halaxa/json-machine

Efficient, easy-to-use and fast JSON pull parser

0.7.0 2021-05-06 11:47 UTC

This package is auto-updated.

Last update: 2021-10-04 20:39:07 UTC


README

JSON Machine

Very easy to use and memory efficient drop-in replacement for inefficient iteration of big JSON files or streams for PHP 5.6+. See TL;DR. No dependencies in production except optional ext-json.

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TL;DR

<?php

use \JsonMachine\JsonMachine;

// this often causes Allowed Memory Size Exhausted
- $users = json_decode(file_get_contents('500MB-users.json'));

// this usually takes few kB of memory no matter the file size
+ $users = JsonMachine::fromFile('500MB-users.json');

foreach ($users as $id => $user) {
    // just process $user as usual
}

Random access like $users[42] or counting results like count($users) is not possible by design. Use above-mentioned foreach and find the item or count the collection there.

Requires ext-json if used out of the box. See Decoders.

Introduction

JSON Machine is an efficient, easy-to-use and fast JSON stream/pull/incremental/lazy (whatever you name it) parser based on generators developed for unpredictably long JSON streams or documents. Main features are:

  • Constant memory footprint for unpredictably large JSON documents.
  • Ease of use. Just iterate JSON of any size with foreach. No events and callbacks.
  • Efficient iteration on any subtree of the document, specified by Json Pointer
  • Speed. Performance critical code contains no unnecessary function calls, no regular expressions and uses native json_decode to decode JSON document items by default. See Decoders.
  • Parses not only streams but any iterable that produces JSON chunks.
  • Thoroughly tested. More than 100 tests and 700 assertions.

Parsing JSON documents

Itearting a collection

Let's say that fruits.json contains this really big JSON document:

// fruits.json
{
    "apple": {
        "color": "red"
    },
    "pear": {
        "color": "yellow"
    }
}

It can be parsed this way:

<?php

use \JsonMachine\JsonMachine;

$fruits = JsonMachine::fromFile('fruits.json');

foreach ($fruits as $name => $data) {
    // 1st iteration: $name === "apple" and $data === ["color" => "red"]
    // 2nd iteration: $name === "pear" and $data === ["color" => "yellow"]
}

Parsing a json array instead of a json object follows the same logic. The key in a foreach will be a numeric index of an item.

If you prefer JSON Machine to return objects instead of arrays, use new ExtJsonDecoder() as decoder which by default decodes objects - same as json_decode

<?php

use JsonMachine\JsonDecoder\ExtJsonDecoder;
use JsonMachine\JsonMachine;

$objects = JsonMachine::fromFile('path/to.json', '', new ExtJsonDecoder);

Parsing a subtree

If you want to iterate only results subtree in this fruits.json:

// fruits.json
{
    "results": {
        "apple": {
            "color": "red"
        },
        "pear": {
            "color": "yellow"
        }
    }
}

use Json Pointer "/results" as the second argument:

<?php

use \JsonMachine\JsonMachine;

$fruits = JsonMachine::fromFile("fruits.json", "/results");
foreach ($fruits as $name => $data) {
    // The same as above, which means:
    // 1st iteration: $name === "apple" and $data === ["color" => "red"]
    // 2nd iteration: $name === "pear" and $data === ["color" => "yellow"]
}

Note:

Value of results is not loaded into memory at once, but only one item in results at a time. It is always one item in memory at a time at the level/subtree you are currently iterating. Thus, the memory consumption is constant.

Parsing nested values in arrays

The JSON Pointer spec also allows to use a hyphen (-) instead of a specific array index. JSON Machine interprets it as a wildcard which matches any array index (not any object key). This enables you to iterate nested values in arrays without loading the whole item.

Example:

// fruitsArray.json
{
    "results": [
        {
            "name": "apple",
            "color": "red"
        },
        {
            "name": "pear",
            "color": "yellow"
        }
    ]
}

To iterate over all colors of the fruits, use the JSON pointer "/results/-/color".

Getting single scalar values

You can parse sigle scalar value anywhere in the document the same way as a collection. Consider this example:

// fruits.json
{
    "lastModified": "2012-12-12",
    "apple": {
        "color": "red"
    },
    "pear": {
        "color": "yellow"
    },
    // ... gigabytes follow ...
}

Get the single value of lastModified key like this:

<?php

use \JsonMachine\JsonMachine;

$fruits = JsonMachine::fromFile('fruits.json', '/lastModified');
foreach ($fruits as $key => $value) {
    // 1st and final iteration:
    // $key === 'lastModified'
    // $value === '2012-12-12'
}

When parser finds the value and yields it to you, it stops parsing. So when a single scalar value is in the beginning of a gigabytes-sized file or stream, it just gets the value from the beginning in no time and with almost no memory consumed.

