/ 
Data-Intern-Shared-0.04
/ Data::Intern::Shared

NAME

Data::Intern::Shared - shared-memory string interning table for Linux

SYNOPSIS

use Data::Intern::Shared;

# up to 1M distinct strings, 32 MB of string bytes, anonymous mapping
my $in = Data::Intern::Shared->new(undef, 1_000_000, 32 << 20);

my $id = $in->intern("alice");   # 0  (assigns and stores the string once)
$in->intern("bob");              # 1
$in->intern("alice");            # 0  (same bytes -> same id)

my $same = $in->id_of("alice");  # 0, or undef if never interned
my $str  = $in->string(0);       # "alice"
$in->exists("carol");            # false

# pair with Data::SortedSet::Shared (int64 members) for a string-keyed ZSET:
$zset->add($in->intern($key), $score);
my @names = map { $in->string($_) } $zset->rev_range_by_rank(0, 9);

DESCRIPTION

A string interning table in shared memory: it maps arbitrary byte strings to dense uint32 ids (0, 1, 2, ... in interning order) and back. Each distinct string is stored once in an append-only arena; interning the same bytes again returns the same id.

It exists so that string-keyed shared structures can store a cheap fixed-size id while the string itself is held once, and -- because the table lives in shared memory -- so that several processes agree on the same string<->id mapping (a per-process Perl hash cannot do that). In particular it turns the int64-keyed Data::SortedSet::Shared into a string-keyed sorted set: intern the key, store the id, map ids back to strings on the way out.

Lookups are O(1): an open-addressed forward hash (xxhash) finds the id; a dense id -> arena offset array gives the string back. A write-preferring futex rwlock with dead-process recovery guards mutation, so many processes may intern and look up concurrently.

Strings are interned by their byte content (encode wide/utf8 strings first). Interning is permanent: ids are stable for the life of the table; there is no per-string removal (see "LIMITS"). Linux-only. Requires 64-bit Perl.

METHODS

Constructors

my $in = Data::Intern::Shared->new($path, $max_strings, $arena_bytes, $mode);
my $in = Data::Intern::Shared->new(undef, $max_strings);          # anonymous
my $in = Data::Intern::Shared->new_memfd($name, $max_strings, $arena_bytes);
my $in = Data::Intern::Shared->new_from_fd($fd);
my $ro = Data::Intern::Shared->new_readonly($path);   # frozen file, read-only

$path is the backing file (undef for an anonymous mapping); $max_strings is the id/string capacity; $arena_bytes is the total string-bytes capacity and is optional (defaults to $max_strings * 32, with a 64-byte floor, capped at 4 GB). When reopening an existing file or memfd, the stored header wins and the caller's sizes are ignored. Backing files are created with mode 0600 (owner-only) by default; pass an octal $mode (e.g. 0666, applied exactly via fchmod -- not narrowed by umask) to allow cross-user sharing. $mode applies only when the file is created -- it is ignored when attaching to an existing file, and for anonymous and memfd tables. 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").

Interning

my $id = $in->intern($str);   # id (>=0); undef if the id space or arena is full
$in->id_of($str);             # id, or undef if $str was never interned
$in->string($id);             # the string, or undef if $id is out of range
$in->exists($str);
$in->clear;                   # forget everything (all ids invalidated)

intern returns the (existing or newly assigned) id, or undef if either the id space ($max_strings) or the arena ($arena_bytes) is exhausted -- an already-interned string always succeeds since it needs no new id or storage. $str is taken by its bytes; a string containing wide characters croaks (encode it first). The empty string and strings with embedded NULs are valid keys.

Introspection and lifecycle

$in->count; $in->max_strings; $in->arena_used; $in->arena_bytes; $in->stats;
$in->path; $in->memfd; $in->sync; $in->unlink;     # or Class->unlink($path)

count is the number of distinct interned strings (also the next id to be assigned). sync flushes the mapping to its backing store (a no-op for anonymous and memfd tables, which have none); unlink removes the backing file (also callable as Class->unlink($path)); path returns the backing path (undef for anonymous, memfd, or fd-reopened tables) and memfd the backing descriptor -- the memfd of a new_memfd table or the dup'd fd of a new_from_fd table, and -1 for file-backed or anonymous tables.

SHARING ACROSS PROCESSES

The table lives in a shared mapping, shared the same three ways as the rest of the family: a backing file (every process calls new($path, ...) on the same path), an anonymous mapping inherited across fork, or a memfd whose descriptor is passed to an unrelated process (over a UNIX socket via SCM_RIGHTS, or via /proc/$pid/fd/$n) and reopened with new_from_fd($fd). Because the mapping is shared, every process resolves a given string to the same id and can turn any id back into the string -- which is the whole point.

# producer and consumer agree on ids with no coordination
my $in = Data::Intern::Shared->new(undef, 100_000);   # before fork
unless (fork) { my $id = $in->intern("session-42"); ...; exit }
# parent: $in->id_of("session-42") yields the child's id; string($id) agrees

STATS

stats() returns a hashref: count, max_strings, hash_slots, hash_load (occupied fraction of the forward hash), arena_used, arena_bytes, arena_load, ops (running count of intern calls), mmap_size (bytes), frozen (1 if sealed by freeze, else 0), and readonly (1 if this handle is a read-only view, else 0).

LIMITS

  • Permanent interning. There is no per-string removal; ids never change. This is ideal for a bounded key universe (usernames, symbols, paths): add/remove churn of the same key in a consuming structure never grows the arena. For an unbounded stream of unique strings the arena grows until full; clear is the only reset.

  • Byte keys. Strings are interned by byte content; encode wide strings first.

  • Fixed sizes. $max_strings (<= 2^30) and $arena_bytes (<= 4 GB) are set at construction and cannot grow.

FROZEN (READ-ONLY) MODE

A file-backed table 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 $in = Data::Intern::Shared->new("/tmp/dict.intern", 1_000_000, 32 << 20);
$in->intern($_) for @known_strings;
$in->freeze;                 # seal: now immutable, and $in itself is read-only
# ... copy /tmp/dict.intern to another host ...

# consumer (any process, same architecture): read-only, lock-free
my $ro = Data::Intern::Shared->new_readonly("/tmp/dict.intern");
my $id  = $ro->id_of("alice");
my $str = $ro->string($id);

freeze takes the write lock, marks the table permanently immutable (there is no unfreeze -- rebuild the file to change it), and flushes the seal to disk. A frozen table rejects every mutator (intern, clear) with a croak, and a read-write reopen (new($path, ...) or new_from_fd) of a sealed file is refused -- so a shipped artifact can never be silently mutated out from under its readers.

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 table's forward hash, reverse array and arena are immutable, id_of, string, exists, count, arena_used 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. The arena and tables are append-only and never rewritten in place, so a crash leaves the table consistent up to the last completed intern. 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.

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 intern 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.

SEE ALSO

Data::SortedSet::Shared (the int64-keyed sorted set this interns keys for), 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.