Dynos and the Dyno Manager

Last Updated: 15 June 2015

dyno dyno manager memory

Table of Contents

This article applies to the new dyno types on Heroku. If your application is still using the traditional dyno types, please refer to the Traditional Dynos article instead. The traditional dyno types offer different behavior than what is documented here.


A dyno is a lightweight Linux container that runs a single user-specified command. A dyno can run any command available in its default environment (what we supply in the Cedar stack) or in your app’s slug (a compressed and pre-packaged copy of your application and its dependencies).

For information about dyno pricing, see the Heroku pricing overview.

Terminology: Containerization is a virtualization technology that allows multiple isolated operating system containers to be run on a shared host. A process in one container cannot see anything about another process in another container.

Types of dynos

Heroku runs dynos in three different ways:

  • Web Dynos: Web dynos are dynos of the “web” process type that is defined in your Procfile. Only web dynos receive HTTP traffic from Heroku’s routers.

  • Worker Dynos: Worker dynos can be of any process type declared in your Procfile, other than “web”. Worker dynos are typically used for background jobs, queueing systems, and timed jobs. You can have multiple kinds of worker dynos in your application. For example, one for urgent jobs and another for long-running jobs. For more information, see Worker Dynos, Background Jobs and Queueing.

  • One-off Dynos: One-off dynos are temporary dynos that can run detached, or with their input/output attached to your local terminal. They’re loaded with your latest release. They can be used to handle administrative tasks, such as database migrations and console sessions. They can also be used to run occasional background work, as with Heroku Scheduler. For more information, see One-Off Dynos

Once a web or worker dyno is started, the dyno formation of your app will change (the number of running dynos of each process type) - and subject to dyno lifecycle, Heroku will continue to maintain that dyno formation until you change it. One-off dynos, on the other hand, are only expected to run a short-lived command and then exit, not affecting your dyno formation.

The Dyno Manager

The dyno manager keeps dynos running automatically; so operating your app is generally hands-off and maintenance free.


To scale horizontally (scale out), add more dynos. For example, adding more web dynos allows you to handle more concurrent HTTP requests, and therefore higher volumes of traffic. For more information, see Scaling Your Dyno Formation.

To scale vertically (scale up), use bigger dynos. The maximum amount of RAM available to your application depends on the dyno type you use. For more information, see Dyno Types.

Both horizontal and vertical scale are features of the professional dynos, and are not available to free or hobby dynos.


Applications with multiple running dynos will be more redundant against failure. If some dynos are lost, the application can continue to process requests while the missing dynos are replaced. Typically, lost dynos restart promptly, but in the case of a catastrophic failure, it can take more time. Multiple dynos are also more likely to run on different physical infrastructure (for example, separate AWS Availability Zones), further increasing redundancy.

Isolation and security

Dynos execute in complete isolation from one another, even when on the same physical infrastructure. This provides protection from other application processes and system-level processes consuming all available resources. The dyno manager uses a variety of technologies to enforce this isolation, most notably LXC for subvirtualized resource and process table isolation, independent filesystem namespaces, and the pivot_root syscall for filesystem isolation. These technologies provide security and evenly allocate resources such as CPU and memory in Heroku’s multi-tenant dyno types.

Ephemeral filesystem

Each dyno gets its own ephemeral filesystem, with a fresh copy of the most recently deployed code. During the dyno’s lifetime its running processes can use the filesystem as a temporary scratchpad, but no files that are written are visible to processes in any other dyno and any files written will be discarded the moment the dyno is stopped or restarted. For example, this occurs any time a dyno is replaced due to application deployment and approximately once a day as part of normal dyno management.

IP addresses

When running multiple dynos, apps are distributed across several nodes by the dyno manager. Access to your app always goes through the routers. As a result, dynos don’t have static IP addresses. While you can never connect to a dyno directly, it is possible to originate outgoing requests from a dyno. However, you can count on the dyno’s IP address changing as it gets restarted in different places.

