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Have you been able to replace your database implementation transparently because of the use of the repository pattern? While this is a controversial question and topic, I will explain why it doesn’t need to be. Sure, you’re creating an abstraction, but an abstraction around what?

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Repository Pattern

First, we need to agree on the definition of the Repository Pattern. I will use the definition in the Patterns of Enterprise Application Architecture.

Conceptually, a Repository encapsulates the set of objects persisted in a data store and the operations performed over them, providing a more object-oriented view of the persistence layer.

I think everyone can agree on that definition, but I think some of the confusion is around what a “set of objects” means. What type of objects are we talking about?

I’ve found two ways people think about objects in a repository. The first is thinking of them as a data model. It represents how you persist data in your data store and the mapping that goes along.

The second group of people comes at it from a Domain Driven Design perspective and thinks about a domain model of aggregates composed of entities and value objects.

There are generally, these two sides view a repository differently, hence why the use of a repository can become a hot topic.

Differences

On the surface, you might think of an aggregate or a data model with some hierarchy as being the same. But there’s a difference.

In the example above, I have a repository that fetches a basket and its collection of basket items. This object model hierarchy sure looks like an aggregate.

However, the difference between an aggregate and a data model is behaviors.

Behavior

With an aggregate, you’re exposing behaviors to make state changes. An aggregate represents a consistency boundary. The root entity is the entry point to invoke behaviors that will make changes to the state of the aggregate as a whole. Because of this, it’s also the place where business rules are enforced.

Here’s an example.

While this is a trivial example, AddItem ensures that only one item in our _items collection exists for a catalogItemId. Because items themselves don’t know about other items, this is why our Basket is the aggregate root.

If we worked with a data model, we could have this logic in a transaction script or somewhere higher up the call stack, but the point of our aggregate is to hold all the logic to where we make state changes.

Data

So, if you’re making state changes and need to encapsulate those behaviors within an aggregate, what situation would you want a data model?

Typically, you want to query data for UI purposes. In that case, we’ve established a difference between needing to make state changes and querying data. Guess where we’ve landed? You guessed it, CQRS.

So, when you start thinking about the difference between the two, why would I want to use a repository that’s returning an aggregate when I don’t need to make state changes?

Sure, you can use an aggregate for queries if you expose the data within them, but do you need all that data? Your aggregate is backed by all the data used for state changes to ensure consistency. Queries are often specialized for their use case. They don’t always need all the data. Ultimately, you’d often be over-fetching data if you returned an aggregate for query purposes.

You’ll likely conclude that then you only need a repository for commands. If a repository returns an aggregate, and an aggregate is used for state changes, that only happens when executing a command.

Queries are a use-case. As mentioned, it is often very specialized for a specific need to fetch specific data. Using a repository to over-fetch data and then having to transform it into you use case can add a lot of indirection and complexity.

Let your queries define how they retrieve data. Does this mean not abstracting your data access? It can. Or it might not. You make that decision. Now, you’re talking about coupling between queries and your database. Do you need an abstraction if you have ten queries coupled to your database? Do you need an abstraction if you have 1000 queries coupled to your database? Your answers may differ.

Repository Pattern

Thanks to David Fowler for tweeting this, as it inspired this video/blog. The answers to this question are going to be all over the place. Some don’t abstract or use the repository pattern because they think or have never replaced an underlying database. Another group of people always abstract and use the repository pattern because they have replaced their underlying database.

I think the key thing to be thinking about, however, is your use case. If you’re using a repository for commands and returning aggregates, that repository/abstraction will look very different than an abstraction that’s for querying data in a very specific way and/or doing transformations while querying the database and not in memory once data is returned.

A repository for an aggregate might have only a few methods. GetById, Save. There may be a few more, but you know exactly the aggregate you need to call. You’re not querying and filtering by other data elements, it’s generally by the key.

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Developer-level members of my Patreon or YouTube channel get access to a private Discord server to chat with other developers about Software Architecture and Design and access to source code for any working demo application I post on my blog or YouTube. Check out my Patreon or YouTube Membership for more info.

Related Links

• Aggregate (DDD) isn’t hierarchy & relationships
• Aggregate Design: Using Invariants as a Guide
• All our aggregates are wrong

When should you use blocking or non-blocking API calls? Blocking synchronous communication vs non-blocking asynchronous communication? Should commands fire and forget? What’s the big deal of making blocking synchronous calls between services?

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Check out my YouTube channel where I post all kinds of content that accompanies my posts including this video showing everything in this post.

Communication

There are 3 different forms of communication that I’m generally thinking of. Commands, Queries, and Events. I’m going to cover all three and the pros/cons of using blocking synchronous or non-blocking asynchronous with each of them.

Commands

With commands, it’s all about users’/clients expectations. When the command is performed, do they expect to get a result immediately and the execution be fully complete? Because of this expectation, often times commands are blocking synchronous calls.

If the client was requesting a state change to the service, the expectation is that if they want to then perform a query afterward, they would expect to read their write. There would be no eventual consistency involved. Blocking synchronous calls can make sense here.

Where asynchronous commands can be useful is if the execution time is going to be longer than the user might want to wait. This is an issue with blocking calls as the client could be waiting to get a response back to know if the command is completed. As an example, if the command had to fetch and mutate a lot of data in a database that could take a long time. Or perhaps it has to send 1000s of emails which could take a lot of time to execute.

Another situation where async commands are helpful is when you want to capture the command and just tell the user it has been accepted. You can then process the command asynchronously and then let the user know asynchronously that it has been completed. An example of this is placing an Order on an e-commerce site. When the user places an order, it doesn’t immediately create the order and charge their credit card, it simply creates a command that is placed on a queue.

Once the message has been sent to the broker, the client is then told the order has been accepted (not created). Then asynchronously the process of creating and charging the customer’s credit card can occur.

Once the asynchronous work has been completed, typically you’d receive an email letting you know the order has been created. Depending on your context and system, something like WebSockets can also be helpful for pushing notifications to the client to let them know when asynchronous work has been completed. Check out my post on Real-Time Web by leveraging Event Driven Architecture

Queries

Almost all queries are naturally request-response. They will be blocking calls by default because the purpose of a query is to get a result.

This is pretty straightforward when a service is interacting with a database or any of its own infrastructure within a logical boundary, such as a cache.

Where this totally falls apart is when you have services that are physically deployed independently and they are calling each other using blocking synchronous communication (eg, HTTP, gRPC).

There are many different issues with service-to-service direct RPC style communication. I cover a bunch of them in my post REST APIs for Microservices? Beware!

One issue is latency. Because you don’t have really any visibility into the entire call stack without something like distributed tracing, you can end up with very slow queries because there are so many RPC calls being made between services to build up the result of a query.

The second issue and the worst offender is tight coupling. If a service is failing or unavailable, your entire system could be down if you aren’t handling all these failures appropriately. If Service D has availability issues, this will affect service A indirectly because it’s calling service C which requires service D.

Congratulations, you’ve built a distributed monolith! Or as I like to call it, a distributed turd pile.

So how do you handle queries since they are naturally blocking and synchronous? Well, what you’re trying to do is view composition.

There are different ways to tackle this problem, one of which is to have a gateway or BFF (Backend-for-Frontend) that does the composition. Meaning it is the one responsible for making the calls to all the services and then composing the results back out to the client.

This isn’t the only way to do UI/ViewModel composition. There are other approaches such as moving the logic to the client or having each logical boundary own various UI components. Regardless, each service is independent and does not make calls to other services.

As for availability, if Service C is unavailable, that portion of the ViewModel/UI can be handled explicitly.

There is one option that I won’t go into in this post, which is asynchronous request-reply. This allows you to leverage a message broker (queues) but still have the client/caller block until it gets a reply message. Check out my post Asynchronous Request-Response Pattern for Non-Blocking Workflows.

Events

It may seem obvious (or not) but events generally should be processed asynchronously. The biggest reason why is because of isolation.

If you publish an event in a blocking synchronous way, this means that the service calling the consumers when the event occurs has to deal with the failures of the consumer.

If there were two consumers, and the first consumer handled the event correctly, but the second consumer failed, the service/caller would need to handle that failure so it doesn’t bubble up back to the client. Also without any isolation, if the consumer takes a long time to process the event, this affects the entire blocking call from the client. The more consumers, the longer the entire call will take form the client. You want each consumer to execute/handle the event in isolation.

The publisher should be unaware of the consumers.

It doesn’t care if there are any consumers or if there are 100. It simply publishes events.

Blocking or Non-Blocking API calls?

Hopefully, this illustrated the various ways that blocking synchronous and non-blocking asynchronous can be applied to Commands, Queries, and Events.

Join!

Developer-level members of my YouTube channel or Patreon get access to a private Discord server to chat with other developers about Software Architecture and Design as well as access to source code for any working demo application that I post on my blog or YouTube. Check out the YouTube Membership or Patreon for more info.

You also might like

• REST APIs for Microservices? Beware!
• Asynchronous Request-Response Pattern for Non-Blocking Workflows
• RPC vs. Messaging – which is faster?