NAME
Data::BloomFilter::Shared - shared-memory Bloom filter for Linux
SYNOPSIS
use Data::BloomFilter::Shared;
# sized for 1_000_000 items at a 1% false-positive rate, anonymous mapping
my $bf = Data::BloomFilter::Shared->new(undef, 1_000_000, 0.01);
$bf->add("alice");
$bf->add("bob");
$bf->contains("alice"); # 1 (probably present)
$bf->contains("carol"); # 0 (definitely absent)
# "have I seen this before?" -- add returns 1 the first time, 0 after
my $first = $bf->add("event-42"); # 1: probably new
my $again = $bf->add("event-42"); # 0: all its bits were already set
# bulk add in a single lock acquisition
my $new = $bf->add_many([ map { "user-$_" } 1 .. 1000 ]);
# merge another filter of identical geometry (bitwise OR -> union)
my $other = Data::BloomFilter::Shared->new(undef, 1_000_000, 0.01);
$other->add_many([ map { "user-$_" } 500 .. 1500 ]);
$bf->merge($other);
# share across processes via a backing file
my $shared = Data::BloomFilter::Shared->new("/tmp/seen.bloom", 1_000_000, 0.01);
# freeze and ship: query it read-only (lock-free) on other machines
$shared->freeze;
my $ro = Data::BloomFilter::Shared->new_readonly("/tmp/seen.bloom");
$ro->contains("event-42");DESCRIPTION
A Bloom filter in shared memory: a compact, fixed-size structure for probabilistic set membership. You add items to it, then ask whether an item is in the set. The answer is either "definitely not present" or "probably present": the filter has no false negatives (if you added it, contains always returns true) but a tunable rate of false positives (it may occasionally report an item as present that was never added). It never stores the items themselves, only a bit array, so memory is proportional to the configured capacity and false-positive rate, not to the size of the items.
Each item is hashed once with XXH3 (128-bit); the two 64-bit halves drive k probe positions into a power-of-two bit array via Kirsch-Mitzenmacher double hashing. add sets those k bits; contains reports present only if all k are set. The number of hashes k and the bit-array size are derived from the capacity and fp_rate you request.
Because the bit array lives in a shared mapping, several processes share one filter: any process that opens the same backing file, inherits the anonymous mapping across fork, or reopens a passed memfd, sees the others' additions and contributes its own. A write-preferring futex rwlock with dead-process recovery guards mutation, so many processes may add and contains concurrently. Two filters of identical geometry can be combined with merge (bitwise OR), which yields a filter whose membership is the union of the two input sets.
Items are added and tested by their byte content; wide-character strings (any codepoint above 255) cause a "Wide character" croak -- encode such strings to bytes first (for example with Encode::encode_utf8). Linux-only. Requires 64-bit Perl.
METHODS
Constructors
my $bf = Data::BloomFilter::Shared->new($path, $capacity, $fp_rate);
my $bf = Data::BloomFilter::Shared->new(undef, 1_000_000); # fp_rate 0.01
my $bf = Data::BloomFilter::Shared->new_memfd($name, $capacity, $fp_rate);
my $bf = Data::BloomFilter::Shared->new_from_fd($fd);
my $ro = Data::BloomFilter::Shared->new_readonly($path); # frozen file, read-only$path is the backing file (undef or omitted for an anonymous mapping). $capacity is the number of items you expect to add; it must be at least 1. $fp_rate is the target false-positive rate at that capacity; it is optional, defaults to 0.01 (1%), and must be strictly between 0 and 1. new and new_memfd croak if $capacity is less than 1 or $fp_rate is out of range.
From $capacity and $fp_rate the filter derives its geometry: k = round(-log2(fp_rate)) hashes (clamped to the range 1..32), and a bit array of m = next_power_of_two(ceil(capacity * k / ln 2)) bits (with a floor of 64 bits). Rounding the bit count up to a power of two means the realised false-positive rate at capacity is typically at or below the configured target. When reopening an existing file or memfd, the stored geometry wins and the caller's $capacity/$fp_rate do not resize it -- but they are still range-checked, so an out-of-range value croaks. new_memfd creates a Linux memfd (transferable via its memfd descriptor); new_from_fd reopens one in another process. The descriptor you pass is duplicated (F_DUPFD_CLOEXEC), so it stays yours to close and closing it does not disturb the handle. new_readonly opens a frozen file read-only for lock-free querying (see "FROZEN (READ-ONLY) MODE").
