我是卡夫卡的原作者之一。Kafka可以很好地作为事件来源的日志。它是容错的，可扩展到巨大的数据大小，并有一个内置的分区模型。

在LinkedIn上，我们将它用于该表单的几个用例。例如，我们的开源流处理系统Apache Samza内置了对事件源的支持。

我认为你很少听说使用Kafka进行事件源，主要是因为事件源术语似乎在Kafka最流行的消费网络空间中并不流行。

我在这里写过一些关于卡夫卡风格的用法。

所有现有的答案似乎都很全面，但有一个术语问题，我想在我的答案中解决这个问题。

什么是事件来源?

似乎如果你看五个不同的地方，你会得到这个问题的五个不同答案。

然而，如果你看一下Greg Young在2010年的论文，它从第32页开始就很好地总结了这个想法，但它没有包含最终的定义，所以我自己大胆地阐述了它。

事件源是一种持久化状态的方法。不是由于状态突变而将一个状态替换为另一个状态，而是持久化表示该突变的事件。因此，您总是可以通过读取所有实体事件并按顺序应用这些状态变化来获得实体的当前状态。通过这样做，当前实体状态变成了该实体所有事件的左侧折叠。

什么是“好的”事件存储(数据库)?

任何持久性机制都需要执行两个基本操作:

将新的实体状态保存到数据库中 从数据库检索实体状态

这就是Greg谈论实体流概念的地方，其中每个实体都有自己的事件流，由实体id唯一标识。当您有一个数据库，它能够通过实体id读取所有实体事件(读取流)时，使用Event Sourcing不是一个困难的问题。

As Greg's paper mentions Event Sourcing in the context of CQRS, he explains why those two concepts play nicely with each other. Although, you have a database full of atomic state mutations for a bunch of entities, querying across the current state of multiple entities is hard work. The issue is solved by separating the transactional (event-sourced) store that is used as the source of truth, and the reporting (query, read) store, which is used for reports and queries of the current system state across multiple entities. The query store doesn't contain any events, it contains the projected state of multiple entities, composed based on the needs for querying data. It doesn't necessarily need to contain snapshots of each entity, you are free to choose the shape and form of the query model, as long as you can project your events to that model.

出于这个原因，“合适的”事件数据库需要支持所谓的_real-time订阅，它将向要投射的查询模型交付新的(和历史的，如果我们需要重播的话)事件。

We also know that we need the entity state in hand when making decisions about its allowed state transition. For example, a money transfer that has already been executed, should not be executed twice. As the query model is by definition stale (even for milliseconds), it becomes dangerous when you make decisions on stale data. Therefore, we use the most recent, and totally consistent state from the transactional (event) store to reconstruct the entity state when executing operations on the entity.

有时，您还希望从数据库中删除整个实体，这意味着删除其所有事件。例如，这可能是符合gdpr的要求。

那么，作为事件存储的数据库需要哪些属性才能使事件源系统正常工作呢?就几个:

使用实体id作为键，将事件附加到有序的、只能追加的日志中 使用实体id作为键，按顺序加载单个实体的所有事件 删除给定实体的所有事件，使用实体id作为键 支持实时订阅项目事件以查询模型

卡夫卡是什么?

Kafka是一个高度可伸缩的消息代理，基于仅追加日志。Kafka中的消息是根据主题生成的，现在一个主题通常包含一个单独的消息类型，以便更好地使用模式注册表。主题可以是CPU -load，其中我们为许多服务器生成CPU负载的时间序列测量。

Kafka主题可以分区。分区允许并行地生成和使用消息。消息只在一个分区内排序，通常需要使用一个可预测的分区键，这样Kafka就可以跨分区分发消息。

现在，让我们看一下清单:

Can you append events to Kafka? Yes, it's called produce. Can you append events with the entity id as a key? Not really, as the partition key is used to distribute messages across partitions, so it's really just a partition key. One thing mentioned in another answer is optimistic concurrency. If you worked with a relational database, you probably used the Version column. For NoSQL databases you might have used the document eTag. Both allow you to ensure that you update the entity that is in the state that you know about, and it hasn't been mutated during your operation. Kafka does not provide you with anything to support optimistic concurrency for such state transitions. Can you read all the events for a single entity from a Kafka topic, using the entity id as a key? No, you can't. As Kafka is not a database, it has no index on its topics, so the only way to retrieve messages from a topic is to consume them. Can you delete events from Kafka using the entity id as a key? No, it's impossible. Messages get removed from the topic only after their retention period expires. Can you subscribe to a Kafka topic to receive live (and historical) events in order, so you can project them to your query models? Yes, and because topics are partitioned, you can scale out your projections to increase performance.

