Journey 8: Epilogue: Lessons Learned
How good was our map? How far did we get? What did we learn? Did we get lost?
"This land may be profitable to those that will adventure it." Henry Hudson
This chapter summarizes the findings from our journey. It highlights what we feel were the most significant lessons we learned along the way, suggests some things we would do differently if we were embarking on the journey with our newfound knowledge, and points out some future paths for the Contoso Conference Management System.
You should bear in mind that this summary reflects our specific journey; not all of these findings will necessarily apply to your own CQRS journeys. For example, one of our goals was to explore how to implement the CQRS pattern in an application that is deployed to Windows Azure and that makes use of the scalability and reliability of the cloud. For our project, this meant using messaging to enable multiple role types and instances to communicate with each other. It may be that your project does not require multiple role instances or is not deployed to the cloud and therefore may not need to use messaging so extensively (or at all).
We hope these findings do prove useful, especially if you are just starting out with CQRS and event sourcing.
What did we learn?
This section describes the key lessons we learned. They are not presented in any particular order.
At the start of our journey, one of our notions about the CQRS pattern was that by separating the read and write sides of the application we could optimize each for performance. This perspective is shared by many in the CQRS community, for example:
"CQRS taught me that I can optimize reads and writes separately and I don't have to manually denormalize into flat tables all the time."
— Kelly Sommers – CQRS Advisor
This was borne out in practice during our journey and we benefited significantly from this separation when we did need to solve a performance issue.
During the last stage of our journey, testing revealed a set of performance issues in our application. When we investigated them, it turned out they had less to do with the way we had implemented the CQRS pattern and more to do with the way we were using our infrastructure. Discovering the root cause of these problems was the hard part; with so many moving parts in the application, getting the right tracing and the right data for analysis was the challenge. Once we identified the bottlenecks, fixing them turned out to be relatively easy, largely because of the way the CQRS pattern enables you to clearly separate different elements of the system, such as reads and writes. Although the separation of concerns that results from implementing the CQRS pattern can make it harder to identify an issue, once you have identified one, it is not only easier to fix, but also easier to prevent its return. The decoupled architecture makes it simpler to write unit tests that reproduce issues.
The challenges we encountered in tackling the performance issues in the system had more to do with the fact that our system is a distributed, message-based system than the fact that it implements the CQRS pattern.
Chapter 7, "Adding Resilience and Optimizing Performance" provides more information about the ways we addressed the performance issues in the system and makes some suggestions about additional changes that we would like to make, but didn't have time to implement.
Implementing a message-driven system is far from simple
Our approach to infrastructure on this project was to develop it as needed during the journey. We didn't anticipate (and had no forewarning of) how much time and effort we would need to create the robust infrastructure that our application required. We spent at least twice as much time as we originally planned on many development tasks because we continued to uncover additional infrastructure-related requirements. In particular, we learned that having a robust event store from the beginning is essential. Another key idea we took away from the experience is that all I/O on the message bus should be asynchronous.
|Although our event store is not production-ready, the current implementation gives a good indication of the type of issues you should address if you decide to implement your own event store.|
Although our application is not large, it illustrated clearly to us the importance of having end-to-end tracing available, and the value of tools that help us understand all of the message flows in the system. Chapter 4, "Extending and Enhancing the Orders and Registrations Bounded Context," describes the value of tests in helping us understand the system, and discusses the messaging intermediate language (MIL) created by Josh Elster, one of our advisors.
|It would also help if we had a standard notation for messaging that would help us communicate some of the issues with the domain experts and people outside of the core team.|
In summary, many of the issues we met along the way were not related specifically to the CQRS pattern, but were more related to the distributed, message-driven nature of our solution.
|We found that partitioning our service bus by using different topics to transport events published by different aggregates helped us to achieve scalability. For more information, see Chapter 7, "Adding Resilience and Optimizing Performance." Also, see these blog posts: "Windows Azure Storage Abstractions and their Scalability Targets" and "Best Practices for Performance Improvements Using Service Bus Brokered Messaging."|
The cloud has challenges
Although the cloud provides many benefits, such as reliable, scalable, off-the-shelf services that you can provision with just a few mouse clicks, cloud environments also introduce some challenges:
- You may not be able to use transactions everywhere you want them because the distributed nature of the cloud makes ACID (atomicity, consistency, isolation, durability) transactions impractical in many scenarios. Therefore, you need to understand how to work with eventual consistency. For examples, see Chapter 5, "Preparing for the V1 Release," and the section Options to reduce the delay in the UI in Chapter 7, "Adding Resilience and Optimizing Performance."
