Modern applications often need to deliver information to the browser as it becomes available, rather than waiting until the entire controller action finishes.

Examples include:

• Live progress updates
• Long-running exports
• Real-time dashboards
• AI-generated responses
• Build/deployment logs
• Notifications
• Server-side status updates
• Streaming large files
• Server-Sent Events (SSE)

Rails provides this capability through ActionController::Live.

Rails 8.1 also exposes a particularly useful companion class:

ActionController::Live::SSE

Together, they provide a relatively simple way to implement HTTP streaming and Server-Sent Events directly from a Rails controller.

One important clarification: ActionController::Live itself is not new in Rails 8.1. It has existed for several Rails versions. However, Rails 8.1 continues to provide and refine the streaming infrastructure, including configuration around execution-state sharing. The examples below are based on the Rails 8.1 API.

What is ActionController::Live?

Normally, a Rails controller behaves conceptually like this:

Browser
   |
   | HTTP request
   v
Rails Controller
   |
   | execute entire action
   |
   | generate complete response
   v
Browser receives response

For example:

def report
  result = generate_report
  render json: result
end

The browser generally waits until the action has generated its response.

With ActionController::Live, Rails can instead stream pieces of the response while the action is still executing:

Browser
   |
   | HTTP request
   v
Rails Controller
   |
   | write chunk #1
   |--------------------> Browser
   |
   | write chunk #2
   |--------------------> Browser
   |
   | write chunk #3
   |--------------------> Browser
   |
   | finish

Rails documents ActionController::Live as a module that allows controller actions to stream data to the client as it is written.

Basic ActionController::Live Example

A minimal controller looks like this:

class StreamsController < ApplicationController
  include ActionController::Live

  def show
    response.headers["Content-Type"] = "text/plain"

    5.times do |i|
      response.stream.write "Chunk #{i + 1}\n"
      sleep 1
    end
  ensure
    response.stream.close
  end
end

The important part is:

include ActionController::Live

and then:

response.stream.write(...)

Instead of constructing one large response, the controller writes directly to the response stream.

What happens internally?

Rails executes the streaming action in a separate thread so that the response can begin flowing to the client while the controller continues producing data. Rails 8.1 uses a dedicated cached thread-pool executor for live controller processing.

That distinction is extremely important for production applications.

What is Server-Sent Events?

ActionController::Live is the general streaming mechanism.

SSE is a specific protocol built on top of HTTP streaming.

Server-Sent Events allow the server to continuously send events to a browser over a long-lived HTTP connection.

The browser uses the standard JavaScript API:

const source = new EventSource("/events");
source.onmessage = event => {
  console.log(event.data);
};

The communication is one-way:

Server --------------------> Browser

Unlike WebSockets:

Server <-------------------> Browser

SSE is therefore a good choice when the browser mainly needs to listen for server-side updates rather than continuously send messages back to the server. The browser’s EventSource API maintains the persistent connection and automatically handles reconnection.

ActionController::Live::SSE

Rails provides:

ActionController::Live::SSE

to make SSE formatting easier.

Instead of manually writing:

event: update
data: {"status":"processing"}

Rails can generate the SSE format for you.

The class accepts a stream:

sse = ActionController::Live::SSE.new(response.stream)

and then:

sse.write({ status: "processing" })

Rails converts non-string objects to JSON and writes them using SSE formatting.

Building a Rails SSE Endpoint

Let’s build a realistic example.

Controller

class NotificationsController < ApplicationController
  include ActionController::Live

  def index
    response.headers["Content-Type"] = "text/event-stream"
    response.headers["Cache-Control"] = "no-cache"

    sse = ActionController::Live::SSE.new(
      response.stream,
      retry: 3000
    )

    10.times do |i|
      sse.write(
        {
          message: "Notification #{i + 1}",
          timestamp: Time.current.iso8601
        },
        event: "notification",
        id: i + 1
      )

      sleep 2
    end
  ensure
    sse&.close
  end
end

Rails’ SSE implementation supports three primary options:

:event
:retry
:id

event identifies the event type, retry tells the browser how long to wait before reconnecting, and id becomes the event identifier used for Last-Event-ID on reconnect.

