PUSH Technologies - An Overview
Template:ShortDescription Template:Infobox PublicTech
PUSH technologies allow servers to deliver updates to clients instantly when data changes, without requiring repeated polling. This article compares the six major PUSH mechanisms—Server-Sent Events, WebSockets, MQTT, Long‑Polling, Web Push API, and the Web Push Protocol (RFC 8030)—explaining how each works, where they fit, and how to choose between them.
What “PUSH” Means
PUSH reverses the traditional HTTP request/response flow by enabling servers to send updates the moment something meaningful happens. This requires:
- a persistent or semi‑persistent channel
- a subscription mechanism
- a delivery engine
- event triggers
Summary Comparison Table
| Technology | Directionality | Ideal Use Cases | Complexity |
|---|---|---|---|
| SSE | Server→Client | Notifications, dashboards, workflow updates | Low |
| WebSockets | Full‑duplex | Chat, collaboration, presence | Medium–High |
| MQTT | Pub/Sub | IoT, sensors, distributed systems | Medium |
| Long‑polling | Simulated PUSH | Legacy fallback | Low |
| Web Push API | Server→Browser (via Service Worker) | User notifications, app background updates | Medium |
| Web Push Protocol (RFC 8030) | Server→Push Service→Browser | Reliable delivery, offline notifications | Medium |
Major PUSH Technologies
Server‑Sent Events (SSE)
Lightweight one‑way PUSH over a long‑lived HTTP connection.
WebSockets
Full‑duplex real‑time communication.
MQTT
Pub/sub protocol designed for IoT and telemetry.
Long‑Polling
Held‑open requests until change.
Web Push API
Browser-side mechanism for receiving push notifications.
Web Push Protocol (RFC 8030)
Server-to-push-service protocol enabling browser push delivery.
Universal PUSH Lifecycle
1. Discovery/Intent 2. Channel Establishment 3. Subscription Registration 4. Delivery
When to Use What
- Use SSE for simple one‑way notifications.
- Use WebSockets for collaborative apps.
- Use MQTT for IoT/distributed messaging.
- Use Long‑polling for legacy compatibility.
- Use Web Push API for browser notifications.
- Use Web Push Protocol when interacting with push services.
Deep Dive: Server‑Sent Events (SSE)
Server‑Sent Events (SSE) is a server → client streaming technology defined in the HTML Living Standard. Clients receive UTF‑8 encoded events over a long‑lived HTTP connection via the JavaScript `EventSource` interface. (Spec: WHATWG HTML — Server‑Sent Events)
How it Works
- Client opens a long‑lived HTTP GET request.
- Server streams events using `text/event-stream`.
- Events support `data:`, `id:` and `event:` fields.
- Browsers auto‑reconnect with `Last-Event-ID`.
Example
id: 583
event: order_update
data: {"order":123,"status":"shipped"}
Deep Dive: WebSockets
WebSockets provide full‑duplex, low‑latency communication over a single TCP connection, standardised by IETF RFC 6455.