- You may want to re-examine your assumptions about how to organize your application into different tiers. For example, see the discussion around in-process, synchronous commands in Chapter 7, "Adding Resilience and Optimizing Performance."
- You must take into account not only the latency between the browser or on-premises environment and the cloud, but also the latency between the different parts of your system that are running in the cloud.
- You must take into account transient errors and be aware of how different cloud services might implement throttling. If your application uses several cloud services that might be throttled, you must coordinate how your application handles being throttled by different services at different times.
|We found that having a single bus abstraction in our code obscured the fact that some messages are handled locally in-process and some are handled in a different role instance. To see how this is implemented, look at the ICommandBus interface and the CommandBus and SynchronousCommandBusDecorator classes. Chapter 7, "Adding Resilience and Optimizing Performance" includes a discussion of the SynchronousCommandBusDecorator class.|
A complex cloud environment can make it harder to run quick tests during development. A local test environment may not mimic the behavior of the cloud exactly, especially with respect to performance and throttling.
|The multiple build configurations in our Visual Studio solution were partially designed to address this, but also to help people downloading and playing with the code to get started quickly.|
CQRS is different
At the start of our journey we were warned that although the CQRS pattern appears to be simple, in practice it requires a significant shift in the way you think about many aspects of the project. Again, this was borne out by our experiences during the journey. You must be prepared to throw away many assumptions and preconceived ideas, and you will probably need to implement the CQRS pattern in several bounded contexts before you begin to fully understand the benefits you can derive from the pattern.
An example of this is the concept of eventual consistency. If you come from a relational database background and are accustomed to the ACID properties of transactions, then embracing eventual consistency and understanding its implications at all levels in the system is a big step to take. Chapter 5, "Preparing for the V1 Release" and Chapter 7, "Adding Resilience and Optimizing Performance" both discuss eventual consistency in different areas of the system.
In addition to being different from what you might be familiar with, there is also no single correct way to implement the CQRS pattern. We made more false starts on pieces of functionality and estimated poorly how long things would take due to our unfamiliarity with the pattern and approach. As we become more comfortable with the approach, we hope to become faster at identifying how to implement the pattern in specific circumstances and improve the accuracy of our estimates.
|The CQRS pattern is conceptually simple; the devil is in the details.|
Another situation in which we took some time to understand the CQRS approach and its implications was during the integration between our bounded contexts. Chapter 5, "Preparing for the V1 Release," includes a detailed discussion of how the team approached the integration issue between the Conference Management and the Orders and Registrations bounded contexts. This part of the journey uncovered some additional complexity that relates to the level of coupling between bounded contexts when you use events as the integration mechanism. Our assumption that events should only contain information about the change in the aggregate or the bounded context proved to be unhelpful; events can contain additional information that is useful to one or more subscribers and helps to reduce the amount of work that a subscriber must perform.
The CQRS pattern introduces additional considerations for how to partition your system. Not only do you need to consider how to partition your system into tiers, but also how to partition your system into bounded contexts, some of which will contain implementations of the CQRS pattern. We revised some of our assumptions about tiers in the last stage of our journey, bringing some processing into our web roles from the worker role where it was originally done. This is described in Chapter 7, "Adding Resilience and Optimizing Performance" in the section that discusses moving some command processing in-process. Partitioning the system into bounded contexts should be done based on your domain model: each bounded context has its own domain model and ubiquitous language. Once you have identified your bounded contexts, you can then identify in which bounded contexts to implement the CQRS pattern. This affects how and where you need to implement integration between these isolated bounded contexts. Chapter 2, "Decomposing the Domain," introduces our decisions for the Contoso Conference Management System.
|A single process (role instance in our deployment) can host multiple bounded contexts. In this scenario, you don't necessarily need to use a service bus for the bounded contexts to communicate with each other.|
Implementing the CQRS pattern is more complex than implementing a traditional (create, read, update, delete) CRUD-style system. For this project, there was also the overhead of learning about CQRS for the first time, and creating a distributed, asynchronous messaging infrastructure. Our experiences during the journey have clearly confirmed to us why the CQRS pattern is not a top-level architecture. You must be sure that the costs associated with implementing a CQRS-based bounded context with this level of complexity are worth it; in general, it is in high-contention, collaborative domains that you will see the benefits of the CQRS pattern.
