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


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
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')):

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):

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):

# Skip all records that are smaller than 128 bytes
for record in ArchiveIterator(stream, min_content_length=128):

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):

# Skip records without a block digest
for record in ArchiveIterator(stream, func_filter=has_block_digest):

# Skip records that are not WARC/1.1
for record in ArchiveIterator(stream, func_filter=is_warc_11):

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'):

# 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)):

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):


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 =

    # Consume and return the remaining record bytes
    body +=

    # Or: Consume rest of stream without allocating a buffer for it (i.e., skip over)

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.


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.

ClueWeb Notes

FastWARC is a standards-compliant WARC parser. Unfortunately, the ClueWeb authors were somewhat creative with the standard. If you work with these datasets, you will inevitably notice certain defects in the files that result in premature stream aborts. This applies to both the old ClueWeb09 as well as the new ClueWeb22. Following is a list of known ClueWeb WARC defects and how to work around them:


The ClueWeb09 uses a non-standard WARC version and the WARC headers are separated by LF instead of CRLF line breaks. In addition, some header values contain line breaks without proper continuation indents and non-ASCII characters. To prevent FastWARC from choking on this creative WARC flavour, pass strict_mode=False to your ArchiveIterator.


ClueWeb22 WARCs are a bit more predictable than ClueWeb09 WARCs, but have non-trivial defects nonetheless.

First, the initial warcinfo records are missing the required Content-Length header, so we have to rely on heuristics to determine where the record ends. If strict_mode=False is set and the WARC is read from a GZip-compressed stream, FastWARC will attempt to use the internal buffer boundaries for determining the record end. If you are reading the WARC as an uncompressed file, FastWARC has to seek forward to the next valid WARC/1.1 version line. In this case the record body will be skipped as empty. Without strict_mode=False, FastWARC will stop after the first header block.

Second, all records are of type response with Content-Type: application/http; msgtype=response, yet they contain only the HTML body and not the full HTTP response (the correct record type would be resource with Content-Type: text/html). This incorrect type description will trigger FastWARC’s automatic HTTP parsing, which will result in empty or incomplete record bodies. To avoid this, explicitly set parse_http=False.


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


$ 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


$ 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.