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RailsConf 2019 - Event Sourcing made Simple by Philippe Creux

Philippe Creux
Senior Software Developer at Kickstarter
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RailsConf 2019
April 30, 2019, Minneapolis, USA
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About speaker

Philippe Creux
Senior Software Developer at Kickstarter

I am a software developer. I deeply care about shipping software that’s simple, robust and cost-effective.I had the opportunity to work with talented people on a variety of (mostly web) projects.At IBM (2007-08), I fell in love with Ruby on Rails, TDD and Agile methodologies.At Versapay (2009-11), we built an electronic fund transfer platform and shipped the open source project ActiveAdmin.At Reverb (2011-12), we relied on ActiveAdmin to build Enterprise Software for Photography Studios, Logistic Firms and the Pharmaceutical Industry.In 2013, I co-founded brewhouse - the best Rails Agency in Vancouver. In two years, we shipped a dozen of web applications, helped 4 development teams get there codebase to the next level while working on Goodbits, the best application to build curated email newsletters in minutes.Late 2016, we’ve decided that brewhouse should focus solely on Goodbits and most of the engineering team was hired by Unbounce. We've designed and shipped a new subscription and billing platform to enable Unbounce to launch its new pricing on March 14th, 2017.I've joined Kickstarter in April 2017 to "help bring creative projects to life".I’ve been sharing my learnings and practices on various blog posts and built several open source projects.I am currently running the Vancouver Ruby meet-up. Prior to that, I ran the meet-ups Lyon.rb and Agile Valence, hosted the podcast “Parlons Ruby” and co-organized the Ruby Lyon Conference in 2012.

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RailsConf 2019 - Event Sourcing made Simple by Philippe Creux


Event Sourcing provides a full history of actions allowing us to understand how we get got there. Events can be replayed to backfill a new column, to fix a bug or to travel back in time. It is often described as a complex pattern that requires immutable databases, micro services and asynchronous communication.

In this talk, I will introduce you to Event Sourcing and present the simple framework we’ve built at Kickstarter. It runs on our Rails monolith, uses ActiveRecord models and a SQL database. And yet, it gives us super powers.

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All right, that's so let's get started talks about even so sing and how we made it seem to bring you raise app called to dues and then we had a Motorola called task just asked to come to England together is completing a task action called competes in the task comforter and we're doing a tribute to true and then addressed Lake Rangers the new task. Jason. Now and you requirements comes in the new requirements that we actually want to record that competition dates to when was the task completed? Because

we want to see your face of that. They got to the user and it is because we want to send an email we chat every day of they call the things you've accomplished during the day, but let's get that at things tell me something that we often do in Ruby on Rails applications. So we just said the competed at * * 2 time. Now when it was a task is completed to add that calendar to do that. We create a raised migration migration it right over every single task that I already completed

update. Completed now. What you got value V time updated our times and that's the closest we might have to steal. That's not perfect. Now, we want to know who completed and now we assign the completed by that you to the current user comes to that terrain key to all tasks. Well, we basically don't know who completed the task before end. The once again the data is lost. Notice of some event tracking right because as soon as big an application goes into production,

we need to track some events on it. So we add event task ID user ID in this action, but we actually have that every single action since almost every single comforter in a notice at some email notifications now that it's leaking milk to use your platform that you can assign a task to people Etc. You want to get an email whenever someone else come visit as you've created. That's what Justin Bieber of code is all about. And we also want an activity feed right be great. If we can have date that activity feed the police in this new line which way to naturally

item with the subject has completed. Now when we redid that feature of the activity feed well, we don't have any past activities. So when we were used the future, we stop he's just a blank slate how you could write. Somebody custom scripts backfield. They must be a better way. My name is the Philly crew. I work for Kickstarter. And today is a special day for Kickstarter because we have today. Free feels like because and this is available today. Anyway

Kickstarter all DeSoto Health 162,000 a creative projects come to life 16 million people back to project to revise a total of 94.2 billion dollars for this project project projects Like Cards Against Humanity all the table to watch the dollar store thousands and thousands of Lakes Mall glasses project Wight comes too late. The thin documentary is Dance Project art projects comic books sellers a great place for that website today, because to celebrate our

tenth anniversary download of Easter eggs and Konami Code type of thing on the website. List to make the web weird again. All right, even still sitting Made Simple. Let's get to it. So obvious sequence of events. Let me illustrate that. We have the Appalachian State at the top and event at the bottom and that happens. We treat a new event to ask created with the title Reaper sides. We didn't apply that events to the application state. So we just create a new task with the title people sides.

