Processing terabyte large LZ4 files in a streaming fashion with resumption and random access.

## Intro

LZ4 is lossless compression algorithm, providing compression speed at 400 MB/s per core, scalable with multi-core CPU. It features an extremely fast decoder, with speed of multiple GB/s per core, typically reaching RAM speed limits on multi-core systems lz4.

LZ4 is often used to compress large files (hundreds of GBs), e.g., Censys uses it to archive Internet-wide TLS scans - large JSON files. LZ4 typically reduces such JSON file to 20-50% of the original size.

curl --head returns Content-Length: 242919507913 which is roughly 226.24 GB reducing the file size to 23% of the original size. JSON is quite redundant (field names) making it a good subject to compression.

## Motivation

Imagine you have to process 1TB json.lz4 file with one JSON record per line with the limited amount of memory and disk storage. You definitely cannot download the file or load it to the memory. Fortunately, LZ4 is designed so the streaming processing is possible so you only need to choose a library that supports it.

There is a lz4 package available on RHEL/Ubuntu to work with LZ4 archives. Fetching the first record from the compressed json can be done like this:

\$> curl -s -L 'https://ph4r05.deadcode.me/static/lz4/certificates.20171002T020001.15.json.lz4' | lz4 -d -c - | head -n 1
{"fingerprint_sha256":"f9000003ebc6a586e0076ee11f446cf2a844461bfee6e9d2735fabfd99aba231","raw":"MIIDUTCCAjmgAwIBAgIFFIdXdAkwDQYJKoZIhvcNAQELBQAwUjELMAkG ....


The problem is when you have processed already 800 GB and sudden network glitch occurs or there is some firewall problem or system reboots. There is currently no way to resume the processing so you basically have to download the whole file again.

I will demonstrate solutions in Python, using my version of a py-lz4framed library, the forked version of the py-lz4framed original repository with few my additions which I explain in this post.

## Stream processing

LZ4 decompressor accepts a file-like object with read() method. It can be a locally opened file or a network socket. The naive way to read a remote file in a streaming fashion is to open a remote connection and pass the socket object to the decompressor like this:

import requests, lz4framed
r = requests.get(url, stream=True, allow_redirects=True)
for idx, chunk in enumerate(lz4framed.Decompressor(r.raw)):
print(chunk)


If there is a timeout/connection reset or other network glitch either the exception is thrown or empty data is returned from the read() which causes decompressor to fail.

## Temporal outage - reconnect wrapper

LZ4 decompressor holds state internally in the memory. One solution to survive network glitches without a need to modify the LZ4 library is to wrap the network file-like object with the object transparently reconnecting to the data source on error.

In this way, LZ4 decompressor is fed with the uninterrupted data stream. When an error on reading occurs the wrapping object blocks, retries to connect and reads data again. Once data is loaded it is passed to the decompressor. So even if there is 30 minutes blackout the read resumes eventually.

I’ve implemented this solution here: ReconnectingLinkInputObject. It wraps requests library to open the network connection and read from it. If there is an exception or unexpected end of data it back-offs, reconnects and uses Range HTTP header to request unprocessed part of the data from the server.

import requests, lz4framed, input_obj
for idx, chunk in enumerate(lz4framed.Decompressor(iobj)):
print(chunk)


A disadvantage of this approach is in-memory only state. When system reboots the state is lost and you have to start from the beginning.

To use this library you can either use my GitHub repository https://github.com/ph4r05/input_objects or install the python library via pip:

pip install input_objects


## Advanced approach - state checkpoint

In order to be able to resume after OS crash we need to backup the decompressor state somehow.

## LZ4 Format

LZ4 file consists of frames (top-level format) which contains LZ4 blocks with data. The frame format is the following:

• Magic bytes and end mark are fixed 4 bytes marks
• Frame descriptor defines basic setting of the LZ4 algorithm - explained below.
• LZ4 data blocks are present multiple times
• The checksum is xxHash of the data blocks - if enabled in the frame descriptor.

Usually, there is only one frame encapsulating the whole data in the LZ4 file. The frame format is described in detail in frame-format reference.

### Frame descriptor

Frame descriptor contains basic flags defining the processing of the LZ4 archive.

• Block independence: 1 means blocks are independent and can be decoded separately (i.e., the LZ4 decoding dictionary is not shared among blocks). 0 means blocks are dependent on each other (up to the LZ4 window size - 64 KB).

• Block checksum: 1 means each block in the frame is followed by its 4B xxHash checksum.

• Content size: 1 means uncompressed content size is stored in the frame descriptor.

• Block Maximum Size: maximum size of an uncompressed block, helps with allocating memory. Options: 64 KB, 256 KB, 1 MB, 4 MB.

### LZ4 data blocks

Data blocks stored in the frame have the following structure:

Data part contains the LZ4 compressed data, specified in the block-format reference. For the scope of this blog post the internal structure of the block is not important as we work with them as opaque data units processed by low-level LZ4 engine.

### Decompressor state

Here is the main state of the decompressor in C. It is here just for demonstration purposes to demonstrate the complexity of the state (and its size). You don’t have to understand the fields meaning.

struct LZ4F_dctx_s {
LZ4F_frameInfo_t frameInfo;  // basic frame info
U32    version;
U32    dStage;
U64    frameRemainingSize;
size_t maxBlockSize;    // max 4MB block
size_t maxBufferSize;   // maxBlockSize + 128 kB in linked mode
BYTE*  tmpIn;           // memory buffer for input
size_t tmpInSize;
size_t tmpInTarget;
BYTE*  tmpOutBuffer;    // memory buffer for output
const BYTE*  dict;      // decompress dictionary, 64 kB, offset to tmpOutBuffer
size_t dictSize;
BYTE*  tmpOut;          // output buffer, offset to tmpOutBuffer
size_t tmpOutSize;      // size of the data in the output buffer
size_t tmpOutStart;     // start of the data in the output buffer
XXH32_state_t xxh;      // checksum state 48 B (frame-wise)
};  /* typedef'd to LZ4F_dctx in lz4frame.h */


As you can see the state has quite simple structure. There are mainly:

• Two main memory buffers - tmpIn, tmpOutBuffer.
• The decoding dictionary for LZ4 decompressor is stored in the out buffer.
• The state of the checksum xxHash engine.
• and a copy of the frame header.

