Experimental Physics and
Industrial Control System

Siniša Veseli <[email protected]> · Thu, 19 Jul 2018 17:33:45 -0500

Hi,

Judging from Michael's comments, documentation, epics meeting slides,

etc. it looks like these new APIs are indeed meant to replace what we

had since v4.5 or so. While I am not necessarily opposed to such change,

I do have a few points I'd like to make:

1) EPICS in itself is probably not the easiest software to learn and

use. Having two sets of C++ APIs does not help and can be terribly

confusing. If I were new to Argonne, and someone told me to write an

EPICS C++ client for the first time, and then I were to discover that in

epics base we have client.h and pvaClient.h that seem to be completely

different, that we have two sets of examples, different documentation,

and in some case different behavior between the clients, I would ask

myself "What is going on here, and who the heck designed all of this...".

2) Maintaining and supporting two sets of high-level APIs is not

feasible in the long run. Resources that we have for epics development

are scarce, and those should be used wisely. This is something that

anyone sitting on the council should be concerned with. Why would we

have Marty or anyone else spend lots of his time on pvaClient and

pvDatabase if we are going to get rid of it in 6 months or in a year? I

would personally rather spend my time on something that will have a

lasting impact on the project, rather than fixing a bug or two in

something that is viewed as temporary.

3) I would hope that EPICS development project has some sort of coherent

strategy in terms of what gets developed and why, what are the

recommended user interfaces, etc. Module owners have a wide latitude and

control in terms of how they want things to be done. However, in a case

where someone feels that a new high-level C++ API is needed because the

old one is just not good, or if they wanted to make API changes that

affect users or other downstream modules, I imagine there would be some

group discussions and planning around that, so that we all know what is

going on. We may not all agree about particular set of changes, but at

least everyone would get their say and the group would build some sort

of consensus around the direction that the project is taking.

It seems like there was not much discussion in this particular case. New

interfaces appeared in the base code at some point, and are finding

their way slowly through the rest of the system, so now we have two sets

of APIs, and that's just how it is... Depending on who you talk to

first, you might go this way or the other for developing your own code,

and hope that the path you've chosen will not be deprecated in the next

release. This just does not seem to be a good thing as far as EPICS

users are concerned.

I am not arguing to maintain some sort of status quo here, but I am not

sure I completely agree with how are changes being introduced at the

moment. If new client/server APIs are going to be significantly better,

if we need to change the way we use PV requests in our code, etc., that

would all be fine with me, as long as this was discussed, consequences

were understood, transition plan for the old code was developed, etc.

This email is not meant to offend anyone, if I did so, then I am sorry

and I apologize in advance.

I do appreciate Michael's clarifications on what happened and how things

are at the moment.

Sinisa

On 7/19/18 1:42 PM, Marty Kraimer wrote:

On 07/18/2018 08:05 PM, Michael Davidsaver wrote:

On 07/18/2018 01:20 PM, Siniša Veseli wrote:

Hi,

I realize I have not been attending the working group meetings

lately, so I apologize in advance if I missed something.

Reading through some of the github issues that seem to be getting a

lot of attention recently, it seems like we are getting a completely

new set of PVA client/server C++ APIs, which raises several

questions in my mind:

1) Are those APIs becoming the "official" EPICS PVA client/server

APIs? In other words, if I am starting a new C++ project based on

EPICS7, should I start with the new interfaces, or work with the

ones available in the most recent 7.0.1.1 release?

My new client API (pva/client.h) is actually in the 7.0.1.1 release. 

I wrote this last summer.  It is the result of my

experiences using the ChannelProvider API with the PVA gateway

prototype, the P4P client,

the CLI tools, and in unittest code for QSRV and the PVA gateway.

pva/client.h is my attempt to distill out the code common to all of

these use cases.

Since 7.0.1.1 I've successfully used it in the new PVA link support,

some of the CLI tools, and recently

rewritten the P4P client around it.  In doing this I didn't have to

make any incompatible changes, just to

add a few missing pieces and make bug fixes.  So from this point I

expect it will remain stable.  I plan to

apply it to the PVA gateway the next time I iterate.