JavaScript Client

The browser can consume the endpoint using EventSource.

const source = new EventSource("/notifications");
source.addEventListener("notification", event => {
  const data = JSON.parse(event.data);
  console.log(data.message);
  console.log(data.timestamp);
});
source.onerror = error => {
  console.error("SSE connection error", error);
};

The browser automatically opens a persistent HTTP connection.

When Rails sends:

event: notification
id: 1
data: {"message":"Notification 1","timestamp":"..."}

the browser invokes:

source.addEventListener("notification", ...)

The SSE wire format is based on text fields such as event, data, id, and retry, with an empty line terminating each event.

ActionController::Live vs ActionController::Live::SSE

This distinction is worth remembering.

Think of it like this:

ActionController::Live
        |
        +---- response.stream.write
        |
        +---- send_stream
        |
        +---- SSE
                 |
                 +---- event
                 +---- data
                 +---- id
                 +---- retry

Streaming a Large CSV

ActionController::Live is not limited to SSE.

A very practical use case is exporting a large dataset.

Rails 8.1 exposes send_stream, specifically for generating data progressively rather than buffering the entire file in memory.

For example:

class ReportsController < ApplicationController
  include ActionController::Live

  def export
    send_stream(
      filename: "users.csv",
      type: "text/csv"
    ) do |stream|

      stream.write "id,email,created_at\n"

      User.find_each do |user|
        stream.write(
          "#{user.id},#{user.email},#{user.created_at.iso8601}\n"
        )
      end
    end
  end
end

This is much better than:

csv = User.find_each.map do |user|
  ...
end

send_data csv

for a very large export.

The second approach potentially builds a large amount of data in memory.

The streaming approach allows Rails to send the output progressively.

A Very Interesting Use Case: AI Streaming

Another practical use case is streaming generated text.

Imagine an AI API returns tokens incrementally:

Hello
Hello, I
Hello, I can
Hello, I can help
Hello, I can help you
...

Instead of waiting for the complete response:

response = ai_client.generate(...)
render json: response

you could expose a streaming endpoint:

class AiController < ApplicationController
  include ActionController::Live

  def stream
    response.headers["Content-Type"] = "text/event-stream"

    sse = ActionController::Live::SSE.new(
      response.stream,
      event: "token"
    )

    ai_client.stream(prompt) do |token|
      sse.write(
        {
          content: token
        }
      )
    end
  ensure
    sse&.close
  end
end

The browser can then update the UI immediately as chunks arrive.

This is one of the reasons HTTP streaming has become particularly relevant for modern applications.

Real-Time Notifications

A very common architecture is:

                    +----------------+
                    | Rails Server   |
                    +--------+-------+
                             |
                             | SSE
                             |
                    +--------v-------+
                    | Browser       |
                    +----------------+

For example:

class NotificationsController < ApplicationController
  include ActionController::Live

  def stream
    response.headers["Content-Type"] = "text/event-stream"

    sse = ActionController::Live::SSE.new(
      response.stream,
      event: "notification"
    )

    loop do
      notification = Notification.pending.first

      if notification
        sse.write(
          {
            id: notification.id,
            message: notification.message
          },
          id: notification.id
        )
      else
        # Heartbeat
        sse.write(": keep-alive")
      end

      sleep 2
    end
  ensure
    sse&.close
  end
end

However, this example introduces an important architectural question.

Where does the event come from?

Polling the database inside every open SSE request is usually not a scalable architecture.

For production systems, you will typically want an event source such as:

Database
   |
   v
Redis / PubSub / Message Broker
   |
   v
Rails SSE endpoint
   |
   v
Browser

That is a much better design than repeatedly querying the database from every connected client.

Heartbeats Matter

Long-lived HTTP connections can be terminated by proxies, load balancers, or infrastructure when no data is transferred for a while.

SSE supports comment messages such as:

: heartbeat

which browsers ignore as application events but still receive as stream traffic.

The SSE format explicitly allows comment lines, and they can be used to keep connections alive.

In Rails:

sse.write(": heartbeat")

or, depending on how you implement the stream, write an SSE comment directly to response.stream.

For an application with long periods of inactivity, heartbeat strategy should be considered part of your production design.

Reconnection and Last-Event-ID

One of the most useful SSE features is event IDs.

Suppose Rails sends:

id: 101
event: order_update
data: {"status":"paid"}

The browser remembers the last event ID.

If the connection is interrupted, the browser may reconnect and send:

Last-Event-ID: 101

Rails’ SSE class supports the id field specifically for this scenario.

