPjqvKWvreo1G4ReicrB3XNTSgSxv7wZFSDFzhhN1r78Z'></a></center><hr>\n\nLet’s start by diving right into an abstruse example of the low-level lisp-like language (LLL).\n\n    { \n    [[69]] (caller ) \n    (return 0 (lll \n        (when (= (caller) @@69) \n            (for {} (< @i (calldatasize)) [i](+ @i 64) \n                [[ (calldataload @i) ]] (calldataload (+ @i 32)) \n            ) \n        ) \n    0)) \n    } \n\nAs the name suggests, LLL is a low level language. It operates more closely to the EVM bytecode than higher level languages like Solidity or Vyper.\n\nLLL has direct access to storage and memory of the EVM so you can determine exactly where your data sits. When you use an EVM opcode in LLL, it translates directly to the bytecode representation of that opcode. In fact, all EVM opcodes are available to LLL.\n\nIt produces smaller binaries that Solidity. As an example, reimplementing uPort's registry contract in LLL. The Solidity implementation comes in at 591 bytes, compiled with the optimize flag. Admittedly, this is pretty small. But the LLL implementation compiles to 171 bytes. So in this case the LLL version is about 70% smaller than the Solidity version. That matters a lot when a contract will be accessed many times. It also makes deployment less costly. This is an extreme case, of course. Generally LLL binaries are about 30 to 40% smaller than the equivalent in Solidity.\n\nLLL is considered lisp like because of its syntax. There are many parenthesis compared to other smart contract languages. LLL uses symbolic expression. Symbolic expressions are a notation for nested list data.\n\nLisp developers generally use indentation to indicate code blocks.\n\nLLL also uses prefix notation, where operators are placed to the left of their operands. Infix notation is the more familiar notation (2 + 3), which looks like (+ 2 3) in prefix notation. The expression (+ @i 64) adds i and 64.\n\n“Symbolic expressions are a means of representing semistructured data in human readable text form mostly composed of symbols and lists and extensively used in the Lisp programming language.” -yourdictionary.com\n\n# An example of LLL\n\n;; check that the caller is the node owner \n\n(def ‘only-node-owner (node) \n   \t(when (!= caller) (get-owner-node)) \n    (panic)) \n\nLLL uses semicolons for comments.\n\ndef starts the definition of an LLL macro. Macros can be used to clarify your code. They aren't even close to the power of Lisp macros. They're more like the C macro system. They do a simple substitution.\n\nHere we're defining a macro called only-node-owner. This will be used as a modifier to ensure that only the node owner can execute a block of code. So when only-node-owner is used in the source, the macro will be looked up and its definition will be compiled into the bytecode.\n\nMacros can take parameters. In this case the node's ID.\n\nThe rest of the code is what will be substituted at compile time when the macro is invoked. Here you can see the use of symbolic expressions and prefix notation. Everything is composed of lists defined within matching parentheses. Prefix notation can be seen with the use of !=. That operator comes first, and its operands follow. One of the != operands uses another macro I defined that's not shown here, called get-owner. So you can see that it's easy to build up complex code but keep things clear with the use of macros.\n\nThere is actually an LLL contract deployed on the mainnet. The registry contract of the Ethereum Name service (ENS) was written in LLL mainly because of the gas savings due to its size and the brevity of its code. It's the contract in the ENS system that's called more than any other.\n\nCheck out the github (https://github.com/ethereum/ens) for details about the LLL contracts as well as the Solidity counterparts.\n\n<hr><a href='https://d.tube/#!/v/goam/539ewdz0'> ▶️ DTube</a><br /><a href='https://ipfs.io/ipfs/QmYbfaJomGaPNEUgUG9TPn9PvZ25bzx1gZnm5DQNVTMLfs'> ▶️ IPFS</a>",
      "json_metadata": "{\"video\":{\"info\":{\"title\":\"Ethereum #53: LLL\",\"snaphash\":\"QmTkbwq6hZwjBCrSFJriVgQXLCctkQX4wJSfSiX3Qrt4Zz\",\"author\":\"goam\",\"permlink\":\"539ewdz0\",\"duration\":84.752834,\"filesize\":8943271,\"spritehash\":\"QmUb9n9PGGRj2byjBt4jozBuE3V2rZXA28bUwVgac2j95W\"},\"content\":{\"videohash\":\"QmYbfaJomGaPNEUgUG9TPn9PvZ25bzx1gZnm5DQNVTMLfs\",\"description\":\"Let’s start by diving right into an abstruse example of the low-level lisp-like language (LLL).\\n\\n    { \\n    [[69]] (caller ) \\n    (return 0 (lll \\n        (when (= (caller) @@69) \\n            (for {} (< @i (calldatasize)) [i](+ @i 64) \\n                [[ (calldataload @i) ]] (calldataload (+ @i 32)) \\n            ) \\n        ) \\n    0)) \\n    } \\n\\nAs the name suggests, LLL is a low level language. It operates more closely to the EVM bytecode than higher level languages like Solidity or Vyper.\\n\\nLLL has direct access to storage and memory of the EVM so you can determine exactly where your data sits. When you use an EVM opcode in LLL, it translates directly to the bytecode representation of that opcode. In fact, all EVM opcodes are available to LLL.\\n\\nIt produces smaller binaries that Solidity. As an example, reimplementing uPort's registry contract in LLL. The Solidity implementation comes in at 591 bytes, compiled with the optimize flag. Admittedly, this is pretty small. But the LLL implementation compiles to 171 bytes. So in this case the LLL version is about 70% smaller than the Solidity version. That matters a lot when a contract will be accessed many times. It also makes deployment less costly. This is an extreme case, of course. Generally LLL binaries are about 30 to 40% smaller than the equivalent in Solidity.\\n\\nLLL is considered lisp like because of its syntax. There are many parenthesis compared to other smart contract languages. LLL uses symbolic expression. Symbolic expressions are a notation for nested list data.\\n\\nLisp developers generally use indentation to indicate code blocks.\\n\\nLLL also uses prefix notation, where operators are placed to the left of their operands. Infix notation is the more familiar notation (2 + 3), which looks like (+ 2 3) in prefix notation. The expression (+ @i 64) adds i and 64.\\n\\n“Symbolic expressions are a means of representing semistructured data in human readable text form mostly composed of symbols and lists and extensively used in the Lisp programming language.” -yourdictionary.com\\n\\n# An example of LLL\\n\\n;; check that the caller is the node owner \\n\\n(def ‘only-node-owner (node) \\n   \\t(when (!