As for the (very new) server API.  This does not attempt to be fully

general purpose, eg. it won't be

enough to build a gateway.  Instead, it is a direct decedent of the

server code common to QSRV and the unittests

of the PVA gateway prototype, which is will replace.  This common

code is what has held these two otherwise

unrelated projects in the same source repository.

So far it's only been use behind the P4P server API.  I'll consider

it stable once it has been successfully

used a few more times (by myself, or others).  So I suppose you could

call it a "technology preview" in the

upcoming 7.0.2 release (a step I think should be mandatory for all

new APIs).

As for "official"-ness; these APIs will be released, and I will

continue to use them in my own projects,

and as a tool to test the underlying PVA client and server code

(hopefully finding bugs before you do).

Should you use them?

I'd like to get feedback from people like yourself who also have

experience with both the low-level

ChannelProvider API, as well as Marty's pvaClientCPP/pvDatabaseCPP. 

My goals are that these new APIs should be easier to use,

harder to break, and perform just as well.  If I'm not meeting these

goals, then I'll do something about it.

2) If #1 is true, what becomes of the current pvaClientCPP and

pvDatabaseCPP APIs? Will those be deprecated? Or are we going to

have two separate sets of high-level C++ APIs?

As you allude to in #3, Marty and I disagree about pvaClientCPP and

pvDatabaseCPP, of which I am not a fan.

I don't know what his plans are, but I have not, and will not,

dissuade him from continuing to develop these modules.

So yes, it looks like there will be two sets.

3) It looks like there are some rather significant differences in

behavior between the new and existing interfaces. Have those

differences been summarized anywhere?

No.  I can only refer you to my documentation.  See "Client API" and

"Server API", and particularly the "API usage Examples".

http://epics-base.github.io/pvAccessCPP/

I would imagine that making API changes like this, and for a project

that numerous sites depend on, requires some sort of process:

somebody comes up with a proposal and plan, there is a discussion

about advantages/disadvantages and about the consequences for the

existing code and for the future development, and (most importantly)

there is an agreement between the developers as far as the direction

that project will be taking. If this was discussed in one of the v4

meetings already, sorry that I missed it, and I would appreciate if

someone points me to the document where all of this has been written

down.

My motivation for this tool smithing is that I found the existing

options quite awkward, error prone,

and unlikely to change.  I know that others have similar

experiences.  I developed some patterns and

utilities to cope with this.  I also dislike copy+pasting code

between projects.  These new APIs are

the end result of this process.  I know they are useful to me. I

think they will be useful for others,

but only time will tell.

On a personal note, few years back, after we all decided that the

new client API (pvaClientCPP) will be implemented, I had to put in a

fair amount of work to replace my own high-level API with the

official one, and I would not be exactly looking forward to yet

another iteration of the same. However, if this was already

discussed and agreed upon, then so be it.

If you're happy with pvaClientCPP, then I see no immediate reason for

you to change.  The low level

ChannelProvider API on which all of these high level APIs are based

will be maintained (warts and all)

for the foreseeable future.

Eventually, I hope that you will conclude that these new APIs would

make your job easier.  But only you

can be the judge of this.

Michael is creating a new client  API for pvAccess.

https://github.com/epics-base/pvAccessCPP/blob/master/src/client/pva/client.h

He has used this API as the basis for
1) pvTools. In particular pvget and pvput
2) pva2pva. Both qsrv and gateway
3) p4p

The issues

https://github.com/epics-base/pvAccessCPP/issues/116

and

https://github.com/epics-base/pva2pva/issues/21

Show that pvTools and qsrv have broken the exceptions of existing

clients and servers.

In order to support both the existing clients and Michael's new client

API then there will have to be two implementation of at least pvTools

and qsrv. If we continue with two client API's there will probably be

a need for more dual implementations.

Marty

--
Siniša Veseli
Scientific Software Engineering & Data Management
Advanced Photon Source
Argonne National Laboratory
[email protected]
(630)252-9182

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System