|Analyzing the business requirements, building a useful model, maintaining the model, expressing it in code, and implementing it using the CQRS pattern all take time and cost money. If this is the first time you have implemented the CQRS pattern, you'll also have the overhead of investing in your infrastructure elements such as message buses and event stores.|
Event sourcing and transaction logging
We had some debate about whether or not event sourcing and transaction logging amount to the same thing: they both create a record of what happened, and they both enable you to recreate the state of your system by replaying the historical data. The conclusion was that the distinguishing feature is that events capture intent in addition to recording the facts of what happened. For more detail on what we mean by intent, see Chapter 4, "A CQRS and ES Deep Dive," in the Reference Guide.
Involving the domain expert
Implementing the CQRS pattern encourages involvement of the domain expert. The pattern enables you to separate out the domain on the write side and the reporting requirements on the read side and to separate these from infrastructure concerns. This separation makes it easier to involve the domain expert in those aspects of the system where his expertise is most valuable. The use of domain-driven design concepts such as bounded contexts and the ubiquitous language also help to focus the team and to foster clear communication with the domain expert.
Our acceptance tests proved to be an effective way to involve the domain expert and capture his knowledge. Chapter 4, "Extending and Enhancing the Orders and Registrations Bounded Context," describes this testing approach in detail.
|As a side-effect, these acceptance tests also contributed to our ability to handle our pseudo-production releases quickly because they enabled us to run a full set of tests at the UI level to verify the behavior of the system in addition to the unit and integration tests.|
In addition to helping the team define the functional requirements of the system, the domain expert should also be involved in evaluating the trade-offs between consistency, availability, durability, and costs. For example, the domain expert should help to identify when a manual process is acceptable and what level of consistency is required in different areas of the system.
|Developers have a tendency to try to lock everything down to transactions to guarantee full consistency, but sometimes it's just not worth the effort.|
When to use CQRS
Now that we are at the end of our journey, we can suggest some of the criteria you should evaluate to determine whether or not you should consider implementing the CQRS pattern in one or more bounded contexts in your application. The more of these questions you can answer positively, the more likely it is that applying the CQRS pattern to a given bounded context will benefit your solution:
- Does the bounded context implement an area of business functionality that is a key differentiator in your market?
- Is the bounded context collaborative in nature with elements that are likely to have high levels of contention at run time? In other words, do multiple users compete for access to the same resources?
- Is the bounded context likely to experience ever-changing business rules?
- Do you have a robust, scalable messaging and persistence infrastructure already in place?
- Is scalability one of the challenges facing this bounded context?
- Is the business logic in the bounded context complex?
- Are you clear about the benefits that the CQRS pattern will bring to this bounded context?
|These are rules of thumb, not hard and fast rules.|
What would we do differently if we started over?
This section is a result of our reflection on our journey and identifies some things we'd do differently and some other opportunities we'd like to pursue if we were starting over with the knowledge of the CQRS pattern and event sourcing that we now have.
Start with a solid infrastructure for messaging and persistence
We'd start with a solid messaging and persistence infrastructure. The approach we took, starting simple and building up the infrastructure as required meant that we built up technical debt during the journey. We also found that taking this approach meant that in some cases, the choices we made about the infrastructure affected the way we implemented the domain.
|From the perspective of the journey, if we had started with a solid infrastructure, we would have had time to tackle some of the more complex parts of the domain such as wait-listing.|
Starting with a solid infrastructure would also enable us to start performance testing earlier. We would also do some more research into how other people do their performance testing on CQRS-based systems, and seek out performance benchmarks on other systems such as Jonathan Oliver's EventStore.
One of the reasons we took the approach that we did was the advice we received from our advisors: "Don't worry about the infrastructure."
Leverage the capabilities of the infrastructure more
Starting with a solid infrastructure would also allow us to make more use of the capabilities of the infrastructure. For example, we use the identity of the message originator as the value of the session ID in Windows Azure Service Bus when we publish an event, but this is not always the best use of the session ID from the perspective of the parts of the system that process the event.
As part of this, we'd also investigate how the infrastructure could support other special cases of eventual consistency such as timing consistency, monotonic consistency, "read my writes," and self-consistency.