= caller) (get-owner-node)) \\n    (panic)) \\n\\nLLL uses semicolons for comments.\\n\\ndef starts the definition of an LLL macro. Macros can be used to clarify your code. They aren't even close to the power of Lisp macros. They're more like the C macro system. They do a simple substitution.\\n\\nHere we're defining a macro called only-node-owner. This will be used as a modifier to ensure that only the node owner can execute a block of code. So when only-node-owner is used in the source, the macro will be looked up and its definition will be compiled into the bytecode.\\n\\nMacros can take parameters. In this case the node's ID.\\n\\nThe rest of the code is what will be substituted at compile time when the macro is invoked. Here you can see the use of symbolic expressions and prefix notation. Everything is composed of lists defined within matching parentheses. Prefix notation can be seen with the use of !=. That operator comes first, and its operands follow. One of the != operands uses another macro I defined that's not shown here, called get-owner. So you can see that it's easy to build up complex code but keep things clear with the use of macros.\\n\\nThere is actually an LLL contract deployed on the mainnet. The registry contract of the Ethereum Name service (ENS) was written in LLL mainly because of the gas savings due to its size and the brevity of its code. It's the contract in the ENS system that's called more than any other.\\n\\nCheck out the github (https://github.com/ethereum/ens) for details about the LLL contracts as well as the Solidity counterparts.\",\"tags\":[\"ethereum\",\"blockchain\",\"developer\",\"education\"],\"video240hash\":\"QmVcmwo6guCHajyaJJdMopssie3gUr1pLzcvXD6sjJHW3W\",\"video480hash\":\"QmZutwFXqcXaULNwyUU2kaMQLCstS4p6PijuPfLNbH2xBd\"}},\"tags\":[\"ethereum\",\"blockchain\",\"developer\",\"education\",\"dtube\"],\"app\":\"dtube/0.7\"}",
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      "body": "<center><a href='https://d.tube/#!/v/goam/o416ruwy'><img src='https://ipfs.io/ipfs/QmeiE7bfzUFuxRFtVa7wSU5mUn1hHifwCencTghspJyyLc'></a></center><hr>\n\n# Formal Verification\n\nThere is a dire need for more secure methods of smart contract development within the blockchain community. \n\nThere have been hundreds of millions of dollars worth of crypto assets lost or stolen due to insecure smart contracts, and there are thousands of smart contracts on the blockchain with known vulnerabilities.\n\nPart of the problem is due to the current state of developer tools and programming languages such as Solidity. \n\nThe goal is to make writing secure smart contracts as easy and accessible as possible. One route to this goal is via formal verification. \n\nFrom Wikipedia, formal verification is the act of proving or disproving the correctness of intended algorithms underlying a system with respect to a certain formal specification or property, using formal methods of mathematics.\n \nSaid another way, it is a way to prove that a program is correct for all inputs. This can ensure that a hacker cannot modify the contract to an unintended state.\n\nUsing formal verification, smart contract proofs can be checked by machines. Verification requires specific programming languages and features that are not currently present in the Ethereum ecosystem. \n\nRead more about formal verification as it applies to computer science more generally on Wikipedia: https://en.wikipedia.org/wiki/Formal_verification\n\nhttps://ethereum.stackexchange.com/questions/11092/what-is-formal-verification-and-why-is-it-important-for-smart-contracts\nhttps://github.com/pirapira/ethereum-formal-verification-overview\nhttps://ethereum.stackexchange.com/questions/11092/what-is-formal-verification-and-why-is-it-important-for-smart-contracts\nhttps://github.com/pirapira/ethereum-formal-verification-overview\nhttps://securify.ch/\n\n<hr><a href='https://d.tube/#!/v/goam/o416ruwy'> ▶️ DTube</a><br /><a href='https://ipfs.io/ipfs/QmXDt8uMFJBd2Kby5gwbyDFnBuqB16NHHMJ7oNzxVURVg1'> ▶️ IPFS</a>",
      "json_metadata": "{\"video\":{\"info\":{\"title\":\"Ethereum #52: Formal Verification\",\"snaphash\":\"QmYGtxs44yhdSu8QXvETse7LUSkDcviU5pPJAJPfXCzYVq\",\"author\":\"goam\",\"permlink\":\"o416ruwy\",\"duration\":53.823855,\"filesize\":4577276,\"spritehash\":\"QmSmHTnhidj6TyW7dDUrGUjb2bTP7oj6R6kw3KQzaYpAwj\"},\"content\":{\"videohash\":\"QmXDt8uMFJBd2Kby5gwbyDFnBuqB16NHHMJ7oNzxVURVg1\",\"description\":\"# Formal Verification\\n\\nThere is a dire need for more secure methods of smart contract development within the blockchain community. \\n\\nThere have been hundreds of millions of dollars worth of crypto assets lost or stolen due to insecure smart contracts, and there are thousands of smart contracts on the blockchain with known vulnerabilities.\\n\\nPart of the problem is due to the current state of developer tools and programming languages such as Solidity. \\n\\nThe goal is to make writing secure smart contracts as easy and accessible as possible. One route to this goal is via formal verification. \\n\\nFrom Wikipedia, formal verification is the act of proving or disproving the correctness of intended algorithms underlying a system with respect to a certain formal specification or property, using formal methods of mathematics.\\n \\nSaid another way, it is a way to prove that a program is correct for all inputs. This can ensure that a hacker cannot modify the contract to an unintended state.\\n\\nUsing formal verification, smart contract proofs can be checked by machines. Verification requires specific programming languages and features that are not currently present in the Ethereum ecosystem. \\n\\nRead more about formal verification as it applies to computer science more generally on Wikipedia: https://en.wikipedia.org/wiki/Formal_verification\\n\\nhttps://ethereum.stackexchange.com/questions/11092/what-is-formal-verification-and-why-is-it-important-for-smart-contracts\\nhttps://github.com/pirapira/ethereum-formal-verification-overview\\nhttps://ethereum.stackexchange.com/questions/11092/what-is-formal-verification-and-why-is-it-important-for-smart-contracts\\nhttps://github.com/pirapira/ethereum-formal-verification-overview\\nhttps://securify.ch/\",\"tags\":[\"ethereum\",\"developer\",\"education\",\"blockchain\"],\"video240hash\":\"QmUaqXdzgUR8yaQqsxnr6F39SFpB5SJVr8dEpRp3gPNe2k\",\"video480hash\":\"QmYt8qvM8FBr3DNECEyHkasTtfAmKyZpZr4xcuWQKKCpAY\"}},\"tags\":[\"ethereum\",\"developer\",\"education\",\"blockchain\",\"dtube\"],\"app\":\"dtube/0.7\"}",