Adding and testing
my $new = $bf->add($item); # 1 if probably new, else 0
my $added = $bf->add_many(\@items); # count of items that were probably new
my $in = $bf->contains($item); # 1 if probably present, 0 if definitely absent
$bf->clear; # reset to empty (all bits 0)add hashes $item (taken by its bytes; wide characters croak, encode first) and sets its k bits, returning 1 if the item was probably new -- that is, if at least one of its bits was previously unset -- and 0 if all k bits were already set (the item was probably added before). A return of 0 is therefore a "probably seen this already" signal, subject to the same false-positive rate as contains. add_many takes an array reference and does the whole batch under a single write lock, returning how many of its elements were probably new.
contains returns 1 if the item is probably present and 0 if it is definitely absent. The contract is asymmetric and is the whole point of a Bloom filter: a 0 is exact (the item was never added), while a 1 may be a false positive (some other items happened to set all of this item's bits). There are never false negatives: any item you have added always reports as present.
Merging
$bf->merge($other);Folds $other's bit array into $bf by bitwise OR, so $bf then reports as present the union of the two filters' item sets (still with no false negatives). Both filters must have identical geometry -- the same number of bits and the same number of hashes, which follows from constructing both with the same $capacity and $fp_rate (merge croaks on a mismatch). $other is read under its own lock into a private snapshot first, so merging is deadlock-free even if two processes merge each other concurrently; $other is not modified.
Introspection and lifecycle
$bf->capacity; $bf->bits; $bf->hashes; $bf->fp_rate; $bf->count; $bf->stats;
$bf->path; $bf->memfd; $bf->sync; $bf->unlink; # or Class->unlink($path)capacity is the configured item capacity; bits is the bit-array size in bits (a power of two); hashes is the number of hash probes k; fp_rate is the configured target false-positive rate. count returns an estimate of the number of distinct items added, computed from the fraction of bits set (accurate while the filter is not saturated, and capped at capacity once it is). sync flushes the mapping to its backing store (a no-op for anonymous and memfd filters, which have none); unlink removes the backing file (also callable as Class->unlink($path)) and croaks if the removal fails -- except when the file is already gone, which is what you asked for; it is likewise a no-op when there is no backing file (anonymous or memfd); path returns the backing path (undef for anonymous, memfd, or fd-reopened filters) and memfd the backing descriptor -- the memfd of a new_memfd filter or the dup'd fd of a new_from_fd filter, and -1 for file-backed or anonymous filters.
STATS
stats() returns a hashref describing the filter:
capacity-- the configured item capacity.fp_rate-- the configured target false-positive rate.bits-- the bit-array size in bits (a power of two).hashes-- the number of hash probeskper item.bits_set-- how many bits are currently set.count-- the estimated number of distinct items added.fill_ratio--bits_set / bits, between 0 and 1. As this approaches 0.5 the filter is near its designed capacity and the realised false-positive rate approaches the configured target; well above that the rate climbs.ops-- running count of mutating operations (add,add_many,merge,clear).mmap_size-- bytes of the shared mapping.frozen-- 1 if the filter has been sealed byfreeze(immutable), else 0.readonly-- 1 if this handle is a read-only view (fromnew_readonly, or the handle that calledfreeze), else 0.
SHARING ACROSS PROCESSES
The filter lives in a shared mapping, shared the same three ways as the rest of the family: a backing file (every process calls new($path, ...) on the same path with matching capacity and rate), an anonymous mapping inherited across fork, or a memfd whose descriptor is passed to an unrelated process (over a UNIX socket via SCM_RIGHTS, or via /proc/$pid/fd/$n) and reopened with new_from_fd($fd). Because the mapping is shared, every process adds into and tests against the same bit array, so membership reflects the union of what all of them have added.
# producer and consumer share one filter with no coordination
my $bf = Data::BloomFilter::Shared->new(undef, 100_000, 0.01); # before fork
unless (fork) { $bf->add_many([ map { "ev-$_" } 1 .. 1000 ]); exit }
wait;
print $bf->contains("ev-500") ? "seen\n" : "no\n"; # seen -- the child's addFROZEN (READ-ONLY) MODE
A file-backed filter can be frozen and then shipped to other machines, where consumers open it read-only and query it with no locking at all.
# producer: build, freeze, ship the file
my $bf = Data::BloomFilter::Shared->new("/tmp/seen.bloom", 1_000_000, 0.01);
$bf->add_many(\@known);
$bf->freeze; # seal: now immutable, and $bf itself is read-only
# ... copy /tmp/seen.bloom to another host ...
# consumer (any process, same architecture): read-only, lock-free
my $ro = Data::BloomFilter::Shared->new_readonly("/tmp/seen.bloom");
$ro->contains($item) for @queries;freeze takes the write lock, marks the filter permanently immutable (there is no unfreeze -- rebuild the file to change it), and flushes the seal to disk. A frozen filter rejects every mutator (add, add_many, merge, clear) with a croak, and a read-write reopen (new($path, ...)) of a sealed file is refused -- so a shipped artifact can never be silently mutated out from under its readers. That protection is enforced by the reader: the seal is a header flag that 0.04 and earlier do not know about, and the on-disk format version is deliberately unchanged so those releases can still open files written here. A pre-0.05 build therefore opens a sealed file read-write and can modify it, so keep producers and consumers on 0.05 or later if you rely on the seal. freeze itself is not idempotent: the handle that seals the file becomes a read-only view of it, so calling freeze on that handle again croaks.
