If your goal is to write IOCs, see Input-Output Controllers (IOCs).

Caproto includes three implementations of a Channel Access server for three different Python concurrency libraries:

  • asyncio (built in to Python)

  • curio

  • trio

To learn more about concurrency in Python (and in general) we recommend these introductory resources, suggested by a caproto user:

The IOC code abstracts out the particular server implementation, so IOC authors do not need to interact with the server API directly. The low-level server API is still experimental and subject to change, and it is not yet documented here. We refer interested developers to the source code in caproto.asyncio.server, caproto.curio.server, and caproto.trio.server.

Running multiple servers on one host

For service discovery EPICS primarily relies on UDP broadcast (rsrv also supports multicast, this is not (yet) supported by caproto). To achieve this all of the servers processes on a host bind the canonical port (default 5064) with the SO_REUSEADDR and SO_REUSEPORT settings. These settings allow multiple sockets (across different processes or threads) to bind the same interface and port, see this SO post for more details. When configured this way the UDP sockets:

  • load balance between IOC processes for uni-cast messages (the exact method of load balancing changed with the 3.9 Linux kernel)

  • sent to all IOC processes for broadcast messages

Because you can not broadcast to if you bind your IOCs to the localhost interface you will be able to talk to at most 1 of them from any given client.

If you want to put non-broadcast IPs in EPICS_CA_ADDR_LIST then you can not run more than one IOC per host (because search requests will be load balanced).

If you have a host with more than one interface and you want to bind your IOCs to a specific interface doing so by specifying the broadcast address will work, however in that case any uni-cast searches (ex a specific IP in tho EPICS_CA_ADDR_LIST will be ignored). Binding to the specific interface using the ip adderesss will result in the messages to the broadcast address being ignored and uni-cast messages being load-balanced.

The EPICS wiki has some additional details about how to set up iptables to work around this issue.