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Supersedes 0032: REST api() with the Encore client is no longer the transport rule. Services talk through transport drivers.
Amended by 0260: in-process or RPC is chosen by topology inside the driver; the consumer’s code is the same.

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Context

0032 chose REST api() plus the Encore client for service calls. The owner’s audit answer Q2 (mem L545, 2026-10-20) moves integration events and typed calls behind drivers, so the same service code runs in SaaS and on-prem.

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Decision

  1. Integration events and typed calls go through drivers. In SaaS: Encore Pub/Sub and RPC. On-prem: a Postgres queue and HTTP.
  2. Kafka, NATS or Redpanda may come later as an event-bus driver. This ADR does not adopt them.
  3. There is no hand-written gRPC.
  4. Clients are generated (ADR 0158, wave 1 part 3), and ids follow 0135.

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Consequences

  • A service calls another service through a generated client and an event, never through another service’s source.
  • On-prem needs no broker. The Postgres queue carries events there.

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Enforcement

planned — the boundaries rule bans direct imports of another service’s source, and gen emits the clients only through the drivers.

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Alternatives

  • REST plus the Encore client only (0032): superseded, because on-prem has no Encore runtime.
  • Kafka as the default: not chosen; it is a later driver.

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Supersedes and sources

Supersedes: 0032. Sources: owner audit answer Q2 (stackra-decisions-2026-10-19.md L545, 2026-10-20).