那么，为什么人们一直这样做呢?

I believe that the reason why a lot of people claim that Kafka is a good choice to be an event store for event-sourced systems is that they confuse Event Sourcing with simple pub-sub (you can use a hype word "EDA", or Event-Driven Architecture instead). Using message brokers to fan out events to other system components is a pattern known for decades. The issue with "classic" brokers as that messages are gone as soon as they are consumed, so you cannot build something like a query model that would be built from history. Another issue is that when projecting events, you want them to be consumed in the same order as they are produced, and "classic" brokers normally aim to support the competing consumers pattern, which doesn't support ordered message processing by definition. Make no mistake, Kafka does not support competing consumers, it has a limitation of one consumer per one or more partitions, but not the other way around. Kafka solved the ordering issue, and historical messages retention issue quite nicely. So, you can now build query models from events you push through Kafka. But that's not what the original idea of Event Sourcing is about, it's what we today call EDA. As soon as this separation is clear, we, hopefully, stop seeing claims that any append-only event log is a good candidate to be an event store database for event-sourced systems.

我一直在思考这个QA问题。我觉得现有的答案不够细致，所以我加上了这个。

TL,博士。是或否，取决于您的事件源使用情况。

我知道有两种主要的事件源系统。

下游事件处理器= Yes

在这种系统中，事件发生在现实世界中，并被记录为事实。例如仓库系统，以跟踪产品的托盘。基本上没有冲突事件。一切都已经发生了，即使它是错的。(即托盘123456装在卡车A上，但原计划装在卡车b上)然后，通过报告机制检查事实是否有例外。Kafka似乎非常适合这种下游事件处理应用程序。

在这种情况下，Kafka的人提倡将其作为事件源解决方案是可以理解的。因为它非常类似于它已经在点击流中使用的方式。然而，人们使用术语事件来源(相对于流处理)可能指的是第二种用法……

应用程序控制的真相来源=否

这类应用程序在用户请求通过业务逻辑时声明自己的事件。Kafka在这种情况下不太适用，主要有两个原因。

缺乏实体隔离

此场景需要为特定实体加载事件流的能力。这样做的常见原因是为用于处理请求的业务逻辑构建一个瞬态写模型。这样做在卡夫卡中是不切实际的。使用每个实体主题可以实现这一点，但是当存在数千或数百万个实体时，这是不可能的。这是由于Kafka/Zookeeper的技术限制。

以这种方式使用瞬态写模型的主要原因之一是使业务逻辑更改变得廉价且易于部署。

建议Kafka使用每类型主题(topic-per-type)，但这将需要为该类型的每个实体加载事件，以获取单个实体的事件。由于您无法通过日志位置来判断哪些事件属于哪个实体。即使使用快照从已知的日志位置开始，如果需要对快照进行结构更改以支持逻辑更改，则可能会产生大量事件。

缺乏冲突检测

Secondly, users can create race conditions due to concurrent requests against the same entity. It may be quite undesirable to save conflicting events and resolve them after the fact. So it is important to be able to prevent conflicting events. To scale request load, it is common to use stateless services while preventing write conflicts using conditional writes (only write if the last entity event was #x). A.k.a. Optimistic Concurrency. Kafka does not support optimistic concurrency. Even if it supported it at the topic level, it would need to be all the way down to the entity level to be effective. To use Kafka and prevent conflicting events, you would need to use a stateful, serialized writer (per "shard" or whatever is Kafka's equivalent) at the application level. This is a significant architectural requirement/restriction.

主要原因:设备存在问题

添加2021/09/29

Kafka is designed to solve giant-scale data problems. An app-controlled source of truth is a smaller scale, in-depth solution. Using event sourcing to good effect requires crafting events and streams to match the business processes. This usually has a much higher level of detail than would be generally useful to at-scale consumers. Consider if your bank statement contained an entry for every step of a bank's internal transaction processes. A single deposit or withdrawal could have many entries before it is confirmed to your account. The bank needs that level of detail to process transactions. But it's mostly inscrutable bank jargon (domain-specific language) to you, unusable for reconciling your account. Instead, the bank publishes separate events for consumers. These are course-grained summaries of each completed transaction. These summary events are what consumers know as "transactions" on their bank statement.

