Metainfo
Meta Information
As an RPC framework, Kitex services communicate with each other through protocols described by IDL (thrift, protobuf, etc.). The interface defined in an IDL determines the data structures that could be transmitted between the client and the server.
However, in the production environment, we somehow may need to send special information to a remote server and that information is temporary or has an unstable format which can not be explicitly defined in the IDL. Such a situation requests the framework to be capable of sending meta information.
NOTE MUST use the underlying transport protocol that supports passthrough of meta information，such as TTHeader, gRPC, HTTP。
To decouple with the underlying transport protocols, and interoperate with other frameworks, Kitex does not provide APIs to read or write meta information directly. Instead, it uses a stand-alone library metainfo to support meta information transmitting.
Forward Meta Information Transmitting
Package metainfo provides two kinds of API for sending meta information forward – transient and persistent. The former is for ordinary needs of sending meta information; while the later is used when the meta information needs to be kept and sent to the next service and on, like a log ID or a dying tag. Of course, the persistent APIs works only when the next service and its successors all supports the meta information tarnsmitting convention.
A client side example:
import "github.com/bytedance/gopkg/cloud/metainfo"
func main() {
...
ctx := context.Background()
cli := myservice.MustNewClient(...)
req := myservice.NewSomeRequest()
ctx = metainfo.WithValue(ctx, "temp", "temp-value") // attach the meta information to the context
ctx = metainfo.WithPersistentValue(ctx, "logid", "12345") // attach persistent meta information
resp, err := cli.SomeMethod(ctx, req) // pass the context as an argument
...
}
A server side example:
import (
"context"
"github.com/bytedance/gopkg/cloud/metainfo"
)
var cli2 = myservice2.MustNewClient(...) // the client for next service
func (MyServiceImpl) SomeMethod(ctx context.Context, req *SomeRequest) (res *SomeResponse, err error) {
temp, ok1 := metainfo.GetValue(ctx, "temp")
logid, ok2 := metainfo.GetPersistentValue(ctx, "logid")
if !(ok1 && ok2) {
panic("It looks like the protocol does not support transmitting meta information")
}
println(temp) // "temp-value"
println(logid) // "12345"
// if we need to call another service
req2 := myservice2.NewRequset()
res2, err2 := cli2.SomeMethod2(ctx, req2) // pass the context to other service for the persistent meta information to be transmitted continuously
...
}
Backward Meta Information Transmitting
Some transport protocols also support backward meta information transmitting. So Kitex supports that through metainfo, too.
A client side example:
import "github.com/bytedance/gopkg/cloud/metainfo"
func main() {
...
ctx := context.Background()
cli := myservice.MustNewClient(...)
req := myservice.NewSomeRequest()
ctx = metainfo.WithBackwardValues(ctx) // mark the context to receive backward meta information
resp, err := cli.SomeMethod(ctx, req) // pass the context as an argument
if err == nil {
val, ok := metainfo.RecvBackwardValue(ctx, "something-from-server") // receive the meta information from server side
println(val, ok)
}
...
}
A server side example:
import (
"context"
"github.com/bytedance/gopkg/cloud/metainfo"
)
func (MyServiceImpl) SomeMethod(ctx context.Context, req *SomeRequest) (res *SomeResponse, err error) {
ok := metainfo.SendBackwardValue(ctx, "something-from-server-key", "something-from-server-value")
if !ok {
panic("It looks like the protocol does not support transmitting meta information backward")
}
...
}
Kitex gRPC metadata
Kitex gRPC scenarios can also use metainfo. But note that the key of the CGI gateway style interface in the format of uppercase + ‘_’ needs to be satisfied.
In addition to metainfo usage, it is also compatible with the original metadata transmission method. But the two cannot be mixed.
Similar to native gRPC, the forward pass is implemented through metadata. Reverse transmission is sent back through Header or Trailer, the specific usage is as follows:
Forward
Client send settings:
ctx := metadata. AppendToOutgoingContext(ctx, "k1", "v1", "k1", "v2", "k2", "v3")
// unary scene
resp, err := client. SayHello(ctx, req)
// stream scene
stream, err := client. CallStream(ctx)
Server receives:
// unary scene
md, ok := metadata. FromIncomingContext(ctx)
// stream scene
md, ok := metadata.FromIncomingContext(stream.Context())
Backward
Unary
In the unary scenario, the server sends meta information to the client as follows:
Server settings:
nphttp2. SendHeader(ctx, metadata. Pairs("k1", "v1"))
nphttp2. SetHeader(ctx, metadata. Pairs("k1", "v1"))
nphttp2. SetTrailer(ctx, metadata. Pairs("k2", "v2"))
Client receives:
// set in advance
var header, trailer metadata.MD
ctx = nphttp2.GRPCHeader(ctx, &header)
ctx = nphttp2. GRPCTrailer(ctx, &trailer)
// RPC Call
resp, err := client. SayHello(ctx, req)
// get header and trailer
log.Println("header is ", header)
log.Println("trailer is ", trailer)
Streaming
In the Streaming scenario, the server sends meta information to the client as follows: Server sends:
stream.SetHeader(metadata. Pairs("k1", "v1"))
stream.SetTrailer(metadata. Pairs("k2","v2"))
Client receives:
// After stream call
md, _ := stream.Header()
md = stream.Trailer()