Joerg Hampel is a guest blogger. As the owner of Hampel Software Engineering and CLA and LabVIEW Champion, his biggest interest lies in team development best practices.
Ever wanted to use the API of a software module in an application when the module itself is actually running in another application on another device, connected via network? No? Well, imagine these scenarios:
Remote control of headless applications
One of the three pillars of my company‘s business is using LabVIEW Real-Time and LabVIEW FPGA to build embedded, network-enabled control devices and device drivers. Most of these applications tend to run as headless, stand-alone units, but all of them need configuration, visualisation or user input from time to time.
We always start out with a Real-Time application that has its own user interface for operating the application in its early stages (and even longer, sometimes). When it’s time to start developing the Windows application that should connect remotely, one often wishes to be able to reuse at least part of the code that’s already running and proved to be working. Even better than reusing would be using, right?
Remote access of (shared) functionality
There is an abundance of examples where you would want or need to access a shared resource remotely:
- A piece of hardware you need to talk to from a device running LabVIEW Real-Time, but with only drivers for Windows available
- A service (like a database) living in its own network segment, and IT allows access only from one gateway computer, but you need access from multiple clients
- A target that you select during runtime, such as when you are having a shared software control panel for multiple devices
In all of these cases, wouldn’t it be great if you didn’t have to worry about where the functionality will be executed? Wouldn’t it be great to just drop the DQMH requests into your block diagram and just use the module’s functions?
use the API of a DQMH module even if the module itself is running on a different, distributed system
The DQMH Generic Networking Module is a DQMH Singleton module altered to allow for (near) zero-coupled networking functionality. The motivation behind this implementation is to use the API of a DQMH module even if the module itself is running on a different, distributed system. Both the caller and the callee are absolutely oblivious of whether they are running on the same or on different systems.
The DQHM Generic Networking Module can be made a proxy. By loading a NET-TX clone module and redirecting communication, all requests are sent via TCP to another DQHM Generic Networking Module running in another application and/or another PC accessible via TCP/IP.
Process remote requests
The DQHM Generic Networking Module can be made to accept messages via network. By loading a NET-RX clone module, the module accepts messages from other DQHM Generic Networking Modules running in another application and/or on another PC via TCP/IP communication.
1. Generalize communication
In order to be able to communicate in a generic way, we need to separate the message data and its type information from the framework’s communication mechanisms. This allows us to send and receive messages without knowing about their actual contents, and that’s the prerequisite for separating the module’s actual use code from the networking code.
All notifications for requests are changed to variant datatype. This allows for generic access to the notifier reference. The actual datatype of the event’s notification lives inside the variant.
Helper VIs take care of the Notifier reference management: HSE_DQMH_Add Notifier To Variant.vi is used in the event handling loop to store the notifier reference as an attribute of the message variant, and HSE_DQMH_Get Notifier From Variant.vi is used in the message handling loop to extract the notifier reference from the message variant.
2. Modify Message Queue
A new MessageQueue class inherits from Delacor_Lib_QMH_Message Queue.lvclass, and overwrites the Message Queue to contain additional values:
- member variable “relay via network?”
- member variables for network configuration (ip address, port, timeout)
3. Network Communication
A DQMH Generic Networking module doesn’t have to know about how its messages are forwarded, received or answered.
When configured to forward requests, it dynamically loads a NET-TX clone module, which handles the actual forwarding:
When configured to receive remote requests, it dynamically loads a NET-RX module, which takes care of receiving messages and forwarding them to the module itself:
Example: Database Communication
For the following description, let’s imagine we have to talk to a database. We assume that we have two applications: One application is running locally, the other remotely (on another computer), and they are connected to each other via TCP. Only the remote application has access to the database, the local application cannot reach it due to network limitations.
Both applications make use of the same DQMH database module (DB module), but they use it differently:
- The local application is only using the API of the module. The local DB module forwards all requests to the remote application and also receives replies from the remote application for events with responses.
- The remote application is oblivious to the fact that its module is used by other applications. The remote DB module serves requests from other applications exactly the same way as requests from within the application it’s running in.
- The local module is configured to relay messages via network
- during configuration, a NetworkSender (NET-TX) module is started dynamically
- the calling module’s message queue object is handed to the NET-TX Start Module.vi
- The remote module is configured to receive networked messages
- during configuration, a NetworkReceiver (NET-RX) module is started dynamically
- the NetworkReceiver creates a TCP Listener and opens a port for incoming messages
- the calling module’s message queue object is handed to the NET-RX Start Module.vi
- Local: DQMH Request Event with Notification (eg. Execute Query ( select * from table ))
- Local DB module: DQMH_Event_Main sends the message data as variant
- Notification refnum (datatype variant) is set as an attribute of the message data variant
- Local DB module: Delacor_lib_QMH_Dequeue_Message.vi knows that the local module is configured to relay messages and thus calls the case “Send via Network”
- checks message (string) against blacklist of messages that are not forwarded (eg ‘Exit’)
- The original message (string) is appended as attribute to the message data (variant)
- Local DB module: The case “Send via Network” calls the “Send via Network” request event of the NetworkSender (NET-TX) module
- The message data (variant) is the payload of the NET-TX’s “Send via Network” request event
- Local NetworkSender module: DQMH_MSG_CASES “Send via Network”
- The original message is flattened to string and sent via TCP/IP
- TCP read waits for response
- Remote NetworkReceiver module: DQMH_EVENT_HELPER spawns a reentrant TCP service VI for each client that connects
- reads the data from the network and unflattens it to variant
- If the variant contains an attribute “notifier” (the original one from the local requester), creates a new notifier and overwrites the invalid one in the the received variant
- reads the message name from the variant attribute
- sends the message directly via the caller’s message queue to the Remote DB module’s MHL
- Wait for notification
- Remote DB module: DQMH_MSG_CASES
- executes the called case
- reads the notifier reference from the variant attribute
- sends the notification from TCP service VI
- Remote NetworkReceiver module: TCP service VI
- receives the notification from DB.DQMH_MSG_CASES
- sends the variant that was sent with the notification back via TCP
- Local NetworkSender module: DQMH_MSG_CASES “Send via Network”
- reads data from TCP, which is unflattened to variant
- If the original message variant contained an attribute “notifier”, the received data is sent via the original notifier refnum back to the request VI
Where to go from here?
If you want to give it a try yourself, you can find the source code on gitlab.com. Pull the repo, check out the example, and be sure to let me know what went well for you and what not so much. Just be aware that this is still a work-in-progress. The code is based on DQMH 4.0, so don’t forget to update your DQMH to the latest version if you haven’t done so already.
I’m also very much looking forward to hearing other people’s thoughts on the whole idea and on our implementation so far. So please feel free to leave your feedback in the comments section or contact me directly at firstname.lastname@example.org. I’d really appreciate it.
Happy generic networking,