I'm quite new to DDD and Event Driven architectures. And after searching this issue I did not manage to get a clear answer by myself, so I'll appreciate if someone can clarify this.
Suppose I currently have an app with 2 bounded contexts. ContextA emits some InterestingEvent that propagates through an integration event to ContextB. This allows ContextB to have its own representation about some eventual state each time this event is emitted and picked up.
So far so good.
But this happy path assumes that ContextA and ContextB have always existed or ContextB has always been subscribed to the InterestingEvent.
Suppose now ContextA existed a while before ContextB... or they both existed, but ContextB was not subscribed initially to InterestingEvent. As a result from this, a number of InterestingEvents have already happened when ContextB goes online or subscribes to the event.
What are the strategies for ContextB to deal with this "first sync" or "past events" situation?
Some crazy thoughts:
Is ContextA suppose to replay all events ContextB requires to be up to date? That could be problematic in the sense that some of this events may have side effects in other contexts as well.
Do ContextB may ask for an update batch from ContextA at bootstrap? Kind of a fat-full-state Godzilla event that carries all the state ContextB needs to be on sync?
Thanks in advance.
What are the strategies for ContextB to deal with this "first sync" or "past events" situation?
The most common strategy I've seen in expert discussion is that new subscribers pull event histories, rather than expecting them to be pushed.
In other words, the basic idea is that the thing-being-subscribed-to is an append-only ordered sequence of events; as the subscriber, I track some token that identifies the most recent event that I have seen, and when I'm ready for new events I ask the provider "Have there been any events since [token]?"
For new subscribers, the basic idea is the same, except that we use some special token/message that means "I haven't seen any events yet".
Of course, event histories are not the only way you can copy information across a boundary; in some domains, it will make sense to ask for a snapshot of information aka import aka bulk transfer. That representation of information can, of course, include in it some metadata that the representation is valid as at [token], and the subscriber, after processing the import, and then ask if there have been any updates since token.
Recommended viewing: Polyglot Data (Greg Young, 2014)
Related
I'm new to event sourcing and have some question on my mind. Here example diagram.
Let say we have 2 instances of service BookShop and 2 instances of service Wallet.
User ask BookService_1 to buy him a book. This book service creates event BuyBookRequestCreated and push it to event bus. Event bus emits this event to two instances of service Wallet. Two instances now try to reserve enough money from user wallet and they both emit event BookMoneyReserved? Now on the other side two instances of BookShop services recive 2 events and they both try to emit event BookBought? Or maybe eventbus will send BuyBookRequestCreated to only one subscriber? But then what happends when this selected service fails?
How this pattern is handlen from API consumer point of view? If I make call to some API to buy me a book I expect it to "return 200" when the book is bought. In event sourcing pattern there is no awaiting for response from other services so in case if some other service must emit an event to complete book purchase there is no way of telling the customer if his purchase actually faild or not.
I'm a little lost in the whole microservice world. On one hand we have Grpc, protobufs and service mesh but on the other we have event sourcing and event driven architecture. When to use which? And from what I see and can understand I could use event sourcing and grpc togheter? I could just use grpc to communicate beetwen services and save events as a form of state persistance or maybe I just completly didn't get it and should read thru articles again?
Thanks for help.
I don't understand event sourcing
That's not your fault. The literature sucks.
Here example diagram.
OK, so the most important lesson I can give you about that diagram: it has nothing to do with event sourcing. It has quite a bit to do with messaging, and distributed systems, and event driven. But event sourcing is a different animal.
At the high level, a fundamental concern is that distributed systems are imperfect. So we need to accept that as a constraint in our designs, and work with it. Pat Helland's Memories, Guesses, and Apologies is a good starting point.
In normal operation, we would never have two wallet services doing the same work. They might be sharing the work (in much the same way that the Book Services share the work from the load balancer).
One way that you might share the work is to assign to each message a unique number; top wallet processes the odd numbered messages and ignores the even numbered messages, bottom wallet processes the even numbered messages and ignores the odd numbered messages.
Of course, normal operation is what you want, but not necessarily what you get -- after all, distributed systems are imperfect. For some kinds of problems, there are comfortable patterns - locking, or idempotent message handling -- and your system can continue to deliver business value.
For other kinds of problems, the answer is that some human being gets on the phone, and tells somebody else that there has been a mistake, and can we work things out?
This is a general architectural question on ES. The concern is generally about the need to keep a great amount of business non-important events, that affect intermediate state, though we definitely won't care about them (will just ignore them) at the end of the day.
Say we have a User, that has a list of items (i.e Tasks), and the user may quite often add/remove/edit different fields of a task. If we are building ES, we should treat each update as an individual event for example TaskNameChange, TaskCommentChange etc, or we may have one event TaskModified whatever. In our case tasks state changes are actually not important for us, is we don't get much from task change history, from the business standpoint we will ever care about only last ones (for example last TaskNameChange), but we should anyway track and record all the events.
