Introduction
PM2 is a great tool that can help you manage not just your processes when they are running but also env types, vars, and configs. I still find many questions around the basic operation of the tool around the internet such as stackoverflow and other boards. So this article will cover how I setup most of my projects using PM2; short, sweet, and to the point.
Setting up a quick app
First let's create a quick API that returns hello world as a response. Move into an empty directory and create a file called API.js and put the following into it.
Starting up PM2
Within the project directory install PM2 as a package globally and into your project dependencies npm i pm2 -g; npm i pm2 --save
You have now installed all you need to setup production level process running and environment control ( with enough configs and a pipeline of course ). With the latest versions of PM2 the tool comes with the ecosystem command that will help us generate what we need to create and application definition for our small API. However to see some immediate action you can just run : pm2 start API.js
Confirm that your API start by visiting localhost:3000/ again and checking if the text "Hello World!" is rendered. You can see that your process starts up and you get a nice looking table print out with some facts about your process. You can shut down your process by running pm2 delete API .
Note: Delete will remove the application entirely from PM2's registry; however you can also use pm2 stop API and it will stop the application but not remove it so you can use PM2 start API to start it up again. In our case however we want PM2 to forget about our process since we are going to create an ecosystem config for it and start it that way.
Setting up ecosystem.js
Now that we have shown that pm2 will start our API, and that our API still returns what we expect we can setup a more maintainable and useful way to start our app. Within the project app run pm2 ecosystem
This will generate a application definition with a few other things inside of a ecosystem.js file that it created. Lets go through the sections that get generated and a quick over view of what we can do with them.
Apps Section
This section is arguably the most important for getting your application up and running. This is where you will define how your application runs, environment specific vars, logging behaivors, and more. In addition you can define multiple apps in the same ecosystem file; this can be used to start up co-processors, log streamers, queue managers, and more.
I will go over a few of the fields that can be used with an app definition that I think are some of the most common or useful.
Instances - Int
The number of app instance to be launched. This can be set to -1 to start as many processes as the system has CPU cores subtract 1. I use this in docker setups a lot because I can allow the application to consume the entire container since that is what it is dedicated to.
node_args - String Array
This is an array of arguments that will be passed to the actual node execution which allows you to pass things like the --harmony flags for older node version or things such as debug flags.
error_file, out_file, pid_file - String ( directory/file path )
These values point to the directory and file name that you want pm2 to export the generated logs from your application to. This is valuable again in the containerization scenario when you want your logs to go specific places to be picked up by log aggregation systems.
max_restarts - Int
This is the number of consecutive unstable restarts (less than 1sec interval or custom time via min_uptime) before your app is considered errored and is stopped from being restarted any more. This is a great option to allow your application to show that it can't connect to mission critical services or API's at start up time. I currently use this as a flag that something is very wrong when a new deploy goes out.
max_memory_restart - String
This option allows a max memory limit to be set that, when hit, causes PM2 to restart you application automatically. Useful to ensure a rogue process doesn't bring down boxes if they suddenly get a ton of load or the process simply goes off the rails. The value of this field is a String that uses the normal volume types M = megabyte, b = byte, so if you wanted the max memory to be 400 megabytes then the value here would be "400M".
min_uptime - String
This option lets pm2 know that your application should be up and stable for x amount of seconds before being considered started instead of in the starting state. This can be important if your project has to connect to a service and timeouts are an issue or there is a lengthy read operation on disk, etc.
env - Object Key/Val
The env object is what allows you to define environment level vars based on the environment flag you give PM2 at startup time. the plain env field is an object that contains values that will be available all of the time regardless of the environment that is passed to pm2.
Any field that follows the pattern env_${myEnvironmentName} is considered a valid env setting and the values defined within will only be available when the --env flag us used and a valid key that matches the following rules is found. For example a env_production env object then allows you to use the following command pm2 start ecosystem.js --env production
Note I think it is worth mentioning how you get to these environment variables. All of these keys will be available via the global process variable. So to get the env NODE_ENV to see what mode the application has been started in you would use process.env.NODE_ENV.
