I like things to be easy

The Situation

You've got a large JSON file, in my case 21MB, that takes a while to parse. You want to have a web service running that can load this file into memory and server data from it. Nice and simple right? Sure the initial data read might take a few seconds but after that the service can just trundle along.

Here's a simple example using Cro.

use JSON::Fast;
use Cro::HTTP::Router;
use Cro::HTTP::Server;

constant DATAFILE = 'data.json';
my $data = from-json($data)

my $application = route {
    get -> 'count' {
        content 'text/plain', "{$data.keys.elems}\n";
    }
    get -> 'keys' {
        content 'text/plain', "{$data.keys.join(",")}\n";
    }
    get -> $uid {
        not-found unless $data{$uid}:exists;
        content 'text/plain', "{$data{$uid}.keys.join(",")}\n";
    }
    get -> $uid, $cid {
        not-found unless ($data{$uid}:exists) && ($data{$uid}{$cid}:exists);
        content 'text/plain', "{$data{$uid}{$cid}}\n";
    }
}

my Cro::Service $server = Cro::HTTP::Server.new:
    :host<localhost>, :port<5000>, :$application;

$server.start;
react whenever signal(SIGINT) {
    $server.stop;
    exit;
}

(Most of this is cribbed from the Cro docs to be honest.)

$data is read at the start and then shared between our threads. (Note it's a two level data structure but that's not that important for the example). The data is immutable and everything is fine.

The wrinkle

Every once in a while the data file gets updated. When this happens we have a bunch of rules we need to apply :

it's OK for a request to server stale data for a bit
request should return in a timely fashion (so no 3 second waits to read our data file allowed)
the server should always serve response (so we can't just reboot the server).

Now we could fire up a new server and swap them in a proxying system, that wouldn't be too hard. But what if we could hot swap the data and keep the server running?

A solution

It might not be the solution, feel free to comment with others but here's what I came up with.

unit class JSONDataWatcher;

use JSON::Fast;

subset DataFilePath of Str:D where *.IO:e && *.IO.f;

has DataFilePath $!datafile;
has $.data;
has Lock $!lock;

method !update-data {
    my $read;
    try {
        $read = from-json( $!datafile.IO.slurp );
    }
    $!lock.protect( {$!data = $read} ) unless $!;
}

submethod BUILD( DataFilePath:D :$!datafile ) {
    $!lock = Lock.new();
    self!update-data();
    start react {
        whenever $!datafile.IO.watch() {
           self!update-data();
        }
    }
}

Here was have a JSONDataWatcher class. You give it a file path and it does a couple of things :

Parses the JSON file and puts it in it's $.data value
Sets up a watcher on the file path if the file changes it tries to reload and update the data.
- If the data parsing fails (which if the file hasn't finished being written could well happen) we ignore it and keep using the old data
- If it parses OK we lock the data attribute while we update it. This will give us a small blip when doing reads but I figure it's safer.

Then we can update our server so :

use JSONDataWatcher;
use Cro::HTTP::Router;
use Cro::HTTP::Server;

constant DATAFILE = 'data.json';
my $w = JSONDataWatcher( datafile => DATAFILE );

my $application = route {
    get -> 'count' {
        content 'text/plain', "{$w.data.keys.elems}\n";
    }
    get -> 'keys' {
        content 'text/plain', "{$w.data.keys.join(",")}\n";
    }
    get -> $uid {
        not-found unless $w.data{$uid}:exists;
        content 'text/plain', "{$w.data{$uid}.keys.join(",")}\n";
    }
    get -> $uid, $cid {
        not-found unless ($w.data{$uid}:exists) && ($data{$uid}{$cid}:exists);
        content 'text/plain', "{$w.data{$uid}{$cid}}\n";
    }
}

my Cro::Service $server = Cro::HTTP::Server.new:
    :host<localhost>, :port<5000>, :$application;

$server.start;
react whenever signal(SIGINT) {
    $server.stop;
    exit;
}

Notes

This is all a bit rough and ready, there's some error checking I'd add. But I thought it would be interesting to show what you can do with Raku. Note that apart from the data parsing everything in the JSONDataWatcher class is core to the language.

Enjoy and stay safe.

Blog

I like things to be easy

Simon Proctor

The Situation

The wrinkle

A solution

Notes

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