Your controller can inspect it:

last_id = request.headers["Last-Event-ID"]

and resume appropriately:

updates = OrderUpdate.where("id > ?", last_id.to_i)
updates.find_each do |update|
  sse.write(
    update.attributes,
    id: update.id,
    event: "order_update"
  )
end

This is significantly more robust than treating every reconnect as a completely new stream.

The Most Important ActionController::Live Caveat: Threads

This is probably the most important thing to understand before introducing ActionController::Live.

Rails executes the streaming action in a separate thread.

Therefore:

class MyController < ApplicationController
  include ActionController::Live
  def stream
    # Runs in streaming execution context
  end
end

should not be treated exactly like a normal synchronous controller action.

Rails explicitly warns that streaming actions need to be thread-safe and should not share unsafe mutable state between threads.

Avoid patterns such as:

@@shared_state = {}
@@shared_state[user_id] = ...

or other mutable global/class-level state unless it is deliberately designed for concurrent access.

Prefer:

Redis
Database
Message broker
Thread-safe abstractions

for shared state.

Rails 8.1: Execution State Sharing

Rails 8.1 exposes:

config.action_controller.live_streaming_excluded_keys

which controls which execution-state keys should not be copied into the streaming thread.

By default, Rails shares execution state from the parent thread.

One important example involves Active Record connection routing.

Rails documents this configuration for cases such as:

ActiveRecord::Base.connected_to(role: :reading) do
  ...
end

where the streaming thread might otherwise inherit the parent’s database connection context.

For example:

config.action_controller.live_streaming_excluded_keys =
  [:active_record_connected_to_stack]

This is a more advanced Rails 8.1 consideration, but it demonstrates an important point:

Streaming is not just “normal controller code with response.stream.write.”

Execution context matters.

Headers Must Be Set Before Streaming

Once you start writing to the stream:

response.stream.write(...)

the response can be committed.

After the response is committed, you cannot safely modify headers.

Rails specifically documents that calling write or close commits the response.

Therefore do this:

response.headers["Content-Type"] = "text/event-stream"
response.headers["Cache-Control"] = "no-cache"
sse = ActionController::Live::SSE.new(response.stream)
sse.write(...)

Not:

sse.write(...)
response.headers["Cache-Control"] = "no-cache"

The second version is too late.

Always Close the Stream

This is another critical rule.

Always ensure the stream closes:

ensure
  sse&.close
end

or:

ensure
  response.stream.close
end

Rails explicitly warns that failing to close the stream can leave the socket open indefinitely.

A production implementation should therefore almost always look like:

begin
  # streaming work
ensure
  # close stream
end

Handling Client Disconnects

A browser can disappear at any time.

For example:

User closes tab
       |
       v
SSE connection disappears
       |
       v
Rails stream encounters disconnect

Rails exposes:

ActionController::Live::ClientDisconnected

for client disconnect situations.

You can handle it explicitly when appropriate:

rescue ActionController::Live::ClientDisconnected
  Rails.logger.info("SSE client disconnected")
ensure
  sse&.close
end

For long-running streams, disconnect handling is especially important because you don’t want server-side work continuing unnecessarily after the browser is gone.

Proxy and Middleware Buffering

A common mistake is to test streaming locally and assume production will behave identically.

You might write:

response.stream.write "hello"
sleep 5
response.stream.write "world"

and expect:

hello

to appear immediately.

But an intermediary could buffer the response.

Possible intermediaries include:

Browser
   |
Load Balancer
   |
Reverse Proxy
   |
Nginx
   |
Rails

Rails itself documents that response buffering can interfere with streaming, including interaction with Rack::ETag in relevant Rack versions.

Therefore streaming should always be tested through the same infrastructure path used in production.

SSE vs WebSockets vs Polling

This is one of the most important architectural decisions.

Use SSE when:

Server -> Browser

is the dominant requirement.

Examples:

Order status
Build progress
Notifications
Stock updates
Live dashboard
AI text streaming
Import progress

Use WebSockets when:

Server <-> Browser

needs continuous two-way communication.

Examples:

Chat
Multiplayer applications
Collaborative editing
Interactive sessions

Use normal HTTP when:

You simply need:

request -> response

There is no reason to introduce streaming complexity for an ordinary CRUD endpoint.