Again my concern is that we should record and keep a great amount of business meaningless events in event store.
Has anyone met such situation and what are ideas about it?
Has anyone met such situation and what are ideas about it?
Horses for courses
If the costs associated with keeping a complete event backed history of your document exceed the business value that you can accrue from that history, then don't design your system to keep all of the history. Set up a document store, on each save overwrite the previous version of the document, and get on with it.
Greg Young: a whole system based on event sourcing is an anti pattern.
I'm new to EDA and I've read a lot about benefits and would probably be interested to apply it during my next project but still haven't understood something.
When raising an event, which pattern is the most suited:
Name the event "CustomerUpdate" and include all information (updated or not) about the customer
Name the event "CustomerUpdate" and include only information that have really been updated
Name the event "CustomerUpdate" and include minimum information (Identifier) and/or a URI to let the consumer retrieves information about this Customer.
I ask the question because some of our events could be heavy and frequent.
Thx for your answers and time.
Name the event "CustomerUpdate"
First let's start with your event name. The purpose of an event is to describe something which has already happenned. This is different from a command, which is to issue an instruction for something yet to happen.
Your event name "CustomerUpdate" sounds ambiguous in this respect, as it could be describing something in the past or something in the future.
CustomerUpdated would be better, but even then, Updated is another ambiguous term, and is nonspecific in a business context. Why was the customer updated in this instance? Was it because they changed their payment details? Moved home? Were they upgraded from silver to gold status? Events can be made as specific as needed.
This may seem at first to be overthinking, but event naming becomes especially relevant as you remove data and context from the event payload, moving more toward skinny events (the "option 3" from your question, which I discuss below).
That is not to suggest that it is always appropriate to define events at this level of granularity, only that it is an avenue which is open to you early on in the project which may pay dividends later on (or may swamp you with thousands of event types).
Going back to your actual question, let's take each of your options in turn:
Name the event "CustomerUpdate" and include all information (updated
or not) about the customer
Let's call this "pattern" the Fat message.
Fat messages (also called snapshots) represent the state of the described entity at a given point in time with all the event context present in the payload. They are interesting because the message itself represents the contract between service and consumer. They can be used for communicating changes of state between business domains, where it may be preferred that all event context be present during message processing by the consumer.
Advantages:
Self consistent - can be consumed entirely without knowledge of other systems.
Simple to consume (upsert).
Disadvantages:
Brittle - the contract between service and consumer is coupled to the message itself.
Easy to overwrite current data with old data if messages arrive in the wrong order (hint: you can mitigate this by using the event sourcing pattern)
Large.
Name the event "CustomerUpdate" and include only information that have
really been updated
Let's call this pattern the Delta message.
Deltas are similar to fat messages in many ways, though they are generally more complex to generate and consume. A good example here is the JSONPatch standard.
Because they are only a partial description of the event entity, deltas also come with a built-in assumption that the consumer knows something about the event being described. For this reason they may be less suitable for sending outside a business domain, where the event entity may not be well known.
Deltas really shine when synchronising data between systems sharing the same entity model, ideally persisted in non-relational storage (eg, no-sql). In this instance an entity can be retrieved, the delta applied, and then persisted again with minimal effort.
Advantages:
Smaller than Fat messages
Excels in use cases involving shared entity models
Portable (if based on a standard such as jsonpatch, or to a lesser extent, diffgram)
Disadvantages:
Similar to the Fat message, assumes complete knowledge of the data entity.
Easy to overwrite current data with old data.
Complex to generate and consume (except for specific use cases)
Name the event "CustomerUpdate" and include minimum information
(Identifier) and/or a URI to let the consumer retrieves information
about this Customer.
Let's call this the Skinny message.
Skinny messages are different from the other message patterns you have defined, in that the service/consumer contract is no longer explicit in the message, but implied in that at some later time the consumer will retrieve the event context. This decouples the contract and the message exchange, which is a good thing.
This may or may not lend itself well to cross-business domain communication of events, depending on how your enterprise is set up. Because the event payload is so small (usually an ID with some headers), there is no context other than the name of the event on which the consumer can base processing decisions; therefore it becomes more important to make sure the event is named appropriately, especially if there are multiple ways a consumer could handle a CustomerUpdated message.
Additionally it may not be good practice to include an actual resource address in the event data - because events are things which have already happened, event messages are generally immutable and therefore any information in the event should be true forever in case the events need to be replayed. In this instance a resource address could easily become obsolete and events would not be re-playable.
Advantages:
Decouples service contract from message.
Information about the event contained in the event name.
Naturally idempotent (with time-stamp).
Generally tiny.
Simple to generate and consume.
