Re: EPICS V3 PV name length limits and how to get around them

[Michael Davidsaver via Core-talk <core-talk@aps.anl.gov>]

Hi Ben,

> The CA client library could be extended to handle PV names longer than
> say 29 bytes by hashing them transparently. So what goes over the
> network is only the hashed name, but the programs that /use/ the CA
> client library need not know or care.

I'm a bit perplexed that overcoming an implementation limit of name
length should involve what sounds like effectively a protocol change.
In that it would prevent old and new clients/server from interoperating.
Am I missing something?


On 11/19/18 6:30 AM, Benjamin Franksen via Core-talk wrote:
> Hi everyone
> 
> the EPICS version 3 database will probably stay with us for some time
> yet, considering all the existing device supports and drivers.
> Similarly, Channel Access is not going to die soon, given that many
> sites still use ancient display managers and other clients that aren't
> easily ported to a new protocol.
> 
> We (Götz and me) just had an idea how to circumvent the existing
> limitations in CA and in the EPICS (V3) database regarding the length of
> record names and more generally PVs and therefore link fields. The idea
> is quite simple: instead of the name itself, allow to use a hash sum of
> the name, concretely the SHA1 of the clear text name, ASCII encoded
> using Base64. The clear text name can then have arbitrarily many
> characters. SHA1 has 20 bytes, so its Base64 encoding uses at most 27
> bytes. This leaves enough room for field name, link options, and a
> special character that signifies that this is a hashed name and not a
> clear text name. The IOC can internally use a table to allow look up of
> the clear text name given the hashed name. However, note that conversion
> between hashed and clear text names on the IOC is purely for convenience
> e.g. when using command line tools like dbpr or dbl. It is not needed
> for record processing or CA.
> 
> The CA client library could be extended to handle PV names longer than
> say 29 bytes by hashing them transparently. So what goes over the
> network is only the hashed name, but the programs that /use/ the CA
> client library need not know or care.
> 
> Cheers
> Ben
>

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