DApp Design Patterns

Transcript

[Music] patterns by the gang of war is uh is is very famous and also outside of the field of developing you have patterns so if you want to if you want to knit a Celtic pillow I wouldn't know where to start so first I Google and then I get this kind of stuff I I'm not sure what it is it looks like brain F codes but probably it tells me how to uh how to knit a pillow so what what is a pattern a pattern is a is a reusable solution for common occurring problems um that you

can that you can apply it's not some boilerplate piece of code that you just copy paste and you're done with it you apply it to your own codex but you you try to reuse it you try to to see when does it make sense when does it make sense and in the the the the pattern description the pattern language we mention when is the good problem to apply this pattern when you shouldn't do it and if you're applying it what are the considerations and of course some some code examples um yeah Microsoft guys with their their Azure Cloud they also released all kinds of patterns so in all kinds of software engineering Fields you have this pattern system so probably the next version of this slides we'll have the blockchain pattern in here at as well so when when looking at the

patterns we discovered three areas we have data patterns transaction and value patterns and we have action and control patterns probably there are much more different categories but these are the three distinctions that we came up with um and I'm going to mention six examples from this uh this set actually this morning we also we we've seen the fan in and fan out pattern from the Block apps demo we have seen the the escrow example earlier so already during this day you have me you have witnessed a couple patterns without even realizing it so for each of these patterns we'll mention well of course the name of it a definition of the problem so when when can you apply this pattern and then if you're going to apply it what kind of

context what kind of considerations what kind of constraints are there to uh to be assure of followed by an actual solution of uh of this pattern and a couple examples out of the all the code that is publicly available on getp already and you see these patterns are quite often related to each other so if you use one pattern you might as well consider another pattern or even use them in combination of each other so first of all the the data patterns two two examples the first one is the the name registry it's um so you you need this if you're looking for for a human readable name for a ethereum address

um and you might need this human real name because you want to display it in your user interface maybe you have a wallet application and you want to show the person that you're transferring something to or you have a game and you want to show the nickname of the person that was uh just just winning your your contest or because your dep consists of a a cluster of contracts that are related with each other you want to make a reference from one contract to the other but this relationship isn't known uh up front so you want to refer to the other contract by name instead of hardcoding the address in your your contract um and of course that has the advantage that if you update the name registry you can also update the code of your of your code

construct um and you can e even apply hierarchic patterns here where you get some kind of subdomain registry so you can register uh you can appoint a registry for a for a domain and then for every I don't know for every household within within in an area there would be another reference to another name registry for the individual people living there of course there's some issues with this uh you have the traditional kind of problems that you're also seeing in the the DNS uh name registry so you have this name squatting potential um what happens if you lose your key is this name going to be lost and we're going to expire it maybe you can out bid somebody else so there's all these complex things

to to think of as well so a solution here is to create uh a contract a registry and this could either be a global one that you use in all your ethereum applications or it could be one specific for your context or for your application and it has a a reference from a from a name to an address it's just a big hashmap that just looks in the contract storage for the given name what is the address that is associated with it and then it Returns the address and you can extend this by giving a reverse lookup so you can query it for address address and then you can see what is the name that belongs to this uh to this address and a good example here is the is the name the name rare contract that

is uh actually implemented by by gin I think it's in the the de bin GitHub project in the ethereum um repository uh and here you have the the name register implementation so you can take a look at this and see how how G uh how G does it so a second data pattern is this data feed smart contracts they cannot reach out to the real world you cannot make a HTTP call and and see what a website returns as a value you cannot directly connect the camera to it so can make a picture so you need you need to actually feed the data to your contracts to get some kind of value out of it so what if

if you're making application that needs to know the weather for a certain area or or it needs to know an exchange rate so you can exchange your ether for Bitcoin Vis Versa um and yeah there's this limitation of the ethereum virtual machine where because it's deterministic you cannot look outside and fetch data uh into the into the blockchain by itself so what you need to do you actually need to feed this data into ethereum onto the blockchain and then once it is there every other application on the ethereum blockchain can actually use it and reuse it and use it in fashions that you really didn't think of yourself uh before

so typically you'll need an automated process some kind of demon that frequently publishes updates to this data feed so maybe once every day what the weather was or what the exchange rate was or more frequent or less frequent uh depending on your on your usage um and of course you can you can publish this as a free service but you can always you can also uh ask for a fee if people are getting the data out of this data feed so of course there's some cost associating with with setting up such a process that keeps updating keeps pushing data to the to the ethereum contract at least you need to pay the the gas and also some kind of Hosting uh fee so you could actually charge every

contract that is using this and this could be a a viable Revenue model for a for data feed um yeah I've given you these uh examples already and a much more advanced example is the the BTC relay uh contracts where it's actually storing blocks and transactions from the Bitcoin onto ethereum so then you can use this on the ethereum uh blockchain as well and and as an example here we have the the price fre the the price feed example from Nick Dodson I think he's also here or might be presenting later this week so here they're pushing uh fiat currency uh prices on the uh on the

on the on the blockchain and then you can use that for some kind of set up some kind of trade some set set up some kind of escrow without having to rely on the exchange rates of The Ether itself so looking at these transaction and value related patterns two two examples uh the asset token we you've seen that in Peter's presentation so you need to have some kind of representation from a either virtual or physical asset uh on the blockchain so you can for crowdfunding purposes so you can you can sell it or maybe so you can you can lease your car or you can you can trade it with with somebody else or just you

can track it for auditing purposes and typically these these assets um are are are equal are equal in kind so you can enumerate that them and you can have multiple of them and if you for this if you use a standard interface and there's the the The Meta Meta currency uh API description on the ethereum wiki if you use the same kind of interface for this then you can use all kinds of different wallets and exchanges to to to track your uh sub currency to track your assets without the the wallet itself having to specifically support you um so what you do you create a contract and