Disadvantages:
Consumer must make additional call to retrieve event context - requires explicit knowledge of other systems.
Event context may have become obsolete at the point where the consumer retrieves it, making this approach generally unsuitable for some real-time applications.
When raising an event, which pattern is the most suited?
I think the answer to this is: it depends on lots of things, and there is probably no one right answer.
Update from comments: Also worth reading, a very old, classic, blog post on messaging: https://learn.microsoft.com/en-gb/archive/blogs/nickmalik/killing-the-command-message-should-we-use-events-or-documents (also here: http://vanguardea.com/killing-the-command-message-should-we-use-events-or-documents/)
Martin Fowler gave a great talk about "The Many Meanings of Event-Driven Architecture" (the content is based on this paper) in which he mentioned the Event-Carried State Transfer pattern.
It seems to be close to your second option "Delta message" with the difference that it doesn't try to describe an entity, but instead describe a named business fact that happened and carry over all the necessary data to understand this fact.
I don't think it matters how you have modeled your persistence layer when it comes to designing domain events. Likewise, I don't think it matters how your consumer has modeled its own persistence layer when designing domain events.
Thus, I don't think it's wise to put as an advantage the fact that you can apply the event as a patch directly on your data (from a consumer point of view), because it pushes the producer to design their events given the persistence model of a consumer.
In that case, I would tend to think that you're designing persistence patches, instead of domain events.
What do you think?
I am designing some events that will be raised when actions are performed or data changes in a system. These events will likely be consumed by many different services and will be serialized as XML, although more broadly my question also applies to the design of more modern funky things like Webhooks.
I'm specifically thinking about how to describe changes with an event and am having difficulty choosing between different implementations. Let me illustrate my quandry.
Imagine a customer is created, and a simple event is raised.
<CustomerCreated>
<CustomerId>1234</CustomerId>
<FullName>Bob</FullName>
<AccountLevel>Silver</AccountLevel>
</CustomerCreated>
Now let's say Bob spends lots of money and becomes a gold customer, or indeed any other property changes (e.g.: he now prefers to be known as Robert). I could raise an event like this.
<CustomerModified>
<CustomerId>1234</CustomerId>
<FullName>Bob</FullName>
<AccountLevel>Gold</AccountLevel>
</CustomerModified>
This is nice because the schema of the Created and Modified events are the same and any subscriber receives the complete current state of the entity. However it is difficult for any receiver to determine which properties have changed without tracking state themselves.
I then thought about an event like this.
<CustomerModified>
<CustomerId>1234</CustomerId>
<AccountLevel>Gold</AccountLevel>
</CustomerModified>
This is more compact and only contains the properties that have changed, but comes with the downside that the receiver must apply the changes and reassemble the current state of the entity if they need it. Also, the schemas of the Created and Modified events must be different now; CustomerId is required but all other properties are optional.
Then I came up with this.
<CustomerModified>
<CustomerId>1234</CustomerId>
<Before>
<FullName>Bob</FullName>
<AccountLevel>Silver</AccountLevel>
</Before>
<After>
<FullName>Bob</FullName>
<AccountLevel>Gold</AccountLevel>
</After>
</CustomerModified>
This covers all bases as it contains the full current state, plus a receiver can figure out what has changed. The Before and After elements have the exact same schema type as the Created event. However, it is incredibly verbose.
I've struggled to find any good examples of events; are there any other patterns I should consider?
You tagged the question as "Event Sourcing", but your question seems to be more about Event-Driven SOA.
I agree with #Matt's answer--"CustomerModified" is not granular enough to capture intent if there are multiple business reasons why a Customer would change.
However, I would back up even further and ask you to consider why you are storing Customer information in a local service, when it seems that you (presumably) already have a source of truth for customer. The starting point for consuming Customer information should be getting it from the source when it's needed. Storing a copy of information that can be queried reliably from the source may very well be an unnecessary optimization (and complication).
Even if you do need to store Customer data locally (and there are certainly valid reasons for need to do so), consider passing only the data necessary to construct a query of the source of truth (the service emitting the event):
<SomeInterestingCustomerStateChange>
<CustomerId>1234</CustomerId>
</SomeInterestingCustomerStateChange>
So these event types can be as granular as necessary, e.g. "CustomerAddressChanged" or simply "CustomerChanged", and it is up to the consumer to query for the information it needs based on the event type.
There is not a "one-size-fits-all" solution--sometimes it does make more sense to pass the relevant data with the event. Again, I agree with #Matt's answer if this is the direction you need to move in.
Edit Based on Comment
I would agree that using an ESB to query is generally not a good idea. Some people use an ESB this way, but IMHO it's a bad practice.
Your original question and your comments to this answer and to Matt's talk about only including fields that have changed. This would definitely be problematic in many languages, where you would have to somehow distinguish between a property being empty/null and a property not being included in the event. If the event is getting serialized/de-serialized from/to a static type, it will be painful (if not impossible) to know the difference between "First Name is being set to NULL" and "First Name is missing because it didn't change".
