Live streaming log files have fascinated me for a long time now. I saw the use of live streaming log when I deployed an app inNetlify for the first time. While deploying, Netlify displays the server log right in the browser so that as a user, we know what's happening in the background.

If you are confused on what I am talking about right now, you can also replicate that behavior if you open the log file with the commandtail -f prepended to the file name like this:tail -f log/development.log

Now if you fire the rails server and access any route, that changes will be appended to the file and shown in the bash where we have openedlog/development.log.

Backstory

In one of the projects I am working on, we have invoicing module and we can create invoices with one click from the browser. Invoicing can take a long time to complete and user will have to wait there without knowing what's going in the background. That was when I began to wonder, what if we also try same thing like Netlify and show the logs to user as it happens in our Rails app server, that will be so cool.

Then I began my research and found this gem of atutorial fromAaron Patterson himself.

It was a 9 years old tutorial but had what we needed to start with. He streams some static code and not the actual file content but that was the start to know more about streaming in Rails. After a day of more research and trial and error, I got the live streaming for the log file to the browser from Rails app working.

Implementation

Let's see step by step how I implemented live log streaming in the browser from Rails App.

Step 1: Create a new Rails app

rails new file-streaming-app

Step 2: Generate a controller for streaming files manually

touch live_file_streams_controller.rb

Add the following code inside

classLiveFileStreamsController<ApplicationControllerend

We are not using rails generator because it also generates view and helpers; which we don't need in this tutorial.

Step 3: Add route for rendering the view

resources:live_streams,only:[]docollectiondoget:log_fileendend

Step 4: Stream with "response.stream"

To enable streaming in our Rails app, we will be usingresponse.stream fromActionController::Streaming.

classLiveStreamsController<ApplicationControllerdeflog_file5.times{response.stream.write"hello world\n"sleep0.2}response.stream.closeendend

Read more about response streaming inofficial Rails documentation.

Step 5: View response in browser

• Fire rails serverrails s
• Go tolocalhost:3000/live_streams/log_file
• You will see "hello world" printed 5 times in the browser
• Response is printed at the same time, even though we used sleep function in betweenresponse.write

Let's print them one by one next.

Step 6: IncludeActionController::Live for live streaming response

classLiveStreamsController<ApplicationControllerincludeActionController::Livedeflog_file5.times{response.stream.write"hello world\n"sleep0.2}response.stream.closeendend

ActionController::Live adds streaming functionality to all actions inside the controller.

Step 7: Response stays the same. What happened?

There is a bug inside the rack gem which is sending response at once instead of live streaming. You can find the issue discussionhere.

As suggested in one of thecomments in the discussion, let's add "Last-Modified" inresponse.headers with current time.

Let's also add "Content-Type" toresponse.headers with "text/event-stream" so that our response are actually streamed and displayed one by one.

deflog_fileresponse.headers['Content-Type']='text/event-stream'# hack due to new version of rack not supporting sse and sending all response at once: https://github.com/rack/rack/issues/1619#issuecomment-848460528response.headers['Last-Modified']=Time.now.httpdate5.times{response.stream.write"hello world\n"sleep0.2}response.stream.closeend

You should be able to see "hello world" printed one by one like below:

Wow! We live streamed something! 🤩

Step 8: Server side events

From Aaron's blog:

If you’ve never heard of Server-Sent Events (from here on we will be calling them SSEs), it’s a feature of HTML5 that allows long polling, but is built in to the browser. Basically, the browser keeps a connection open to the server, and fires an event in JavaScript every time the server sends data.

You can read further about ithere

Step 9: Create "file_streaming_app/sse.rb"

To emit events and format the response instead of inside controller, we will be creating a new class calledfile_streaming_app/sse insidelib folder.

Create the file with:touch lib/file_streaming_app/sse.rb

Add following to it:

require'json'moduleFileStreamingAppclassSSEdefinitialize(io)@io=ioenddefwrite(object)@io.write"#{JSON.dump(object)}"enddefclose@io.closeendendend

Step 10: Use "SSE" class inside the controller

require'file_streaming_app/sse'classLiveStreamsController<ApplicationControllerdeflog_fileresponse.headers['Content-Type']='text/event-stream'# hack due to new version of rack not supporting sse and sending all response at once: https://github.com/rack/rack/issues/1619#issuecomment-848460528response.headers['Last-Modified']=Time.now.httpdatesse=FileStreamingApp::SSE.new(response.stream)5.times{sse.write('hello world')sleep0.5}ensuresse.closeendend

Response shouldn't have much different apart fromhello world changed to"hello world".

NOTE: Only copy changed lines (Don't override the controller)

Next, we will stream our actual log file.