How do you say go back to the UI keeps on the calendar icon and it sets a due date to April 20th. We stole that events in database and then we apply that eventually application state. Wait a second cousin free presides those peepers lights. So these are go back and update the title in the UI that creates a new event and we end up like that even I'm the one to bring my slides. So I assigned the task of myself lecrae's new event application State due date to

April 24th and apply that to the application State and finally as of this morning. My slides are finished the task as completed apply that to the application state. No rain here. We only see events with data but even and have like a lot of metadata a lie to them you can store when the event was created who created the event what device is using IP address. It's coming from the contact that you have in your controller at that point in time. You can proceed that in the database because you might need it at some point.

The good news that you use even so sing Everyday on you at least use a tool that has to make event sourcing flavor to it every day. You know what that tool should be Get yes. When you think about gets your current working directory ubt the Triant state of your application and as you create new comments, and as you move between Comet that's how you creating event. So each event with Joe commit message my daily, why doesn't happen what happened? And then the d f is how the change happened when you get very nice things from can we play

you can Branch off you can go back in time and that we see what we're talking about here to pay the kid get for data. Let's go through the key benefits of even so seeing as you've seen you or history of a different event that happened. We basically understand how we got there and it makes it a lot easier when you get events are coming from different users or maybe from webhooks and 8 year olds having Lake East full history helps to take that back thing for you today.

When you get a phone call because the date us doesn't seem that right? Well, you know what happened and you can get a good sense of how we got there. You can replay events to backfill new attributes. As I said even have a lot of data and a lot of me and you tend to where you change the function that applies that even to the Appalachian State replay all the event and your data is in that. You can repay events to recover lost records because you have that like full log of events. You can basically just get rid of your application State destroyed everything we play all

events and gravitation state is back there. Which is great when you run migration and it just feels like a different perspective on your data did everything and you can't get it back. You can replay events to fix the new constitution state is has been some late race conditions going on and maybe the way you are applying an event to the state was incorrect can just fix that replay event in the station back in the competition state. And you can obviously travel back in time. If you replay old events at Lake a certain point then you can

look at your application State at that point in time to get the time machine. You guys go forward only answer your data warehouse now events is like a first-class citizen. If not, like we do not trailing kind of fake event with this track even if stain is like the event is the things we can just like forwarded to your data warehouse and they can use that as well and actual event. And as you add new event to your application do Pastors get forwarded to the day our house. You can forward them to you or monitoring solution. Whatever that is and you

can then have checks in Charlton Tetra make high-level application event, since you can check that the building. There is at least one pledge every minute way more than that. And you can apply events to build tailored projections still in the same way. You can have your account even supplied to an accountant. Scription Etc collections. You can live events in Falls from them to get Natalie feet how to get a daily report. You can even get to Something Sweet you can be a

projection to the. Specific two different needs of the speaker different type of users and add event to get graded. They just update those projections and you just have to read from going to raise that you tend to build something from the crazy SQL queries to be awesome report. You don't need to do that anymore. I don't even talk rated you can just a date. In-Place industry from it Events can also be used to backfill your data warehouse tables. So I talked about the fact that you can forward those events to your data warehouse. But once you have

that in place, if you happen to set that up, maybe 6 months or a year in you can always send eventual data warehouse to back to your old data. When you realize that brand new activity feed feature. Well, if you have events, you got to Randall's events through that actually feed mechanism to get deactivated feed from day one. 11% is great for distributed systems. And most of the talks that you can see on Evenflo single. Most of the literature is around this with the distance. You can have my event sending events around.

but you don't need a microservice architecture all this architecture to take advantage of events Austin event for let's talk about even so singer Pink's daughter and how we try to make it simple. About two years ago. We tried we started a new product called drip and rip is a subscription-based platforms to support Raiders on an ongoing basis. We do that just was having a new product was late and very goods. Way to experiment with even so the requirements of the even something framers were putting together.