Connection Scalability Is Different

A normal HTTP request may live for:

100 ms
500 ms
2 seconds

An SSE connection may remain open for:

5 minutes
30 minutes
several hours

That changes your capacity model.

Suppose:

10,000 users

each maintain an SSE connection.

That means your infrastructure potentially needs to support:

10,000 long-lived connections

You therefore need to think about:

Web server capacity
Worker/thread usage
File descriptors
Load balancers
Reverse proxies
Timeout configuration
Connection limits
Memory
Monitoring

There is also a browser-level consideration: SSE uses persistent HTTP connections, and connection limits can matter especially under HTTP/1.1; HTTP/2 changes the connection model by multiplexing streams.

Be Careful with Active Record Connections

A particularly important Rails concern is database connection usage.

Consider:

loop do
  users = User.where(active: true)
  ...
  sleep 1
end

inside every SSE request.

If you have hundreds or thousands of clients, you can easily end up with poor database behavior.

A better architecture is usually:

            Event Producer
                 |
       +---------+---------+
       |                   |
     Redis              Broker
       |                   |
       +---------+---------+
                 |
            Rails SSE
                 |
              Browser

The SSE request should ideally wait for events, rather than continuously hammer the database.

A Better Production Architecture

For example, imagine an order-management application.

When an order changes:

Order updated
     |
     v
Publish "order.updated"
     |
     v
Redis / PubSub
     |
     v
SSE connection
     |
     v
Browser updates UI

The Rails controller becomes primarily responsible for:

Connection
   ↓
Subscribe
   ↓
Receive event
   ↓
Serialize event
   ↓
Write SSE
   ↓
Repeat

rather than:

Connection
   ↓
Query database
   ↓
Sleep
   ↓
Query database
   ↓
Sleep
   ↓
Query database

That distinction becomes very important at scale.

Testing an SSE Endpoint

A browser test is useful, but curl is often even more convenient during development.

For example:

curl -N http://localhost:3000/notifications

The -N option prevents curl from buffering output, making the stream easier to observe.

You should see events arrive progressively:

event: notification
id: 1
data: {"message":"Notification 1"}
event: notification
id: 2
data: {"message":"Notification 2"}

This is a very useful debugging technique.

Testing ActionController::Live

For controller tests, streaming requires more consideration than a typical controller action because the response is not necessarily generated as one complete body.

The key things to test are:

Content-Type
Event names
Event IDs
Payload format
Connection termination
Client disconnect handling
Error handling

For example, conceptually:

assert_equal "text/event-stream", response.media_type

and verify that the generated body contains expected SSE fields.

For more complex streaming behavior, integration/system-level testing is generally more valuable than testing only internal controller implementation details.

A Clean SSE Controller Pattern

For a production-style controller, I prefer keeping the controller small:

class EventsController < ApplicationController
  include ActionController::Live

  def index
    prepare_stream_headers

    sse = ActionController::Live::SSE.new(
      response.stream,
      retry: 3_000
    )

    event_stream.each do |event|
      sse.write(
        event.payload,
        event: event.type,
        id: event.id
      )
    end
  rescue ActionController::Live::ClientDisconnected
    Rails.logger.info("SSE client disconnected")
  ensure
    sse&.close
  end

  private

  def prepare_stream_headers
    response.headers["Content-Type"] = "text/event-stream"
    response.headers["Cache-Control"] = "no-cache"
  end

  def event_stream
    # Redis / PubSub / broker subscription
  end
end

The controller handles HTTP concerns, while the event source is delegated elsewhere.

That separation becomes especially valuable when the event system grows.

Advantages of ActionController::Live

Lower time-to-first-byte

The server can start sending data before the complete operation has finished.

Lower memory usage for large streams

You don’t necessarily need to construct the entire response in memory first.

Native HTTP

There is no requirement for a completely different networking protocol.

SSE is simple for browser clients

The browser already provides:

EventSource

Automatic SSE reconnect behavior

SSE includes protocol support for reconnecting and event IDs.

Fits naturally into Rails controllers

You can continue using Rails authentication, routing, controllers, and application services while introducing streaming only where needed.

Disadvantages

Streaming is not free.

Threading complexity

ActionController::Live executes the action in a separate thread.

Long-lived connections

Unlike conventional requests, connections may remain open for long periods.