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      "author": "goam",
      "body": "<center><a href='https://d.tube/#!/v/goam/1nw1q9sw'><img src='https://ipfs.io/ipfs/QmXTjNmE2rbTGgHANkBvN23GeBJv9kJYMQpFJ4fEf8sZg7'></a></center><hr>\n\nThere are several design patterns and best practices that have been developed.\n\n# Fail early and fail loud\n\n   //Bad code, do not emulate\n   function silentFailIfZero(uint num) public view returns (uint){\n       if(num != 0){\n           return num;\n       }\n   }\n\nThis function will fail without throwing an exception. This is a bad practice because it is not immediately clear whether the function executed properly or not.\n\n   function throwsErrorIfZero(uint num) public view returns (uint){\n       require(num != 0);\n       return num;\n   }\n\nThis function checks the condition required for execution as early as possible in the function body and throws an exception if the condition is not met. This is a good practice to reduce unnecessary code execution in the event that an exception will be thrown.\n\n# Restricting Access\n\nYou cannot prevent people or computer programs from reading your contracts’ state. The state is publicly available information.\n\nYou can restrict other contracts’ access to the state by making state variables private.\n\ncontract C1 {\n uint private internalNum;\n}\n\nYou can restrict function access so that only specific addresses are permitted to execute functions.\n\nThis is useful for allowing only designated users, or other contracts to access administrative methods, such as changing ownership of a contract, implementing an upgrade or stopping the contract. \n\nhttps://i.imgur.com/1L2eAn0.png\n\nIt can be useful to restrict function access to owners, a more general admin class or to any stakeholder in the system.\n\n# Auto Deprecation\n\nhttps://i.imgur.com/JfXhR0R.png\n\nThe auto deprecation design pattern is a useful strategy for closing contracts that should expire after a certain amount of time. \n\nThis can be useful when running alpha or beta testing for your smart contracts.\n\nRemember that using timestamps such as the now keyword are subject to manipulation by the block miners in a 30-second window. \n\n# Mortal\n\nhttps://i.imgur.com/4tXBASn.png\n\nImplementing the mortal design pattern means including the ability to destroy the contract and remove it from the blockchain.\n\nYou can destroy a contract using the selfdestruct keyword. The function to do it is often called kill.\n\nIt takes one parameter which is the address that will receive all of the funds that the contract currently holds.\n\nAs an irreversible action, restricting access to this function is important.\n\n# Pull over Push Payments\n\nNavigate to this fund splitter contract on github.\n\nThere are a few key takeaways from this contract. There is a separation of function logic. The split() function handles the accounting and divides the msg.value sent with the transaction. Another function, withdraw(), allows accounts to transfer their balance from the contract to their account.\n\nThis pattern is also called the withdrawal pattern. It protects against re-entrancy and denial of service attacks that we will cover in the next lesson.\n\n# Circuit Breaker\n\nCircuit Breakers are design patterns that allow contract functionality to be stopped. This would be desirable in situations where there is a live contract where a bug has been detected. Freezing the contract would be beneficial for reducing harm before a fix can be implemented.\n\nCircuit breaker contracts can be set up to permit certain functions in certain situations. For example, if you are implementing a withdrawal pattern, you might want to stop people from depositing funds into the contract if a bug has been detected, while still allowing accounts with balances to withdraw their funds.\n\ncontract CircuitBreaker {\n    bool public stopped = false;\n    modifier stopInEmergency { require(!stopped); _; }\n    modifier onlyInEmergency { require(stopped); _; }\n    function deposit() stopInEmergency public { … }\n    function withdraw() onlyInEmergency public { … } \n}\n\nIn a situation such as this, you would also want to restrict access to the accounts that can modify the stopped state variable, maybe to the contract owner (such as multisig wallet) or a set of admins.\n\n# State Machine\n\nContracts often act as a state machine, which means that they have certain stages in which they behave differently or in which different functions can be called. A function call often ends a stage and transitions the contract into the next stage (especially if the contract models interaction). It is also common that some stages are automatically reached at a certain point in time.\n\nThe Colony token weighted voting protocol implemented this design pattern to manage the poll state.\n\nhttps://i.imgur.com/GCfGdZx.png\n\nAdmins can only add poll options in the poll creation stage. Votes can only be submitted when the poll was active. The poll can only be resolved after the poll close time has been reached.\n\n# Speed Bump\n\nSpeed bumps slow down actions so that if malicious actions occur, there is time to recover. \n\nhttps://i.imgur.com/tzY3GYg.png\n\nFor example, The DAO required 27 days between a successful request to split the DAO and the ability to do so. This ensured the funds were kept within the contract, increasing the likelihood of recovery.\n\nhttps://solidity.readthedocs.io/en/develop/common-patterns.html\nhttps://github.com/cjgdev/smart-contract-patterns\nhttps://gist.github.com/critesjosh/80d41928db2310684bc7660aa45873da\n\n<hr><a href='https://d.tube/#!/v/goam/1nw1q9sw'> ▶️ DTube</a><br /><a href='https://ipfs.io/ipfs/QmYq91ZRPXh8wbT8rTMMss3qEvSTe4fmiJUmd3KMLB3goQ'> ▶️ IPFS</a>",