It has to be very simple to learning to use because we had to ship trip in six months. So we didn't have a lot of time to mess around with things in to train people. You have to be very easy to remove because just an experiment so is things went wrong and because we had that tight deadlines should be easiest way to remove that whole thing and just go back to Lake Rudolph. Desertion in the end is just a custom framework. That's only like a hundred lines of code and realize a lot on a cube record and a cube record pull back and they see events are regular verbs and once

again with only change the way that we knew taedatea. We didn't change the way we special day are all the way that we read data from the database. We don't need mine. Let's be able to do that together. Let's get back to the section where we come Pizza task. Thanks a big difference. What we're doing is that considered an event which is the task completed snacky record noodle recipe made with the task and we give you some metadata here. We just giving What event look like is basically? It is inherited from the task base event that has old behavior that that you need for that event and all it

has is that apply method and Supply method is responsible for updating the state of being the task. Basically, when you create an event that persist even to the database and they need an app to create call we simply reply that event to the task and then we saved the time. That's all we do and that's how the task happen to become completed. You get the day that we haven't database that's the task even stable and they see all events have like an ID that stayed with the tasks. They have a type of computer doesn't have any data. I ain't has made a date on

which current is only the user ID and a timestamp of like when it was created at Libraries in Eureka requirements we want to record when the task was completed in the chance that in the event. And now instead of sitting the completed a tribute to True reset completed at to the created a timestamp very important to study to the created because if you happen to replay event later on you want to make sure that you always stay at the same time stand. That's why we use created at and not

time. Now him. Way Comes to adding that new connan to database in the in the migration all we have to do is to replay events so that likes of six lines of code to replay events will be the same everywhere biscuit eater rate over every single task Wheelock it starts to make sure that we don't like having some tea and the right place and we basically iterate over every single event for that given task and we applied even to the task and then we saved the task today at base and I just doing this the completed a

timestamp is now back to you. Now we also want to record through computer task. Same thing. The control doesn't change or we are changing. Is that apply method in the controller? Are basically extract the idea of the user from the metadata the switch beans saving from day one. So we just said the company by ID to the user ID. Waze comes to the migration to the data migration. We just added to the database and then we replay events the exact same way that we would play them before and we now have that completed by ID set back

Shield. Next step is to create emails and for that we need before we actress. Reactors Ultra good when an event is created not what is repaid and they allow you to perform all the side effects. If you need they can be associated with one or minivan types and they can be off 18th Street. I used to pick a side effects and they can create other events in trim. What is adenine a notification that we will register a dispatcher has a very simple. It allows you to associate a reactor. We didn't even

go here when the new task completed event is created. We trigger the reactor notify completed and that's reactor is just a lenda. It's just something that takes that response to the method core and takes an event as an attribute so that you can be a class if you want to. I'm here whenever task to be completed. That's one. We actually be triggered and an email notification be sent. It's now up some even fucking well days can just meet to create a new reactor cold forward event

associate with the task completed and given an event it real and take between most bases together the date of the event send that to even bus event name from the event last name. Annapolis add an activity feed once again just a new a new reactor trade activity entry and in turn we create an activity entry in the database. Now this track order things there is something going on. We have the base event at the top table and then a task base event and Daenerys from that the task completed so we can whiskey tell the dispatcher to

dispatch or events to the forward event reactor and all the tasks events to the create a convenient reactor to Oswego and those we create more events. We don't have to do anything for those new event to be forwarded to our even less than just happened. You guys are so pretty easily decide to run a reactor. Asynchronously. You just have to switch that cured agreement for from trigger to a sink and that we are q a job that accept psychic will perform and they see right now and I think that

about three-quarters of our events run. They should go back to the first example of the task controller the weight restriction of us. Like the comforter was finding a task to task sending an email reading activity feed and sending events today even bus and obtain the task and then reading the results from the database. The new approach we create an event that's called task completed and that's even to update task. I'm on the side. I will send out the email and

forward and create activities in trees. As you can see or we spent here is the way that we right there another way that we read data. So after 1 year of even so sinh hundred I can say one year after we launched the different types of 50 reactors and about 800,000 aggregate records and 500,000 event records. All the first of all replaying events is just awesome Windows a fantastic to be able to repay events to back feel to be on you at some point since when is a subscription-based platform to find things when we needed to do face when subscription was

