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tonyg / racket-reloadable


Support for code-reloading for long-running racket programs (such as web-servers).


Code-reloading for Racket

Illustration of reloadable program

Racket's built-in dynamic-rerequire does the heavy lifting, but doesn't give a high-level interface to help us build reloadable servers. This package fills in that gap.


A complete example of a website written using the Racket web-server is available at The site uses this module to support runtime code-reloading.

  • main.rkt is the permanent part of the server
  • site.rkt is the reloadable part of the server


  1. Split your server into a permanent and a reloadable part
  2. Decide which pieces of state in the reloadable part should be persistent
  3. Use indirection to access the reloadable part from the permanent part
  4. Decide how and when to reload code

Splitting the server

It's easiest to make the permanent part of your program as small as possible. This is because if a module is required by the permanent part of your program, directly or indirectly, then it will never be reloaded, even if it is also required by the reloadable part.

Any modules required from the permanent part of your program are effectively included in the permanent part of the program.

For example, say your program is started from main.rkt, which will be the permanent part of the application, with the bulk of the program functionality in the reloadable part, features.rkt. Then your main.rkt should be something along the lines of the following:

#lang racket
(require reloadable)
(define main (reloadable-entry-point->procedure
              (make-reloadable-entry-point 'start-program "features.rkt")))

where start-program is provided from features.rkt. It is important that we do not require features.rkt from main.rkt! Instead, that is taken care of by the entry-point machinery in this package.

You must call reload! at least once before accessing a new entry-point's value.

You must also ensure that there are no stray .zo files for the reloadable part of your program. If any such .zo files exist, they will interfere with code loading.

Persistent state

Your features.rkt module may have global variables. Some of these should be initialised every time the module is reloaded, but others should only be initialised once, at server startup time.

Global variables that should be reinitialised on every code reload do not need to be declared differently:

(define module-variable-initialised-every-time
  (begin (printf "Reinitialising module-variable-initialised-every-time!\n")

Global variables that should be initialised only once, at server startup, should be declared using make-persistent-state:

(define some-persistent-variable
  (make-persistent-state 'some-persistent-variable
                         (lambda ()
                           (printf "Initialising some-persistent-variable!\n")

Note that the first argument to make-persistent-state must be unique across the entire Racket instance. This is arguably a bug: ideally, it'd only need to be unique to a particular module. A future version of this library may fix this.

Read and write persistent state values like you would parameters:

;; Access it
(printf "Current some-persistent-variable value: ~a"
;; Set it to a new value
(some-persistent-variable (compute-new-value))

Use #:prefab structs for persistent state

Make sure you use #:prefab structs for your persistent state:

(struct my-state-vector (field1 field2) #:prefab)

If you use non-prefab structs for persistent state, any newly-loaded code won't be able to recognise structs that were created by previous versions of the code.

The reason for this is that non-prefab structs in Racket are generative, meaning that each time your code is reloaded, a new set of struct types are created.

This transcript shows the problem:

Welcome to Racket v6.1.1.4.
-> (struct x () #:transparent)
-> (define x1 (x))
-> (struct x () #:transparent)
-> (define x2 (x))
-> (x? x2)
-> (x? x1)
-> (equal? x1 x2)

With prefab structs, however, the problem goes away:

Welcome to Racket v6.1.1.4.
-> (struct x () #:prefab)
-> (define x1 (x))
-> (struct x () #:prefab)
-> (define x2 (x))
-> (x? x2)
-> (x? x1)
-> (equal? x1 x2)

Struct definitions from the permanent part of your program will never be reloaded, of course, so this warning doesn't apply to them. Pre- and post-reload routines share the same struct definition in that case. Likewise, struct definitions whose instances never survive across a code-reloading (i.e. that are never placed in the program's persistent state) can be non-prefab.

Accessing reloadable code from permanent code

Use the entry points you create with make-reloadable-entry-point (which you may also retrieve after they are created by calling lookup-reloadable-entry-point).

Each time reload! is called, the reloadable-entry-point-value of each entry point is recomputed from the new versions of each module.

  • If an entry point holds a procedure, you can

    • extract its value and call it directly, or
    • use reloadable-entry-point->procedure to convert an entry-point into a general procedure that reflects the calling conventions of the underlying procedure.
  • If an entry point holds any other kind of value, you can use reloadable-entry-point-value to access it.

Controlling code reloading

Direct calls to reload! force immediate reloading of any changed code, subject to the caveats about the split between the permanent and reloadable parts of your program given above.

In addition, by default, the reloadable part of your program is scanned constantly for changes, and whenever the system notices that a .rkt file in the reloadable part of your program has changed, it will automatically be recompiled and reloaded.

To disable this automatic scanning, call

(set-reload-poll-interval! #f)

If automatic scanning is disabled, then calls to reload! will be the only way to make code reloading happen.

Copyright and License

Copyright © 2014 Tony Garnock-Jones

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
GNU Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public License
along with this program.  If not, see <>.