Network interfaces

The dyno manager allocates each dyno a separate network interface. Dynos are only reachable from outside Heroku via the routers at their assigned $PORT. Individual processes within a dyno can bind to any address or port they want and communicate among them using e.g. standard TCP. The external networking interface (i.e.: eth0) for each dyno will be part of a /30 private subnet in the range, such as or Processes within one dyno don’t share IPs or subnets with other dynos, nor can they observe TCP session state of other dynos.

CLI commands for dyno management

To view and modify your app’s dyno settings, you can use the Heroku CLI that is included with the Heroku toolbelt.

Task Example See Also
List the dynos for an app heroku ps Scaling
Start worker dynos. (Look at your Procfile to see the worker process types that are defined for your app) heroku ps:scale worker=2 Scaling
Stop a particular dyno type heroku ps:stop worker Scaling
Stop a particular dyno heroku ps:stop worker.2 Scaling
Restart all dynos heroku ps:restart Dyno Manager
Restart a particular dyno type heroku ps:restart web Dyno Manager
Restart a particular dyno heroku ps:restart web.1 Dyno Manager
Scale horizontally (Add more dynos) heroku ps:scale web=2 Scaling
Scale horizontally by incrementing the current number of dynos heroku ps:scale web+5 Scaling
Scale different dyno types horizontally at the same time heroku ps:scale web=1 worker=5 Scaling
Scale vertically (Use bigger dynos) heroku ps:resize worker=standard-2x Dyno Types
Scale horizontally and vertically at the same time. This example scales the number of web dynos to 3 and resizes them to performance heroku ps:scale web=3:performance Dyno Types
Get help for the heroku ps command heroku ps --help
Launch a one-off dyno that runs bash in a console heroku run bash One-Off Dynos
Launch a one-off dyno that runs the “worker” process type that is present in your application’s Procfile heroku run worker One-Off Dynos
View logs heroku logs or heroku logs --tail Logging

It is also possible to modify some of your app’s dyno settings with the Heroku Dashboard.

Dyno sleeping

Only free dynos will sleep. For more information, see Dyno Sleeping.


The .profile file

During startup, the container starts a bash shell that runs any code in $HOME/.profile before executing the dyno’s command. You can put bash code in this file to manipulate the initial environment, at runtime, for all dyno types in your app.

The .profile script will be sourced after the app’s config vars. To have the config vars take precedence, use a technique like that shown here with LANG.

# add vendor binaries to the path
export PATH=$PATH:$HOME/vendor/bin

# set a default LANG if it does not exist in the environment
export LANG=${LANG:-en_US.UTF-8}

For most purposes, config vars are more convenient and flexible than .profile. You need not push new code to edit config vars, whereas .profile is part of your source tree and must be edited and deployed like any code change.

Local environment variables

The Dyno Manager sets up a number of default environment variables that you can access in your application.

  • If the dyno is a web dyno, the $PORT variable will be set. The dyno must bind to this port number to receive incoming requests.

The $DYNO variable is experimental and subject to change or removal. Also, $DYNO is not guaranteed to be unique within an app. For example, during a deploy or restart, the same dyno identifier could be used for two running dynos. It will be eventually consistent, however.

  • The $DYNO variable will be set to the dyno identifier. e.g. web.1, worker.2, run.9157.


After the .profile script is executed, the dyno executes the command associated with the process type of the dyno. For example, if the dyno is a web dyno, then the command in the Procfile associated with the web process type will be executed.

Any command that’s executed may spawn additional processes.

Orphan processes within a dyno will be regularly reaped to prevent the accumulation of zombie/defunct processes.

Process/thread limits

The maximum number of processes/threads that can exist in a dyno at any one time depends on dyno type:

  • free, hobby and standard-1x dynos support no more than 256
  • standard-2x dynos support no more than 512
  • performance dynos support no more than 32768

These limits include all processes and threads, whether they are executing, sleeping or in any other state. Note that the dyno counts threads and processes towards this limit. For example, a standard-1x dyno with 255 threads and one process is at the limit, as is a dyno with 256 processes.

