Lessons Learned to Build a Dapp On a Light Client by Thibaut Sardan (Devcon5)

I want to talk about our lessons that we learned by building a decentralized application on top of write client who view here is fairly like who you know what is a light client like kind of if you think you know what like fairly well what's a like land ok it's like half of the person so I will be a bit slow at the beginning just to make sure we are all on board on why we want to use a light client actually by the way I will use the words light client light node for me it's the same because we're talking about a node so just two words about myself i'm t-bo I'm working at 30 I'm fronting dev and I work on pretty feather which I will talk about right now I work also on parity signer which I had to talk about before pretty interesting you should check it out and right now I'm actually working a lot on polka dot J's and and UI so this talk is about three things we want to understand first why we want to build or use like lines then I will show you how to hack on a library that we made that's called like Jess and then in the end I'll talk I'll show you a bit more about feather this application that we built on top of a light client you you can see I use light node inside an electron app right now like clients same thing so let's start why light clients are important so first of all you probably all know but running the full node is not as simple as it could seem or as simple as we hope it would be when people build a node in there in first place we thought ok everybody should be able to run a full node on a laptop right why it's not really possible anymore so what do what people do when you want to build a light clients are sorry when you want to build and test centralized application you do not connect to full node you connect actually to intra or to a third-party service that provides you full node to access the blockchain and the thing is infrared super awesome like you can build an application connect to the blockchain within a couple minutes that's really really nice like it helps turn the community to grow but there are many problems in there yeah those are numbers that I mean actually kind of odd numbers it's probably much higher now how many taps have been developed using infra how many billions of requests infrareds actually serving the problem you can all see that we call these decentralized applications but we're all connecting to one server here like or one set of server that is owned by a company that it's not decentralized that's that's actually not the right thing to do right so so what is the light client it's a node that stores header but will remain trustless so it's a note so it's something that does access the blockchain and can give you many information about the blockchain can help you do almost anything a full node would help you do from a decentralized application perspective but it is not decentralized it is sorry decentralized and it is not centralized it is it is just and it does not so it's not as secure the full node I will not talk about this in his talk because it's just it gets too complex but it still is trustless so you cannot get fake data or something you know it's decentralized and it it actually verifies all the requests and all the information that that gets so just to cope oh yeah numbers to show the differences between a full node and a light node or light client the sync time is a couple seconds if you're lucky if you're not so lucky maybe a couple minutes but we're not talking about hours or days right as long as you have internet I'm not talking about this Wi-Fi here because it could be could be it's not easy I tried it I did sync a light note in the and send for instance with my 40 you right it took a couple minutes but it was it was doing tubule the state size so how many gigabytes or whatever are you going to download on your mobile phone or on your PC with a light client is going to be zero it does not source the state with a full node of course that's its job it's gonna have gigabytes and gigabytes of state and the database that you're gonna have again on your on your desktop or on your mobile it's going to be tens of gigabytes people are talking about terabytes I won't go into this debate but anyway for full notes and like client its we're talking about couple hundreds of megabytes so what is the like line or why do we want to use a like lines so today you have on the right the notes the full nodes that are connected to each other and they form the ethereum network and you have your decentralized application that is there and sits there and when you want to connect when you connect to say in here you connect to one of these nodes or let's say a set of note that is owned by a company and you trusted this person so this person can actually turn bad and what they can do is they cannot change your transactions or something they cannot you know spend your money on your behalf or something that's okay but still they can track you they can know exactly who you are with your IP they can see okay this person is using this address all the time like they can do many mean mean stuff or they can censor you right infrared they are nice we know them and they actually want to get decentralized more they had talks about it I'm just saying this is not the right way to onboard people we all know that okay we want to care about decentralization in the space that's why we want to bring people more people to the space so let's let's do it right and what we can do is so the the new dots here or like lines so they are connected to full nodes and what we want to do is actually embed a light client in our tab this is what we did and this is what we want actually to let people do more so to let people and better light client and use a light clients in their centralized application we both library that is called light yes so it's super light it's for light client everything is right and here's what here's how a stack that you would have today for your normal app so basically you have your UI in JavaScript monstro using react angular whatever then you embed web through J's which you want to use to just send transactions and do stuff on the blockchain this web 3js one of the first thing you do with web to jess is setting the provider you have new provider and there you most probably use some URL from infra with the token that you have and then infer us or this centralized service node that you have to trust in this case we'll talk to a theorem and what we want to do so yeah these guys can eventually censor you or or track you and so what we what we want to do with this light J's is actually instead of what three J's you use this light Jess and this will talk to a light node on your own inside your app actually and you will talk directly to the ethereum network so the difference here is that you remove write the centralized node and you put the light node actually inside the tap right so this like Jess library is a high-level reactive and easy to use library we'll talk about it's just a bit we are whoa how do you install it just like any other real library you probably insulted a it's from the party name face and slash like dot J's so let's have a look a bit at some code I hope you can see that all right so that's a really simple sample code so you can see that we import light and we import balance off and block number and those balance often drop number have a dollar behind and it means there are these are observable and just like we would do with web 3gs today you would just do light set provider with your provider and then you can do with these observables Rock number you can subscribe for instance and this will give you a block number that you can log and so in this in this little program we'll just subscribe with the block number and just block the current block number and we do kind of the same with balance of so balance off you want to say which address want to see the balance off and then I don't know if you