When I asked myself the same question as the OP, I wanted to know if Kafka was a scaling option for event sourcing. But perhaps a better question is whether it makes sense for my event sourced solution to operate at a giant scale. I can't speak to every case, but I think often it does not. When this scale enters the picture, like with the bank statement example, the granularity of events tends to be different. My event sourced system should probably publish course-grained events to the Kafka cluster to feed at-scale consumers rather than use Kafka as internal storage.

Scale can still be needed for event sourcing. Strategies differ depending on why. Often event streams have a "done" or "no-longer-useful" state. Archiving those streams is a good answer if event size/volume is a problem. Sharding is another option -- a perfect fit for regional- or tenant-isolated scenarios. In less siloed scenarios, when streams are arbitrarily related in a way that can cross shard boundaries, sharding is still the move (partition by stream ID). But there are no order guarantees across streams, which can make the event consumer's job harder. For example, the consumer may receive transaction events before it receives events describing the accounts involved. The first instinct is to "just use timestamps" to order received events. But it is still not possible to guarantee perfect occurrence order. Too many uncontrollable factors. Network hiccups, clock drift, cosmic rays, etc. Ideally you design the consumer to not require cross-stream dependencies. Have a strategy for temporarily missing data. Like progressive enhancement for data. If you really need the data to be unavailable instead of incomplete, use the same tactic. But keep the incomplete data in a separate area or marked unavailable until it's all filled in. You can also just attempt to process each event, knowing it may fail due to missing prerequisites. Put failed events in a retry queue, processing next events, and retry failed events later. But watch out for poison messages (events).

总结

你能强迫卡夫卡为一个应用程序控制的真相来源工作吗?当然，如果你足够努力，足够深入地融入。但这是个好主意吗?不。

---

每条评论更新

该评论已被删除，但问题是这样的:人们用什么来存储事件?

似乎大多数人都将自己的事件存储实现放在现有数据库之上。对于非分布式场景(如内部后端或独立产品)，如何创建基于sql的事件存储是有详细文档的。在各种各样的数据库之上还有很多可用的图书馆。还有一个EventStoreDB，它就是为此目的而构建的。

In distributed scenarios, I've seen a couple of different implementations. Jet's Panther project uses Azure CosmosDB, with the Change Feed feature to notify listeners. Another similar implementation I've heard about on AWS is using DynamoDB with its Streams feature to notify listeners. The partition key probably should be the stream id for best data distribution (to lessen the amount of over-provisioning). However, a full replay across streams in Dynamo is expensive (read and cost-wise). So this impl was also setup for Dynamo Streams to dump events to S3. When a new listener comes online, or an existing listener wants a full replay, it would read S3 to catch up first.

我目前的项目是一个多租户场景，我在Postgres的基础上开发了自己的项目。像Citus这样的东西似乎适合于可伸缩性，按帐篷+流进行分区。

Kafka在分布式场景中仍然非常有用。将每个服务的关键事件公开给其他服务并不是一个简单的问题。事件存储通常不是为此而构建的，但这正是Kafka所擅长的。每个服务都有自己的内部真相来源(可以是事件、BNF、图表等)，然后听Kafka来知道“外部”发生了什么。该服务将公共事件发布给Kafka，以通知外界它遇到的有趣的事情。

是的，Kafka在事件源模型特别是CQRS中工作得很好，但是你在为主题设置ttl时要小心，并始终记住Kafka不是为这个模型设计的，但是我们可以很好地使用它。

我认为你应该看看axon框架以及他们对Kafka的支持

Kafka是一个消息系统，它与事件存储有许多相似之处，但引用他们的介绍:

Kafka集群保留所有已发布的消息——不管它们是否 在可配置的时间段内被消耗。例如，如果 保留期设定为两天，然后是之后的两天 消息发布后，就可以使用了 将被丢弃以释放空间。卡夫卡的表演是有效的 常数的数据大小，所以保留大量的数据不是一个 问题。

因此，尽管消息可能被无限期地保留，但预期它们将被删除。这并不意味着您不能使用它作为事件存储，但使用其他东西可能会更好。请查看EventStoreDB作为替代方案。

更新

卡夫卡文档:

事件源是一种应用程序设计风格，其中状态更改被记录为按时间顺序排列的记录序列。Kafka对非常大的存储日志数据的支持使它成为这种风格的应用程序的绝佳后端。

更新2

使用Kafka进行事件来源的一个问题是所需主题的数量。通常在事件源中，每个实体(如用户、产品等)都有一个事件流(主题)。这样，可以通过重新应用流中的所有事件来重新构建实体的当前状态。每个Kafka主题由一个或多个分区组成，每个分区存储为文件系统上的一个目录。随着znode数量的增加，也会有来自ZooKeeper的压力。