      "json_metadata": "{\"video\":{\"info\":{\"title\":\"Ethereum #50: Design Patterns\",\"snaphash\":\"QmQ2bomN9MREqyaG6rGEKscRUYJccyBHMTBGgYAiL3mBmS\",\"author\":\"goam\",\"permlink\":\"1nw1q9sw\",\"duration\":101.215782,\"filesize\":9966972,\"spritehash\":\"QmULXzFNvujDfuSC6Cxrt5VRoXaCSiUuzKr6HTQe2zRB3G\"},\"content\":{\"videohash\":\"QmYq91ZRPXh8wbT8rTMMss3qEvSTe4fmiJUmd3KMLB3goQ\",\"description\":\"There are several design patterns and best practices that have been developed.\\n\\n# Fail early and fail loud\\n\\n   //Bad code, do not emulate\\n   function silentFailIfZero(uint num) public view returns (uint){\\n       if(num != 0){\\n           return num;\\n       }\\n   }\\n\\nThis function will fail without throwing an exception. This is a bad practice because it is not immediately clear whether the function executed properly or not.\\n\\n   function throwsErrorIfZero(uint num) public view returns (uint){\\n       require(num != 0);\\n       return num;\\n   }\\n\\nThis function checks the condition required for execution as early as possible in the function body and throws an exception if the condition is not met. This is a good practice to reduce unnecessary code execution in the event that an exception will be thrown.\\n\\n# Restricting Access\\n\\nYou cannot prevent people or computer programs from reading your contracts’ state. The state is publicly available information.\\n\\nYou can restrict other contracts’ access to the state by making state variables private.\\n\\ncontract C1 {\\n uint private internalNum;\\n}\\n\\nYou can restrict function access so that only specific addresses are permitted to execute functions.\\n\\nThis is useful for allowing only designated users, or other contracts to access administrative methods, such as changing ownership of a contract, implementing an upgrade or stopping the contract. \\n\\nhttps://i.imgur.com/1L2eAn0.png\\n\\nIt can be useful to restrict function access to owners, a more general admin class or to any stakeholder in the system.\\n\\n# Auto Deprecation\\n\\nhttps://i.imgur.com/JfXhR0R.png\\n\\nThe auto deprecation design pattern is a useful strategy for closing contracts that should expire after a certain amount of time. \\n\\nThis can be useful when running alpha or beta testing for your smart contracts.\\n\\nRemember that using timestamps such as the now keyword are subject to manipulation by the block miners in a 30-second window. \\n\\n# Mortal\\n\\nhttps://i.imgur.com/4tXBASn.png\\n\\nImplementing the mortal design pattern means including the ability to destroy the contract and remove it from the blockchain.\\n\\nYou can destroy a contract using the selfdestruct keyword. The function to do it is often called kill.\\n\\nIt takes one parameter which is the address that will receive all of the funds that the contract currently holds.\\n\\nAs an irreversible action, restricting access to this function is important.\\n\\n# Pull over Push Payments\\n\\nNavigate to this fund splitter contract on github.\\n\\nThere are a few key takeaways from this contract. There is a separation of function logic. The split() function handles the accounting and divides the msg.value sent with the transaction. Another function, withdraw(), allows accounts to transfer their balance from the contract to their account.\\n\\nThis pattern is also called the withdrawal pattern. It protects against re-entrancy and denial of service attacks that we will cover in the next lesson.\\n\\n# Circuit Breaker\\n\\nCircuit Breakers are design patterns that allow contract functionality to be stopped. This would be desirable in situations where there is a live contract where a bug has been detected. Freezing the contract would be beneficial for reducing harm before a fix can be implemented.\\n\\nCircuit breaker contracts can be set up to permit certain functions in certain situations. For example, if you are implementing a withdrawal pattern, you might want to stop people from depositing funds into the contract if a bug has been detected, while still allowing accounts with balances to withdraw their funds.\\n\\ncontract CircuitBreaker {\\n    bool public stopped = false;\\n    modifier stopInEmergency { require(!stopped); _; }\\n    modifier onlyInEmergency { require(stopped); _; }\\n    function deposit() stopInEmergency public { … }\\n    function withdraw() onlyInEmergency public { … } \\n}\\n\\nIn a situation such as this, you would also want to restrict access to the accounts that can modify the stopped state variable, maybe to the contract owner (such as multisig wallet) or a set of admins.\\n\\n# State Machine\\n\\nContracts often act as a state machine, which means that they have certain stages in which they behave differently or in which different functions can be called. 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This ensured the funds were kept within the contract, increasing the likelihood of recovery.\\n\\nhttps://solidity.readthedocs.io/en/develop/common-patterns.html\\nhttps://github.com/cjgdev/smart-contract-patterns\\nhttps://gist.github.com/critesjosh/80d41928db2310684bc7660aa45873da\",\"tags\":[\"ethereum\",\"developer\",\"blockchain\",\"education\"],\"video240hash\":\"QmdUY2rQr3bC9ZBjRVgPSHYdttPH27XAu6r2hiBtdZKQBE\",\"video480hash\":\"QmVSKzS9vbytcXCPvASJoVp2hT9yH8unKKWxPHPiQ43Dpz\"}},\"tags\":[\"ethereum\",\"developer\",\"blockchain\",\"education\",\"dtube\"],\"app\":\"dtube/0.7\"}",
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  "op_in_trx": 0,
  "timestamp": "2018-06-29T04:29:42",
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{
  "block": 23736821,
  "op": [
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    {
      "author": "goam",
      "body": "<center><a href='https://d.tube/#!/v/goam/vxss8el8'><img src='https://ipfs.io/ipfs/QmXh5g4bJJ8aRQDTmawSwVeU3toXYPLmQQU6V7Gn3PmB4r'></a></center><hr>\n\nThe first set of dangers that we are going to look at fall in the class of race conditions.\n\nReentrancy attacks can be problematic because calling external contracts passes control flow to them. The called contract may end up calling the smart contract function again in a recursive manner.\n\n// INSECURE\nmapping (address => uint) private userBalances;\n\nfunction withdrawBalance() public {\n    uint amountToWithdraw = userBalances[msg.sender];\n    require(msg.sender.call.value(amountToWithdraw)()); // At this point, the caller's code is executed, and can call withdrawBalance again\n    userBalances[msg.sender] = 0;\n}\n\nThis is the type of attack that destroyed the DAO in mid 2016. \n\nIf you can’t remove the external call, the next simplest way to prevent this attack is to do the internal work before making the external function call. \n\nmapping (address => uint) private userBalances;\n\nfunction withdrawBalance() public {\n    uint amountToWithdraw = userBalances[msg.sender];\n    userBalances[msg.sender] = 0;\n    require(msg.sender.call.value(amountToWithdraw)()); // The user's balance is already 0, so future invocations won't withdraw anything\n}\n\nOr to use the withdrawal design pattern and separate the contract accounting logic and the transfer logic.