PubSubRT - an industrial real-time pub/sub server

What is PSRT

PSRT is a pub/sub real-time telemetry protocol, optimized for industrial needs: providing low latency, dealing with slow channels and large payloads.

PSRT can process 100K+ messages on a single node with very low latencies (<1ms). Speeds are reasonable (1K+ ops/sec) even with enormous (1MB+) payloads.

PSRT is designed to work in wide-area networks, where connections can be slow, unstable and clients must be authenticated. If you need a local-network or local-host solution with even lower latencies, higher speeds, more features, but zero security, try ELBUS.

Topic subscriptions in PubSubRT are processed with B-tree algorithms, which allows the server to handle hundred thousands subscriptions without any speed loss.

Why not MQTT?

We love MQTT. And we use MQTT a lot. There are cases where MQTT ideally fits requirements. However, for some it does not satisfy our speed and reliability needs and produces additional overhead. That is why we invented PSRT and use it as the primary protocol for EVA ICS in large enterprise setups.

What is the difference?

• PSRT is the protocol, optimized for large (65K+) message payloads
• No QoS - all messages are always delivered to subscribers only once, so consider it is always QoS=2 if use MQTT measurements
• No retain topics. Retains usually require disk writes, which produce additional overhead
• No OP-ACK loops. All control operations are fast, synchronous and atomic
• Two TCP sockets: one for control ops and the second one for incoming messages. This makes clients a bit more complicated, but allows to process incoming messages without any extra overhead. Additionally, with two sockets control op acknowledgements and incoming message data can be mixed, which is important when large messages are processed on slow channels
• Devices and nodes, which do not need subscriptions, can use either a single TCP control socket or work without any connection established, using UDP datagrams with or without acknowledge from the server
• PSRT is almost 100% logically compatible with MQTT, so software can be switched to it and vice versa with only a couple of lines of code

What is the same?

PSRT is logically the same as MQTT: same format for topics, same format for topic masks etc:

• path/to/topic - an individual topic (subscribe / publish)
• path/to/# - all topics under the specified path (subscribe)
• path/+/some/+/topic - all topics matching the mask ("+" for any subtopic)

About the repository

This repository contains PubSubRT server, implementing PSRT (open-source version without the cluster module), command-line client and Rust client library. The cluster code is not open-source, but may be opened in the future as well.

Other repositories:

• psrt-py - Python client library (sync), SDK is semi-compatible with paho-mqtt

Installation

Use binaries from https://github.com/alttch/psrt/releases. For Debian/Ubuntu and other deb-based distros, *.deb packages can be used.

Build from source

• Install Rust
• Build the server and cli:

shell cargo build --features server,cli --release

Configuration files

If installed manually, get configuration files from https://github.com/alttch/psrt/tree/main/make-deb/etc/psrtd or use files from test-configs directory (need to be edited before use).

If installed from deb-package, configuration files are stored in /etc/psrtd directory.

The official port numbers, assigned by IANA for PSRT are 2873 TCP/UDP. It is recommended to keep these ports on servers to let clients connect / push data using defaults.

Problems

• If any problems occur, try running psrtd with -v argument to get verbose logging in terminal.

• The most common problem is timeout disconnect. The server timeout MUST be higher than the slowest expected client timeout.

Authentication

PubSubRT uses the standard htpasswd format, use any htpasswd-compatible tool (with -B flag for bcrypt).

Passwords file and ACL can be reloaded on-the-flow. Use either kill -HUP $SERVER_PID or systemctl reload psrtd (if systemd service is configured).

Statistical data

Overview (web interface / API)

Users with admin rights can obtain statistical data using a web browser (by default, at http://localhost:8880).

The data can also be obtained in JSON for 3rd-party apps at:

shell curl http://localhost:8880/status

If the anonymous user has no admin rights, URI requires login and password (HTTP basic auth).

Most used topics

By executing psrt-cli with --top argument, the most used topics can be monitored in console in real-time. Use "s" key to switch sorting between message count and bytes.

Cargo crate (async client)

https://crates.io/crates/psrt

Protocol specifications

https://github.com/alttch/psrt/blob/main/proto.md

UDP encryption

PSRT supports symmetrical encrypted UDP frames (see the protocol specifications). Currently supported encryption modes: AES128-GCM and AES256-GCM.

To enable UDP encryption, add to "auth" section of the main config:

yaml auth: # ........ key_file: keys.yml

The keys file has the following format and there can be only one encryption key per user:

yaml user1: <aes_key> user2: <aes_key>

where aes_key is a random 32-byte (for AES128 only first 16 bytes are used) hex sequence, which can be generated, e.g. with:

shell head -c16384 /dev/urandom|sha256sum|awk '{ print $1 }'

Enterprise version

Download packages from https://pub.bma.ai/psrt-enterprise/

Cluster setup instructions

The Enterprise version can be tested in "unlimited trial" mode. Feel free to download testing key files. Each key file is bound to the specific host name, so the system host names in "unlimited trial" PSRT Enterprise clusters must be "node1", "node2" and "node3".

About the authors

Bohemia Automation / Altertech is a group of companies with 15+ years of experience in the enterprise automation and industrial IoT. Our setups include power plants, factories and urban infrastructure. Largest of them have 1M+ sensors and controlled devices and the bar raises higher and higher every day.