Your problem isn't the monolith.
It's the data.

Penultimate Markets, 2022. One deployment. 120 engineers. 8 markets.

01·Ownership

Aggregates own their invariants.

DIAGRAM: Order aggregate root

Inside boundary: OrderLine, ShippingAddress, Money
Outside boundary (referenced as IDs only): CustomerId, ProductId

01·Ownership

What this looks like in practice

Somewhere in the tax codebase…

var order = orderRepo.findById(id);
if (order.state() != DRAFT)
    throw new IllegalStateException();

var subtotal = order.lines().stream()
    .map(l -> l.price().multiply(l.qty()))
    .reduce(ZERO, BigDecimal::add);

order.lines().forEach(l -> {
    var rate = rateFor(order.address());
    l.setTaxRate(rate);
    l.setTaxAmount(l.price().multiply(rate));
});

order.setSubtotal(subtotal);
order.setState(TAXED);
orderRepo.save(order);

orderService.applyTaxes(
    orderId, taxBreakdown);

Tell, don't ask.

02·Contracts

Other teams consume contracts, not tables.

02·Contracts

The contract is the boundary.

-- private to orders-team
CREATE TABLE orders (
  order_id       UUID        PRIMARY KEY,
  customer_id    UUID        NOT NULL,
  state          order_state NOT NULL,
  placed_at      TIMESTAMPTZ NOT NULL,
  currency       CHAR(3)     NOT NULL,
  total_minor    BIGINT      NOT NULL,
  fraud_score    SMALLINT,        -- internal
  channel        TEXT,
  channel_meta   JSONB,           -- internal
  fulfillment_id UUID,
  ml_segment     TEXT,            -- internal
  updated_at     TIMESTAMPTZ NOT NULL
);
CREATE TABLE order_lines       ( ... );
CREATE TABLE order_status_log  ( ... );
CREATE INDEX ON orders (customer_id, placed_at DESC);

syntax = "proto3";
package penultimate.orders.events.v3;

// Owner: orders-team
// Compatibility: BACKWARD (Buf in CI)
message OrderPlaced {
  reserved 9, 10;   // removed: discount_code, promo_id

  string             order_id        = 1;
  string             customer_id     = 2;
  int64              placed_at_ms    = 3;
  string             currency        = 4;
  Address            shipping        = 5;
  repeated OrderLine lines           = 6;
  Money              total_amount    = 7;
  Source             source          = 8;
  optional string    idempotency_key = 11;
}
// internal-only fields excluded:
//   fraud_score, channel_meta, ml_segment

Versioned. Owned by orders-team. Published with a guarantee.

03·CI fitness

Fitness functions for data boundaries.

@AnalyzeClasses(packages = "com.penultimate")
public class DataBoundaryFitnessFunctions {

  @ArchTest
  static final ArchRule no_cross_domain_persistence_access =
    noClasses().that().resideInAPackage("..tax..")
      .should().accessClassesThat()
      .resideInAPackage("..orders.persistence..")
      .because("Tax reads OrderPlaced events. Tax does not query Order's tables.");

  @ArchTest
  static final ArchRule writes_only_via_aggregate_root =
    noClasses().that().resideOutsideOfPackage("..orders.domain..")
      .should().callMethod(OrderRepository.class, "save", Order.class)
      .because("Order's invariants live in Order. No exceptions.");

  @ArchTest
  static final ArchRule events_versioned_and_immutable =
    classes().that().areAnnotatedWith(DomainEvent.class)
      .should().haveSimpleNameEndingWith("V1")
      .orShould().haveSimpleNameEndingWith("V2")
      .andShould().haveOnlyFinalFields();
}

Three rules. One CI build. The four failure modes — blocked at the door.

03·CI fitness

Other fitness functions you'll add

04·Safe migrations

Migrations expand. Then contract.

Every step is reversible. None of them is a flag day.

Enforced in CI: an Expand without its Contract is a failed build.

05·Decompose along seams

The pressure was not traffic. It was lifecycle.

05·Decompose along seams

Same colors. Same domains. Same teams.

Before (Year 4)

One outer boundary, 8 domains inside as colored regions.
Solid lines between aggregates.
Event arrows loop internally.

After (Year 7)

Three extracted services: Search, Tax, Payments.
Remaining monolith: Identity, Catalog, Orders, Fulfillment, Pricing.
Versioned event contracts as labelled arrows.

05·Decompose along seams

Aggregates become extraction boundaries.

DIAGRAM: Search extraction

Search subscribes to ProductPublished, OrderPlaced.
Year 4: events in-process.  ·  Year 5: events over Kafka.
Same code, different transport.

05·Decompose along seams

Same contract. Bigger blast radius.

DIAGRAM: Read model contract — side by side

Year 4: consumed by 1 internal team.
Year 7: consumed by 6 internal teams + 2 external integrations.
With SLA, version policy, deprecation window.

05·Decompose along seams

Process managers replace transactions.

DIAGRAM: Order placement, before and after

Year 4: single DB transaction across Order, Payment, Tax, Inventory.
Year 7: process manager — PlaceOrder → AuthorizePayment → ApplyTaxes → ReserveInventory.
Each step has compensation.

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