\n\nAnother thing to be aware of is potential cross function race conditions. This can be problematic if your contract has multiple functions that modify the same state.\n\n// INSECURE\nmapping (address => uint) private userBalances;\n\nfunction transfer(address to, uint amount) {\n    if (userBalances[msg.sender] >= amount) {\n       userBalances[to] += amount;\n       userBalances[msg.sender] -= amount;\n    }\n}\n\nfunction withdrawBalance() public {\n    uint amountToWithdraw = userBalances[msg.sender];\n    require(msg.sender.call.value(amountToWithdraw)()); // At this point, the caller's code is executed, and can call transfer()\n    userBalances[msg.sender] = 0;\n}\n\nAsk yourself, if multiple contract functions are called, what happens?\n\nThere are several ways to mitigate these problems.\n\nIt is generally a good idea to handle your internal contract state changes before calling external contracts, such as in the withdrawal design pattern. Use battle tested design patterns and learn from other people’s mistakes and heed their advice.\n\nA more complex solution could implement mutual exclusion, or a mutex. This allows you to lock a state and only allow changes by the owner of the lock.\n\nYou can dig deeper into known attacks such as these here: https://consensys.github.io/smart-contract-best-practices/known_attacks/#race-conditions42\n\n# Transaction Ordering and Timestamp Dependence\n\nThe previous examples of race conditions involved an attacker executing malicious code in a single transaction. Here we focus on how transactions are included in the blockchain and considerations around the process.\n\nTransactions that are broadcast to the network but have not yet been included in a block are in the mempool.\n\nMiners choose the order in which to include transactions from the mempool into a block that they are mining.\n\nAlso, since transactions are in the mempool before they make it into a block, anyone can know what transactions are about to occur on the network. \n https://i.imgur.com/I2WfEwL.png\n\nThis can be problematic for things like decentralized markets.\n\nProtecting against this is difficult and you will likely need to devise contract specific solutions.\n\nDecentralized markets can mitigate concerns by implementing batch auctions or using a pre-commit scheme, where the details are submitted after the transaction is committed. \n\n# Integer Overflow and Underflow\n\nIntegers can underflow or overflow in the EVM.\n\nThe max value for an unsigned integer is 2 ^ 256 - 1, which is roughly 1.15 times 10 ^ 77. If an integer overflows, the value will go back to 0. For example, a variable called score of type uint8 storing a value of 255 that is incremented by 1 will now be storing the value 0.\n\nYou may or may not have to worry about integer overflow depending on your smart contract.\n\nA variable that can be set by user input may need to check against overflow, whereas it is infeasible that a variable that is incremented will ever approach this max value.\n\nUnderflow is a similar situation, but when a uint goes below its minimum value it will be set to its maximum value.\n\nBe careful with smaller data types like uint8, uint16, etc… they can more easily reach their maximum value\n\n# Denial of Service\n\nhttps://i.imgur.com/OPAYpA3.png\n\nAnother danger of passing execution to another contract is a denial of service attack.\n\nIn the provided example, the highestBidder could be another contract and transferring funds to the contract triggers the contract’s fallback function. If the contract’s fallback always reverts, the Auction contract’s bid function becomes unusable - it will always revert. The bid function requires the transfer operation to succeed to fully execute.\n\nThe contract at the provided address throws an exception, execution halts and the exception is passed into the calling contract and prevents further execution.\n\nThis problem is avoidable using the withdrawal pattern.\n\n# Force Sending Ether\n\nAnother danger is using logic that depends on the contract balance. \n\nBe aware that it is possible to send ether to a contract without triggering its fallback function. \n\nUsing the selfdestruct function on another contract and using the target contract as the recipient will force the destroyed contract’s funds to be sent to the target.\n\nIt is also possible to precompute a contracts address and send ether to the address before the contract is deployed.\n\nThe contract’s balance will be greater than 0 when it is finally deployed.\n\nhttps://eprint.iacr.org/2016/1007.pdf\nhttps://solidity.readthedocs.io/en/develop/bugs.html\nhttps://medium.freecodecamp.org/a-hacker-stole-31m-of-ether-how-it-happened-and-what-it-means-for-ethereum-9e5dc29e33ce\n\n<hr><a href='https://d.tube/#!/v/goam/vxss8el8'> ▶️ DTube</a><br /><a href='https://ipfs.io/ipfs/QmRBajdRTbkDHSVfEupjJNuwhWr2rGRUDAuNbxs283X3p6'> ▶️ IPFS</a>",
      "json_metadata": "{\"video\":{\"info\":{\"title\":\"Ethereum #51: Exploits and Dangers\",\"snaphash\":\"QmbD2DD7QC8UeXAM7x6JJipxid6zyiv11MbxVggFMVGsms\",\"author\":\"goam\",\"permlink\":\"vxss8el8\",\"duration\":105.627574,\"filesize\":10840919,\"spritehash\":\"Qmb9jWn4LB8uvu3U32q9BFBe4XJj56ieu1t2s9PbqMRrL7\"},\"content\":{\"videohash\":\"QmRBajdRTbkDHSVfEupjJNuwhWr2rGRUDAuNbxs283X3p6\",\"description\":\"The first set of dangers that we are going to look at fall in the class of race conditions.\\n\\nReentrancy attacks can be problematic because calling external contracts passes control flow to them. The called contract may end up calling the smart contract function again in a recursive manner.\\n\\n// INSECURE\\nmapping (address => uint) private userBalances;\\n\\nfunction withdrawBalance() public {\\n    uint amountToWithdraw = userBalances[msg.sender];\\n    require(msg.sender.call.value(amountToWithdraw)()); // At this point, the caller's code is executed, and can call withdrawBalance again\\n    userBalances[msg.sender] = 0;\\n}\\n\\nThis is the type of attack that destroyed the DAO in mid 2016. \\n\\nIf you can’t remove the external call, the next simplest way to prevent this attack is to do the internal work before making the external function call. \\n\\nmapping (address => uint) private userBalances;\\n\\nfunction withdrawBalance() public {\\n    uint amountToWithdraw = userBalances[msg.sender];\\n    userBalances[msg.sender] = 0;\\n    require(msg.sender.call.value(amountToWithdraw)()); // The user's balance is already 0, so future invocations won't withdraw anything\\n}\\n\\nOr to use the withdrawal design pattern and separate the contract accounting logic and the transfer logic.