FastWARC
FastWARC is a high-performance WARC parsing library for Python written in C++/Cython. The API is inspired in large parts by WARCIO, but does not aim at being a drop-in replacement. FastWARC supports compressed and uncompressed WARC/1.0 and WARC/1.1 streams. Supported compression algorithms are GZip and LZ4.
FastWARC belongs to the ChatNoir Resiliparse toolkit for fast and robust web data processing.
Why FastWARC and not WARCIO?
WARCIO is a fantastic tool for reading and writing WARCs, but it is implemented entirely in Python and thus becomes rather inefficient for large web crawls at the tera- or petabyte scale where a few seconds of additional processing time add up quickly. FastWARC solves these performance issues by being written in efficient, low-level C++. We also took the opportunity to add support for LZ4, a much, much (!) faster compression algorithm than GZip, which unfortunately is the only compression algorithm mentioned in the WARC specification (and thus also the only one supported by WARCIO, although it wouldn’t be a big deal to add that).
FastWARC’s design goals are high speed, a low and fixed memory footprint, and simplicity. For the latter reason, we decided against adding support for the legacy ARC format. If you need that kind of backwards compatibility, use WARCIO instead.
Installing FastWARC
Pre-built FastWARC binaries can be installed from PyPi:
pip install fastwarc
Attention
The Linux binaries are provided solely for your convenience. Since they are built on the very old manylinux base system for better compatibility, their performance isn’t optimal (though still better than WARCIO).
For best performance, see the next section on how to build FastWARC yourself.
Building FastWARC From Source
You can compile FastWARC either from the PyPi source package or directly from the Github repository, though in any case, you need to install all required build-time dependencies first. On Ubuntu, this is done as follows:
sudo apt install build-essential python3-dev zlib1g-dev liblz4-dev
To build and install FastWARC from PyPi, run
pip install --no-binary fastwarc fastwarc
That’s it. If you prefer to build directly from the GitHub repository instead, run:
# Clone repository
git clone https://github.com/chatnoir-eu/chatnoir-resiliparse.git
cd chatnoir-resiliparse
# Optional: Create a fresh venv
python3 -m venv venv && source venv/bin/activate
pip install -e fastwarc
To build the wheels without installing them, run:
pip wheel -e fastwarc
# Or:
pip install build && python -m build --wheel fastwarc
Iterating WARC Files
The central class for stream-processing WARC files is fastwarc.warc.ArchiveIterator
:
from fastwarc.warc import ArchiveIterator
for record in ArchiveIterator(open('warcfile.warc.gz', 'rb')):
print(record.record_id)
This will iterate over all records in the file and print out their IDs. You can pass any file-like Python object to ArchiveIterator
, for either an uncompressed or a GZip- or LZ4-compressed WARC. FastWARC will try to auto-detect the stream format, but if you know the compression algorithm beforehand, you can speed up the process a little by explicitly passing a GZipStream
or LZ4Stream
object instead:
from fastwarc.stream_io import *
# GZip:
stream = GZipStream(open('warcfile.warc.gz', 'rb'))
# LZ4:
stream = LZ4Stream(open('warcfile.warc.lz4', 'rb'))
As a further optimization for local files, it is recommended that you use a FileStream
instead of a Python file object. FileStream
is a native file reader that circumvents the entire Python I/O stack for better performance:
from fastwarc.stream_io import *
stream = GZipStream(FileStream('warcfile.warc.gz', 'rb'))
Filtering Records
FastWARC provides several ways in which you can filter and efficiently skip records you are not interested in. These filters are checked very early in the parsing process, right after the WARC header block has been read. Multiple types of filters can be combined.
Record Type Filter
If you want only records of a certain type, you can skip all other records efficiently by specifying a bitmask of the desired record types:
from fastwarc.warc import ArchiveIterator, WarcRecordType
for record in ArchiveIterator(stream, record_types=WarcRecordType.request | WarcRecordType.response):
pass
This will skip all records with a WARC-Type
other than request
or response
.
Content-Length Filter
You can automatically skip any records whose Content-Length
exceeds or is lower than a certain value:
from fastwarc.warc import ArchiveIterator
# Skip all records that are larger than 500 KiB
for record in ArchiveIterator(stream, max_content_length=512000):
pass
# Skip all records that are smaller than 128 bytes
for record in ArchiveIterator(stream, min_content_length=128):
pass
Function Filter
If the above-mentioned filter mechanisms are not sufficient, you can pass a function object that accepts as its only parameter a WarcRecord
and returns a bool
value as a filter predicate. This filter type is much slower than the previous filters, but probably still more efficient than checking the same thing later on in the loop. Be aware that since the record body hasn’t been seen yet, you cannot access any information beyond what is in the record headers.
FastWARC comes with a handful of existing filters that you can use:
from fastwarc.warc import *
# Skip any non-HTTP records
for record in ArchiveIterator(stream, func_filter=is_http):
pass
# Skip records without a block digest
for record in ArchiveIterator(stream, func_filter=has_block_digest):
pass
# Skip records that are not WARC/1.1
for record in ArchiveIterator(stream, func_filter=is_warc_11):
pass
The full list of pre-defined function filters is: is_warc_10()
, is_warc_11()
, has_block_digest()
, has_payload_digest()
, is_http()
, is_concurrent()
. Besides these, you can pass any Python callable that accepts a WarcRecord
and returns a bool
:
# Skip records which haven't been identified as HTML pages
for record in ArchiveIterator(stream, func_filter=lambda r: r.headers.get('WARC-Identified-Payload-Type') == 'text/html'):
pass
# Skip records without any sort of digest header
for record in ArchiveIterator(stream, func_filter=lambda r: has_block_digest(r) and has_payload_digest(r)):
pass
Digest Filter
This is the only filter that is executed after the content is available and will skip any records without or with an invalid block digest:
for record in ArchiveIterator(stream, verify_digests=True):
pass
Warning
This is the most expensive filter of all and it will create an in-memory copy of the whole record. See Verifying Record Digests for more information on how digest verification works.
Record Properties
The ArchiveIterator
returns objects of type WarcRecord
, which have various properties:
for record in ArchiveIterator(stream):
record.headers # Dict-like object containing the WARC headers
record.record_id # Shorthand for record.headers['WARC-Record-ID']
record.record_type # Shorthand for record.headers['WARC-Type']
record.record_date # Parsed record.headers['WARC-Date']
record.content_length # Effective record payload length
record.stream_pos # Record start offset in the (uncompressed) stream
record.is_http # Boolean indicating whether record is an HTTP record
record.http_headers # Dict-like object containing the parsed HTTP headers
record.http_content_type # Plain HTTP Content-Type without charset
record.http_charset # HTTP charset from the Content-Type header (if any)
record.http_date # Parsed HTTP Date header
record.http_last_modified # Parsed HTTP Last-Modified header
record.reader # A BufferedReader for the record content
# Read and return up to 1024 bytes from the record stream
body = record.reader.read(1024)
# Consume and return the remaining record bytes
body += record.reader.read()
# Or: Consume rest of stream without allocating a buffer for it (i.e., skip over)
record.reader.consume()
As you can see, HTTP request and response records are parsed automatically for convenience. If not needed, you can disable this behaviour by passing parse_http=False
to the ArchiveIterator
constructor to avoid unnecessary processing. record.reader
will then start at the beginning of the HTTP header block instead of the HTTP body. You can parse HTTP headers later on a per-record basis by calling record.parse_http()
as long as the BufferedReader
hasn’t been consumed at that point.
Verifying Record Digests
If a record has digest headers, you can verify the consistency of the record contents and/or its HTTP payload:
for record in ArchiveIterator(stream, parse_http=False):
if 'WARC-Block-Digest' in record.headers:
print('Block digest OK:', record.verify_block_digest())
if 'WARC-Payload-Digest' in record.headers:
record.parse_http() # It's safe to call this even if the record has no HTTP payload
print('Payload digest OK:', record.verify_payload_digest())
Note that both verify_block_digest()
and verify_payload_digest()
will simply return False
if the headers do not exist, so check that first. Also keep in mind that the block verification will fail if the reader has been (partially) consumed, so automatic HTTP parsing has to be turned off for this to work.
Warning
Calling either of these two methods will create an in-memory copy of the remaining record stream to preserve its contents for further processing (that’s why verifying the HTTP payload digest after verifying the block digest worked in the first place).
If your records are very large, you need to ensure that they fit into memory entirely (e.g. by checking record.content_length
). If you do not want to preserve the stream contents, you can set consume=True
as a parameter. This will avoid the creation of a stream copy altogether and fully consume the rest of the record instead.
Benchmarks
Depending on your CPU, your storage speed, and the WARC compression algorithm, you can typically expect speedups between 1.3x and 6.5x over WARCIO.
The FastWARC CLI comes with a benchmarking tool for measuring WARC record decompression and parsing performance on your own machine. The benchmarking results can be compared directly with WARCIO. Here are three example runs on an AMD Ryzen Threadripper 2920X (with NVMe SSD) over five Common Crawl WARCs:
Uncompressed WARC:
$ fastwarc benchmark CC-MAIN-*.warc --bench-warcio
Benchmarking read performance from 5 input path(s)...
FastWARC: 630,245 records read in 5.81 seconds (108,487.93 records/s).
WARCIO: 630,245 records read in 37.19 seconds (16,945.51 records/s).
Time difference: -31.38 seconds, speedup: 6.40
GZip WARC:
$ fastwarc benchmark CC-MAIN-*.warc.gz --bench-warcio
Benchmarking read performance from 5 input path(s)...
FastWARC: 630,245 records read in 60.52 seconds (10,413.38 records/s).
WARCIO: 630,245 records read in 97.56 seconds (6,460.06 records/s).
Time difference: -37.04 seconds, speedup: 1.61
LZ4 WARC:
$ fastwarc benchmark CC-MAIN-*.warc.lz4
Benchmarking read performance from 5 input path(s)...
FastWARC: 630,245 records read in 12.65 seconds (49,825.44 records/s).
(Direct comparison not possible, since WARCIO does not support LZ4.)
The read benchmarking tool has additional options, such as reading WARCs directly from a remote S3 data source using Boto3.