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Welcome to PCP

PCP: Clojure Processor -- Like drugs but better


Too long have we hustled to deploy Clojure websites. Too long have we spun up one server instance per site. Too long have we reminisced about the simplicty of deploying PHP. Today we enjoy the benefits of both. Welcome to PCP.


Active development. Stabilizing.
Latest version: v0.0.2


  • An easy to use, drop-in Clojure replacement for php scripts
  • Allow multiple websites to be hosted on a single $5 VPS


  • Performance. PCP should be sufficient for prototypes and small websites (< 1400 req/s)

Demo site

You can view a demo site built in PCP here:


  1. Quick start
  2. How PCP works
  3. System requirements
  4. Project structure and requiring files
  5. Environment variables and secrets
  6. Core PCP namespace
  7. Other built into PCP
  8. Performance
  9. Deploying PCP
  10. Roadmap

Quick start

Install pcp via the installer script:

$ bash -c "$(curl -sSL"

Query the PCP service status:

$ pcp service status

Create a new project:

$ pcp new project-name

When you create a new project some example code is generated as well which, for convenience, you can view using the local server built in to the pcp utility. This local server behaves as your pcp site would when deployed with nginx.

$ cd project-name
$ pcp -s public/

Navigate to http://localhost:3000/ and voilà.

This is the corresponding code.

(ns index
  (:require [pcp :as pcp] ;this is the core namespace with various useful functions
            [ :as a] ;this is another name space in the same project. 
            [garden.core :refer [css]])) ;this is 3rd party namespace from the hosted environment

(def resp 
    [:html {:style "font-family: 'Source Sans Pro', Arial, Helvetica, sans-serif;"
            :lang "en"}
        [:title "PCP website"]
          (css [:html   { :text-align "center"}])
          (css [ { :background-color "#EEE" 
                          :font-size "13px"
                          :margin-top "40px"
                          :padding "10px" 
                          :text-align "left" 
                          :width "300px"}])
          (css [:strong { :font-size "13px"}])
          (css [:code   { :font-size "12px" :font-weight "normal"}])
          (css [:.main  { :display "flex"
                          :flex-direction "column"
                          :justify-content "center"
                          :align-items "center"
                          :min-height "90vh" 
                          :font-weight "normal"}])]]
            {:src "//" 
             :width "200px"}]
          [:h1 "Your PCP site is up and running."]
          [:p "Your json endpoint is here " [:a {:href "/api/info.clj"} "here"] 
            "Learn more about pcp at " [:a {:href a/repo :target "_blank"} "here"]
            "Happy scripting!"]
            [:strong "Request uri "] [:span (str (-> pcp/request :request-uri))]
            [:strong "Request method "] [:span (str (-> pcp/request :request-method))]
            [:strong "Request scheme "] [:span (str (-> pcp/request :request-scheme))]
            [:strong "User agent "]
            [:code (str (-> pcp/request :http-user-agent))]

(pcp/response 200 resp "text/html")            

When navigating to your site, if you see Connection refused (Connection refused) it means the PCP service not running. It could still be booting or be down.

You can find instructions on replacing php and deploying to production here

How PCP works

PCP has two parts. The utility is a simple binary, built with GraalVM, that allows you to work effectively with pcp.

PCP: Clojure Processor -- Like drugs but better

Usage: pcp [option] [value]

  new [project]           Create a new pcp project in the [project] directory
  service [stop/start]    Stop/start the PCP service
  passphrase [project]    Set passphrase for [project]
  secret [path]           Add and encrypt secrets at . or [path]
  secret [path]           Add and encrypt secrets at . or [path]
  -e, --evaluate [path]   Evaluate a clojure file using PCP
  -s, --serve [root]      Start a local server at . or [root]
  -v, --version           Print the version string and exit
  -h, --help              Print the command line help

The heavy lifting is done by an PCP service. This server runs on port 9000 and receives requests from the local pcp server or from nginx. The PCP service is a uberjar that runs as a daemon.

System requirements

PCP is designed to work on Linux and OSX. It requires the following to work:

  • bash
  • java 8+
  • systemd / launchd
  • unzip
  • persitent disk space
  • nginx (for production deployment)

Project structure and requiring files

Requiring files in PCP mostly works as it does in Clojure, the primary exception being that a project cannot require a file outside of its root. Given a project like this:

.                           # project root    
├── .pcp      
│   └── a24d...cfdcb8.npy  # encrypted secret, touch this and bad things will happen    
├── public                 # server root    
│   ├── index.clj          
│   └── login.clj    
│   └── dashboard.clj    
│   └── logic    
│       └── orders.clj    
│       └── payments.clj    
│       └── stock.clj    
│       └── users.clj    
├── pcp.edn                # pcp configuration file    
└──              # surely you don't need a comment to tell you what this is.    

Your dashboard could require business logic as follows

(ns dashboard
  (:require [logic.users :as lu]
            [logic.orders :as lo]
            [logic.stock :as ls]
            [logic.payments :as lp]))

Environment variables and secrets

To allow API keys, tokens, and the like to be stored and retrieved securely, PCP encrypts them and stores them in the project. Secrets are created using the PCP CLI. They are encrypted using the passphrase selected. Here is an example of the process.

