Full documentation for the latest released version can be found at: https://www.rubydoc.info/gems/progressor

Here's an example long-running task:

Product.find_each do |product|
  next if product.not_something_we_want_to_process?
  product.calculate_interesting_stats
end

In order to understand how it's progressing, we might add some print statements:

Product.find_each do |product|
  if product.not_something_we_want_to_process?
    puts "Skipping product: #{product.id}"
    next
  end

  puts "Working on product: #{product.id}"
  product.calculate_interesting_stats
end

This gives us some indication of progress, but no idea how much time is left. We could take a count and maintain a manual index, and then eyeball it based on how fast the numbers are adding up. Progressor automates that process:

progressor = Progressor.new(total_count: Product.count)

Product.find_each do |product|
  if product.not_something_we_want_to_process?
    progressor.skip(1)
    next
  end

  progressor.run do |progress|
    puts "[#{progress}] Product #{product.id}"
    product.calculate_interesting_stats
  end
end

Each invocation of run measures how long its block took and records it. The yielded progress parameter is an object that can be to_s-ed to provide progress information.

The output might look like this:

...
[0038/1000, (004%), t/i: 0.5s, ETA: 8m:00s] Product 38
[0039/1000, (004%), t/i: 0.5s, ETA: 7m:58s] Product 39
[0040/1000, (004%), t/i: 0.5s, ETA: 7m:57s] Product 40
...

You can check the documentation for the Progressor class for details on the methods you can call to get the individual pieces of data shown in the report.

Initializing a Progressor with a provided total_count: parameter gives you a limited sequence, which can give you not only a progress report, but an estimation of when it'll be done:

[<current loop>/<total count>, (<progress>%), t/i: <time per iteration>, ETA: <time until it's done>]

The calculation is done by maintaining a list of measurements with a limited size, and a list of averages of those measurements. The average of averages is the "time per iteration" and it's multiplied by the remaining count to produce the estimation.

I can't really say how reliable this is, but it seems to provide smoothly changing estimations that seem more or less correct to me, for similarly-sized chunks of work per iteration.

Not providing a total_count: parameter leads to less available information:

progressor = Progressor.new

(1..100).each do |i|
  progressor.run do |progress|
    sleep rand
    puts progress
  end
end

A sample of output might look like this:

...
11, t: 5.32s, t/i: 442.39ms
12, t: 5.58s, t/i: 446.11ms
...

The format is:

<current>, t: <time from start>, t/i: <time per iteration>

For ActiveRecord and other iterable collections, it's possible to skip some boilerplate. For example, you might start from this:

Record.not_processed.find_each do |record|
  record.process
end

In order to add measurements, you could instantiate a Progressor etc like in the above example, or you could do this:

Progressor::Iteration.find_each(Record.not_processed) do |record|
  record.process
end

And it'll add some default print statements. Check the API documentation or the code for details.

Apart from total_count, which is optional and affects the kind of sequence that will be stored, you can provide min_samples and max_samples. You can also provide a custom formatter:

progressor = Progressor.new({
  total_count: 1000,
  min_samples: 5,
  max_samples: 10,
  formatter: -> (p) { p.eta }
})

The option min_samples determines how many loops the tool will wait until trying to produce an estimation. A higher number means no information in the beginning, but no wild fluctuations, either. It needs to be at least 1 and the default is 1.

The option max_samples is how many measurements will be retained. Those measurements will be averaged, and then those averages averaged to get a time-per-iteration estimate. A smaller number means giving more weight to later events, while a larger one would average over a larger amount of samples. The default is 100.

The formatter is a callback that gets a progress object as an argument and you can return your own string to output on every loop. Check LimitedSequence and UnlimitedSequence for the available methods and accessors you can use.

A very similar tool is the gem ke. It provides its estimation by maintaining the median quartile range of the stored measurements, removing outliers. It also automates the output of the progress report, only printing it every N loops. Depending on your needs and preferences, it might be better for your use case.