Another idea we'd like to explore is the use of the infrastructure to support migration between versions. Instead of treating migration in an ad-hoc manner for each version, we could consider using a message-based or real-time communication process to coordinate bringing the new version online.
Adopt a more systematic approach to implementing process managers
We began to implement our process manager very early in the journey and were still hardening it and ensuring that its behavior was idempotent in the last stage of the journey. Again, starting with some solid infrastructure support for process managers to make them more resilient would have helped us. However, if we were starting over, we'd also wait to implement a process manager until a later stage in the journey rather than diving straight in.
We began implementing the RegistrationProcessManager class during the first stage in our journey. The initial implementation is described in Chapter 3, "Orders and Registrations Bounded Context." We made changes to this process manager during every subsequent stage of our journey.
Partition the application differently
We would think more carefully at the start of the project about the tiering of the system. We found that the way we partitioned the application into web roles and worker roles as described in Chapter 4, "Extending and Enhancing the Orders and Registrations Bounded Context," was not optimal, and in the last stage of the journey, in Chapter 7, "Adding Resilience and Optimizing Performance," we made some major changes to this architecture as part of the performance optimization effort.
For example, as a part of this reorganization in the last stage of the journey, we introduced synchronous command processing in the web application alongside the pre-existing asynchronous command processing.
Organize the development team differently
The approach we took to learning about the CQRS pattern was to iterate—develop, go back, discuss, and then refactor. However, we may have learned more by having several developers work independently on the same feature and then compare the result; that might have uncovered a broader set of solutions and approaches.
Evaluate how appropriate the domain and the bounded contexts are for the CQRS pattern
We would like to start with a clearer set of heuristics, such as those outlined earlier in this chapter, to determine whether or not a particular bounded context will benefit from the CQRS pattern. We might have learned more if we had focused on a more complex area of the domain such as wait-listing instead of on the Orders and Registrations and Payments bounded contexts.
Plan for performance
We would address performance issues much earlier in the journey. In particular, we would:
- Set clear performance goals ahead of time.
- Run performance tests much earlier in the journey.
- Use larger and more realistic loads.
We didn't do any performance testing until the last stage of our journey. For a detailed discussion of the issues we found and how we addressed them, see Chapter 7, "Adding Resilience and Optimizing Performance."
During the last stage of our journey, we introduced some partitioning on the Service Bus to improve the throughput of events. This partitioning is done based on the publisher of the event, so events published by one aggregate type go to one topic. We'd like to extend this to use multiple topics where we currently have one, perhaps partitioning based on a hash of the order ID in the message (this approach is often referred to as sharding). This would enable greater scale-out for the application.
Think about the UI differently
We felt that the way our UI interacts with the write and read models, and how it handles eventual consistency worked well and met the business requirements. In particular, the way that the UI checks whether a reservation is likely to succeed and modifies its behavior accordingly and the way that the UI allows the user to continue entering data while it waits for the read model to be updated. For more details about how the current solution works, see the section "Optimizing the UI" in Chapter 7, "Adding Resilience and Optimizing Performance."
We'd like to investigate other ways to avoid waiting in the UI unless it's absolutely necessary, perhaps by using browser push techniques. The UI in the current system still needs to wait, in some places, for asynchronous updates to take place against the read model.
Explore some additional benefits of event sourcing
We found during the third stage of our journey, described in Chapter 5, "Preparing for the V1 Release," that modifying the Orders and Registrations bounded context to use event sourcing helped to simplify the implementation of this bounded context, in part because it was already using a large number of events.
In the current journey, we didn't get a chance to explore the further promises of flexibility and the ability to mine past events for new business insights from event sourcing. However, we did ensure that the system persists copies of all events (not just those that are needed to rebuild the state of the aggregates) and commands to enable these types of scenarios in the future.
|It would also be interesting to investigate whether the ability to mine past event streams for new business insights is easier to achieve with event sourcing or other technologies such as database transaction logs or the StreamInsight feature of SQL Server.|
Explore the issues associated with integrating bounded contexts
In our V3 release, all of the bounded contexts are implemented by same core development team. We would like to investigate how easy it is, in practice, to integrate a bounded context implemented by a different development team with the existing system.
This is a great opportunity for you to contribute to the learning experience: go ahead and implement another bounded context (see the outstanding stories in the product backlog), integrate it into the Contoso Conference Management System, and write another chapter of the journey describing your experiences.