Local gRPC using grpcurl in an OCI container

This is the latest installment in a series about using gRPC to interact with the PLCnext Runtime on a PLCnext Control device.

Earlier posts in the series included:

• Remote gRPC using grpcurl
• Secure remote gRPC using grpcurl
• Remote gRPC using reflection

The articles listed above show how to access the gRPC server on a PLCnext Control device from a remote client device. This post gives an example of a local gRPC client - that is, a gRPC client running on a PLCnext Control device, which accesses the gRPC server on the same device.

Other examples of local gRPC clients on PLCnext Control devices can be found here:

• How to create a client for the PLCnext Control gRPC server in C#
• Use gRPC to interface Python scripts with PLCnext Engineer Projects.

Since there is no gRPC client software installed on PLCnext Control devices as standard, this article shows how to run a gRPC client (in this case, grpcurl) in an OCI container using Podman and a suitable image from dockerhub.

Prerequisites

1. A PLCnext Control device (e.g. AXC F 3152), with firmware version 2025.0 or later, with internet access. Currently, the OCI image used in this article is not available for armv7 targets like the AXC F 2152.

2. In a browser, open the Web-based Management (WBM) site on the device and, on the System Services page, make sure the GRPC LOCAL SERVER feature is activated.

Procedure:

1. Open a secure shell session on the PLCnext Control device using the admin user account.

2. In the shell session, execute the following command:

     podman pull docker.io/fullstorydev/grpcurl:latest-alpine
   

   This downloads an OCI image containing the grpcurl command-line tool to the PLCnext Control device.

3. List the gRPC services available on the local PLCnext Control device:

     podman run -it --rm -v /run/plcnext/:/run/plcnext/ --userns=keep-id docker.io/fullstorydev/grpcurl:latest-alpine --plaintext unix:///run/plcnext/grpc.sock list
   

   Response:

   Arp.Device.Interface.Services.Grpc.IDeviceControlService
   Arp.Device.Interface.Services.Grpc.IDeviceInfoService
   Arp.Device.Interface.Services.Grpc.IDeviceSettingsService
   Arp.Device.Interface.Services.Grpc.IDeviceStatusService
   Arp.Io.Axioline.Services.Grpc.IAcyclicCommunicationService
   Arp.Io.Axioline.Services.Grpc.IAxioMasterService
   Arp.Plc.Gds.Services.Grpc.IDataAccessService
   Arp.Plc.Gds.Services.Grpc.IForceService
   Arp.Plc.Gds.Services.Grpc.ISubscriptionService
   Arp.Services.DataLogger.Services.Grpc.IDataLoggerService2
   Arp.Services.NotificationLogger.Services.Grpc.INotificationLoggerService
   Arp.System.Commons.Services.Io.Grpc.IDirectoryService
   Arp.System.Commons.Services.Io.Grpc.IFileService
   Arp.System.Commons.Services.Io.Grpc.IFileSystemInfoService
   Arp.System.Lm.Services.Grpc.ILicenseStatusService
   Arp.System.Nm.Services.Grpc.INotificationManagerService
   Arp.System.Security.Services.Grpc.IAuthenticationService
   Arp.System.Um.Services.Grpc.IAuthorizationInfoService
   Arp.System.Um.Services.Grpc.IPasswordAuthenticationService
   grpc.reflection.v1.ServerReflection
   grpc.reflection.v1alpha.ServerReflection
   

4. (Optional) Create an alias to simplify subsequent grpcurl commands:

     alias grpcurl="podman run -it --rm -v /run/plcnext/:/run/plcnext/ --userns=keep-id docker.io/fullstorydev/grpcurl:latest-alpine --plaintext unix:///run/plcnext/grpc.sock" 
   

5. Retrieve the board temperature from the PLCnext Control device:

   grpcurl -format json -d '{"identifier":"Status.Board.Temperature.Centigrade"}'  Arp.Device.Interface.Services.Grpc.IDeviceStatusService.GetItem
   

   Response:

   {
     "_ReturnValue": {
     "TypeCode": "CT_Int8",
     "Int8Value": 46
     }
   }