if you wear or if you know how to use reactive or rxjs but basically you can get what's the result of balance off and pipe it and do something with this with this result so what we're going to do is this balance is going to be formatted or you have a question so the question is well balanced off will be actually stream and updated and the answer is yes and this is the nice thing without observables and and your you'll see that in your in a second so the balance off will be you know not formatted nicely so what we want to do is from way we want to format it basically and then what we do is we do the same as before we subscribe to this and we just log it so what happens is when you just run this in the console it will just go like this current block blah blah blah my balance and then it updates and then it updates you change the balance boom balance will show the new balance and then it updates and it goes on so this is an observable and well yeah that's or XJS like this is not what we built this specifically we just built on top of this and and this is awesome that's why we will use it so like Jess I said is right it's actually extremely simple we did not go crazy with the amount of thing we wanna support we actually just support what we need and what the community up to now has told us lean it so we have twelve functions and that's it right we have like super basic things the block number you know you can post transaction post row transactions already signed transactions you can get some contracts you can get you know like my balance the version info of the node and suffering this so just to see an overview of the differences between for instance web through Jess and like Jess so the account management the way it's done no sorry there a synchronous management with what we Jess you would use from eyes we use observables the coverage of the our pcs that we that we support we keep it very very simple right and if you need some more please just let us know and of course and this is the very important thing light client like this is built the way we have it in mind is it's built to be used on top of light client and it brings a lot of things that yeah that do not come embedded in what 3GS and I'll talk about it right now so so behind the scene the light yes uses the least possible resources on the light line so what you're gonna do today just before I showed you okay what's the latest what's the latest header frames or what's my balance for instance your balance will not change every second right the block number the block time is five fifteen seconds on therm does not make sense to just bother the node you know every second on fifty five hundred milliseconds right today you can do that with the theorem but it does not bring anything so light client the largest library will do this for you right will actually get just a push notification will not have pull so you will get the the latest information when it's there not when you ask for it because sometimes developers just don't know everything and that's fine and that's why we want it to be light yes this way so it fetches States on you head and not whenever you want so it just gives you new information whenever there is new information actually it handles the subscriptions to to the node for you it handles cash and it keeps the amount of network calls to the minimum but to the I mean it still it's so there's you know you won't be like outdated you won't get how they did information it just does the thing like in an efficient way right so I'll go and the reason why we built all of this elbow is to embed light clients everywhere as much as possible and facilitate a very mass adoption while maintaining the centralization because we do not want to to just trade this off right so because of this we you know probably parity has builds a node and this node can be run as a light node or as a light lines the thing is we wanted to use it and to see how things work and we wanted to embed like let people in that light client everywhere so that's why we will build feather so feather is an application on top of a light client and I will show you just I'll show you in a minute I want to just talk very briefly about the stack so just like we said before so we have you Act UI in react we use light jazz to interact with a light node that is inside an electron app and all this app will interact with directly with the term network without third party service in between so truly decentralized wallet is what better is it used to be not the only one because mist used to be also using a light wallet unfortunately it's not possible a light client sorry and foster only is not available anymore due to other reasons so to my knowledge feather is the only application unfortunately on top of our light clients so it uses light just as I said it's bundled with a parity of theorem right client and it syncs in couple minutes depending on the Wi-Fi we have other interesting libraries that you might want to see one of them is called parity electron it's a small library to start and stop parity I am from a node.js environment and you will see the UX and everything about feather right now so here I just launched feather and this is what happens at the bottom you will see that it's start syncing right now and it just shows you you know like and it shows you there how far you are and shows you an empty page because you have no account so I'm going to check create a new account this looks like a green cat so I'm gonna name that green cat you know and now it's it's getting synced you know like at the bottom you can see sometimes so we show the you know the mnemonic blah blah blah you have to be careful with the mnemonic please type the new Nanak again and then you're gonna have the password and just to secure your account and it's tricky sinking in the in the bar in the background so it's you know it's not extremely fast but it's alright and I can also for instance import an account using I don't know Jason so here my window was a bit not on the right side but I'm just importing a Jason and just shows you know the name and stuff and yeah so by the time I'm done with this process actually I'm done thinking you can see that it's inked and now if I click on an account it takes a bit of time to get the balance again it could be super quick this Wi-Fi was not like awesome okay now it's dated and I have a new C 20 token as the other one the other balance so here for instance I can add I don't know another token so it's gonna be a fake token it's my token so it's just cheap coin and yeah you know at some point you choose updates and to send a transaction so I'm just going to click on this one and I'm just gonna you know say to whatever these amounts I can select the amount way so here does some call right it does like okay how much this transaction is gonna cost etc does some calls in the background and then right using just a transaction like nothing nothing very right so I'm just gonna cut this because it takes some times to send the transaction but you you get it so what do we do now with this application so it has been outdated this is the first time I say it actually it has been operated this summer by trail of bits I'm super happy because there was no big critical problem found we fixed actually all the problem now and we're gonna have a release probably next week with everything fixed and we want to give this to the community if anyone is interested to actually keep this going because this is not something we're gonna go further on we do not have to bandwidth for this and yeah so if anyone's interested please talk to me and we'll be happy to - yeah go on with with feather because I have a bit of time I'm also having a shameless plug here we're hiring a full stack developer we have awesome other stuff to do with light clients with many many things and please talk to me or anyone party that's about it we have a couple minutes for questions thank you so much and yeah if you have any question [Applause] you