$grpc_internal_trailers

Description

The $grpc_internal_trailers variable is utilized within the context of gRPC protocols to provide access to the trailer fields of a gRPC response. Trailers in gRPC are key-value pairs that can be sent at the end of a response, similar to HTTP headers, allowing additional metadata to be communicated alongside the response body. This variable is only populated when the server processes a gRPC request and is capable of handling trailer fields. When processing a gRPC response, if the relevant trailer information is set to be included, it becomes accessible through $grpc_internal_trailers. Therefore, this variable will typically contain a string representation of the trailer fields formatted similarly to HTTP headers, separated by commas or newlines. If the request does not return any trailers, this variable will remain empty. The use of trailers can enhance the efficiency of the communication by allowing the server to send significant information along with the response without adding additional overhead to the body.

Config Example

location /example {
    grpc_pass grpc://backend;
    add_header Trailer 'grpc-status';
    set $status $grpc_internal_trailers;
}