Processing large JSON files in PHP

Question

I am trying to process somewhat large (possibly up to 200M) JSON files. The structure of the file is basically an array of objects.

So something along the lines of:

[
  {"property":"value", "property2":"value2"},
  {"prop":"val"},
  ...
  {"foo":"bar"}
]

Each object has arbitrary properties and does not necessary share them with other objects in the array (as in, having the same).

I want to apply a processing on each object in the array and as the file is potentially huge, I cannot slurp the whole file content in memory, decoding the JSON and iterating over the PHP array.

So ideally I would like to read the file, fetch enough info for each object and process it. A SAX-type approach would be OK if there was a similar library available for JSON.

Any suggestion on how to deal with this problem best?

Answer 1

I've written a streaming JSON pull parser pcrov/JsonReader for PHP 7 with an api based on XMLReader.

It differs significantly from event-based parsers in that instead of setting up callbacks and letting the parser do its thing, you call methods on the parser to move along or retrieve data as desired. Found your desired bits and want to stop parsing? Then stop parsing (and call close() because it's the nice thing to do.)

(For a slightly longer overview of pull vs event-based parsers see XML reader models: SAX versus XML pull parser.)

---

Example 1:

Read each object as a whole from your JSON.

use pcrov\JsonReader\JsonReader;

$reader = new JsonReader();
$reader->open("data.json");

$reader->read(); // Outer array.
$depth = $reader->depth(); // Check in a moment to break when the array is done.
$reader->read(); // Step to the first object.
do {
    print_r($reader->value()); // Do your thing.
} while ($reader->next() && $reader->depth() > $depth); // Read each sibling.

$reader->close();

Output:

Array
(
    [property] => value
    [property2] => value2
)
Array
(
    [prop] => val
)
Array
(
    [foo] => bar
)

Objects get returned as stringly-keyed arrays due (in part) to edge cases where valid JSON would produce property names that are not allowed in PHP objects. Working around these conflicts isn't worthwhile as an anemic stdClass object brings no value over a simple array anyway.

---

Example 2:

Read each named element individually.

$reader = new pcrov\JsonReader\JsonReader();
$reader->open("data.json");

while ($reader->read()) {
    $name = $reader->name();
    if ($name !== null) {
        echo "$name: {$reader->value()}\n";
    }
}

$reader->close();

Output:

property: value
property2: value2
prop: val
foo: bar

---

Example 3:

Read each property of a given name. Bonus: read from a string instead of a URI, plus get data from properties with duplicate names in the same object (which is allowed in JSON, how fun.)

$json = <<<'JSON'
[
    {"property":"value", "property2":"value2"},
    {"foo":"foo", "foo":"bar"},
    {"prop":"val"},
    {"foo":"baz"},
    {"foo":"quux"}
]
JSON;

$reader = new pcrov\JsonReader\JsonReader();
$reader->json($json);

while ($reader->read("foo")) {
    echo "{$reader->name()}: {$reader->value()}\n";
}

$reader->close();

Output:

foo: foo
foo: bar
foo: baz
foo: quux

---

How exactly to best read through your JSON depends on its structure and what you want to do with it. These examples should give you a place to start.