Web dynos

A web dyno must bind to its assigned $PORT within 60 seconds of startup. If it doesn’t, it is terminated by the dyno manager and a R10 Boot Timeout error is logged. Processes can bind to other ports before and after binding to $PORT.

Contact support to increase this limit to 120 seconds on a per-application basis. In general, slow boot times will make it harder to deploy your application and will make recovery from dyno failures slower, so this should be considered a temporary solution.


Automatic dyno restarts

The dyno manager restarts all your app’s dynos whenever you:

Dynos are also restarted at least once per day, in addition to being restarted as needed for the overall health of the system and your app. For example, the dyno manager occasionally detects a fault in the underlying hardware and needs to move your dyno to a new physical location. These things happen transparently and automatically on a regular basis and are logged to your application logs.

Dynos are also restarted if the processes running in the dyno exit. The cases when the processes running in a dyno can exit are as follows:

  • Defect in startup code - If your app is missing a critical dependency, or has any other problem during startup, it will exit immediately with a stack trace.
  • Transient error on a resource used during startup - If your app accesses a resource during startup, and that resource is offline, it may exit. For example, if you’re using Amazon RDS as your database and didn’t create a security group ingress for your Heroku app, your app will generate an error or time out trying to boot.
  • Segfault in a binary library - If your app uses a binary library (for example, an XML parser), and that library crashes, then it may take your entire application with it. Exception handling can’t trap it, so your process will die.
  • Interpreter or compiler bug - The rare case of a bug in an interpreter (Ruby, Python) or in the results of compilation (Java, Scala) can take down your process.

Dyno crash restart policy

A dyno “crash” represents any event originating with the process running in the dyno that causes the dyno to stop. That includes the process exiting with an exit code of 0 (or any other exit code).

The first time a dyno crashes, it will be restarted immediately. If the dyno crashes again, it will be subject to a cool-off period before a restart is attempted. The first cool-off period is 10 minutes, the next one is 20 minutes, then 40 minutes, 60 minutes, 180 minutes and 320 minutes. After the 320 minute cool-off period is reached, restart attempts happen every 320 minutes.

The cool-off period will be reset when the dyno starts correctly, or when you push a new release to your app, or when you restart your app (for example, by typing heroku restart), or when you scale your dynos to 0 and then scale them back up again.


Graceful shutdown with SIGTERM

When the dyno manager restarts a dyno, the dyno manager will request that your processes shut down gracefully by sending them a SIGTERM signal. This signal is sent to all processes in the dyno, not just the process type.

Please note that it is currently possible that processes in a dyno that is being shut down may receive multiple SIGTERMs

The application processes have ten seconds to shut down cleanly (ideally, they will do so more quickly than that). During this time they should stop accepting new requests or jobs and attempt to finish their current requests, or put jobs back on the queue for other worker processes to handle. If any processes remain after ten seconds, the dyno manager will terminate them forcefully with SIGKILL.

When performing controlled or periodic restarts, new dynos are spun up as soon as shutdown signals are sent to processes in the old dynos.

We can see how this works in practice with a sample worker process. We’ll use Ruby here as an illustrative language - the mechanism is identical in other languages. Imagine a process that does nothing but loop and print out a message periodically:

STDOUT.sync = true
puts "Starting up"

trap('TERM') do
  puts "Graceful shutdown"

loop do
  puts "Pretending to do work"
  sleep 3

If we deploy this (along with the appropriate Gemfile and Procfile) and heroku ps:scale worker=1, we’ll see the process in its loop running on dyno worker.1:

$ heroku logs
2011-05-31T23:31:16+00:00 heroku[worker.1]: Starting process with command: `bundle exec ruby worker.rb`
2011-05-31T23:31:17+00:00 heroku[worker.1]: State changed from starting to up
2011-05-31T23:31:17+00:00 app[worker.1]: Starting up
2011-05-31T23:31:17+00:00 app[worker.1]: Pretending to do work
2011-05-31T23:31:20+00:00 app[worker.1]: Pretending to do work
2011-05-31T23:31:23+00:00 app[worker.1]: Pretending to do work