\\n\\nAnother thing to be aware of is potential cross function race conditions. This can be problematic if your contract has multiple functions that modify the same state.\\n\\n// INSECURE\\nmapping (address => uint) private userBalances;\\n\\nfunction transfer(address to, uint amount) {\\n    if (userBalances[msg.sender] >= amount) {\\n       userBalances[to] += amount;\\n       userBalances[msg.sender] -= amount;\\n    }\\n}\\n\\nfunction withdrawBalance() public {\\n    uint amountToWithdraw = userBalances[msg.sender];\\n    require(msg.sender.call.value(amountToWithdraw)()); // At this point, the caller's code is executed, and can call transfer()\\n    userBalances[msg.sender] = 0;\\n}\\n\\nAsk yourself, if multiple contract functions are called, what happens?\\n\\nThere are several ways to mitigate these problems.\\n\\nIt is generally a good idea to handle your internal contract state changes before calling external contracts, such as in the withdrawal design pattern. Use battle tested design patterns and learn from other people’s mistakes and heed their advice.\\n\\nA more complex solution could implement mutual exclusion, or a mutex. This allows you to lock a state and only allow changes by the owner of the lock.\\n\\nYou can dig deeper into known attacks such as these here: https://consensys.github.io/smart-contract-best-practices/known_attacks/#race-conditions42\\n\\n# Transaction Ordering and Timestamp Dependence\\n\\nThe previous examples of race conditions involved an attacker executing malicious code in a single transaction. Here we focus on how transactions are included in the blockchain and considerations around the process.\\n\\nTransactions that are broadcast to the network but have not yet been included in a block are in the mempool.\\n\\nMiners choose the order in which to include transactions from the mempool into a block that they are mining.\\n\\nAlso, since transactions are in the mempool before they make it into a block, anyone can know what transactions are about to occur on the network. \\n https://i.imgur.com/I2WfEwL.png\\n\\nThis can be problematic for things like decentralized markets.\\n\\nProtecting against this is difficult and you will likely need to devise contract specific solutions.\\n\\nDecentralized markets can mitigate concerns by implementing batch auctions or using a pre-commit scheme, where the details are submitted after the transaction is committed. \\n\\n# Integer Overflow and Underflow\\n\\nIntegers can underflow or overflow in the EVM.\\n\\nThe max value for an unsigned integer is 2 ^ 256 - 1, which is roughly 1.15 times 10 ^ 77. If an integer overflows, the value will go back to 0. For example, a variable called score of type uint8 storing a value of 255 that is incremented by 1 will now be storing the value 0.\\n\\nYou may or may not have to worry about integer overflow depending on your smart contract.\\n\\nA variable that can be set by user input may need to check against overflow, whereas it is infeasible that a variable that is incremented will ever approach this max value.\\n\\nUnderflow is a similar situation, but when a uint goes below its minimum value it will be set to its maximum value.\\n\\nBe careful with smaller data types like uint8, uint16, etc… they can more easily reach their maximum value\\n\\n# Denial of Service\\n\\nhttps://i.imgur.com/OPAYpA3.png\\n\\nAnother danger of passing execution to another contract is a denial of service attack.\\n\\nIn the provided example, the highestBidder could be another contract and transferring funds to the contract triggers the contract’s fallback function. If the contract’s fallback always reverts, the Auction contract’s bid function becomes unusable - it will always revert. The bid function requires the transfer operation to succeed to fully execute.\\n\\nThe contract at the provided address throws an exception, execution halts and the exception is passed into the calling contract and prevents further execution.\\n\\nThis problem is avoidable using the withdrawal pattern.\\n\\n# Force Sending Ether\\n\\nAnother danger is using logic that depends on the contract balance. \\n\\nBe aware that it is possible to send ether to a contract without triggering its fallback function. \\n\\nUsing the selfdestruct function on another contract and using the target contract as the recipient will force the destroyed contract’s funds to be sent to the target.\\n\\nIt is also possible to precompute a contracts address and send ether to the address before the contract is deployed.\\n\\nThe contract’s balance will be greater than 0 when it is finally deployed.\\n\\nhttps://eprint.iacr.org/2016/1007.pdf\\nhttps://solidity.readthedocs.io/en/develop/bugs.html\\nhttps://medium.freecodecamp.org/a-hacker-stole-31m-of-ether-how-it-happened-and-what-it-means-for-ethereum-9e5dc29e33ce\",\"tags\":[\"ethereum\",\"developer\",\"blockchain\",\"education\"],\"video240hash\":\"QmU6dW4dSEFQvfk47Ynqbd67UJSnn7pzuBgZsmFuAqjNcF\",\"video480hash\":\"QmQ8wY5z7Htr9L6k9xb4YeZTzsjDWpFmLBpYC1fARNL2A1\"}},\"tags\":[\"ethereum\",\"developer\",\"blockchain\",\"education\",\"dtube\"],\"app\":\"dtube/0.7\"}",
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      "title": "Ethereum #51: Exploits and Dangers"
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  "timestamp": "2018-06-29T04:29:42",
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{
  "block": 23736793,
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      "author": "goam",
