Schema Registry & contracts

1. Your subject is BACKWARD. You need to add a currency field to the order event. How do you do it without breaking anyone, and in what order do you deploy?

Add currency with a default (for example "USD"). Under BACKWARD, a consumer on the new schema can read old records, because the default fills in the value. Follow these steps:

1. Register the new schema.
2. Upgrade consumers first, so they understand the field.
3. Upgrade the producers to emit the field.

The registry rejects a required field with no default at registration.

2. What is the difference between BACKWARD and FORWARD, and when would you deliberately choose FORWARD?

BACKWARD = a new reader reads old data (consumers first). FORWARD = an old reader reads new data (producers first). Choose FORWARD when you cannot coordinate the consumers. For example, many external or independent teams consume the topic. You want to roll out a richer producer immediately. Old consumers keep working because they ignore new fields.

3. Why does each message carry a 4-byte schema id, and not the schema itself?

The reasons are payload size and decoupling. If you embed the full schema in each message, every record gets bigger. It also couples serialization to transport. The id is a stable pointer. The consumer caches the schema that it fetched. So resolution costs one lookup for each new id.

4. Someone proposes NONE compatibility to "ship faster." Argue against it with a concrete failure.

NONE disables the gate, so the registry accepts anything. This includes a rename (amount→total). A rename is a drop plus an add, with no default link. Every running consumer that reads amount receives a resolution failure or a null. This corrupts downstream state. The whole point of the registry is to reject this at CI time. NONE throws that away.

5. How is BACKWARD compatibility like a dbt model contract?

Both fix the shape and let a machine reject drift before it ships. A dbt contract fails the build if a column type changes or a contracted column disappears. BACKWARD fails registration if a change would break a reader. In both, the safe move is to add something with a default.

6. The registry says a schema is compatible, but production still breaks. What class of bug did the registry NOT catch?

Semantic or value bugs. The registry checks that the shape parses under resolution. It does not check that values are sane (amount >= 0, currency in an allowed set, referential integrity). For these checks you need explicit validation or registry rules. They are the on-the-wire equivalent of dbt data tests.