Polyglot was the second gRPC client that I looked at. One of the things that I liked about it is the fact that it does not need users to generate protoset files. It generates the protoset files in flight every time it runs. This, combined with the fact that it is written in Java does have a disadvantage - every time the client makes a call, it has to fire up a JVM, generate protosets and make the request.

You run polyglot by using the distributed polyglot jar. To list all the services available on an endpoint, polyglot can be executed as:

> java -jar ~/Downloads/polyglot.jar --command=list_services \
  --proto_discovery_root=$PROTO_DISCOVERY_ROOT \
  --deadline_ms=3000
[main] INFO me.dinowernli.grpc.polyglot.Main - Polyglot version: 1.6.0
[main] INFO me.dinowernli.grpc.polyglot.Main - Loaded configuration:

manualflowcontrol.StreamingGreeter -> /Users/sdqali/src/grpc/grpc-java/examples/src/proto/hello_streaming.proto
manualflowcontrol.StreamingGreeter/SayHelloStreaming

routeguide.RouteGuide -> /Users/sdqali/src/grpc/grpc-java/examples/src/proto/route_guide.proto
routeguide.RouteGuide/GetFeature
routeguide.RouteGuide/ListFeatures
routeguide.RouteGuide/RecordRoute
routeguide.RouteGuide/RouteChat

helloworld.Greeter -> /Users/sdqali/src/grpc/grpc-java/examples/src/proto/helloworld.proto
helloworld.Greeter/SayHello

The command call can be issued to execute a particular service method:

echo "{'name': 'World'}" | java -jar ~/Downloads/polyglot.jar --command=call \
  --full_method=helloworld.Greeter/SayHello \
  --endpoint=localhost:50051 \
  --proto_discovery_root=$PROTO_DISCOVERY_ROOT \
  --deadline_ms=3000
[main] INFO me.dinowernli.grpc.polyglot.Main - Polyglot version: 1.6.0
[main] INFO me.dinowernli.grpc.polyglot.Main - Loaded configuration:
[main] INFO me.dinowernli.grpc.polyglot.command.ServiceCall - Creating channel to: localhost:50051
[main] INFO me.dinowernli.grpc.polyglot.command.ServiceCall - Using proto descriptors obtained from protoc
[main] INFO me.dinowernli.grpc.polyglot.command.ServiceCall - Creating dynamic grpc client
[main] INFO me.dinowernli.grpc.polyglot.command.ServiceCall - Making rpc with 1 request(s) to endpoint [localhost:50051]
[main] INFO me.dinowernli.grpc.polyglot.grpc.DynamicGrpcClient - Making unary call
[grpc-default-executor-0] INFO me.dinowernli.grpc.polyglot.io.LoggingStatsWriter - Got response message
{
"message": "Hello World"
}

[grpc-default-executor-0] INFO me.dinowernli.grpc.polyglot.io.LoggingStatsWriter - Completed rpc with 1 response(s)

Notice how we are passing path to proto files and polyglot executes protoc on them to get protosets. With reflection turned ON, we no longer need to provide a path to the proto files.

echo "{'name': 'World'}" | java -jar ~/Downloads/polyglot.jar --command=call \
  --full_method=helloworld.Greeter/SayHello \
  --endpoint=localhost:50051 \
  --deadline_ms=3000 \
  --use_reflection=true
[main] INFO me.dinowernli.grpc.polyglot.Main - Polyglot version: 1.6.0
[main] INFO me.dinowernli.grpc.polyglot.Main - Loaded configuration:
[main] INFO me.dinowernli.grpc.polyglot.command.ServiceCall - Creating channel to: localhost:50051
[main] INFO me.dinowernli.grpc.polyglot.command.ServiceCall - Using proto descriptors fetched by reflection
[main] INFO me.dinowernli.grpc.polyglot.command.ServiceCall - Creating dynamic grpc client
[main] INFO me.dinowernli.grpc.polyglot.command.ServiceCall - Making rpc with 1 request(s) to endpoint [localhost:50051]
[main] INFO me.dinowernli.grpc.polyglot.grpc.DynamicGrpcClient - Making unary call
[grpc-default-executor-0] INFO me.dinowernli.grpc.polyglot.io.LoggingStatsWriter - Got response message
{
"message": "Hello World"
}

That is nice, except that reflection does not work for listing services.

java -jar ~/Downloads/polyglot.jar --command=list_services \
  --endpoint=localhost:50051 \
  --deadline_ms=3000 \
  --use_reflection=true
[main] INFO me.dinowernli.grpc.polyglot.Main - Polyglot version: 1.6.0
[main] INFO me.dinowernli.grpc.polyglot.Main - Loaded configuration:
[main] WARN me.dinowernli.grpc.polyglot.Main - Caught top-level exception during command execution
java.lang.IllegalArgumentException: A proto discovery root is required for proto analysis
  at com.google.common.base.Preconditions.checkArgument(Preconditions.java:122)
  at me.dinowernli.grpc.polyglot.protobuf.ProtocInvoker.forConfig(ProtocInvoker.java:36)
  at me.dinowernli.grpc.polyglot.Main.getFileDescriptorSet(Main.java:93)
  at me.dinowernli.grpc.polyglot.Main.main(Main.java:62)
Exception in thread "main" java.lang.RuntimeException: java.lang.IllegalArgumentException: A proto discovery root is required for proto analysis
  at me.dinowernli.grpc.polyglot.Main.main(Main.java:86)
Caused by: java.lang.IllegalArgumentException: A proto discovery root is required for proto analysis
  at com.google.common.base.Preconditions.checkArgument(Preconditions.java:122)
  at me.dinowernli.grpc.polyglot.protobuf.ProtocInvoker.forConfig(ProtocInvoker.java:36)
  at me.dinowernli.grpc.polyglot.Main.getFileDescriptorSet(Main.java:93)
  at me.dinowernli.grpc.polyglot.Main.main(Main.java:62)

This was unexpected, as adding this capability to Polyglot should not be too diffcult, considering that they already support reflection for executing services. This is something that I am interested in implementing.

If you have questions or comments about this blog post, you can get in touch with me on Twitter @sdqali.

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