      "body": "<center><a href='https://d.tube/#!/v/goam/1nw1q9sw'><img src='https://ipfs.io/ipfs/QmXTjNmE2rbTGgHANkBvN23GeBJv9kJYMQpFJ4fEf8sZg7'></a></center><hr>\n\nThere are several design patterns and best practices that have been developed.\n\n# Fail early and fail loud\n\n   //Bad code, do not emulate\n   function silentFailIfZero(uint num) public view returns (uint){\n       if(num != 0){\n           return num;\n       }\n   }\n\nThis function will fail without throwing an exception. This is a bad practice because it is not immediately clear whether the function executed properly or not.\n\n   function throwsErrorIfZero(uint num) public view returns (uint){\n       require(num != 0);\n       return num;\n   }\n\nThis function checks the condition required for execution as early as possible in the function body and throws an exception if the condition is not met. This is a good practice to reduce unnecessary code execution in the event that an exception will be thrown.\n\n# Restricting Access\n\nYou cannot prevent people or computer programs from reading your contracts’ state. The state is publicly available information.\n\nYou can restrict other contracts’ access to the state by making state variables private.\n\ncontract C1 {\n uint private internalNum;\n}\n\nYou can restrict function access so that only specific addresses are permitted to execute functions.\n\nThis is useful for allowing only designated users, or other contracts to access administrative methods, such as changing ownership of a contract, implementing an upgrade or stopping the contract. \n\nhttps://i.imgur.com/1L2eAn0.png\n\nIt can be useful to restrict function access to owners, a more general admin class or to any stakeholder in the system.\n\n# Auto Deprecation\n\nhttps://i.imgur.com/JfXhR0R.png\n\nThe auto deprecation design pattern is a useful strategy for closing contracts that should expire after a certain amount of time. \n\nThis can be useful when running alpha or beta testing for your smart contracts.\n\nRemember that using timestamps such as the now keyword are subject to manipulation by the block miners in a 30-second window. \n\n# Mortal\n\nhttps://i.imgur.com/4tXBASn.png\n\nImplementing the mortal design pattern means including the ability to destroy the contract and remove it from the blockchain.\n\nYou can destroy a contract using the selfdestruct keyword. The function to do it is often called kill.\n\nIt takes one parameter which is the address that will receive all of the funds that the contract currently holds.\n\nAs an irreversible action, restricting access to this function is important.\n\n# Pull over Push Payments\n\nNavigate to this fund splitter contract on github.\n\nThere are a few key takeaways from this contract. There is a separation of function logic. The split() function handles the accounting and divides the msg.value sent with the transaction. Another function, withdraw(), allows accounts to transfer their balance from the contract to their account.\n\nThis pattern is also called the withdrawal pattern. It protects against re-entrancy and denial of service attacks that we will cover in the next lesson.\n\n# Circuit Breaker\n\nCircuit Breakers are design patterns that allow contract functionality to be stopped. This would be desirable in situations where there is a live contract where a bug has been detected. Freezing the contract would be beneficial for reducing harm before a fix can be implemented.\n\nCircuit breaker contracts can be set up to permit certain functions in certain situations. For example, if you are implementing a withdrawal pattern, you might want to stop people from depositing funds into the contract if a bug has been detected, while still allowing accounts with balances to withdraw their funds.\n\ncontract CircuitBreaker {\n    bool public stopped = false;\n    modifier stopInEmergency { require(!stopped); _; }\n    modifier onlyInEmergency { require(stopped); _; }\n    function deposit() stopInEmergency public { … }\n    function withdraw() onlyInEmergency public { … } \n}\n\nIn a situation such as this, you would also want to restrict access to the accounts that can modify the stopped state variable, maybe to the contract owner (such as multisig wallet) or a set of admins.\n\n# State Machine\n\nContracts often act as a state machine, which means that they have certain stages in which they behave differently or in which different functions can be called. A function call often ends a stage and transitions the contract into the next stage (especially if the contract models interaction). It is also common that some stages are automatically reached at a certain point in time.\n\nThe Colony token weighted voting protocol implemented this design pattern to manage the poll state.\n\nhttps://i.imgur.com/GCfGdZx.png\n\nAdmins can only add poll options in the poll creation stage. Votes can only be submitted when the poll was active. The poll can only be resolved after the poll close time has been reached.\n\n# Speed Bump\n\nSpeed bumps slow down actions so that if malicious actions occur, there is time to recover. \n\nhttps://i.imgur.com/tzY3GYg.png\n\nFor example, The DAO required 27 days between a successful request to split the DAO and the ability to do so. This ensured the funds were kept within the contract, increasing the likelihood of recovery.\n\nhttps://solidity.readthedocs.io/en/develop/common-patterns.html\nhttps://github.com/cjgdev/smart-contract-patterns\nhttps://gist.github.com/critesjosh/80d41928db2310684bc7660aa45873da\n\n<hr><a href='https://d.tube/#!/v/goam/1nw1q9sw'> ▶️ DTube</a><br /><a href='https://ipfs.io/ipfs/QmYq91ZRPXh8wbT8rTMMss3qEvSTe4fmiJUmd3KMLB3goQ'> ▶️ IPFS</a>",
      "json_metadata": "{\"video\":{\"info\":{\"title\":\"Ethereum #50: Design Patterns\",\"snaphash\":\"QmQ2bomN9MREqyaG6rGEKscRUYJccyBHMTBGgYAiL3mBmS\",\"author\":\"goam\",\"permlink\":\"1nw1q9sw\",\"duration\":101.215782,\"filesize\":9966972,\"spritehash\":\"QmULXzFNvujDfuSC6Cxrt5VRoXaCSiUuzKr6HTQe2zRB3G\"},\"content\":{\"videohash\":\"QmYq91ZRPXh8wbT8rTMMss3qEvSTe4fmiJUmd3KMLB3goQ\",\"description\":\"There are several design patterns and best practices that have been developed.\\n\\n<strong># Fail early and fail loud</strong>\\n\\n   //Bad code, do not emulate\\n   function silentFailIfZero(uint num) public view returns (uint){\\n       if(num != 0){\\n           return num;\\n       }\\n   }\\n\\nThis function will fail without throwing an exception. This is a bad practice because it is not immediately clear whether the function executed properly or not.\\n\\n   function throwsErrorIfZero(uint num) public view returns (uint){\\n       require(num != 0);\\n       return num;\\n   }\\n\\nThis function checks the condition required for execution as early as possible in the function body and throws an exception if the condition is not met. This is a good practice to reduce unnecessary code execution in the event that an exception will be thrown.\\n\\n# Restricting Access\\n\\nYou cannot prevent people or computer programs from reading your contracts’ state. The state is publicly available information.