Experimental Physics and
Industrial Control System

From: "Kasemir, Kay" <[email protected]>
Date: Thursday 26 September 2019 at 15:04
To: EPICS Core Talk <[email protected]>, Timo Korhonen <[email protected]>
Subject: Re: pvData introduction

Basically good.

I do worry about the 'introspection'. It's confusing, it's part of what makes the currend C++ API so hard, and most important it's hardly necessary.

PVData currently distinguishes between "Field" and "PVField", between the plain description of a structure and an actual container that holds data.

"type double" vs. "double 3.14".

The examples are full of two-step code that first creates the description, then the structure, with two copies of each, one using a "builder" and the other not using a "builder".
In reality, you almost always need the actual container:

·         You want to serve data, so you fill the data container

·         You read data, so you get the container.
OK, in the first step of the read protocol you get the description of the data, but then in the next step you need to fill that with data, so you do create the container right away.

·         You monitor data, so you apply received changes from a 'small' container to the 'full' container that caches the complete structure.

With the new PVA java client and server, I've only implemented the container. The PVAxxx classes all hold data. When you read, i the first step you get the container where the double value = NaN, an array is empty, .., i.e. you get an empty container, but you do get the container which is then filled with data one millisecond later.

OK, for the plain PVInfo command the container remains empty, the values are all 0, NaN, null, empty.

==> This has cut the amount of code in half, maybe even into just 25% because instead of

1) methods for field description,

2) builder for field description,

3) methods for data,

4) builder for data

I only have the builder for data, which in the pvinfo case is never populated with data

---> Much easier to understand, less code to maintain, works just fine.

So in the description, I would not talk too much about the not really necessary distinction between Field and PVField. That's just a side effect of the current implementation that makes it more confusing.

Thanks,

Kay

From: Core-talk <[email protected]> on behalf of Timo Korhonen via Core-talk <[email protected]>
Sent: Thursday, September 26, 2019 8:42 AM
To: EPICS Core Talk <[email protected]>
Subject: [EXTERNAL] pvData introduction