Capacity planning becomes important

Thousands of connected browsers can have a very different infrastructure impact than thousands of short requests.

Reverse-proxy configuration matters

Buffering and timeout behavior can break an otherwise-correct implementation.

Database usage can become dangerous

Naive polling inside every streaming connection can put significant pressure on PostgreSQL.

Operational complexity

Logging, monitoring, disconnects, reconnects, retries, and infrastructure timeouts all become part of the design.

When Should a Rails Developer Use It?

A good decision rule is:

Do I need data before the complete response is available?
            |
           Yes
            |
            v
Does the client only need server -> browser updates?
            |
         +--+--+
         |     |
        Yes    No
         |      |
         v      v
       SSE    WebSocket

For generic data/file streaming:

ActionController::Live

For browser-facing event streams:

ActionController::Live::SSE

For ordinary request/response APIs:

render json:

is usually the better choice.

What a Senior Rails Developer Should Know Before Using It

Before introducing ActionController::Live, I would explicitly answer these questions:

1. How long will the connection remain open?

Seconds?

Minutes?

Hours?

2. How many simultaneous clients could exist?

100?

1,000?

100,000?

3. What is the event source?

Database?

Redis?

Kafka?

Another service?

4. What happens when the client disconnects?

Can the server stop work immediately?

5. How will reconnects work?

Will events be lost?

Do you need id and Last-Event-ID?

6. What happens behind the load balancer?

Does it buffer?

Does it timeout idle connections?

7. Is your code thread-safe?

Remember that Rails executes Live actions in a separate thread.

8. How will you monitor connections?

You should be able to answer:

How many active SSE connections exist?
How long have they been open?
How many disconnected unexpectedly?
How many events are being delivered?
What is the event delivery latency?

Final Example

A compact Rails 8.1 SSE implementation can look like this:

class EventsController < ApplicationController
  include ActionController::Live

  def stream
    response.headers["Content-Type"] = "text/event-stream"
    response.headers["Cache-Control"] = "no-cache"

    sse = ActionController::Live::SSE.new(
      response.stream,
      retry: 3_000
    )

    10.times do |i|
      sse.write(
        {
          message: "Event #{i + 1}",
          timestamp: Time.current.iso8601
        },
        event: "update",
        id: i + 1
      )

      sleep 1
    end
  rescue ActionController::Live::ClientDisconnected
    Rails.logger.info("Client disconnected")
  ensure
    sse&.close
  end
end

And the client:

const events = new EventSource("/events/stream");
events.addEventListener("update", event => {
  const data = JSON.parse(event.data);
  console.log(data.message);
});
events.onerror = error => {
  console.error("Connection error", error);
};

This small example demonstrates the complete concept:

ActionController::Live
        ↓
HTTP streaming
        ↓
ActionController::Live::SSE
        ↓
text/event-stream
        ↓
Browser EventSource
        ↓
Real-time UI updates

Conclusion

ActionController::Live is Rails’ low-level mechanism for streaming HTTP responses while the controller is still producing them.

ActionController::Live::SSE builds on that mechanism to provide a convenient implementation of Server-Sent Events.

The most important distinction is:

Live = streaming mechanism
SSE  = event-stream protocol

For modern Rails applications, this makes ActionController::Live particularly useful for large exports, progressive responses, logs, long-running operations, and AI output, while SSE is a strong fit for server-to-browser real-time updates.

But the real engineering challenge is usually not writing:

sse.write(...)

The difficult part is designing the surrounding system correctly:

Event source
     ↓
Concurrency
     ↓
Connection lifecycle
     ↓
Reconnect strategy
     ↓
Proxy/load-balancer behavior
     ↓
Database/resource usage
     ↓
Observability

That is where ActionController::Live moves from being a simple Rails API feature to a genuine production architecture decision.

References

Rails 8.1 ActionController::Live API: https://edgeapi.rubyonrails.org/classes/ActionController/Live.html

Rails 8.1 ActionController::Live::SSE API: https://api.rubyonrails.org/classes/ActionController/Live/SSE.html

Rails 8.1 release information:https://guides.rubyonrails.org/8_1_release_notes.html

MDN – Server-Sent Events and EventSource:

https://developer.mozilla.org/en-US/docs/Web/API/Server-sent_events/Using_server-sent_events

https://developer.mozilla.org/en-US/docs/Web/API/EventSource

Happy Implementing!