push time

Push time is the moment at which a remote server initiates transmission of data to a client device without the client having requested it. Unlike pull operations, where a device polls a server at intervals to check for updates, a push sends information on the server's schedule. This matters because it reduces latency, saves bandwidth on repeated polling queries, and keeps distributed systems more tightly synchronized.

In electrical and control engineering, push time appears most often in industrial IoT deployments, supervisory control systems, and real-time telemetry. A programmable logic controller (PLC) may push sensor readings to a central historian database at fixed intervals, or a building management system may push alarm notifications to a technician's display the moment a threshold is crossed. The server decides the push time based on application logic: periodic intervals, event triggers, or priority levels.

Push vs. Pull Trade-offs

Pull-based systems require the client to initiate every transaction, which adds overhead if the client checks frequently but wastes nothing if checks are rare. Push eliminates unnecessary queries but requires the server to maintain persistent connections and manage state for each client. In battery-powered field devices, push can reduce power consumption since the device does not wake to check; in high-latency networks, push introduces dependency on reliable delivery mechanisms like message queues or acknowledgment protocols.

Push time in practice depends on the communication protocol. MQTT brokers push messages based on topic subscriptions; HTTP servers use Server-Sent Events or webhooks to approximate push; industrial protocols like OPC UA support both subscription-based push and periodic polling. The actual time delay between the event and delivery includes network propagation, server processing, and any buffering in middleware.

Failures in push delivery are common: network interruptions, client downtime, or server restarts can cause data loss if no retry logic is implemented. This is why critical systems often combine push notifications with occasional pull queries as a safety net, or use persistent message brokers that queue updates until delivery succeeds.