How it Works
WebSockets create a persistent, full‑duplex communication channel between a client and a server. The connection begins as a normal HTTP/1.1 request, but then upgrades to the WebSocket protocol as defined in IETF RFC 6455. This upgrade transforms the traditional request–response cycle into a continuous, bidirectional data stream. [1](https://pirho-my.sharepoint.com/personal/dex_pirho_net/Documents/Microsoft%20Copilot%20Chat%20Files/PUSH_Technologies_Deep_Dives_MediaWiki.txt)
1. The Opening Handshake
The WebSocket workflow starts with an HTTP `GET` request sent from the client, containing special headers that request a protocol upgrade:
GET /ws HTTP/1.1 Host: example.com Upgrade: websocket Connection: Upgrade Sec-WebSocket-Key: (random base64 string) Sec-WebSocket-Version: 13
If the server supports WebSockets, it responds with a `101 Switching Protocols` message:
HTTP/1.1 101 Switching Protocols Upgrade: websocket Connection: Upgrade Sec-WebSocket-Accept: (derived from Sec-WebSocket-Key)
Once this handshake succeeds, **the connection is no longer HTTP** — it becomes a raw WebSocket channel. [1](https://pirho-my.sharepoint.com/personal/dex_pirho_net/Documents/Microsoft%20Copilot%20Chat%20Files/PUSH_Technologies_Deep_Dives_MediaWiki.txt)
2. Framing and Continuous Communication
After the upgrade, both client and server exchange *WebSocket frames* instead of HTTP messages. RFC 6455 defines several frame types, including:
- **Text frames** — UTF‑8 payloads
- **Binary frames** — arbitrary binary data
- **Continuation frames** — fragmented message parts
- **Ping/Pong frames** — keep‑alive and latency checks
- **Close frames** — graceful shutdown
Each message may be a single frame or span multiple frames, allowing very efficient, low‑latency transfers. [1](https://pirho-my.sharepoint.com/personal/dex_pirho_net/Documents/Microsoft%20Copilot%20Chat%20Files/PUSH_Technologies_Deep_Dives_MediaWiki.txt)
3. Full‑Duplex Operation
Once active, a WebSocket connection allows both endpoints to:
- send data **independently**, without waiting for a request
- send messages at any time
- stream events instantly in either direction
This makes the protocol ideal for scenarios such as live chat, collaboration tools, multiplayer updates, and dashboards — where the server needs to push updates without being polled. [1](https://pirho-my.sharepoint.com/personal/dex_pirho_net/Documents/Microsoft%20Copilot%20Chat%20Files/PUSH_Technologies_Deep_Dives_MediaWiki.txt)
4. Maintaining the Connection
To keep the socket healthy:
- Clients or servers send **ping** frames
- The recipient must reply with **pong** frames
- Missed pings often indicate network failure
- The connection may be closed with a controlled **close** frame
These behaviours are explicitly defined in RFC 6455 as part of WebSocket’s reliability model. [1](https://pirho-my.sharepoint.com/personal/dex_pirho_net/Documents/Microsoft%20Copilot%20Chat%20Files/PUSH_Technologies_Deep_Dives_MediaWiki.txt)
5. Closing the Connection
A WebSocket session ends with a `close` frame containing a close code and optional explanation. After a close handshake, both sides must stop sending frames and release the underlying TCP connection. [1](https://pirho-my.sharepoint.com/personal/dex_pirho_net/Documents/Microsoft%20Copilot%20Chat%20Files/PUSH_Technologies_Deep_Dives_MediaWiki.txt)
Framing
Supports text, binary, continuation, ping/pong and close frames.
Deep Dive: MQTT v5
MQTT v5 is a lightweight publish/subscribe messaging protocol optimised for constrained networks; it is an OASIS Standard.
How it Works
MQTT is a lightweight publish/subscribe messaging protocol designed for efficient communication across unreliable or bandwidth‑constrained networks. Its behaviour and wire format are defined in the OASIS MQTT v5.0 Standard, which specifies the client–server roles, message exchange patterns, quality-of-service levels, session control, and metadata features. [1](https://docs.racket-lang.org/rfc6455/index.html)
1. Client–Broker Architecture
Unlike WebSockets or SSE, MQTT does not create a direct connection between publishers and subscribers. Instead, all communication flows through a central **broker**:
- Clients publish messages to named **topics**
- Subscribers express interest in one or more topics
- The broker routes messages from publishers to all matching subscribers
This architecture decouples senders and receivers in space, time, and execution flow. It is core to MQTT’s suitability for IoT, telemetry, and distributed systems. [1](https://docs.racket-lang.org/rfc6455/index.html)
2. Establishing the Connection
MQTT uses a persistent TCP connection (or any reliable, ordered, bi‑directional transport). A client begins by sending a **CONNECT** packet, which includes:
- protocol version
- client identifier
- optional username/password
- session expiry interval
- authentication data
- Will message (optional message sent on disconnect)
If the broker accepts the session, it responds with a **CONNACK** containing reason codes and connection parameters. These values follow the v5.0 specification. [1](https://docs.racket-lang.org/rfc6455/index.html)
3. Subscribing to Topics
To receive messages, the client sends a **SUBSCRIBE** packet specifying:
- topic filters (e.g., `sensors/temperature`)
- desired QoS level
- optional subscription identifiers
- optional user properties
The broker acknowledges with **SUBACK**, including success/failure reason codes for each subscription. Topic filters may include wildcards, enabling expressive and flexible subscription patterns. [1](https://docs.racket-lang.org/rfc6455/index.html)
4. Publishing Messages
Messages are published with a **PUBLISH** packet, containing:
- topic name
- payload
- QoS level
- retain flag
- user properties
- correlation data (for request/response patterns)
The broker forwards the message to all clients subscribed to the topic.
