LargeArrayBuffer

This is for you, if...

...you are working with pretty large arrays that conflict with close to every memory limit you can set. The array elements on the other hand should not be that big, such that you can keep a single element in memory quite easily. If the array gets too big for memory, it will be moved to disk temporarily and transparently. You can still iterate over it as if it was a normal array.

One typical use case would be to load a lot of datasets from a database at once. (There are reasons to prefer this over running multiple queries.) See Usage below for an example for this use case using this library.

Install

Note: This library requires PHP 8.0+!

Use composer to install this library:

composer require nerou/large-array-buffer

There are pretty much no dependencies with some exceptions:

• If you want to use the toJSONFile() method, you need to install ext-json (PHP's PECL JSON extension) as well.
• If you want to use the igbinary serializer, ext-igbinary is required. See php-ext-igbinary.
• If you want to use LZ4 compression, ext-lz4 is required. See php-ext-lz4.

Usage

$pdo = new PDO(/* your database credentials */);
$stmt = $pdo->query('SELECT * FROM SomeDatabaseTable', PDO::FETCH_ASSOC);

$buffer = new LargeArrayBuffer();       // explicit use of LargeArrayBuffer
$buffer = new ArrayBuffer(1000);        // wrapper using `array` until given threshold (item count) is reached,
                                        // then switching to LargeArrayBuffer

while(($dataset = $stmt->fetch()) !== false){  // load one dataset at a time
    $buffer->push($dataset);    // push this dataset to the buffer
}

// ...

foreach($buffer as $item){
    // work with your data here
}

Options

The constructor of LargeArrayBuffer provides some options:

1. You can set the threshold when to move the data to disk. When pushing data to the buffer, it is stored in memory until it gets too large. E.g.: new LargeArrayBuffer(512); to set a 512 MiB threshold.
2. You can choose either the PHP serializer or the igbinary serializer (PHP serializer is default). E.g.: new LargeArrayBuffer(serializer: LargeArrayBuffer::COMPRESSION_IGBINARY);
3. You can enable GZIP or LZ4 compression for the serialized items. Although this is recommended only if your items are pretty big like > 1 KiB each. E.g.: new LargeArrayBuffer(compression: LargeArrayBuffer::COMPRESSION_GZIP);. Note, that LZ4 compression requires ext-lz4 to be installed.

Read from the buffer

There are several options to read the data:

1. Iterate: As you might have seen in the example above, you can iterate over the buffer just like over an array with foreach. foreach($buffer as $item){ /* work with your data here */ }
2. toArray(): If you have a set of buffers which do not fit in memory all together but one at a time, you can use $buffer->toArray() to get an array to work with.
3. toJSONFile(): If you want to write the array in JSON format to some file or resource, use this method. It supports all the json_encode() options and flags.

Buffer stats

There are some stats you can obtain:

1. count(): The number of items in the buffer.
2. getSize(): The number of bytes of the serialized (!) data.

How it works

To put it in one sentence: This library uses php://temp as well as PHP's serialize/unserialize functions to store an array on disk if it gets too large.

Limitations and concerns

• associative arrays are not supported
• the item type needs to be compatible with PHP's serialize/unserialize functions
• since storage drives (even PCIe SSDs) are a lot slower than memory and de-/serialization needs to be done, you trade hard memory overflows for performance losses

Benchmark

A benchmark with 1 million measurements (consisting of DateTimeImmutable, int and float) using PHP 8.1 with 10 iterations comparing a normal array with the LargeArrayBuffer gave the following results (LargeArrayBuffer was configured with a memory limit of 128 MiB):

Action	Serializer	Compression	Consumed time	Consumed memory	Buffer size
Fill array	none	none	1.57 s	490.0 MiB	NA
Iterate over array	none	none	0.27 s	492.0 MiB	NA
Fill buffer	PHP	none	4.27 s	0 B	378.7 MiB
Iterate over buffer	PHP	none	2.85 s	0 B	378.7 MiB
Fill buffer	PHP	GZIP	24.76 s	0 B	192.5 MiB
Iterate over buffer	PHP	GZIP	6.79 s	0 B	192.5 MiB
Fill buffer	PHP	LZ4	4.89 s	0 B	241.0 MiB
Iterate over buffer	PHP	LZ4	2.93 s	0 B	241.0 MiB
Fill buffer	igbinary	none	4.26 s	0 B	319.1 MiB
Iterate over buffer	igbinary	none	3.41 s	0 B	319.1 MiB
Fill buffer	igbinary	GZIP	21.50 s	0 B	173.2 MiB
Iterate over buffer	igbinary	GZIP	4.80 s	0 B	173.2 MiB
Fill buffer	igbinary	LZ4	4.38 s	0 B	195.1 MiB
Iterate over buffer	igbinary	LZ4	3.17 s	0 B	195.1 MiB

Note:

• The peak memory usage using the buffer is about its memory limit. The table shows the memory usage after the specified action.
• PHP seems to cache the array once it is created for the first time, although unset is used. That is why I have not put the average value in the table for this specific value but the maximum (first run).
• The serialized data is smaller than the binary data in memory. I have absolutly no idea why.

To reproduce call bench/benchmark.php.

License

This library is licensed under the MIT License (MIT). Please see LICENSE for more information.