The serialized state corresponds to the decompressor snapshot at a particular reading position in the LZ4 archive. Hence after the decompressor serialized state is restored it can continue from the given position.

I implemented the decompressor state marshalling to the py-lz4framed library. User can periodically call state marshaling method and build a dictionary with snapshots of decompressor state at given reading position of the stream.

Example of the snapshot/checkpoint map in reading positions 10, 20, 30 MB in the LZ4 archive:

[
{
"pos": 10485760,
},
{
"pos": 20971520,
},
{
"pos": 31457280,
}
]


The position -> decompressor state state mapping file can be built on the fly when reading the file for the first time. Note this requires only negligible constant amount of memory to process arbitrarily large files.

When the server crashes or the process is interrupted the mapping file is read in order to resume the operation. The last entry is loaded to $$(\text{pos}_l, \text{state}_l)$$ tuple.

The file is then opened at the position $$\text{pos}_l$$, decompressor state $$\text{state}_l$$ is resumed and the decompressor continues from the snapshot.

The file reading from the given position can be done with seeking in case of a local file or by using the HTTP Range headers if the file is read from a remote server.

Take a look at the test_decompressor_fp_marshalling test for a detailed example of state marshaling and unmarshalling.

### Random access archive

Situation: 800 GB LZ4 encrypted file. You want a random access the file so it can be processed by map-reduce workers or processed in parallel from different offsets.

If you build the LZ4 position -> decompressor state mapping file other processes can use this file to jump to the checkpoints in the file without the need to read the file from the beginning.

This simple way enables you to “random access” LZ4 archive at cost of building the mapping file (one extra read, performed once per file).

You can try building your own mapping file for the LZ4 archive using my GitHub project lz4-checkpoints.

### State size

A position of a decompressor in the LZ4 archive being read heavily influences the size of the serialized decompressor state.

When you marshall the decompressor state in the middle of a block the decompressor state need to contain the whole decompressing state (buffers, dictionary) so it can continue processing the block after resume. It means it can take up to 8 MB (if the block size is 4 MB), give or take some additional data like a checksum.

Building a mapping file naively is obviously space ineffective. But this can be improved significantly.

If the frame format is using non-linked blocks (each block is independent - all Censys archives) then the state size of the decompressor is minimal at the block boundary position.

There is no need to store the block buffers after the block reading has finished. The only size that needs to be stored is the content checksum xxHash state so decompressor can compute the checksum of all stored blocks when finished.

State serialized at the block boundaries takes only 184 bytes in total.

The problem is the block size varies (up to 4 MB) in the frame so we don’t know where the block boundaries are located in the LZ4 archive.

The LZ4 decompressing library has an API giving you so-called size hints after each read. Size hint is a preferred size of the input archive chunk to process in the next read call so the memory allocation in the decompressor is optimal.

In other words, size hint is a number of bytes to read from the input archive so the block is processed in one call. When reading the archive in this way the file is read and processed by the whole blocks, which is the most effective way.

The size hints determine block boundaries when reading the LZ4 archive by chunks. We store the decompressor state exactly at those positions to minimize the size of the mapping file.

### Extended checkpoints

When processing the LZ4 archive you may also find handy to store one more information to the checkpoint - position in the uncompressed data stream (data produced by decompressor). This may be useful when jumping in the data stream - depends on the structure of the compressed file.

[
{
"pos": 10485760,
"upos": 24117248,
},
{
"pos": 20971520,
"upos": 61865984,
},
{
"pos": 31457280,
"upos": 81788928,
}
]


### Uncompressed Data format

In our case, the LZ4 contains line-separated JSON records which are ideal for this kind of random access.

Most of the time the block boundary is not aligned with the json record separator (new line) in the decompressed JSON file. Thus jumping to the checkpoint resumes the reading in the middle of the JSON record.

In that case your logic has to take this into account and find the next record in the uncompressed data stream to resume processing correctly. The position in the stream may help you with the book-keeping of already processed records or to distribute the file processing among multiple workers correctly.

### Further extensions

As you may noticed the checkpointing is not the optimal as in the each checkpoint we store the same information as in previous checkpoints, like LZ4F_frameInfo_t frameInfo, BYTE header[16]; and so on.

It suffices to store just the position of the block boundary and the checksum state (48 B) but this space saving is not significant and poses another complications. In the current state each checkpoint is fully fledged state snapshot independent on each other with only small space overhead.

## Conclusion

In this post I explained the way to handle network outages and interruptions while processing large LZ4 archives.

The decompressor state marshalling technique was proposed and tested in practice. The set of decompressor checkpoints gives also the possibility to random access the LZ4 archive which was not possible before without reading the whole file from the beginning. This also enables parallel processing of large LZ4 archives saving the bandwidth and time.

## Resources:

• py-lz4framed python library for LZ4 processing, with state marshalling
• input_objects python library providing uniform access to file-like objects. Provides also file-like object which reconnects to the source in case of an read error.
• lz4-checkpoints python library for generating LZ4 checkpoint map files.

LZ4 sources: