buy the book ribbon

7: Aggregates and Consistency Boundaries

In this chapter, we’d like to revisit our domain model to talk about invariants and constraints, and see how our domain objects can maintain their own internal consistency, both conceptually and in persistent storage. We’ll discuss the concept of a consistency boundary and show how making it explicit can help us to build high-performance software without compromising maintainability.

Adding the Product aggregate shows a preview of where we’re headed: we’ll introduce a new model object called Product to wrap multiple batches, and we’ll make the old allocate() domain service available as a method on Product instead.

apwp 0701
Figure 1. Adding the Product aggregate

Why? Let’s find out.

Tip

The code for this chapter is in the appendix_csvs branch on GitHub:

git clone https://github.com/cosmicpython/code.git
cd code
git checkout appendix_csvs
# or to code along, checkout the previous chapter:
git checkout chapter_06_uow

Why Not Just Run Everything in a Spreadsheet?

What’s the point of a domain model, anyway? What’s the fundamental problem we’re trying to address?

Couldn’t we just run everything in a spreadsheet? Many of our users would be delighted by that. Business users like spreadsheets because they’re simple, familiar, and yet enormously powerful.

In fact, an enormous number of business processes do operate by manually sending spreadsheets back and forth over email. This "CSV over SMTP" architecture has low initial complexity but tends not to scale very well because it’s difficult to apply logic and maintain consistency.

Who is allowed to view this particular field? Who’s allowed to update it? What happens when we try to order –350 chairs, or 10,000,000 tables? Can an employee have a negative salary?

These are the constraints of a system. Much of the domain logic we write exists to enforce these constraints in order to maintain the invariants of the system. The invariants are the things that have to be true whenever we finish an operation.

Invariants, Constraints, and Consistency

The two words are somewhat interchangeable, but a constraint is a rule that restricts the possible states our model can get into, while an invariant is defined a little more precisely as a condition that is always true.

If we were writing a hotel-booking system, we might have the constraint that double bookings are not allowed. This supports the invariant that a room cannot have more than one booking for the same night.

Of course, sometimes we might need to temporarily bend the rules. Perhaps we need to shuffle the rooms around because of a VIP booking. While we’re moving bookings around in memory, we might be double booked, but our domain model should ensure that, when we’re finished, we end up in a final consistent state, where the invariants are met. If we can’t find a way to accommodate all our guests, we should raise an error and refuse to complete the operation.

Let’s look at a couple of concrete examples from our business requirements; we’ll start with this one:

An order line can be allocated to only one batch at a time.

— The business

This is a business rule that imposes an invariant. The invariant is that an order line is allocated to either zero or one batch, but never more than one. We need to make sure that our code never accidentally calls Batch.allocate() on two different batches for the same line, and currently, there’s nothing there to explicitly stop us from doing that.

Invariants, Concurrency, and Locks

Let’s look at another one of our business rules:

We can’t allocate to a batch if the available quantity is less than the quantity of the order line.

— The business

Here the constraint is that we can’t allocate more stock than is available to a batch, so we never oversell stock by allocating two customers to the same physical cushion, for example. Every time we update the state of the system, our code needs to ensure that we don’t break the invariant, which is that the available quantity must be greater than or equal to zero.

In a single-threaded, single-user application, it’s relatively easy for us to maintain this invariant. We can just allocate stock one line at a time, and raise an error if there’s no stock available.

This gets much harder when we introduce the idea of concurrency. Suddenly we might be allocating stock for multiple order lines simultaneously. We might even be allocating order lines at the same time as processing changes to the batches themselves.

We usually solve this problem by applying locks to our database tables. This prevents two operations from happening simultaneously on the same row or same table.

As we start to think about scaling up our app, we realize that our model of allocating lines against all available batches may not scale. If we process tens of thousands of orders per hour, and hundreds of thousands of order lines, we can’t hold a lock over the whole batches table for every single one—​we’ll get deadlocks or performance problems at the very least.

What Is an Aggregate?

OK, so if we can’t lock the whole database every time we want to allocate an order line, what should we do instead? We want to protect the invariants of our system but allow for the greatest degree of concurrency. Maintaining our invariants inevitably means preventing concurrent writes; if multiple users can allocate DEADLY-SPOON at the same time, we run the risk of overallocating.

On the other hand, there’s no reason we can’t allocate DEADLY-SPOON at the same time as FLIMSY-DESK. It’s safe to allocate two products at the same time because there’s no invariant that covers them both. We don’t need them to be consistent with each other.

The Aggregate pattern is a design pattern from the DDD community that helps us to resolve this tension. An aggregate is just a domain object that contains other domain objects and lets us treat the whole collection as a single unit.

The only way to modify the objects inside the aggregate is to load the whole thing, and to call methods on the aggregate itself.

As a model gets more complex and grows more entity and value objects, referencing each other in a tangled graph, it can be hard to keep track of who can modify what. Especially when we have collections in the model as we do (our batches are a collection), it’s a good idea to nominate some entities to be the single entrypoint for modifying their related objects. It makes the system conceptually simpler and easy to reason about if you nominate some objects to be in charge of consistency for the others.

For example, if we’re building a shopping site, the Cart might make a good aggregate: it’s a collection of items that we can treat as a single unit. Importantly, we want to load the entire basket as a single blob from our data store. We don’t want two requests to modify the basket at the same time, or we run the risk of weird concurrency errors. Instead, we want each change to the basket to run in a single database transaction.

We don’t want to modify multiple baskets in a transaction, because there’s no use case for changing the baskets of several customers at the same time. Each basket is a single consistency boundary responsible for maintaining its own invariants.

An AGGREGATE is a cluster of associated objects that we treat as a unit for the purpose of data changes.

— Eric Evans
Domain-Driven Design blue book

Per Evans, our aggregate has a root entity (the Cart) that encapsulates access to items. Each item has its own identity, but other parts of the system will always refer to the Cart only as an indivisible whole.

Tip
Just as we sometimes use _leading_underscores to mark methods or functions as "private," you can think of aggregates as being the "public" classes of our model, and the rest of the entities and value objects as "private."

Choosing an Aggregate

What aggregate should we use for our system? The choice is somewhat arbitrary, but it’s important. The aggregate will be the boundary where we make sure every operation ends in a consistent state. This helps us to reason about our software and prevent weird race issues. We want to draw a boundary around a small number of objects—the smaller, the better, for performance—that have to be consistent with one another, and we need to give this boundary a good name.

The object we’re manipulating under the covers is Batch. What do we call a collection of batches? How should we divide all the batches in the system into discrete islands of consistency?

We could use Shipment as our boundary. Each shipment contains several batches, and they all travel to our warehouse at the same time. Or perhaps we could use Warehouse as our boundary: each warehouse contains many batches, and counting all the stock at the same time could make sense.

Neither of these concepts really satisfies us, though. We should be able to allocate DEADLY-SPOONs and FLIMSY-DESKs at the same time, even if they’re in the same warehouse or the same shipment. These concepts have the wrong granularity.

When we allocate an order line, we’re interested only in batches that have the same SKU as the order line. Some sort of concept like GlobalSkuStock could work: a collection of all the batches for a given SKU.

It’s an unwieldy name, though, so after some bikeshedding via SkuStock, Stock, ProductStock, and so on, we decided to simply call it Product—after all, that was the first concept we came across in our exploration of the domain language back in [chapter_01_domain_model].

So the plan is this: when we want to allocate an order line, instead of Before: allocate against all batches using the domain service, where we look up all the Batch objects in the world and pass them to the allocate() domain service…​

apwp 0702
Figure 2. Before: allocate against all batches using the domain service
[plantuml, apwp_0702, config=plantuml.cfg]
@startuml
scale 4

hide empty members

package "Service Layer" as services {
    class "allocate()" as allocate {
    }
    hide allocate circle
    hide allocate members
}



package "Domain Model" as domain_model {

  class Batch {
  }

  class "allocate()" as allocate_domain_service {
  }
    hide allocate_domain_service circle
    hide allocate_domain_service members
}


package Repositories {

  class BatchRepository {
    list()
  }

}

allocate -> BatchRepository: list all batches
allocate --> allocate_domain_service: allocate(orderline, batches)

@enduml

…​we’ll move to the world of After: ask Product to allocate against its batches, in which there is a new Product object for the particular SKU of our order line, and it will be in charge of all the batches for that SKU, and we can call a .allocate() method on that instead.

apwp 0703
Figure 3. After: ask Product to allocate against its batches
[plantuml, apwp_0703, config=plantuml.cfg]
@startuml
scale 4

hide empty members

package "Service Layer" as services {
    class "allocate()" as allocate {
    }
}

hide allocate circle
hide allocate members


package "Domain Model" as domain_model {

  class Product {
    allocate()
  }

  class Batch {
  }
}


package Repositories {

  class ProductRepository {
    get()
  }

}

allocate -> ProductRepository: get me the product for this SKU
allocate --> Product: product.allocate(orderline)
Product o- Batch: has

@enduml

Let’s see how that looks in code form:

Our chosen aggregate, Product (src/allocation/domain/model.py)
class Product:

    def __init__(self, sku: str, batches: List[Batch]):
        self.sku = sku  #(1)
        self.batches = batches  #(2)

    def allocate(self, line: OrderLine) -> str:  #(3)
        try:
            batch = next(
                b for b in sorted(self.batches) if b.can_allocate(line)
            )
            batch.allocate(line)
            return batch.reference
        except StopIteration:
            raise OutOfStock(f'Out of stock for sku {line.sku}')
  1. Product’s main identifier is the `sku.

  2. Our Product class holds a reference to a collection of batches for that SKU.

  3. Finally, we can move the allocate() domain service to be a method on the Product aggregate.

Note
This Product might not look like what you’d expect a Product model to look like. No price, no description, no dimensions. Our allocation service doesn’t care about any of those things. This is the power of bounded contexts; the concept of a product in one app can be very different from another. See the following sidebar for more discussion.
Aggregates, Bounded Contexts, and Microservices

One of the most important contributions from Evans and the DDD community is the concept of bounded contexts.

In essence, this was a reaction against attempts to capture entire businesses into a single model. The word customer means different things to people in sales, customer service, logistics, support, and so on. Attributes needed in one context are irrelevant in another; more perniciously, concepts with the same name can have entirely different meanings in different contexts. Rather than trying to build a single model (or class, or database) to capture all the use cases, it’s better to have several models, draw boundaries around each context, and handle the translation between different contexts explicitly.

This concept translates very well to the world of microservices, where each microservice is free to have its own concept of "customer" and its own rules for translating that to and from other microservices it integrates with.

In our example, the allocation service has Product(sku, batches), whereas the ecommerce will have Product(sku, description, price, image_url, dimensions, etc…​). As a rule of thumb, your domain models should include only the data that they need for performing calculations.

Whether or not you have a microservices architecture, a key consideration in choosing your aggregates is also choosing the bounded context that they will operate in. By restricting the context, you can keep your number of aggregates low and their size manageable.

Once again, we find ourselves forced to say that we can’t give this issue the treatment it deserves here, and we can only encourage you to read up on it elsewhere. The Fowler link at the start of this sidebar is a good starting point, and either (or indeed, any) DDD book will have a chapter or more on bounded contexts.

One Aggregate = One Repository

Once you define certain entities to be aggregates, we need to apply the rule that they are the only entities that are publicly accessible to the outside world. In other words, the only repositories we are allowed should be repositories that return aggregates.

Note
The rule that repositories should only return aggregates is the main place where we enforce the convention that aggregates are the only way into our domain model. Be wary of breaking it!

In our case, we’ll switch from BatchRepository to ProductRepository:

Our new UoW and repository (unit_of_work.py and repository.py)

The ORM layer will need some tweaks so that the right batches automatically get loaded and associated with Product objects. The nice thing is, the Repository pattern means we don’t have to worry about that yet. We can just use our FakeRepository and then feed through the new model into our service layer to see how it looks with Product as its main entrypoint:

Service layer (src/allocation/service_layer/services.py)
def add_batch(
        ref: str, sku: str, qty: int, eta: Optional[date],
        uow: unit_of_work.AbstractUnitOfWork
):
    with uow:
        product = uow.products.get(sku=sku)
        if product is None:
            product = model.Product(sku, batches=[])
            uow.products.add(product)
        product.batches.append(model.Batch(ref, sku, qty, eta))
        uow.commit()


def allocate(
        orderid: str, sku: str, qty: int,
        uow: unit_of_work.AbstractUnitOfWork
) -> str:
    line = OrderLine(orderid, sku, qty)
    with uow:
        product = uow.products.get(sku=line.sku)
        if product is None:
            raise InvalidSku(f'Invalid sku {line.sku}')
        batchref = product.allocate(line)
        uow.commit()
    return batchref

What About Performance?

We’ve mentioned a few times that we’re modeling with aggregates because we want to have high-performance software, but here we are loading all the batches when we only need one. You might expect that to be inefficient, but there are a few reasons why we’re comfortable here.

First, we’re purposefully modeling our data so that we can make a single query to the database to read, and a single update to persist our changes. This tends to perform much better than systems that issue lots of ad hoc queries. In systems that don’t model this way, we often find that transactions slowly get longer and more complex as the software evolves.

Second, our data structures are minimal and comprise a few strings and integers per row. We can easily load tens or even hundreds of batches in a few milliseconds.

Third, we expect to have only 20 or so batches of each product at a time. Once a batch is used up, we can discount it from our calculations. This means that the amount of data we’re fetching shouldn’t get out of control over time.

If we did expect to have thousands of active batches for a product, we’d have a couple of options. For one, we could use lazy-loading for the batches in a product. From the perspective of our code, nothing would change, but in the background, SQLAlchemy would page through data for us. This would lead to more requests, each fetching a smaller number of rows. Because we need to find only a single batch with enough capacity for our order, this might work pretty well.

Exercise for the Reader

You’ve just seen the main top layers of the code, so this shouldn’t be too hard, but we’d like you to implement the Product aggregate starting from Batch, just as we did.

Of course, you could cheat and copy/paste from the previous listings, but even if you do that, you’ll still have to solve a few challenges on your own, like adding the model to the ORM and making sure all the moving parts can talk to each other, which we hope will be instructive.

You’ll find the code on GitHub. We’ve put in a "cheating" implementation in the delegates to the existing allocate() function, so you should be able to evolve that toward the real thing.

We’ve marked a couple of tests with @pytest.skip(). After you’ve read the rest of this chapter, come back to these tests to have a go at implementing version numbers. Bonus points if you can get SQLAlchemy to do them for you by magic!

If all else failed, we’d just look for a different aggregate. Maybe we could split up batches by region or by warehouse. Maybe we could redesign our data access strategy around the shipment concept. The Aggregate pattern is designed to help manage some technical constraints around consistency and performance. There isn’t one correct aggregate, and we should feel comfortable changing our minds if we find our boundaries are causing performance woes.

Optimistic Concurrency with Version Numbers

We have our new aggregate, so we’ve solved the conceptual problem of choosing an object to be in charge of consistency boundaries. Let’s now spend a little time talking about how to enforce data integrity at the database level.

Note
This section has a lot of implementation details; for example, some of it is Postgres-specific. But more generally, we’re showing one way of managing concurrency issues, but it is just one approach. Real requirements in this area vary a lot from project to project. You shouldn’t expect to be able to copy and paste code from here into production.

We don’t want to hold a lock over the entire batches table, but how will we implement holding a lock over just the rows for a particular SKU?

One answer is to have a single attribute on the Product model that acts as a marker for the whole state change being complete and to use it as the single resource that concurrent workers can fight over. If two transactions read the state of the world for batches at the same time, and both want to update the allocations tables, we force both to also try to update the version_number in the products table, in such a way that only one of them can win and the world stays consistent.

Sequence diagram: two transactions attempting a concurrent update on Product illustrates two concurrent transactions doing their read operations at the same time, so they see a Product with, for example, version=3. They both call Product.allocate() in order to modify a state. But we set up our database integrity rules such that only one of them is allowed to commit the new Product with version=4, and the other update is rejected.

Tip
Version numbers are just one way to implement optimistic locking. You could achieve the same thing by setting the Postgres transaction isolation level to SERIALIZABLE, but that often comes at a severe performance cost. Version numbers also make implicit concepts explicit.
apwp 0704
Figure 4. Sequence diagram: two transactions attempting a concurrent update on Product
[plantuml, apwp_0704, config=plantuml.cfg]
@startuml
scale 4

entity Model
collections Transaction1
collections Transaction2
database Database


Transaction1 -> Database: get product
Database -> Transaction1: Product(version=3)
Transaction2 -> Database: get product
Database -> Transaction2: Product(version=3)
Transaction1 -> Model: Product.allocate()
Model -> Transaction1: Product(version=4)
Transaction2 -> Model: Product.allocate()
Model -> Transaction2: Product(version=4)
Transaction1 -> Database: commit Product(version=4)
Database -[#green]> Transaction1: OK
Transaction2 -> Database: commit Product(version=4)
Database -[#red]>x Transaction2: Error! version is already 4

@enduml
Optimistic Concurrency Control and Retries

What we’ve implemented here is called optimistic concurrency control because our default assumption is that everything will be fine when two users want to make changes to the database. We think it’s unlikely that they will conflict with each other, so we let them go ahead and just make sure we have a way to notice if there is a problem.

Pessimistic concurrency control works under the assumption that two users are going to cause conflicts, and we want to prevent conflicts in all cases, so we lock everything just to be safe. In our example, that would mean locking the whole batches table, or using SELECT FOR UPDATE—we’re pretending that we’ve ruled those out for performance reasons, but in real life you’d want to do some evaluations and measurements of your own.

With pessimistic locking, you don’t need to think about handling failures because the database will prevent them for you (although you do need to think about deadlocks). With optimistic locking, you need to explicitly handle the possibility of failures in the (hopefully unlikely) case of a clash.

The usual way to handle a failure is to retry the failed operation from the beginning. Imagine we have two customers, Harry and Bob, and each submits an order for SHINY-TABLE. Both threads load the product at version 1 and allocate stock. The database prevents the concurrent update, and Bob’s order fails with an error. When we retry the operation, Bob’s order loads the product at version 2 and tries to allocate again. If there is enough stock left, all is well; otherwise, he’ll receive OutOfStock. Most operations can be retried this way in the case of a concurrency problem.

Read more on retries in [recovering_from_errors] and [footguns].

Implementation Options for Version Numbers

There are essentially three options for implementing version numbers:

  1. version_number lives in the domain; we add it to the Product constructor, and Product.allocate() is responsible for incrementing it.

  2. The service layer could do it! The version number isn’t strictly a domain concern, so instead our service layer could assume that the current version number is attached to Product by the repository, and the service layer will increment it before it does the commit().

  3. Since it’s arguably an infrastructure concern, the UoW and repository could do it by magic. The repository has access to version numbers for any products it retrieves, and when the UoW does a commit, it can increment the version number for any products it knows about, assuming them to have changed.

Option 3 isn’t ideal, because there’s no real way of doing it without having to assume that all products have changed, so we’ll be incrementing version numbers when we don’t have to.[1]

Option 2 involves mixing the responsibility for mutating state between the service layer and the domain layer, so it’s a little messy as well.

So in the end, even though version numbers don’t have to be a domain concern, you might decide the cleanest trade-off is to put them in the domain:

Our chosen aggregate, Product (src/allocation/domain/model.py)
class Product:

    def __init__(self, sku: str, batches: List[Batch], version_number: int = 0):  #(1)
        self.sku = sku
        self.batches = batches
        self.version_number = version_number  #(1)

    def allocate(self, line: OrderLine) -> str:
        try:
            batch = next(
                b for b in sorted(self.batches) if b.can_allocate(line)
            )
            batch.allocate(line)
            self.version_number += 1  #(1)
            return batch.reference
        except StopIteration:
            raise OutOfStock(f'Out of stock for sku {line.sku}')
  1. There it is!

Tip
If you’re scratching your head at this version number business, it might help to remember that the number isn’t important. What’s important is that the Product database row is modified whenever we make a change to the Product aggregate. The version number is a simple, human-comprehensible way to model a thing that changes on every write, but it could equally be a random UUID every time.

Testing for Our Data Integrity Rules

Now to make sure we can get the behavior we want: if we have two concurrent attempts to do allocation against the same Product, one of them should fail, because they can’t both update the version number.

First, let’s simulate a "slow" transaction using a function that does allocation and then does an explicit sleep:[2]

time.sleep can reproduce concurrency behavior (tests/integration/test_uow.py)
def try_to_allocate(orderid, sku, exceptions):
    line = model.OrderLine(orderid, sku, 10)
    try:
        with unit_of_work.SqlAlchemyUnitOfWork() as uow:
            product = uow.products.get(sku=sku)
            product.allocate(line)
            time.sleep(0.2)
            uow.commit()
    except Exception as e:
        print(traceback.format_exc())
        exceptions.append(e)

Then we have our test invoke this slow allocation twice, concurrently, using threads:

An integration test for concurrency behavior (tests/integration/test_uow.py)
def test_concurrent_updates_to_version_are_not_allowed(postgres_session_factory):
    sku, batch = random_sku(), random_batchref()
    session = postgres_session_factory()
    insert_batch(session, batch, sku, 100, eta=None, product_version=1)
    session.commit()

    order1, order2 = random_orderid(1), random_orderid(2)
    exceptions = []  # type: List[Exception]
    try_to_allocate_order1 = lambda: try_to_allocate(order1, sku, exceptions)
    try_to_allocate_order2 = lambda: try_to_allocate(order2, sku, exceptions)
    thread1 = threading.Thread(target=try_to_allocate_order1)  #(1)
    thread2 = threading.Thread(target=try_to_allocate_order2)  #(1)
    thread1.start()
    thread2.start()
    thread1.join()
    thread2.join()

    [[version]] = session.execute(
        "SELECT version_number FROM products WHERE sku=:sku",
        dict(sku=sku),
    )
    assert version == 2  #(2)
    [exception] = exceptions
    assert 'could not serialize access due to concurrent update' in str(exception)  #(3)

    orders = list(session.execute(
        "SELECT orderid FROM allocations"
        " JOIN batches ON allocations.batch_id = batches.id"
        " JOIN order_lines ON allocations.orderline_id = order_lines.id"
        " WHERE order_lines.sku=:sku",
        dict(sku=sku),
    ))
    assert len(orders) == 1  #(4)
    with unit_of_work.SqlAlchemyUnitOfWork() as uow:
        uow.session.execute('select 1')
  1. We start two threads that will reliably produce the concurrency behavior we want: read1, read2, write1, write2.

  2. We assert that the version number has been incremented only once.

  3. We can also check on the specific exception if we like.

  4. And we double-check that only one allocation has gotten through.

Enforcing Concurrency Rules by Using Database Transaction Isolation Levels

To get the test to pass as it is, we can set the transaction isolation level on our session:

Set isolation level for session (src/allocation/service_layer/unit_of_work.py)
DEFAULT_SESSION_FACTORY = sessionmaker(bind=create_engine(
    config.get_postgres_uri(),
    isolation_level="REPEATABLE READ",
))
Tip
Transaction isolation levels are tricky stuff, so it’s worth spending time understanding the Postgres documentation.[3]

Pessimistic Concurrency Control Example: SELECT FOR UPDATE

There are multiple ways to approach this, but we’ll show one. SELECT FOR UPDATE produces different behavior; two concurrent transactions will not be allowed to do a read on the same rows at the same time:

SELECT FOR UPDATE is a way of picking a row or rows to use as a lock (although those rows don’t have to be the ones you update). If two transactions both try to SELECT FOR UPDATE a row at the same time, one will win, and the other will wait until the lock is released. So this is an example of pessimistic concurrency control.

Here’s how you can use the SQLAlchemy DSL to specify FOR UPDATE at query time:

SQLAlchemy with_for_update (src/allocation/adapters/repository.py)
    def get(self, sku):
        return self.session.query(model.Product) \
                           .filter_by(sku=sku) \
                           .with_for_update() \
                           .first()

This will have the effect of changing the concurrency pattern from

to

Some people refer to this as the "read-modify-write" failure mode. Read "PostgreSQL Anti-Patterns: Read-Modify-Write Cycles" for a good overview.

We don’t really have time to discuss all the trade-offs between REPEATABLE READ and SELECT FOR UPDATE, or optimistic versus pessimistic locking in general. But if you have a test like the one we’ve shown, you can specify the behavior you want and see how it changes. You can also use the test as a basis for performing some performance experiments.

Wrap-Up

Specific choices around concurrency control vary a lot based on business circumstances and storage technology choices, but we’d like to bring this chapter back to the conceptual idea of an aggregate: we explicitly model an object as being the main entrypoint to some subset of our model, and as being in charge of enforcing the invariants and business rules that apply across all of those objects.

Choosing the right aggregate is key, and it’s a decision you may revisit over time. You can read more about it in multiple DDD books. We also recommend these three online papers on effective aggregate design by Vaughn Vernon (the "red book" author).

Aggregates: the trade-offs has some thoughts on the trade-offs of implementing the Aggregate pattern.

Table 1. Aggregates: the trade-offs
Pros Cons
  • Python might not have "official" public and private methods, but we do have the underscores convention, because it’s often useful to try to indicate what’s for "internal" use and what’s for "outside code" to use. Choosing aggregates is just the next level up: it lets you decide which of your domain model classes are the public ones, and which aren’t.

  • Modeling our operations around explicit consistency boundaries helps us avoid performance problems with our ORM.

  • Putting the aggregate in sole charge of state changes to its subsidiary models makes the system easier to reason about, and makes it easier to control invariants.

  • Yet another new concept for new developers to take on. Explaining entities versus value objects was already a mental load; now there’s a third type of domain model object?

  • Sticking rigidly to the rule that we modify only one aggregate at a time is a big mental shift.

  • Dealing with eventual consistency between aggregates can be complex.

Aggregates and Consistency Boundaries Recap

Aggregates are your entrypoints into the domain model

By restricting the number of ways that things can be changed, we make the system easier to reason about.

Aggregates are in charge of a consistency boundary

An aggregate’s job is to be able to manage our business rules about invariants as they apply to a group of related objects. It’s the aggregate’s job to check that the objects within its remit are consistent with each other and with our rules, and to reject changes that would break the rules.

Aggregates and concurrency issues go together

When thinking about implementing these consistency checks, we end up thinking about transactions and locks. Choosing the right aggregate is about performance as well as conceptual organization of your domain.

Part I Recap

Do you remember A component diagram for our app at the end of Part I, the diagram we showed at the beginning of [part1] to preview where we were heading?

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Figure 5. A component diagram for our app at the end of Part I

So that’s where we are at the end of Part I. What have we achieved? We’ve seen how to build a domain model that’s exercised by a set of high-level unit tests. Our tests are living documentation: they describe the behavior of our system—​the rules upon which we agreed with our business stakeholders—​in nice readable code. When our business requirements change, we have confidence that our tests will help us to prove the new functionality, and when new developers join the project, they can read our tests to understand how things work.

We’ve decoupled the infrastructural parts of our system, like the database and API handlers, so that we can plug them into the outside of our application. This helps us to keep our codebase well organized and stops us from building a big ball of mud.

By applying the dependency inversion principle, and by using ports-and-adapters-inspired patterns like Repository and Unit of Work, we’ve made it possible to do TDD in both high gear and low gear and to maintain a healthy test pyramid. We can test our system edge to edge, and the need for integration and end-to-end tests is kept to a minimum.

Lastly, we’ve talked about the idea of consistency boundaries. We don’t want to lock our entire system whenever we make a change, so we have to choose which parts are consistent with one another.

For a small system, this is everything you need to go and play with the ideas of domain-driven design. You now have the tools to build database-agnostic domain models that represent the shared language of your business experts. Hurrah!

Note
At the risk of laboring the point—​we’ve been at pains to point out that each pattern comes at a cost. Each layer of indirection has a price in terms of complexity and duplication in our code and will be confusing to programmers who’ve never seen these patterns before. If your app is essentially a simple CRUD wrapper around a database and isn’t likely to be anything more than that in the foreseeable future, you don’t need these patterns. Go ahead and use Django, and save yourself a lot of bother.

In Part II, we’ll zoom out and talk about a bigger topic: if aggregates are our boundary, and we can update only one at a time, how do we model processes that cross consistency boundaries?


1. Perhaps we could get some ORM/SQLAlchemy magic to tell us when an object is dirty, but how would that work in the generic case—for example, for a CsvRepository?
2. time.sleep() works well in our use case, but it’s not the most reliable or efficient way to reproduce concurrency bugs. Consider using semaphores or similar synchronization primitives shared between your threads to get better guarantees of behavior.
3. If you’re not using Postgres, you’ll need to read different documentation. Annoyingly, different databases all have quite different definitions. Oracle’s SERIALIZABLE is equivalent to Postgres’s REPEATABLE READ, for example.