MQTT v5 supports three QoS modes:
- **QoS 0 – At most once** (fire‑and‑forget)
- **QoS 1 – At least once** (acknowledged delivery)
- **QoS 2 – Exactly once** (two‑phase handshake for guaranteed delivery)
These delivery controls are explicitly defined in the v5.0 standard. [1](https://docs.racket-lang.org/rfc6455/index.html)
5. Session & State Management
MQTT v5 introduces advanced session features, including:
- **Session expiry** (persistent or ephemeral sessions)
- **Reason codes** for all acknowledgements
- **Flow control**
- **Request/response correlation**
- **Topic aliases** to reduce packet size
Clients may reconnect and resume sessions depending on the session expiry interval. If a session is persistent, the broker stores pending messages while the client is offline. [1](https://docs.racket-lang.org/rfc6455/index.html)
6. Heartbeats & Liveness
MQTT maintains connection health through the **Keep Alive** timer:
- Client specifies a heartbeat interval
- Broker expects traffic at least once per interval
- If no packets arrive, the broker closes the connection
Clients also monitor **DISCONNECT** frames, which may include explanatory reason codes as defined in the v5.0 specification. [1](https://docs.racket-lang.org/rfc6455/index.html)
7. Closing the Connection
A client may gracefully end the session by sending **DISCONNECT**, optionally specifying:
- session expiry
- user properties
- reason codes (e.g., normal disconnect, error conditions)
The broker may also send DISCONNECT if protocol violations occur. [1](https://docs.racket-lang.org/rfc6455/index.html)
Key Features
QoS levels, user properties, will messages, shared subscriptions.
Deep Dive: Long‑Polling
Long‑polling simulates PUSH over repeated held HTTP requests.
How it Works
Long‑polling is a technique that simulates server‑initiated updates over traditional HTTP. Instead of opening a persistent connection or upgrading to another protocol, the client sends an HTTP request that the server deliberately holds open until new data becomes available. When the server responds, the client immediately opens a new request, creating the illusion of a continuous stream.
Long‑polling is not a formal protocol; rather, it is a behavioural pattern built on top of standard HTTP and widely used before WebSockets, SSE, and Push APIs became available.
1. Client Sends a Long‑Lived Request
The client sends an HTTP request (typically a `GET` or `POST`) to a dedicated long‑poll endpoint. Unlike normal request/response patterns, the server does **not** respond immediately.
This initial request expresses the client’s intent to “wait for updates.”
2. Server Holds the Connection Open
The server keeps the request open until one of the following happens:
- **New data becomes available**
- **A timeout is reached**
- **The server chooses to return an empty/no‑op response**
While the connection is open, the server does not send partial data (unless using chunked transfer, which becomes a different pattern closer to SSE).
3. Server Responds When an Event Occurs
When something meaningful happens — such as a database update, a new message, or a completed job — the server sends a full HTTP response containing the update.
A typical response might be JSON, XML, or any other application payload.
The request then terminates normally.
4. Client Immediately Reconnects
As soon as the client receives the server’s response, it:
1. processes the data 2. opens a **new** long‑poll request
This cycle continues indefinitely.
The reconnect step is what creates the “pseudo‑realtime” behaviour. Even though the server never initiates a network call, the user experience resembles PUSH.
5. Heartbeats and Timeouts
Most implementations include:
- **server‑side timeouts** to avoid holding dead connections
- **client‑side retry intervals**
- optional **heartbeat messages** if no data is available
Timeouts prevent idle connections from exhausting server resources.