Step 11: Add "filewatcher" gem to watch changes in file

To know when file is changed, we will be using file watcher gem.File watcher gem watches the files for different events (or changes) like create, update, delete. It was the best gem I could find for our purpose, I tried other gems like:

• rb-fsevent doesn't fire the event when file is modified in background by rails, had to dotouch log/development.log every time to run the code inside watcher. Also, it didn't support file path, instead we had to always provide folder path.
• ruby-filewatch was working flawlessly but the project was not maintained actively
• listen rails uses this gem to auto load files after change so we don't have to reload server after every change to file. This also acted in the same way as rb-fsevent

gem'filewatcher','~> 1.1.1'# specify latest version here and not 1.1.1, this was the latest at the time of writing this tutorial

Don't forget to install gem withbundle install

Step 12: Create "file_streaming_app/log_file.rb"

To get all lines inside the file in array, we will be creating a new class calledfile_streaming_app/log_file insidelib folder. This should normally have been a util, but to show only newly added lines, we need instance variable to store the last line position, hence we will be creating new class.

Create the file with:touch lib/file_streaming_app/log_file.rb

Add following code to it:

moduleFileStreamingAppclassLogFiledefadded_lines(file_path)file_content=File.open(file_path).readlinesfile_content.last(20)endendend

File.open(file_path).readlines returns all array of all lines inside the file.

For now, we will only print last 20 lines of the file when it is modified, henceadded_lines is doing what we want with.last(20)

Step 13: Stream file content when it is modified

Update controller with the following code:

deflog_fileresponse.headers['Content-Type']='text/event-stream'# hack due to new version of rack not supporting sse and sending all response at once: https://github.com/rack/rack/issues/1619#issuecomment-848460528response.headers['Last-Modified']=Time.now.httpdatesse=FileStreamingApp::SSE.new(response.stream)log_file_path=Rails.root.join('log/development.log').to_sfile=FileStreamingApp::LogFile.new# watch development.log file for changesFilewatcher.new([log_file_path]).watchdo|_file_path,event_type|nextunlessevent_type.to_s.eql?('updated')file_lines=file.added_lines(log_file_path)sse.write(file_lines)endensuresse.closeend

Here, we are usingFileWatcher to watch for changes in the file given in thelog_file_path i.e. we are watching changes insidelog/development.log only.

We only want to stream the content of file when something is added to it, so we are ignoring other event types withnext unless event_type.to_s.eql?('updated')

Finally, we are sending array of lines inside the file to write to browser withsse.write(file_lines)

Step 15: Update "SSE" to print array of file lines

Previously, we were just rendering string and using JSON to dump that data and print to browser. But now, we have array of lines from the file and we need to print them line by line in the browser.

Let's update the SSE class with following code to reflect the changes:

defwrite(file_lines)file_lines.eachdo|line|@io.writelineendend

Step 16: View changes in file in the browser

To emit the event and print the content of file to the browser we will first need to find a way to modify thedevelopment.log.

• Reload the browser where streaming url is open
• In new tab, open rails default view:localhost:3000
• When this page loads, log file will be modified and streaming api will be called, which then renders the last 20 lines from the file to the browser

We have now streamed the file content every time the file is modified, next step for us will be to stream only added lines.

Step 17: Parallel Requests

By default, in Rails development environment, requests are not served parallelly and you may be facing the issue of browser just hanging when trying to open two urls at the same time.

To resolve that, let's add a little hack fromStack Overflow.

Add the following to yourconfig/environments/development.rb

Rails.application.configuredo# other configurationsconfig.middleware.deleteRack::Lockend

Step 18: Stream only changed lines in the log file

For streaming only changed lines, "LogFile" will need to remember the position of the last line in the log file before the change and render lines after that position only.

Let's update theLogFile to make that possible.

classLogFiledefadded_lines(file_path)file_content=File.open(file_path).readlinestotal_lines=file_content.length@last_known_line_position||=initial_line_position(total_lines)start_position=@last_known_line_position@last_known_line_position=total_linesfile_content[start_position,total_lines]endprivatedefinitial_line_position(total_lines)return0iftotal_lines.zero?||total_lines<=20# print last 20 lines from the file if event is emitted for the first timetotal_lines-20endend

initial_line_position returns the start position of the line in the file to display in the browser when the event is emitted for the first time.

@last_known_line_position ||= initial_line_position(total_lines) sets the position of the line in the file during previous event. If the@last_known_line_position is empty,initial_line_position will be used.

file_content[start_position, total_lines] gets array items from the given start and end position and we get lines that were added recently.

Conclusion

If you have reached this section, we have come far together. Congratulations!

Though in this tutorial, we only streamed log file; this implementation applies for streaming any files.

Code of this blog is available atLog File Live Streamer [Github]

Thank you for reading. Happy live streaming!

References

• Is it live? [Aaron Patterson's Blog]

Image Credits

• Cover Image byNadjib BR onUnsplash