The obvious shortcut is:

<?php

use \JsonMachine\JsonMachine;

$fruits = JsonMachine::fromFile('fruits.json', '/lastModified');
$lastModified = iterator_to_array($fruits)['lastModified'];

Single scalar value access supports array indices in json pointer as well.

What is Json Pointer anyway?

It's a way of addressing one item in JSON document. See the Json Pointer RFC 6901. It's very handy, because sometimes the JSON structure goes deeper, and you want to iterate a subtree, not the main level. So you just specify the pointer to the JSON array or object you want to iterate and off you go. When the parser hits the collection you specified, iteration begins. It is always a second parameter in all JsonMachine::from* functions. If you specify a pointer to a non-existent position in the document, an exception is thrown. It can be used to access scalar values as well.

Some examples:

Json Pointer value Will iterate through
"" (empty string - default) ["this", "array"] or {"a": "this", "b": "object"} will be iterated (main level)
"/result/items" {"result":{"items":["this","array","will","be","iterated"]}}
"/0/items" [{"items":["this","array","will","be","iterated"]}] (supports array indices)
"/results/-/status" {"results":[{"status": "iterated"}, {"status": "also iterated"}]} (a hyphen instead of an array index)
"/" (gotcha! - a slash followed by an empty string, see the spec) {"":["this","array","will","be","iterated"]}

Parsing streaming responses from a JSON API

A stream API response or any other JSON stream is parsed exactly the same way as file is. The only difference is, you use JsonMachine::fromStream($streamResource) for it, where $streamResource is the stream resource with the JSON document. The rest is the same as with parsing files. Here are some examples of popular http clients which support streaming responses:

GuzzleHttp

Guzzle uses its own streams, but they can be converted back to PHP streams by calling \GuzzleHttp\Psr7\StreamWrapper::getResource(). Pass the result of this function to JsonMachine::fromStream function, and you're set up. See working GuzzleHttp example.

Symfony HttpClient

A stream response of Symfony HttpClient works as iterator. And because JSON Machine is based on iterators, the integration with Symfony HttpClient is very simple. See HttpClient example.

Tracking the progress

Big documents may take a while to parse. Call JsonMachine::getPosition() in your foreach to get current count of the processed bytes from the beginning. Percentage is then easy to calculate as position / total * 100. To find out the total size of your document in bytes you may want to check:

  • strlen($document) if you parse a string
  • filesize($file) if you parse a file
  • Content-Length http header if you parse a http stream response
  • ... you get the point
<?php

use JsonMachine\JsonMachine;

$fileSize = filesize('fruits.json');
$fruits = JsonMachine::fromFile('fruits.json');
foreach ($fruits as $name => $data) {
    echo 'Progress: ' . intval($fruits->getPosition() / $fileSize * 100) . ' %'; 
}

Decoders

As the third and optional parameter of all the JsonMachine::from* functions is an instance of JsonMachine\JsonDecoder\Decoder. If none is specified, ExtJsonDecoder is used by default. It requires ext-json PHP extension to be present, because it uses json_decode. When json_decode doesn't do what you want, implement JsonMachine\JsonDecoder\Decoder and make your own.

Available decoders

  • ExtJsonDecoder - Default. Uses json_decode to decode keys and values. Constructor takes the same parameters as json_decode.

  • PassThruDecoder - uses json_decode to decode keys but returns values as pure JSON strings. Useful when you want to parse a JSON item with something else directly in the foreach and don't want to implement JsonMachine\JsonDecoder\Decoder. Constructor has the same parameters as json_decode. Example:

<?php

use JsonMachine\JsonDecoder\PassThruDecoder;
use JsonMachine\JsonMachine;

$items = JsonMachine::fromFile('path/to.json', '', new PassThruDecoder);
  • ErrorWrappingDecoder - A decorator which wraps decoding errors inside DecodingError object thus enabling you to skip malformed items instead of dying on SyntaxError exception. Example:
<?php

use JsonMachine\JsonMachine;
use JsonMachine\JsonDecoder\DecodingError;
use JsonMachine\JsonDecoder\ErrorWrappingDecoder;
use JsonMachine\JsonDecoder\ExtJsonDecoder;

$items = JsonMachine::fromFile('path/to.json', '', new ErrorWrappingDecoder(new ExtJsonDecoder()));
foreach ($items as $key => $item) {
    if ($key instanceof DecodingError || $item instanceof DecodingError) {
        // handle error of this malformed json item
        continue;
    }
    var_dump($key, $item);
}

Error handling

Since 0.4.0 every exception extends JsonMachineException, so you can catch that to filter any error from JSON Machine library.

