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event-driven

  • The command bus we removed

    Deep dive10 min read

    A command bus introduced in 2021, and 88 commands with one handler each. A stack trace with four frames of dispatch before the work.

  • Message versioning, and the field added five years ago

    Note1 min read

    An additive-only message policy, and an optional field from 2021 that every consumer now requires. An optional field that everybody depends on is a required field with no…

  • An event with no consumer, published for the last time

    Note1 min read

    An event dispatched on every order since 2021, subscribed to by a module that no longer exists. The monthly report had been correct since the module was discontinued…

  • An outbox relay on two hosts, and the partition key

    Note1 min read

    A second host, and a relay that must not run twice against the same rows. The modulo on a hash of the aggregate identifier preserves per-aggregate ordering while…

  • A consumer that was deleted with its module

    Note2 min read

    A queue consumer removed along with the module it belonged to, and the queue that kept receiving messages. A TTL on a queue turns an unconsumed backlog into…

  • The read model that finally earned its place

    Deep dive10 min read

    A projection deleted in 2023 for being premature, rebuilt in 2025 because the query is now eleven seconds.

  • A consumer that had to be idempotent and finally was

    Note2 min read

    A handler documented as idempotent since 2022, which was not, and a redelivery that proved it. A docblock claiming idempotency is a claim and not a mechanism, and…

  • A domain event with a field that was presentation

    Note2 min read

    An OrderPlaced event carrying a formatted currency string, which is a decision about a locale baked into a permanent record. The formatted string was convenient for the one…

  • A PSR-14 dispatcher, and the listener I could not order

    Note2 min read

    PSR-14 deliberately says nothing about listener priority, which is a principled decision and occasionally an obstacle. Two listeners on the same event where one must observe the other’s…

  • An outbox relay with a second instance, partitioned

    Note1 min read

    A single relay process that had become the throughput ceiling, split into two by partitioning on the aggregate identifier. The modulo on a hash of the aggregate id…