deactivated. It was very easy to update the deactivated events to evade that attribute and then we play all the events to get that are coming in. Replaying events are so fantastic to recover hard Detroit record. When we started working on Driftwood basically destroying records when the user was just trying something and at some point we figured that suppose beta tubes stuffed deed records and I just mocking them with a caddy to eat at timestamp. So we just tend the Detroit event to mock the event as a mock. The

aggregate has destroyed instead of destroying it replayed all the events in the date. I was back in the database. How did all the things all our other guests are audited and here you can see Lego subscription and what we can see them anyway, and basically we can see the whole life cycle of that subscribe from the bottom up at subscription was they created by the user than activated that's a webhook from stripe and then everything else is busy with hooks that we get from stripe activated invoice created invoice collected another invoices

for 9 month was created by the payment failed. So now it's past you and Thrive Faith to collect the payment three times and now this week And it's great to have when you need to be back something for customer support tracking down race conditions, and we actually had some of them some of those issues where we are expecting webhooks to have the same day. They didn't have over expecting webhooks to be figuring a specific order and they weren't for some reason so it was easy to update our apply method to work with that. We pay old events and get subscriptions back in the right state.

We use a combination of study grafana influxdb for monitoring and we basically stand or even meaning that there and ready to be reported on so you need to wait explore things if things go wrong in production, the data is already there and everything is there for us to create new child or to report them things. Miss me no way. We send all events to redshift cluster. And slept we reach the business ideas team have like all the events in hands to be of reports and they don't have to ask her to

add me to send you events that Old Ale. Western Unions reporting maybe six months after we went live and what was very interesting was that we were able to replay the key all those events and send them all to the red blister after the fact that we have all events from day one in the Rochester. United States, we have a nice DSL to basic connect reactions with events. And so because a reactor can create an event and that event entry room can't figure out whether we're actors that could figure order events at a very nice to be able to look at all the events and track down

here. We can see how when you create a subscription to a subscription created even just created a reactor. We try to activate a subscription at least you a subscription activated event between turn on Sanda subscription confirmation and use us who founded station and add an item in the activity feed. Well actually looks like this when you generate it, so it's not as good looking. And I want to highlight the fact that the data to write in different from the day. I learned to read and it's a big thing with even so seeing you can store as many things that use as you want in the event.

So that you can at any point in time take advantage of this data that have you struggle when we work on like a regular database that there is no because it might be useful at some point, but you never know whether it will be on. Same goes if you need to to face more data, then you can always add a new card and repay all events and get the data back in your aggregate. Now it is stuck in my thoughts. Naming is hard and as you know, and here we are talking about with naming invisible events.

And I'll tributes code and you can reflect her and rename things here, you know that wants to fit the name that name we left basically forever. We can spend a lot of time. Finding the right name for your events and your outfits. Updating even schemas was surprisingly not a big deal. A lot of people ask me about that and say Hey, you make more data to it more and more attribute attributes. How do you deal with that? I don't have that most of the time they have an implicit before value. When you add to a subscription while the currency was US Dollars

basically for everything up until the time that you're so you can be 20, then you know that it's lazy to be US tour address for most attributes. Most of them would befall to most new attributes an event for default to the same default value on empty value. The hottest spot in the most complex spouse in my opinion is how you need to distract her request into multiple events. because that even so seeing experiments was just an experimental unit on the front end to be aware of that hole even something going on behind the API

API API music the news update Postman station takes a title a description of published Boolean. We were lady and attachment I am so the hardest part is late to basic infrastructure this into multiple events. To do that we use commands and the recipe of commands is busy to make sure that at the given point in time. You can perform the operation to tell Frank to perform. I mean, he's basically to make a diff between DuQuoin State and then you attribute that to get humid in the user. That's

like a dating a post in the UI and you get to food Palos with title of this weekend published a tribute at cetera. Where are you need to compare with the title has changed and then you admit an event. You don't want to Amy. It's an event every single time. They usually update something that hasn't changed. Until he how you can where you can see how this late one request dispatch into Lake 5 comments and then lose 5 Command Center might or might not generate different events. And that's my opinion. One of the most complex part in our code base.