Skipping malformed items

If there's an error anywhere in a json stream, SyntaxError exception is thrown. That's very inconvenient, because if there is an error inside one json item you are unable to parse the rest of the document because of one malformed item. ErrorWrappingDecoder is a decoder decorator which can help you with that. Wrap a decoder with it, and all malformed items you are iterating will be given to you in the foreach via DecodingError. This way you can skip them and continue further with the document. See example in Available decoders. Syntax errors in the structure of a json stream between the iterated items will still throw SyntaxError exception though.

Parser efficiency

Streams / files

JSON Machine reads a stream (or a file) 1 JSON item at a time and generates corresponding 1 PHP array at a time. This is the most efficient way, because if you had say 10,000 users in JSON file and wanted to parse it using json_decode(file_get_contents('big.json')), you'd have the whole string in memory as well as all the 10,000 PHP structures. Following table shows the difference:

String items in memory at a time Decoded PHP items in memory at a time Total
json_decode() 10000 10000 20000
JsonMachine::from*() 1 1 2

This means, that JsonMachine is constantly efficient for any size of processed JSON. 100 GB no problem.

In-memory JSON strings

There is also a method JsonMachine::fromString(). If you are forced to parse a big string, and the stream is not available, JSON Machine may be better than json_decode. The reason is that unlike json_decode, JSON Machine still traverses the JSON string one item at a time and doesn't load all resulting PHP structures into memory at once.

Let's continue with the example with 10,000 users. This time they are all in string in memory. When decoding that string with json_decode, 10,000 arrays (objects) is created in memory and then the result is returned. JSON Machine on the other hand creates single structure for each found item in the string and yields it back to you. When you process this item and iterate to the next one, another single structure is created. This is the same behaviour as with streams/files. Following table puts the concept into perspective:

String items in memory at a time Decoded PHP items in memory at a time Total
json_decode() 10000 10000 20000
JsonMachine::fromString() 10000 1 10001

The reality is even better. JsonMachine::fromString consumes about 5x less memory than json_decode. The reason is that a PHP structure takes much more memory than its corresponding JSON representation.

Troubleshooting

"I'm still getting Allowed memory size ... exhausted"

One of the reasons may be that the items you want to iterate over are in some sub-key such as "results" but you forgot to specify a json pointer. See Parsing a subtree.

"That didn't help"

The other reason may be, that one of the items you iterate is itself so huge it cannot be decoded at once. For example, you iterate over users and one of them has thousands of "friend" objects in it. Use PassThruDecoder which does not decode an item, get the json string of the user and parse it iteratively yourself using JsonMachine::fromString().

<?php

use JsonMachine\JsonMachine;
use JsonMachine\JsonDecoder\PassThruDecoder;

$users = JsonMachine::fromFile('users.json', '', new PassThruDecoder);
foreach ($users as $user) {
    foreach (JsonMachine::fromString($user, "/friends") as $friend) {
        // process friends one by one
    }
}

"I am still out of luck"

It probably means that the JSON string $user itself or one of the friends are too big and do not fit in memory. However, you can try this approach recursively. Parse "/friends" with PassThruDecoder getting one $friend json string at a time and then parse that using JsonMachine::fromString()... If even that does not help, there's probably no solution yet via JSON Machine. A feature is planned which will enable you to iterate any structure fully recursively and strings will be served as streams.

Running tests

tests/run.sh

This uses php and composer installation already present in your OS installation.

Running tests on all supported PHP platforms

Install docker to your machine and run

tests/docker-run-all-platforms.sh

This needs no php nor composer installation on your machine. Only Docker.

Installation

composer require halaxa/json-machine

or clone or download this repository (not recommended).

Support

Do you like this library? Star it, share it, show it :) Issues and pull requests are very welcome.

License

Apache 2.0

Cogwheel element: Icons made by TutsPlus from www.flaticon.com is licensed by CC 3.0 BY

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