New Channel Access Nameserver

1
New Channel Access Nameserver

• Joan Sage
• 12/4/01

2
Overview

• Background
• Motivation
• Old JLab Nameserver
• New JLab Nameserver
• Results

3
Background

• By default, EPICS does not use a nameserver
• Process variable name resolution achieved via UDP
  broadcasts
• JLab has used a cdev based nameserver for BURT
  and MEDM clients for several years

4
EPICS PV Name Resolution
Client
Broadcast TCP Connection
5
Why Use A Nameserver?

• Reduce IOC CPU load due to broadcast requests for
  name resolution
• Provide PV location information for tools that do
  not require connection
• Improve client connection time

6
Why A New JLab Nameserver?

• Old JLab Nameserver
• Integration with client programs required code
  changes and recompilation
• Required changes to Channel Access Library
• Because of these difficulties, the old nameserver
  was only used with BURT and MEDM

7
New Nameserver

• No modifications needed to client code
• No modifications to Channel Access Library
• Client programs are configured to use nameserver
  by setting EPICS_CA_ADDR_LIST to the IP Address
  of the Nameserver host and by setting
  EPICS_CA_AUTO_ADDR_LIST to NO

8
New Nameserver

• Nameserver reads signal.list files generated by
  IOCs at boot time to fill initial hash table
• Nameserver learns about PVs added to the system
  after startup by broadcasting to fill unknown PV
  requests

9
New Nameserver

• Built on PCAS example code directoryServer
  (Jeff Hills suggestion)
• pvExistTest reimplemented
• Heartbeat CA monitors for each IOC

10
New Nameserver

• Runs as a daemon with automatic restart on
  failure (code stolen from CA Gateway)
• Nameserver Startup Time lt 5 sec for 250K PVs

11
New Nameserver

• Nameserver monitors IOC heartbeat for connection
  events
• When an IOC goes down
• Hashtable updated
• Nameserver monitors for reconnection events
• Nameserver does not respond to clients asking
  for PVs on IOCS that are down
• Deleted and moved PVs are automatically handled

12
New Nameserver
Fallthrough
Client
NS
Nameserver Hash Table IS Updated
Broadcast TCP Connection
13
Results

• Tested name resolution and connection
• 10,465 PVs on 36 IOCs
• Max 300 PVs/IOC
• Without Nameserver 4.39 sec
• With New Nameserver 2.18 sec
• Being used successfully for Jlab operations for 3
  months

14
Side Effects

• An extra computer is required to run the
  Nameserver
• Central database
• Allows generation of a list of PVs that never
  resolve
• Allows identification of duplicate PVs