Here is an example of all the discussed settings for an application definition. There are plenty more options that you can find in the PM2 docs page.
Deployments Section
As a disclaimer I do not use the PM2 deployment tool in any projects currently aside from deploying to some rasberry pi's I have hooked up on my local network. This is because these days all of my CI/CD pipelines use Docker or run through Heroku. That being said I will give you what I know on how to get deployments working for you via PM2.
The use case for using the PM2 deployment tool set fits if you have static servers that are not containerized that you don't want to bring down your systems just to do a small update to your application. This is the case at with my home setup where I have node instances running on Rasberry Pi's.
To setup deployments via PM2 first define your application section with the options you want from the previous section. Then in your ecosystem.js go to the deploy section of the config. Here you will find the two generated deployment options production and dev, these are enviornment configs just like the application definition has; they will be used to define what env your are deploying to. Here there are a few key definitions that need to be flushed out.
user - String
This is the user that the target machine will use to run any commands that are pushed to it via PM2. It must have the approprite permissions to execute the commands ( git pull, npm install, etc ) on the target machine. In addition this is the user that PM2 will attempt to authenticate via ssh using a key on your machine or a key you provide within the deployment configuration ( more on this a little later )
host - String/String Array
The host field can hold a single host or an array of hosts. These hosts can be IPs or hostnames that will get resolved via DNS. The machine doing the deploy must have an SSH key for these machines so that authentication can occur OR a .pem file must be given as part of the deployment config.
key - String
The file location of a .pem file that contains the approprite key to authenticate against the hosts using the User as the username.
ref - String
Ref is the git origin branch that you want to get deployed. Most of the time this will be a "production" branch or something similiar that you merge into when a version has been tagged or something similiar.
repo - String
The git repository URI that the ref branch is in and the hosts have access to pull from.
path - String
The path that the git branch will be downloaded into on the host.
pre-setup - String
This field is (a) command(s) to be run BEFORE the git checkout for the branch is done on the host.
post-setup - String
This field is (a) command(s) to be run AFTER the git checkout for the branch is done on the host. This is generally where you would put your npm install, gulp builds, etc.
post-deploy - String
This field is (a) command(s) to be run AFTER the pre and post setups events. This is where you will put your application restart/start command(s)
pre-deploy-local - String
This field is (a) command(s) that will run on the deployment machine, not the hosts that are being deployed to BEFORE it actually fires off the deployments. This is useful for putting deployment configs, alerts, etc into things like slack, emails, etc.
An example of the all these fields for a production env.
Once you have your application running for the first time on the hosts listed in the production deploy config. From a build server ( or your local box ) you can run pm2 deploy ecosystem.js production .
If everything runs successfully you should see a message similiar to : "Deploy Succesful" , you are all done!
Daemonizing your application with PM2 (2.2 >)
PM2 allows you to configure a startup script that will ensure your application comes back up if for some reason a restart or shut down has occured. This is useful again if you have bare metal you are running on that is not containerized or if for some reason you have a very unstable server setup; or have to do rolling restarts of servers for deployments.
PM2 comes with the pm2 startup command that will out put a command for root to run that will add the approprite config so your application will start on machine startup. You can also pass pm2 an explicit type of startup script to generate if you know your environment OS. Here is the supported options: ubuntu, ubuntu14, ubuntu12, centos, centos6, arch, oracle, amazon, macos, darwin , freebsd, systemd, systemv, upstart, launchd, rcd, openrc.
Here is an example output from the command :
On a build server you can write some simple awk, grep, or regex to extract the command and run it on the machine that is being provisioned.
You can read more about process management via things like init.d and more here.
Wrap up
PM2 is a powerful tool for getting things up and running fast, but it also has the staying power for production level applications. I have many applications both at my day job and my personal applications that run using PM2 and KeyMetrics. Some of the deployment management I feel is antiquated by the rising use of containers and the associated services such as AWS, Google Cloud, Heroku, etc but it still has it's place in situations where you don't have the flexiability of a container driven development environment.
Links :
http://pm2.keymetrics.io/docs/usage/application-declaration/