new_readonly($path) maps the file O_RDONLY / PROT_READ and requires it to be frozen (it croaks on a file that was never freezed). Because a sealed filter's bits and geometry are immutable, contains, count and stats read them directly, taking no reader lock -- the mapping is never written, so a read-only view works from a read-only file descriptor or a read-only filesystem, and any number of processes can share one PROT_READ mapping. frozen and readonly report the two states.
Portability. The on-disk format is native binary (native-endian 64-bit words), so a frozen file may be copied only between machines of the same architecture; a wrong-endian file is rejected at open by the magic check. Copy the file to each consumer -- do not share one file over a network filesystem: the lock is a Linux futex (process-local to one kernel), and the "no live writer" contract assumes a static copy. Linux-only; 64-bit Perl.
SECURITY
Backing files are created with mode 0600 (owner-only) by default, so only the creating user can open and attach them. To share a backing file across users, pass an explicit octal file mode such as 0660 as the last argument to new; the mode is applied when the file is created, and when a file left behind by an interrupted create is re-initialized (see "CRASH SAFETY"); a file already in use keeps its own permissions. The file is opened with O_NOFOLLOW, so a symlink planted at the path is refused, and created with O_EXCL; the on-disk header is validated when the file is attached. Any process you grant write access to a shared mapping is trusted not to corrupt its contents while other processes are using it.
CRASH SAFETY
Mutation is guarded by a futex-based write-preferring rwlock with PID-encoded ownership; if a holder dies, the next contender detects the dead owner and recovers. Each bit set is a single word store, so a crash leaves the filter consistent up to the last completed add. Limitation: PID reuse is not detected (very unlikely in practice).
Reader-slot exhaustion (slotless readers): dead-process recovery attributes a crashed lock holder's contribution through its reader-slot. The slot table holds 1024 entries (one per concurrent reader process). If more than that many reader processes share one mapping at once, a reader that cannot claim a slot proceeds "slotless" -- it still takes the read lock but leaves no per-process record. If such a slotless reader is then killed while holding the read lock, its share of the lock cannot be attributed to a dead process, so writer recovery cannot reclaim it and writers may block until the mapping is recreated. Reaching this needs more than 1024 concurrent reader processes on one mapping plus a crash in the brief read-lock window; the dead-process slot reclaim keeps the table from filling with stale entries, so in practice it is very unlikely. Those preconditions cover the live-process route only. The count lives in the mapping and new validates the geometry, not this transient value, so a backing file damaged at rest -- bit rot, a partial copy, or a process that scribbled on the mapping -- can present a non-zero slotless count and block every writer the same way, with none of the above. If writers hang on a file no live reader is using, recreate it.
An interrupted create is recovered too. A creator killed after the backing file is sized but before its header is committed leaves a full-size, all-zero file. new re-initializes such a file automatically, but only when it is exactly the size the requested geometry needs, is owned by your effective uid, and is still entirely zero -- a file holding data is never re-initialized. If the creator got as far as writing part of the header, the file cannot be told apart from a corrupt one and new croaks with incomplete Bloom filter file left by an interrupted create; remove it and retry. A file left behind by an interrupted create never held data, so removing it is safe -- but a file whose header was corrupted after the fact reaches the same croak, so confirm it is an abandoned create before deleting anything you care about.
Disk space. The backing file is created sparse: new sizes it, but blocks are allocated only as you write, so a large filter costs almost nothing on disk until it is used. The cost of that is a late failure, and how it reaches you depends on the filesystem. Where blocks are allocated at fault time -- tmpfs, so /dev/shm and many /tmp mounts -- a write to a page that cannot be backed raises SIGBUS and kills the process, because an mmap store has no way to report ENOSPC. Where allocation is delayed to writeback (ext4, xfs), the store lands in page cache and the failure appears later: the write is lost, and sync is what reports it, croaking with the underlying error. Keep the filesystem sized for the filter you asked for, and call sync when you need to know your writes reached disk.
SEE ALSO
Data::HyperLogLog::Shared, Data::Intern::Shared, Data::SortedSet::Shared, Data::SpatialHash::Shared, and the rest of the Data::*::Shared family.
AUTHOR
vividsnow
LICENSE
This is free software; you can redistribute it and/or modify it under the same terms as Perl itself.