Rucksack

Attach static files to your compiled crystal binary and access them at runtime.

The attached files are not loaded into memory at any point in time. Reading them at runtime has the same performance characteristics as reading them from the local filesystem.

Rucksack is therefore suitable for true Single File Deployments with virtually zero runtime overhead.

Installation

1. Add the dependency to your shard.yml:

   dependencies:
     rucksack:
       github: busyloop/rucksack
   

2. Run shards install

3. Add the following lines to your .gitignore:

   .rucksack
   .rucksack.toc
   

Usage

To get started best have a peek at the included webserver example.

Here is the code:

require "http/server"
require "rucksack"

server = HTTP::Server.new do |context|
  path = context.request.path
  path = "/index.html" if path == "/"
  path = "./webroot#{path}"

  begin
    # Here we read the requested file from the Rucksack
    # and write it to the HTTP response. By default Rucksack
    # falls back to direct filesystem access in case the
    # executable has no Rucksack attached.
    rucksack(path).read(context.response.output)
  rescue Rucksack::FileNotFound
    context.response.status = HTTP::Status.new(404)
    context.response.print "404 not found :("
  end
end

address = server.bind_tcp 8080
puts "Listening on http://#{address}"
server.listen

# Here we statically reference the files to be included
# once - otherwise Rucksack wouldn't know what to pack.
{% for name in `find ./webroot -type f`.split('\n') %}
  rucksack({{name}})
{% end %}

You can develop and test code that uses Rucksack in the same way as any other crystal code.

Packing the Rucksack

After compiling your final binary for deployment, a small extra step is needed to make it self-contained:

crystal build --release webserver.cr
cat .rucksack >>webserver

The .rucksack-file that we append here is generated during compilation and contains all files that you referenced with the rucksack()-macro.

The resulting webserver executable is now self-contained and does not require the referenced files to be present in the filesystem anymore.

Startup and runtime behavior

By default Rucksack operates in mode 0 (see below).

You can alter its behavior by setting the env var RUCKSACK_MODE to one of the following values:

RUCKSACK_MODE=0 (default)

• Rucksack index is read at startup (very fast)

• The rucksack() macro falls back to direct filesystem access if the rucksack is missing or doesn't contain the requested file

• File checksums are verified once on first access

• If a requested file can be found neither in the Rucksack nor in the local filesystem then Rucksack::FileNotFound is raised.

• This mode ensures your app works not only when the Rucksack has been appended to your executable but also when it's missing and the files are present in the filesystem.

  This is the preferred mode during development as e.g. crystal spec and crystal run will work as expected.

RUCKSACK_MODE=1 (for production)

• Rucksack index is read at startup (very fast)

• Application aborts with exit code 42 if Rucksack is missing or corrupt

• File checksums are verified once on first access

• If a requested file can not be found in the Rucksack then Rucksack::FileNotFound is raised

• Prefer this mode for CI and production. It ensures your app aborts at startup in case the Rucksack is missing, rather than later when trying to access the files.

RUCKSACK_MODE=2 (for the paranoid)

• This is like mode 1, except it also verifies the checksums of all attached files at startup
• Application aborts with exit code 42 if Rucksack is missing or corrupt or if any file in the Rucksack is corrupt
• If you are worried about potential corruption and can tolerate a slightly longer startup delay then this is the mode for you

API

rucksack(path : String).read(output : IO)

Packs the referenced file at compile time and writes it to the given I/O at runtime.

Example:

rucksack("data/hello.txt").read(STDOUT)

Files that get referenced in multiple places are of course packed only once.

Please note that when looking up files dynamically at runtime then they need to be referenced statically at least once elsewhere in your code, otherwise rucksack wouldn't know what to pack.

E.g.:

# Dynamic file lookup at runtime
rucksack(ARGV[0]).read(STDOUT)

# Tell rucksack which files should be packed
rucksack("data/hello.txt")
rucksack("data/world.txt")

Also keep in mind that Rucksack reads your files directly from the executable at runtime, they are not cached in memory. Do not modify the executable on disk while the app is running.

rucksack(path : String).size : UInt64

Returns the size of a packed file.

rucksack(path : String).path : String

Returns the original path of a packed file.

rucksack(path : String).checksum : Slice(UInt8)

Returns the SHA256 of a packed file.

Exceptions

Rucksack::FileNotFound

In mode 0: Is raised when attempting to access a file that exists neither in the Rucksack nor in the filesystem.

In mode 1 and 2: Is raised when attempting to access a file that does not exist in the Rucksack

Rucksack::FileCorrupted

Is raised when the accessed file doesn't match the stored checksum. You will never see this in practice unless your executable gets truncated or modified after packing.

Contributing

1. Fork it (https://github.com/busyloop/rucksack/fork)
2. Create your feature branch (git checkout -b my-new-feature)
3. Commit your changes (git commit -am 'Add some feature')
4. Push to the branch (git push origin my-new-feature)
5. Create a new Pull Request