Forward compatibility

What is forward compatibility

Forward compatibility is the ability of a system to work correctly with newer clients or data than those for which it was originally designed. Simpler: the old server does not break when a new client comes to it; the old consumer does not fall when they encounter a new message.

Forward differs from backward compatibility (when the new system supports old clients) in the direction of responsibility: we design protocols and clients in such a way as to "survive" future extensions without a total upgrade of the entire ecosystem.

Basic principles

1. Tolerant reader & tolerant writer

Reader ignores unknown fields/headers and allows new enum values ​ ​ with the correct fallback.
Writer does not send anything that the server has not explicitly declared as supported (capabilities).

2. Capability negotiation

Explicit exchange of capabilities (features/versions/media types) at the handshake stage. The client adapts its behavior to the server response.

3. Default degradation

The new features are considered optional: if the server/consumer does not support them, the script will still end with a useful minimum (MGC).

4. Stable Core (MGC)

The minimum warranty contract is unchanged; innovations live on as extensions.

5. Error contracts as part of the protocol

Predictable codes/reasons ("feature not supported," "media type unknown") allow the client to automatically roll back to a supported mode.

6. Versions without surprises

Major lines separated; minor extensions do not require a server/consumer upgrade.

Where it matters

Public APIs with long-lived integrations (partners, SDKs in mobile applications).
Event platforms with many independent consumers.
Mobile clients that update more slowly than the backend.
Edge/IoT, where part of the device fleet is rarely flashed.

Implementation Patterns by Style

REST/HTTP

• 'Accept '/media types with parameters ('application/vnd. example. order+json; v=1; profile=risk`).
• 'Prefer: include =... 'for optional blocks.
• Title'X-Capabilities: risk_score,item_details_v2'.

• Sends a request in a basic format, extensions - only if the server has confirmed capability (via OPTIONS/desc/lead endpoint).
• When '415/406/501' is automatically rolled back to a supported format/method.
• Server response: unknown parameters - ignore; extra fields are allowed; stable error format ('type/code/detail/trace _ id').

gRPC / Protobuf

Stable services: new methods/fields - additive; the old server quietly ignores unknown request fields.
Feature discovery: The'GetCapabilities () 'method returns lists of features/limits. The client does not call the v2 method unless the server declares it.
Streaming: fix the order of the minimum set of messages; mark new "frames" with extensions/types that the old client ignores.

GraphQL

Forward-friendly: new fields/types appear on the server - old clients simply do not request them.
Guesses are forbidden: the client must keep the scheme (introspection/codogen) and not send unknown directives/variables.
Degradation: if the server does not know the custom/feature directive, the client builds a request without it.

Event-driven (Kafka/NATS/Pulsar, Avro/JSON/Proto)

FORWARD compatibility of the scheme in the registry: old consumers can read messages written by the new scheme.
Additive fields with defaults: new producers do not break old consumers.
Core vs Enriched: the kernel remains the same, new information is published in '.enriched' or as optional fields.

Design practices

1. Minimum Request Contract (MGC)

The operation should have a "narrow neck" that all servers will support for many years.

2. Feature flags at contract level

Describe the features as named features: 'risk _ score', 'pricing _ v2', 'strong _ idempotency'. The client includes them explicitly.

3. Explicit error codes for "not supported"

HTTP: `501 Not Implemented`, `415 Unsupported Media Type`, детальные `problem+json`.
gRPC: `UNIMPLEMENTED`/`FAILED_PRECONDITION`.
Events: route in DLQ with'reason = unsupported _ feature '.

4. Do not rely on order/complete lists

The client must be ready for new enum values, no new fields, and "additional" properties.

5. Stable identifiers and formats

Do not change the format of ID/partitioning keys within the line - this breaks forward on the side of readers.

6. "Machine-readable" documentation

Host descriptors: OpenAPI/AsyncAPI/Proto descriptors/GraphQL SDL. Customers can verify support for the feature.

Forward compatibility testing

Schema-diff in FORWARD/FULL mode: the new schema validates the old consumer/server.
Client Contract Tests: A new client is executed against an old server with features enabled/disabled.
Golden requests: a set of "new" requests is run through the "old" server; expected degradation without critical errors.
Chaos/latency: timeout/retray check - the new client must correctly survive the worst SLAs of the old server.
Canary: some of the new clients work with the previous server version - we collect telemetry of errors/degradation.

Observability and operational metrics

The percentage of requests/messages with unsupported features and their automatic rollbacks.
Distribution by client versions (User-Agent/metadata/claims).
'UNIMPLEMENTED/501/415'errors and routes in DLQ with' unsupported _ feature '.
Degradation time: p95/p99 for MGC vs "extended" response.

Compatibility modes in the schema registry

FORWARD: the new entry is compatible with the old reader (defaults are needed, optionality).
FULL: и FORWARD, и BACKWARD; convenient for public contracts.
Recommendation: for events - BACKWARD for the producer and FORWARD for the consumer (via a tolerant reader), for external APIs - FULL.

Examples

REST (capabilities + degradation)

1. The client makes'GET/meta/capabilities' → '{"risk_score": false, "price_v2": true} '.
2. On 'POST/orders' sends base fields; 'risk _ score' does not request, because the server cannot do it.
3. If accidentally sent 'Prefer: include = risk _ score', the server responds with 200 without the 'risk _ score' field (or 'Preference-Applied: none') - the client does not crash.

gRPC (discovery)

'GetCapabilities () 'returned method list/feature. The client does not call'CaptureV2 'if it is not present - instead using'Capture' and locally converting the input to a supported view.

Events (FORWARD in registry)

The producer added the 'risk _ score' field (nullable with default). The old consumer ignores him; its logic uses only stable kernel fields.

Antipatterns

Hard client: filters the response by whitelist fields and falls on an unfamiliar property.
Implicit features: The client starts sending a new parameter without checking the capabilities.
Changing ID/key formats within the line → old servers/consumers no longer understand new requests/messages.
Hardwired assumptions about the full enum list (switch without default).
Logging as flow control: parsing error strings instead of contract codes.

Implementation checklist

• MGC defined; new features are marked as optional.
• Capability negotiation (endpoint/metadata/handshake) is described and implemented.
• Clients ignore unfamiliar fields and correctly handle new enum (fallback).
• Error contracts capture "not supported" predictably (HTTP/gRPC/Event).
• The schema registry is set to FORWARD/FULL for the corresponding artifacts.
• Autotests: schema-diff (FORWARD), client vs. old server contract tests, canary.
• Metrics: client version, feature failure, degradation rate, p95 MGC.
• Documentation/SDKs publish a list of features and examples of degradation.

FAQ

How does forward differ from backward in practice?
Backward: the new server does not break old clients. Forward: the old server does not break from new clients (or the old consumer from new messages). Ideally, you reach full.

Do I always need to enter capabilities?
If you expect active evolution without synchronous releases, yes. It's cheaper than holding dozens of major lines.

What about security?
New features should require separate scopes/claims. If the server does not support them, the client should not reduce security, but should abandon the feature.

Is it possible to "guess" support by server version?
Undesirable. It is better to ask explicitly (capabilities) or look at the media type/scheme.

Total

Forward interoperability is the discipline of negotiating opportunities and safely degrading. A stable core, capability negotiation, additive extensions and predictable bugs allow new clients and data to get along with old servers and consumers - without mass releases and night migrations.