All right. That's why I was even so seeing Made Simple. So the key things I want you to remember from T stock. It was that even so saying that the state is the result of a sequence of events events hold data and those reactors can create event Cintron. They can't you take the state if they need to they have to create new event entry. Nikki benefits that you get the full history on the floor dog back ceiling the state of the application is easy Rex is where is your sense of that

event am now. Citizen and you can forward forward them to other services or to even bursts. Thank you. The question is what is the framework is it open? So yes, it's a 150 lines of code framework. So we opened it and I got to Dallas some companies that late take inspiration from it to build a loan for the blog post and at the bottom of the blog post I shared into this issue blankets Target app. You can still find it. It would be under a repository Cold Lake.

Raise to do event sourcing demo or something. The question is how do you document your events that you can keep track of them? That's a question. Dearest I didn't show. I am so that in the presentation bad be ventilated adsl2, Define the data attributes that they take to just not and Baseline beeswax regenerator condition. Fila say that list all the events that we have with the name with also the name that we spent leaving bus with its we should have been different from the actual class names and attributes

and a list of the metadata that you can get as well and busy whenever you run the tests are you put the app that information file is a dead and so the data team take that as a good source of Truth. So the question is do we capture the whole business logic in the creation of that event Way Comes Today a violation at cetera that happens in the comment Sobieski whenever we get an API Cola webhook. That's where you go through a come in and the Kumon would be responsible for any dating

event at the crown point in time at some point. We had violations in the event and we got rid of them because over time the data changes and so you can have an event at plate crate subscription and the only activity has is an amount as I said at some point in time, you might add currency and you might want to very day that the currency is like one of them 12 joints is that we should both well now hold events that don't have that. So we ended up like this

removing all those violations from from the sea bands. Because of the simplify things for engineers what they do know that relations only going Cummins. Check the question is Lake. How do we find a metadata and specially with this example where the user ID and what happens if we delete that user from from the database what we do there's a busy everything is soft destroyed. So we know that the user will never go away from their base and fold drip for various reasons an engine

that rent in the kickstarter. So users are not even supposed that we know that they would never get destroyed anyway. Can you migrate from paper trade? That's a very good question those about it dates your data and then papertrey will take the differences and restore those differences in the database all the way around. And do you play write your paper trays? I just need to back there someday. I don't know. It's a very good question. Oh, wow, that's intense. Have we did we get in the timing issues with the clock on different servers being different meaning that

maybe in the real life events when they take a BC but the weather the timestamp older than is a JCB. Know that we have funeral things that happened with the way that we process are webhooks friends. And when I try files webhooks to us in my life, I remote support groups of the same time and they use either sidekick or delay drop to fire them so they can get a kink out of order and then we actually have a separate service that handles them and then we brought them back to our path phone. And so we have like three or four

layers of Lake background job type of things in the right order. But most of the time if you have a grievance coming at the same time, they all it related to the same kind of event latest subscription is canceled and we might get extra different types of events for the invoices subscription in Boise, Idaho. But you can never get into that issue. Well user could perform the two actions pretty quickly and they would get a kick out of order. What kind of locking mechanism we use for event? We use pessimistic locking. So whenever we receive an event or whenever we

create an event, we open a transaction SQL transaction. We lock the aggregate. We apply the event to the Aggregate and we say Diego. That's right at the exact same time and they have the same time stamp. It might be hard to process them in the right order or replay them in the right order. When we Fest event forgiven aggregate. We stop them by traded a timestamp and ID and that way we make sure that we will repay them in the same way that they will guide to the Aggregate and once again from our

experience when we have very like super strong currency going on on our aggregate, but most of the time when we get latex-free events of the time it's very likely that one eventually I bake wanted to do with another one another and they won't take it with one. Get the question is I mentioned that there is after a year that was on your leg. Help me an event in database and that's because he's not a lot. Okay. Nice drip is stealing private data. Actually I giving away drip too

XOXO at the festival that plays important. So there's not a lot of traffic on that being sad person might not moving forward. We don't know but they sound like what I've seen on the stove and every single webhook strong than greed and striped bass trolling the database and they are they seem to be keeping up with leg vegans of Records. I think that this approach would I make you sound level All right, Mike Smith. We're running out of time. Thank you all so much.

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