Backends

Overview

PACSys provides multiple backends that talk to various central services. The DPM backends connect to the DPM server, DMQ talks to DataBroker/Bunny OAC/DAE, etc.

flowchart TB
    subgraph App[Your Application]
        P[Python + pacsys]
    end

    subgraph CS[Central Services]
        DPM[DPM]
        ACL[ACL CGI]
        DMQ[DataBroker<br>Bunny OAC/DAE]
        DB[PostgreSQL]
        CLX[CLX nodes]
    end

    FE[Frontends]
    OT[Other tasks]

    P -->|DPM/HTTP<br>TCP:6802<br>SDD| DPM
    P -->|DPM/gRPC<br>gRPC:50051<br>Protobuf| DPM
    P -->|HTTPS:443<br>CGI script| ACL
    P -->|DMQ<br>TCP:5672<br>AMQP| DMQ
    P -->|DevDB<br>gRPC:50051<br>Protobuf| DB
    P -->|ACL-over-SSH<br>SSH:22| CLX

    CS --> FE
    CS --> OT

Backend Comparison

Feature	DPM/HTTP	DPM/gRPC	DMQ	ACL
Read	✓	✓	✓	✓
Write	✓	✓	✓	-
Stream	✓	✓	✓	-
Auth	Kerberos²	JWT	Kerberos³	None
Permissions	Role/class	Role	Console class	N/A
Protocol	TCP/SDD¹	gRPC/Protobuf	AMQP/SDD¹	HTTP/CGI
Port	6802	50051	5672	443
Factory	pacsys.dpm()	pacsys.grpc()	pacsys.dmq()	pacsys.acl()

¹ SDD = Self-Describing Data, the binary encoding used by protocol compiler in DMQ and DPM/HTTP ² DPM/HTTP: Kerberos required for writes only, reads work anonymously ³ DMQ: Kerberos required for ALL operations (reads, writes, streaming)

---

Implicit Write Conversion

All write-capable backends (DPM/HTTP, DPM/gRPC, DMQ) automatically adjust DRF strings when you call write():

• READING → SETTING: M:OUTTMP becomes M:OUTTMP.SETTING@N
• STATUS → CONTROL: M:OUTTMP.STATUS becomes M:OUTTMP.CONTROL@N
• Already correct: SETTING, CONTROL, ANALOG, DIGITAL are preserved (just @N is forced)

The @N ("never") event tells the server not to send periodic data back - writes are fire-and-confirm, not subscriptions.

---

Backend recommendations

Scenario	Backend
Good-enough read/write	DPM/HTTP
High-performance with JWT token (ask ACORN when this will work)	DPM/gRPC
High-performance read-only (works now)	DPM/gRPC
Fallback if DPM has issues/bugs	DMQ
Last resort	ACL

---

Multiple Backends

You can use different backends simultaneously:

import pacsys
from pacsys import KerberosAuth

# Quick read via ACL (no auth needed)
with pacsys.acl() as acl:
    current = acl.read("M:OUTTMP")

# Write via DPM/HTTP with auth
with pacsys.dpm(auth=KerberosAuth(), role="testing") as dpm:
    dpm.write("M:OUTTMP", current + 1.0)

# Or write via DMQ (no role needed, just Kerberos)
with pacsys.dmq(auth=KerberosAuth()) as dmq:
    dmq.write("M:OUTTMP", current + 1.0)

# Stream via DPM/HTTP
with pacsys.dpm() as dpm:
    with dpm.subscribe(["M:OUTTMP@p,1000"]) as stream:
        for reading, _ in stream.readings(timeout=30):
            print(reading.value)

# Or stream via DMQ
with pacsys.dmq(auth=KerberosAuth()) as dmq:
    with dmq.subscribe(["M:OUTTMP@p,1000"]) as stream:
        for reading, _ in stream.readings(timeout=30):
            print(reading.value)