basically you track per account how much of these assets they have in uh uh in in the the contract storage and then you set up maybe at least two two functions one to transfer this assets from one account to the other account and one to return the the balance of one of these account holders in there and if you have this you can use this to to create a crowdfunding tokens set up IOU constructions uh track Fiat currencies so US US Dollars Euros whatever or even as uh digix is doing uh keep track of gold certificates on the on the blockchain and here as an example is uh

from slockit they crowdfunding contracts where they also using this this pattern so they're doing some kind of crowdfunding probably will hear more about this this later on but you can transfer these tokens to somebody else and you can also see uh what what your balance is um another another one of these uh uh value and transaction related patterns is the exchange or or market so you have one of these asset tokens so that's actually from the previous example and you want to sell it buy it exchange it for something else so you can set up an exchange on the blockchain itself um you you don't need any kind of central

websites that will do this exchange for you and then lose your money and and and run away with it but you can do all of this on the blockchain and typically both the assets for a one-on-one exchange will be one of these assets on the on the uh on the blockchain or one of the two could be just the The Ether currency itself but that I guess that's less uh less common and then of course the trick is how are you going to do this settlement if you have a huge Ledger um maybe it's very good to have the user indicates which is the the winning pair and and have have it matched that way or you need some kind of external trigger that will be called uh to to do the to the matching

and the and the settlement payouts but that's potentially expensive so what you need to do you need to have a contract you need to keep track of the bits or the Buy orderers keep track of all the asks or the sell orders and have some kind of matching mechanism and once a bit and ask is matched you swap the assets from both account holders uh to the other one an example of this is uh vitalic Cyber diey Markets it's written in serpent instead of solidity so it's a good homework exercise to try to convert that from serpent to uh to solidity and another example is the The Ether x uh exchange so an example from a cyberdine you see there's a buy function

there's a sell function but you can use this this Market this exchange to exchange all kinds of assets that have the same interface without having to upfront Implement support for this in your in your contracts so the the third and final group that we have seen are these action related patterns um event lock so it's a feature of the evm itself you can emit events and they will be persisted on the blockchain and you can use them to to track state or to track when some kind of action happens and have the outside respond to it there's a limitation though with the evm from one contract

you cannot inspect the event lock of itself or of another contract so you can only use this data from the outside but this makes it ideal for user interface purposes or if you have some kind of watch doog process that is looking at these event logs and act uh upon them so yeah it's just a single line of solidity code to emit an event if an action happens and then you have a a listening process or a watching process in your user interface that will update its state uh accordingly so examples of this if a buy order took place May on to update your crowdfunding website if you just least your car key to somebody else

you need to change the the the the codes the public key on the on the lock system if an account is registered maybe some other event takes uh takes place so a good example here is the is the W fund again from uh Nick doson the wund crowdfunding uh uh contracts so yes these events for that are emitted when a new c campaign is created when a contribution is made when a payout happens and even when a refund happens maybe he doesn't even use this these kind of events himself but by just sharing them you allow others to consume them and create all kinds of user interface and interactions with them that you didn't

think of didn't think of yourself so um yeah there's a bit of issue with um ethereum there's no alarm up code you cannot trigger some kind of future event to happen automatically um so it every every action in a contract needs to be triggered by a transaction that is eventually being sent from a from a user so what what if something needs to happen in the future how are you going to to make sure that is that this happens an example for this are if you need some kind of I don't know expiration of your of your um of your lottery tickets doing frequent dividend payouts other kinds of cleanups how are

you going to make sure that this will that this will happen so the solution here is to to reward the collar for actually trigging this mechanism so you create a function anybody can trigger this but once they do this they get rewarded for doing so and that way you hope of course you cannot guarantee that somebody will do it but they have a strong economic incentive for triggering this method and making sure that it will be uh that it will be called so an example here from uh gin uh casino contract is this recycle method which anybody can trigger and if there's some kind of expiration event the person that was trigging this

get rewarded with half a percent of the of the of the funds so makes sense there so combining these things together you can come up with all kinds of use cases so say you have a a betting website where you want to allow your users to bet on soccer scores you you would use a name registry to keep track of the the different usernames so you can have a a top a top scoring list High all Fame on your on your on your site of on your da need some kind of data feed to know which soer events happened and what scoring was was there um event lock so if somebody takes a bet or if you want you want to update again your website or your dep accordingly so you

use event lock to get the output uh from that so already you see with such a system you're combining ing multiple of these patterns together um again for Market if you have some kind of manufacturing site and you do some kind of internal research optimization you can try to centralize this but you can also think about can we do this on the on the blockchain using asset token to indicate resources having a exchange contract where people can bid and ask for for resources and then of course having some kind of incentivized trigger for making sure that frequent updates such as Market settlements happen and another example um is if you want to have your dep interact with with

real Hardware so in this case it's a smart meter in my utility closet that I connect with my ethereum Rasberry Pi I get data out of it um but how how do I know that this device belongs to me so I need some kind of notary data pattern that that that makes this Association I have this data feeds so for every energy consumption update uh a contract is updated with my uh data usage and if I want to display this in my dep I need to emit event locks um using the event lock pattern so what's next for you guys take a look at existing codes in GitHub

existing contracts study them and see what kind of common patterns do you do you see do you you are used and and can be used re use for yourself and also we're trying as a community effort to get this uh stack overflow stack exchange site setup for E ethereum so if you go to Area 51 on the stack exchange please vote for it as of this morning I think we were at n um almost 70% so a couple more votes and we have a community site for all kinds of Q&A all kinds of discussions around best practices about patterns sharing code sharing knowled with each other and this would be a great place to exchange uh

ethereum or Deb patterns as well thank you guys