Based on your comment that this is about synchronization of systems, my recommendation would be to send the full set of data on each change (assuming signal+query is not an option). That leaves the interpretation of the data up to each consuming system, and limits the responsibility of the publisher to emitting a more generic event, i.e. "Customer 1234 has been modified to X state". This event seems more broadly useful than the other options, and if other systems receive this event, they can interpret it as they see fit. They can dump/rewrite their own data for Customer 1234, or they can compare it to what they have and update only what changed. Sending only what changed seems more specific to a single consumer or a specific type of consumer.
All that said, I don't think any of your proposed solutions are "right" or "wrong". You know best what will work for your unique situation.
Events should be used to describe intent as well as details, for example, you could have a CustomerRegistered event with all the details for the customer that was registered. Then later in the stream a CustomerMadeGoldAccount event that only really needs to capture the customer Id of the customer who's account was changed to gold.
It's up to the consumers of the events to build up the current state of the system that they are interested in.
This allows only the most pertinent information to be stored in each event, imagine having hundreds of properties for a customer, if every command that changed a single property had to raise an event with all the properties before and after, this gets unwieldy pretty quickly. It's also difficult to determine why the change occurred if you just publish a generic CustomerModified event, which is often a question that is asked about the current state of an entity.
Only capturing data relevant to the event means that the command that issues the event only needs to have enough data about the entity to validate the command can be executed, it doesn't need to even read the whole customer entity.
Subscribers of the events also only need to build up a state for things that they are interested in, e.g. perhaps an 'account level' widget is listening to these events, all it needs to keep around is the customer ids and account levels so that it can display what account level the customer is at.
Instead of trying to convey everything through payload xmls' fields, you can distinguish between different operations based on -
1. Different endpoint URLs depending on the operation(this is preferred)
2. Have an opcode(operation code) as an element in the xml file which tells which operation is to used to handle the incoming request.(more nearer to your examples)
There are a few enterprise patterns applicable to your business case - messaging and its variants, and if your system is extensible then Enterprise Service Bus should be used. An ESB allows reliable handling of events and processing.
This should be easy to follow, but after some reading I still can find an answer.
So, say that the user needs to change his mobile number, to accomplished that, we might have a command as: ChangedUserMobileNumber
holding the new number. The domain responsible for handling the command will perform the change in the aggregate and publish an event: UserMobilePhoneChanged
There is a subscriber for that event in another domain, which also holds the user mobile number in its aggregate but according to our software architect, events can not old any data so what we end up is rather stupid to say the least:
The Domain 1, receives the command to update the mobile number, the number is updated and one event is published, also, because the event cannot hold data, the command handler in the Domain 1 issues yet another command which is sent to Domain 2. The subscriber of that event lives in Domain 2 too, we then have a Saga to handle both the event and the command.
In terms of implementation we are using NServiceBus, so we have this saga to handle these message and in it we have this line of code, where the entity.IsMobilePhoneUpdated field stored in a saga entity is changed when the event is handeled.
bool isReady = (entity.IsMobilePhoneUpdated && entity.MobilePhoneNumber != null);
Effectively the Saga is started by both the command and the event raised in the Domain 1, and until this condition is met, the saga is kept alive.
If it was up to me, I would be sending the mobile number in the event itself, I just want to get a few other opinions on this.
Thanks
I'm not sure how a UserMobilePhoneChanged event could be useful in any way unless it contained the new phone number. User asks to change a number, the event shoots out that it has. Should be very simple indeed. Why does your architect say that events shouldn't contain any information?
In the first event based system i've designed events also had no data. I also did enforce that rule. At the time that sounded like a clever decision. After a while i realised that it was dumb, and i was making a lot of workarounds because of it. Also this caused a lot of querying form the event subscribers, even for trivial data. I had no problem changing this "rule" after i realised i'm doing it wrong.
Events should have all the data required to make them meaningful. Also they should only have the data that makes sense for that event. ( No point in having the user address in a ChangePhoneNumber message )
If your architect imposes such a restriction, it's not going to be easy to develop a CQRS system. How are the read models updated? Since the events have no data then you either query something to get the data ( the write side ? ) of find some way of sending a command to the read model ( then what's the point of publishing events? ). To fix your problem you should try to have a professional discussion with this architect, preferably including other tech heads and without offending anybody try to get him to relax this constraint.
On argument you could use is Event Sourcing. Event Sourcing is complementary to CQRS and would not make sense without events that have data. Even more when using event sourcing, the only data you have is the data stored in the events. Even if you don't actually implement event sourcing you can use it's existence as a reason for events to have data.
There is little point in finding a technical solution to a people problem.