\\n\\nYou can restrict other contracts’ access to the state by making state variables private.\\n\\ncontract C1 {\\n uint private internalNum;\\n}\\n\\nYou can restrict function access so that only specific addresses are permitted to execute functions.\\n\\nThis is useful for allowing only designated users, or other contracts to access administrative methods, such as changing ownership of a contract, implementing an upgrade or stopping the contract. \\n\\nhttps://i.imgur.com/1L2eAn0.png\\n\\nIt can be useful to restrict function access to owners, a more general admin class or to any stakeholder in the system.\\n\\n# Auto Deprecation\\n\\nhttps://i.imgur.com/JfXhR0R.png\\n\\nThe auto deprecation design pattern is a useful strategy for closing contracts that should expire after a certain amount of time. \\n\\nThis can be useful when running alpha or beta testing for your smart contracts.\\n\\nRemember that using timestamps such as the now keyword are subject to manipulation by the block miners in a 30-second window. \\n\\n# Mortal\\n\\nhttps://i.imgur.com/4tXBASn.png\\n\\nImplementing the mortal design pattern means including the ability to destroy the contract and remove it from the blockchain.\\n\\nYou can destroy a contract using the selfdestruct keyword. The function to do it is often called kill.\\n\\nIt takes one parameter which is the address that will receive all of the funds that the contract currently holds.\\n\\nAs an irreversible action, restricting access to this function is important.\\n\\n# Pull over Push Payments\\n\\nNavigate to this fund splitter contract on github.\\n\\nThere are a few key takeaways from this contract. There is a separation of function logic. The split() function handles the accounting and divides the msg.value sent with the transaction. Another function, withdraw(), allows accounts to transfer their balance from the contract to their account.\\n\\nThis pattern is also called the withdrawal pattern. It protects against re-entrancy and denial of service attacks that we will cover in the next lesson.\\n\\n# Circuit Breaker\\n\\nCircuit Breakers are design patterns that allow contract functionality to be stopped. This would be desirable in situations where there is a live contract where a bug has been detected. Freezing the contract would be beneficial for reducing harm before a fix can be implemented.\\n\\nCircuit breaker contracts can be set up to permit certain functions in certain situations. For example, if you are implementing a withdrawal pattern, you might want to stop people from depositing funds into the contract if a bug has been detected, while still allowing accounts with balances to withdraw their funds.\\n\\ncontract CircuitBreaker {\\n    bool public stopped = false;\\n    modifier stopInEmergency { require(!stopped); _; }\\n    modifier onlyInEmergency { require(stopped); _; }\\n    function deposit() stopInEmergency public { … }\\n    function withdraw() onlyInEmergency public { … } \\n}\\n\\nIn a situation such as this, you would also want to restrict access to the accounts that can modify the stopped state variable, maybe to the contract owner (such as multisig wallet) or a set of admins.\\n\\n# State Machine\\n\\nContracts often act as a state machine, which means that they have certain stages in which they behave differently or in which different functions can be called. A function call often ends a stage and transitions the contract into the next stage (especially if the contract models interaction). It is also common that some stages are automatically reached at a certain point in time.\\n\\nThe Colony token weighted voting protocol implemented this design pattern to manage the poll state.\\n\\nhttps://i.imgur.com/GCfGdZx.png\\n\\nAdmins can only add poll options in the poll creation stage. Votes can only be submitted when the poll was active. The poll can only be resolved after the poll close time has been reached.\\n\\n# Speed Bump\\n\\nSpeed bumps slow down actions so that if malicious actions occur, there is time to recover. \\n\\nhttps://i.imgur.com/tzY3GYg.png\\n\\nFor example, The DAO required 27 days between a successful request to split the DAO and the ability to do so. This ensured the funds were kept within the contract, increasing the likelihood of recovery.\\n\\nhttps://solidity.readthedocs.io/en/develop/common-patterns.html\\nhttps://github.com/cjgdev/smart-contract-patterns\\nhttps://gist.github.com/critesjosh/80d41928db2310684bc7660aa45873da\",\"tags\":[\"ethereum\",\"developer\",\"blockchain\",\"education\"],\"video240hash\":\"QmdUY2rQr3bC9ZBjRVgPSHYdttPH27XAu6r2hiBtdZKQBE\",\"video480hash\":\"QmVSKzS9vbytcXCPvASJoVp2hT9yH8unKKWxPHPiQ43Dpz\"}},\"tags\":[\"ethereum\",\"developer\",\"blockchain\",\"education\",\"dtube\"],\"app\":\"dtube/0.7\"}",
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      "title": "Ethereum #50: Design Patterns"
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{
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      "author": "goam",
      "body": "<center><a href='https://d.tube/#!/v/goam/1nw1q9sw'><img src='https://ipfs.io/ipfs/QmXTjNmE2rbTGgHANkBvN23GeBJv9kJYMQpFJ4fEf8sZg7'></a></center><hr>\n\nThere are several design patterns and best practices that have been developed.\n\n# Fail early and fail loud\n\n   //Bad code, do not emulate\n   function silentFailIfZero(uint num) public view returns (uint){\n       if(num != 0){\n           return num;\n       }\n   }\n\nThis function will fail without throwing an exception. This is a bad practice because it is not immediately clear whether the function executed properly or not.\n\n   function throwsErrorIfZero(uint num) public view returns (uint){\n       require(num != 0);\n       return num;\n   }\n\nThis function checks the condition required for execution as early as possible in the function body and throws an exception if the condition is not met. This is a good practice to reduce unnecessary code execution in the event that an exception will be thrown.\n\n# Restricting Access\n\nYou cannot prevent people or computer programs from reading your contracts’ state. 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Freezing the contract would be beneficial for reducing harm before a fix can be implemented.\n\nCircuit breaker contracts can be set up to permit certain functions in certain situations. 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