6. Handling Multiple Clients
Because each long‑poll request consumes one connection, servers need to manage concurrency carefully:
- thread‑per‑request models scale poorly
- event‑driven servers (Node.js, NGINX, or Perl event loops) handle long‑polling more efficiently
- horizontal scaling and load balancing are common in production
This is one of the limitations that motivated the creation of WebSockets and SSE.
7. Closing the Connection
Either the client or server may close the connection at any time. If the client closes unexpectedly, the server detects the failure and cleans up resources.
If the server closes due to timeout, a well‑behaved client simply reconnects.
Summary
Long‑polling maintains compatibility with older browsers and servers because it uses plain HTTP, but it is less efficient and less scalable than modern PUSH technologies like SSE, WebSockets, and dedicated push services. It remains a valid fallback technique for legacy environments or systems with restricted networking options.
Deep Dive: Web Push API (W3C)
The Push API enables web apps to receive messages when not active, via Service Workers and a browser push service. (W3C latest published version)
How it Works
The Web Push API enables web applications to receive push messages from a server even when the webpage is not open, using a Service Worker as the delivery endpoint. It is defined by the W3C Push API specification and relies on a browser‑specific push service to route messages from the application server to the user agent. [1](https://archive.org/details/rfc8030)
1. Service Worker Registration
To participate in Web Push, a site must first register a **Service Worker**. Service Workers run independently of webpage lifecycles, allowing notifications to be received when no tab is active. This background execution environment is required before a push subscription can be created. [2](https://www.ns.kogakuin.ac.jp/manabe/pdf/ItoPush.pdf)
2. Creating a Push Subscription
The web application calls `PushManager.subscribe()` from within the Service Worker or client script. This process involves:
- generating an application‑specific key pair
- requesting user permission for notifications
- establishing options such as `userVisibleOnly`
- registering with the browser’s push service
The result is a **PushSubscription** containing:
- a unique endpoint URL
- public encryption keys
- metadata required for the server to send push messages
This endpoint is managed by the push service (e.g., Google, Mozilla, Apple), not the website. [2](https://www.ns.kogakuin.ac.jp/manabe/pdf/ItoPush.pdf)
3. Application Server Sends a Push Message
When the backend needs to notify the user, it sends an encrypted push message to the subscription endpoint. This request always passes through the browser vendor’s **push service**, which:
- validates the request
- stores the message if needed
- forwards it when the user agent is reachable
This routing behaviour is aligned with the specification’s definition of a push service as an intermediary for message delivery. [1](https://archive.org/details/rfc8030)
4. Push Service Delivers the Message
When the user agent becomes reachable, the push service delivers the message. If the user is offline, the push service may temporarily store the message until delivery is possible, allowing the application to receive updates even after downtime. This supports use cases where alerts must be delivered regardless of whether the application is currently open. [3](https://github.com/w3c/push-api)
5. Service Worker Handles the Push Event
Once the browser receives the push message, it launches (or activates) the Service Worker and fires the `push` event. Within this handler, the application can:
- parse the message payload
- update local data
- show a system notification using `ServiceWorkerRegistration.showNotification()`
- trigger background logic
This enables websites to deliver timely messages without maintaining persistent connections. [2](https://www.ns.kogakuin.ac.jp/manabe/pdf/ItoPush.pdf)
6. Displaying Notifications
If the push message requires user-visible activity, the Service Worker displays a notification. This is mandated by some browser quotas: silent pushes may be limited, whereas notifications reset quotas and improve reliability. [2](https://www.ns.kogakuin.ac.jp/manabe/pdf/ItoPush.pdf)
7. Subscription Lifecycle Management
Subscriptions may expire or be invalidated by:
- user revoking permission
- push service endpoint rotation
- browser data being cleared
- service worker replacement
Applications should periodically verify and refresh subscriptions to maintain long‑term delivery reliability. [1](https://archive.org/details/rfc8030)
Deep Dive: Web Push Protocol (RFC 8030)
RFC 8030 defines the HTTP‑based protocol used by application servers to send push messages to push services, which then deliver them to user agents.
How it Works
- App server sends (encrypted) message to a push service over HTTP/2.
- Push service stores and forwards to the user agent when reachable.
- Browser wakes the Service Worker `onpush` handler to process the message.
