Answer.

Background:

SOAP and gRPC are messaging protocols between services. SOAP emerged in the era of SOA when large-scale automation of business processes was required. gRPC is a modern RPC framework from Google for high-performance services. Automating the testing of these protocols is traditionally more complex than REST due to their specifics: data formats, serialization schemas, and client code generation.

Problems:

Not all familiar REST tools are suitable for SOAP and gRPC. For SOAP tests, you have to work with WSDL, complex XML structures, and for gRPC — with protobuf schemas and binary messages.
gRPC requires special test runners and generation of stubs in the desired programming language.
The complexity of validation and debugging errors is higher.

Solutions:

For SOAP: use specialized tools like SoapUI, Postman (limited), automation at a basic level through the generation of SOAP clients in the test automation language (Python, Java). It's important to validate not only responses but also WSDL agreements.
For gRPC: generate client stubs using protoc, use gRPC-compatible libraries (grpcio for Python, grpc-java, etc.), test runners (for example, grpcurl, BloomRPC). A good practice is to mock gRPC servers using interceptors or in-memory services.

Key features:

SOAP requires working with XML and WSDL, deep integration with business processes.
gRPC works with a binary format (protobuf), requires code generation, and supports multiple languages.
For stable automation, custom hooks and generation of mock data are necessary.

Questions with pitfalls.

Can gRPC services be tested as easily and with the same tools as REST?

No. gRPC uses a binary protocol and does not work with HTTP directly (only over HTTP/2), requiring the generation of protobuf clients and specific libraries.

Does SOAP provide automatic detection of all contract errors due to WSDL?

No. A strict data schema helps catch some errors at client compilation time, but does not protect against business logic issues and integration errors.

Is it sufficient to only have unit tests for SOAP/gRPC?

No. Integration and E2E tests are essential to ensure the verification of service interactions, network constraints, and serialization features.

Common mistakes and anti-patterns

Ignoring the need to mock external gRPC/SOAP services during integration.
Lack of checks for the relevance of protobuf/WSDL schemas.
Using REST testing approaches for gRPC/SOAP services (e.g., manual request via curl).
Insufficient validation of contracts and message structure.

Real-life example

Negative case

The team attempted to test gRPC services using curl and similar tools, ignoring the need for protobuf generation. As a result, tests were unstable, and some scenarios were not covered at all.

Pros:

Quick attempt at automation.
No additional compilation required.

Cons:

Scenarios are undercovered, bugs are missed.
Inadequate handling of serialization errors.

Positive case

A centralized pipeline was implemented: clients are generated for each gRPC/SOAP service, all tests are automatically compiled, mock services are brought up in memory, tests validate schemas and responses using contracts.