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ActiveRecord Under a Fiber Reactor

Written by Claude Fable 5.1 on 2026-09-01, last edited 2026-09-03 · History · Prompts

The Problem

On February 4, 2023, we flipped one line in the Rails config of Hello Weather from :fiber back to :thread. The comment next to it said “Beware ActiveRecord::ConnectionTimeoutError if changing to :fiber”. Two days later we deleted the line, pinned Rails to 7.0.x, and listed the errors from that weekend’s database outage in the commit message. The setting didn’t come back without a workaround until August 2024.

The setting is config.active_support.isolation_level. Rails keeps some state per request, like which database connection the request is using, and this setting tells Rails what counts as “the current request”: the current thread or the current fiber. Under Puma each request runs on its own thread, so the thread is the obvious answer. Under Falcon every request in a worker process is a fiber (a lightweight task Ruby switches between), and all of them run on one thread. That’s where the two answers split, and ActiveRecord’s connection pool is the first place it hurts.

The pool tracks who holds which connection with a lease, one per “current request”. Here’s how it looks the lease up, quoted from the Rails 8.1 source:

# activesupport/lib/active_support/isolated_execution_state.rb (trimmed)
def isolation_level=(level)
  @scope = case level
           when :thread then Thread
           when :fiber  then Fiber
           end
end

def context
  @scope.current
end

# activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb (trimmed)
def connection_lease
  @leases[ActiveSupport::IsolatedExecutionState.context]
end

def lease_connection
  lease = connection_lease
  lease.connection ||= checkout
  lease.sticky = true
  lease.connection
end

Under :thread, every request fiber on a Falcon worker gets the same Thread.current, so they all share one lease and one connection. Their queries wait on each other, and per-request state like the query cache leaks between requests that should never see each other. Under :fiber, each request gets its own lease. That’s the right behavior, and it’s also why our first attempt failed. In Rails 7.0 and 7.1, once a request took a lease it kept the connection until the request finished. A worker with thirty requests in flight wanted thirty connections from a pool of five. The rest waited five seconds and then raised ActiveRecord::ConnectionTimeoutError.

The Solution

There wasn’t one fix. The setting went through four states over four years, and each change waited on something outside the app changing first.

On, then off

We added the setting on August 27, 2022, on Rails 7.0.3 and Falcon 0.42, as one line with no comment. The app had been on Falcon for a year by then, and we’d picked Falcon because it runs each request in its own fiber.

The February 4, 2023 rollback is the commit with the warning comment. The follow-up on February 6 removed the line, pinned Rails, and listed four errors from the outage: ActiveModel::MissingAttributeError on an attribute with an empty name, ActiveRecord::UnknownPrimaryKey on a table that plainly had one, Async::Stop inside a find_by, and the connection timeout. The first two mean the schema cache was corrupted, which happens when two fibers write to shared state at the same time. The commit said “I’m not sure if some changes to Rails may have caused the issue or not”. We decided to run :thread and watch the Rails issues about fiber-safe pools.

By early 2024 Falcon set :fiber by default. Our January 30, 2024 upgrade to Rails 7.1 still kept the app on :thread, with a comment linking six discussions, including the Rails pull request that later became the 7.2 connection-handling change. The cost of staying on :thread is the one above: concurrent requests on a worker share a connection and share state. It caused no visible trouble at low traffic, but it was still wrong.

Back on, behind a scoped lease

On February 2, 2024 we switched back to :fiber. The commit cited socketry/falcon#219 and gave three reasons the outage wouldn’t repeat: a connection pooler now sat between the app and Postgres, the database was bigger, and the app no longer held a connection for the whole request. That last one was the workaround. Every request reads two database records up front, the account and its settings. We moved those two reads into a with_connection block in the request object’s constructor:

class ForecastRequest
  def initialize(args)
    @api_key = args[:api_key]
    initialize_database_records_with_connection
  end

  # Re: isolation_level fiber https://github.com/socketry/falcon/pull/219
  # A workaround until Rails improves connection handling: the connection
  # is released at the end of this block, not at the end of the request.
  def initialize_database_records_with_connection
    ActiveRecord::Base.connection_pool.with_connection do
      account
      settings
    end
  end

  def account
    @_account ||= Account.find_by(api_key: @api_key)
  end

  def settings
    @_settings ||= Settings.find_by(account: account)
  end
end

The block works because of the ||= on account and settings. Both records are loaded and cached while the connection is held. The rest of the request spends its time waiting on upstream weather sources and never touches the pool again. A request held a connection for two queries instead of its whole lifetime, so a five-connection pool could serve far more than five requests.

The limit is that nothing enforced it. Any query that ran later in the request would hold a connection the old way until the request ended, and nothing would tell you.

Rails 7.2 deletes the workaround

Rails 7.2 changed how model queries get their connection. Each query now runs inside with_connection, which checks out a connection for that query and returns it afterward, unless something has already leased one for the request. In 8.1 the pool’s with_connection reads like this:

def with_connection(prevent_permanent_checkout: false)
  lease = connection_lease
  if lease.connection
    yield lease.connection
  else
    begin
      yield lease.connection = checkout
    ensure
      release_connection(lease) unless lease.sticky
    end
  end
end

The sticky flag is the difference. lease_connection, and the old ActiveRecord::Base.connection that calls it, mark the lease sticky, and the connection stays out until the request finishes. A plain query leaves it unset, so the connection goes back in the ensure. Every query now did on its own what our constructor block did, so the August 13, 2024 upgrade to Rails 7.2 deleted the block. The same commit added one line:

# config/application.rb
config.active_record.permanent_connection_checkout = :disallowed
config.active_support.isolation_level = :fiber

permanent_connection_checkout controls what happens when code calls ActiveRecord::Base.connection. The default, true, allows it. :deprecated warns, and :disallowed raises. Here’s the check in 8.1:

# activerecord/lib/active_record/connection_handling.rb (trimmed)
def connection
  pool = connection_pool
  if pool.permanent_lease?
    case ActiveRecord.permanent_connection_checkout
    when :disallowed
      raise ActiveRecordError, "Called deprecated `ActiveRecord::Base.connection` method. " \
                               "Either use `with_connection` or `lease_connection`."
    end
    pool.lease_connection
  else
    pool.active_connection
  end
end

For a fiber-per-request app this is the setting that matters. A permanent lease means a request holds a connection until it finishes, and you can’t see that in production until the pool runs dry during a spike. With :disallowed, the first code path that would take one raises in development, with the caller in the backtrace. The limit is that it only guards Base.connection. lease_connection is still allowed, and so is an open transaction, so a long transaction on the request path still holds a connection for as long as it runs.

An unlimited pool behind a pooler

Once leases were scoped to queries, the pool size no longer capped how many requests could run at once. It only capped how many queries could run at the same instant. In December 2025 a job wrote roughly 9,300 rows in under a second while the request path was writing hit counters, and the default pool of five ran out. The timeouts came back. The fix was to stop enforcing a limit on the client side:

# config/database.yml
default: &default
  adapter: postgresql
  encoding: unicode
  max_connections: -1

The real limit lives in the connection pooler in front of Postgres. Rails opens connections only when needed, so an unlimited pool never opens more than the process is actually using. Rails 8.1 renamed pool to max_connections and documents -1 as unlimited. Our first attempt wrote max_connections: nil. YAML reads that as the string "nil", to_i turns it into zero, and a pool of zero times out on every checkout in every environment. We fixed it the same day, which is why the value is -1 and not null.

The limit is that this only works with a pooler that caps connections on the server side. Without one, an unlimited client pool under a fiber server can use up every Postgres connection from a single dyno.

The setting catches a gem regression

On July 26, 2026 a routine dependency update took Solid Queue to 1.5.0, and the job dashboard started returning 500. The new release had added a module that called ActiveRecord::Base.connection, the form that :disallowed refuses. Under the default setting that call would have held a connection for the length of each dashboard request and nobody would have noticed. Here it raised on every page load and the backtrace named the gem file. We pinned the gem below 1.5 with the upstream link on the same line, and submitted rails/solid_queue#769. The fix shipped in 1.5.1, and on August 3 we dropped the pin and the row in the plans file that had been waiting for that release.

The regression test loads the dashboard, and its comment says what it’s for:

class JobsTest < ActionDispatch::IntegrationTest
  # Catches gems breaking under permanent_connection_checkout = :disallowed
  test "jobs dashboard loads" do
    get "/jobs", headers: basic_auth_headers
    assert_response :success
  end
end

The limit is that it covers one page of one gem. A different gem calling Base.connection from a job would show up in the worker’s error reporter, not in CI.

A thread inside the reactor

One write path leaves the reactor on purpose. A buffered counter mode, turned on by an environment variable and off by default, collects per-request increments in a class-level hash and writes them from a separate thread. Under a fiber server, a plain Thread.new gets none of the setup a request gets: no executor, no query cache, and a lease keyed by a fiber Rails has never seen. So the flush wraps itself:

def flush_entries(entries)
  return if entries.empty?

  Rails.application.executor.wrap do
    ActiveRecord::Base.connection_pool.with_connection do
      entries.each { |key, counters| Counter.add(key, **counters) }
    end
  end
rescue StandardError => exception
  ErrorReporter.capture(exception)
end

executor.wrap runs the same hooks a request gets, including releasing whatever the block leased when it ends. with_connection limits the checkout to the flush. It’s two lines, and it’s the right two lines for any thread a fiber app starts on purpose. The excerpt is trimmed from a class that also handles a deferred mode and a flush threshold.

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Lessons Learned