Restart the dyno, which causes the dyno to receive SIGTERM:

$ heroku restart worker.1
Restarting worker.1 process... done

$ heroku logs
2011-05-31T23:31:26+00:00 app[worker.1]: Pretending to do work
2011-05-31T23:31:28+00:00 heroku[worker.1]: State changed from up to starting
2011-05-31T23:31:29+00:00 heroku[worker.1]: Stopping all processes with SIGTERM
2011-05-31T23:31:29+00:00 app[worker.1]: Graceful shutdown
2011-05-31T23:31:29+00:00 heroku[worker.1]: Process exited

Note that app[worker.1] logged “Graceful shutdown” (as we expect from our code); all the dyno manager messages log as heroku[worker.1].

If we modify worker.rb to ignore the TERM signal, like so:

STDOUT.sync = true
puts "Starting up"

trap('TERM') do
  puts "Ignoring TERM signal - not a good idea"

loop do
  puts "Pretending to do work"
  sleep 3

Now we see the behavior is changed:

$ heroku restart worker.1
Restarting worker.1 process... done

$ heroku logs
2011-05-31T23:40:57+00:00 heroku[worker.1]: Stopping all processes with SIGTERM
2011-05-31T23:40:57+00:00 app[worker.1]: Ignoring TERM signal - not a good idea
2011-05-31T23:40:58+00:00 app[worker.1]: Pretending to do work
2011-05-31T23:41:01+00:00 app[worker.1]: Pretending to do work
2011-05-31T23:41:04+00:00 app[worker.1]: Pretending to do work
2011-05-31T23:41:07+00:00 heroku[worker.1]: Error R12 (Exit timeout) -> Process failed to exit within 10 seconds of SIGTERM
2011-05-31T23:41:07+00:00 heroku[worker.1]: Stopping all processes with SIGKILL
2011-05-31T23:41:08+00:00 heroku[worker.1]: Process exited

Our process ignores SIGTERM and blindly continues on processing. After ten seconds, the dyno manager gives up on waiting for the process to shut down gracefully, and kills it with SIGKILL. It logs Error R12 to indicate that the process is not behaving correctly.

Memory behavior

The maximum amount of RAM available to your application depends on the dyno type you use. The dyno manager will restart your dyno and log an R15 error if the memory usage of a:

  • free, hobby or standard-1x dyno reaches 2.5GB, five times its quota.
  • standard-2x dyno reaches 5GB, five times its quota.
  • performance dyno reaches 12GB, two times its quota.

Using a dyno type that is too small might cause constant memory swapping, which will degrade application performance. Application metrics data, including memory usage, is available via the Metrics tab of the Heroku Dashboard. You can also measure memory with log-runtime-metrics. Memory usage problems might also be caused by memory leaks in your app. If you suspect a memory leak, memory profiling tools can be helpful.

Using small amounts of swap space and infrequent memory swapping are usually not problems. Even when your application hasn’t reached its memory limit, it’s common to see small amounts of memory being swapped to disk as the operating system manages memory and available disk cache.

Connecting to external services

Applications running on dynos can connect to external services. Heroku can run apps in multiple regions, so for optimal latency run your services in the same region as the app.

Dynos and requests

A single dyno can serve thousands of requests per second, but performance depends greatly on the language and framework you use.

A single-threaded, non-concurrent web framework (like Rails 3 in its default configuration) can process one request at a time. For an app that takes 100ms on average to process each request, this translates to about 10 requests per second per dyno, which is not optimal.

Single threaded backends are not recommended for production applications because of their inefficient handling of concurrent requests. Choose a concurrent backend whenever developing and running a production service.

Multi-threaded or event-driven environments like Java, Unicorn, EventMachine, and Node.js can handle many concurrent requests. Load testing your app is the only realistic way to determine request throughput.