$ pcp secret
Set an encrypted secret variable for project: pcp-demo
Please ensure you use the same passphrase for all your secrets in this project
and that you add your passphrase to your production server using:
  pcp passphrase pcp-demo
Secret value: ghp_eR17e5vHq0Sdmj22oracJd0Y7je1IM3g7oPV7yT7sq31
Passphrase: my-super-secure-passphrase-that-i-will-also-store-on-the-server    

Symmetric encryption is used, so the passphrase needs to be added to the server too.

$ pcp passphrase pcp-demo
Set passphrase for project: pcp-demo
This passphrase will be used for decrypting secrets at runtime.
Passphrase: my-super-secure-passphrase-that-i-will-also-store-on-the-server    

PCP namespace

The following functions are part of the core PCP namespace and are made available for convenience.


(pcp/persist :cache-key f-on-miss)
This macro allows expensive operations to only be recomputed if they are not in the cache.
On a cache miss, the f-on-miss is computed, its value stored in the cache and returned. Caches are isolated across projects. Pages in the same project share cache keys. The cache uses a TTL (30 min) as its invalidation strategy.


(pcp/clear :cache-key)
Removes a key from the cache


The request map. The request map conforms to the ring spec.


(pcp/response status body mime-type)
A convenience function for generating a response map. Response map conforms to the ring spec.


(pcp/redirect target)
A convenience function for generating a redirect. Response map conforms to the ring spec.


(pcp/render-html options & content)
Renders html from Clojure data structures using hiccup


(pcp/secret "SECRET_NAME")
Retrieves a secret from the project. The secret is read from disk. It may be worthwhile using pcp/persist to improve performance. See Environment variables and secrets for more info.


Returns the current time in milliseconds (according to your server).


(pcp/slurp "../file-name")
Opens a reader on f and reads all its contents, returning a string. Cannot access files higher than project root (i.e. server root -1). Does not currently accept any optional arguments.


(pcp/slurp-upload "/var/tmp/temp-file-name")
(pcp/slurp-upload (-> pcp/request :params :uploaded :tempfile))
Opens a reader on f and reads all its contents, returning a string. Cannot access files higher up that project root (i.e. server root -1). Does not currently accept any optional arguments.


(pcp/spit "../file-name" "like drugs but better")
Opposite of slurp. Opens f with writer, writes content, then closes f. Cannot access files higher than project root (i.e. server root -1). Does not currently accept any optional arguments.

Other built-in namespaces

In addition to the core clojure namespaces in sci, the following namespaces are also available.

  • clojure.string
  • clojure.core.async
  • clojure.edn
  • cheshire.core
  • selmer.parser
  • selmer.filters
  • org.httpkit.client
  • org.httpkit.sni-client
  • next.jdbc
  • honeysql.core
  • honeysql.helpers
  • postal.core
  • tick.alpha.api
  • buddy.sign.jwt
  • buddy.sign.jwe
  • buddy.core.hash
  • buddy.core.codecs
  • buddy.core.keys
  • buddy.auth.backends
  • buddy.auth.middleware
  • buddy.hashers
  • garden.core
  • garden.stylesheet
  • garden.units
  • konserve.core
  • konserve.filestore
  • konserve-jdbc.core


A modest PCP website can handle its own pretty well.
This test was run for 90 minutes with 500 VUs and 1400 req/s. Of the 7.7M requests generated and 0 failed.

You can see the spike in CPU and bandwidth on the $5 droplet, but the CPU never saturates.

These are by no means comprehensive benchmarks but give you a sense of what a PCP server can withstand for a simple use case. Going above 1400 req/s generally results in bad things happening. You can test your own site using k6 with the instructions in loadtest.js

Deploying PCP

You can find instructions on deploying your PCP site with nginx here.

Or you could just use this nifty button to deploy to DigitalOcean (experimental).

Deploy to DO

We both get credits for DigitalOcean if you end using their services so be a mate.

Roadmap & releases

Release 0.0.1

  • Run arbitrary scripts
  • Generate HTML with hiccup
  • Invoke scripts from Nginx
  • Emulate Nginx environment locally
  • Store secrets in projects
  • Restrict pcp/slurp and pcp/spit to project root
  • Cache expensive operations
  • Expose request data to scripts
  • Require external namespaces as in Clojure
  • Generate new project from CLI
  • Document PCP
  • Load test a production deployment

Release 0.0.2

  • Perfomance improvements.
  • Only read source file from disk if it is not in cache (LRU) or has been modified
  • Add pcp/slurp-upload for reading temporary files
  • Add pcp/redirect for convenience redirect

Release 0.1.0

  • Store passphrases with konserve
  • Add sponsorship button
  • Deploy without ssh-ing into VPS
  • More robust CLI processing
  • Automate deployment from repo

Release 0.2.0

  • Add PEDL (PCP, Nginx, Datahike Server, and Linux) stack template on Digital Ocean.
  • Other things that even the wisest cannont tell; things that were, things that are, and some things... that have not yet come to pass.

Special thanks

For the guidance and examples, special thanks to


Copyright © 2020 Alexander Oloo

PCP is licensed under the MIT License.