606 lines
18 KiB
Lua
606 lines
18 KiB
Lua
-- Copyright (C) Yichun Zhang (agentzh)
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-- Copyright (C) Shuxin Yang
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--[[
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This module implements a key/value cache store. We adopt LRU as our
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replace/evict policy. Each key/value pair is tagged with a Time-to-Live (TTL);
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from user's perspective, stale pairs are automatically removed from the cache.
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Why FFI
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-------
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In Lua, expression "table[key] = nil" does not *PHYSICALLY* remove the value
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associated with the key; it just set the value to be nil! So the table will
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keep growing with large number of the key/nil pairs which will be purged until
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resize() operator is called.
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This "feature" is terribly ill-suited to what we need. Therefore we have to
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rely on FFI to build a hash-table where any entry can be physically deleted
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immediately.
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Under the hood:
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--------------
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In concept, we introduce three data structures to implement the cache store:
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1. key/value vector for storing keys and values.
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2. a queue to mimic the LRU.
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3. hash-table for looking up the value for a given key.
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Unfortunately, efficiency and clarity usually come at each other cost. The
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data strucutres we are using are slightly more complicated than what we
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described above.
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o. Lua does not have efficient way to store a vector of pair. So, we use
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two vectors for key/value pair: one for keys and the other for values
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(_M.key_v and _M.val_v, respectively), and i-th key corresponds to
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i-th value.
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A key/value pair is identified by the "id" field in a "node" (we shall
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discuss node later)
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o. The queue is nothing more than a doubly-linked list of "node" linked via
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lrucache_pureffi_queue_s::{next|prev} fields.
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o. The hash-table has two parts:
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- the _M.bucket_v[] a vector of bucket, indiced by hash-value, and
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- a bucket is a singly-linked list of "node" via the
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lrucache_pureffi_queue_s::conflict field.
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A key must be a string, and the hash value of a key is evaluated by:
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crc32(key-cast-to-pointer) % size(_M.bucket_v).
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We mandate size(_M.bucket_v) being a power-of-two in order to avoid
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expensive modulo operation.
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At the heart of the module is an array of "node" (of type
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lrucache_pureffi_queue_s). A node:
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- keeps the meta-data of its corresponding key/value pair
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(embodied by the "id", and "expire" field);
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- is a part of LRU queue (embodied by "prev" and "next" fields);
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- is a part of hash-table (embodied by the "conflict" field).
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]]
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local ffi = require "ffi"
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local bit = require "bit"
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local ffi_new = ffi.new
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local ffi_sizeof = ffi.sizeof
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local ffi_cast = ffi.cast
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local ffi_fill = ffi.fill
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local ngx_now = ngx.now
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local uintptr_t = ffi.typeof("uintptr_t")
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local c_str_t = ffi.typeof("const char*")
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local int_t = ffi.typeof("int")
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local int_array_t = ffi.typeof("int[?]")
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local crc_tab = ffi.new("const unsigned int[256]", {
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0x00000000, 0x77073096, 0xEE0E612C, 0x990951BA, 0x076DC419, 0x706AF48F,
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0xE963A535, 0x9E6495A3, 0x0EDB8832, 0x79DCB8A4, 0xE0D5E91E, 0x97D2D988,
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0x09B64C2B, 0x7EB17CBD, 0xE7B82D07, 0x90BF1D91, 0x1DB71064, 0x6AB020F2,
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0xF3B97148, 0x84BE41DE, 0x1ADAD47D, 0x6DDDE4EB, 0xF4D4B551, 0x83D385C7,
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0x136C9856, 0x646BA8C0, 0xFD62F97A, 0x8A65C9EC, 0x14015C4F, 0x63066CD9,
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0xFA0F3D63, 0x8D080DF5, 0x3B6E20C8, 0x4C69105E, 0xD56041E4, 0xA2677172,
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0x3C03E4D1, 0x4B04D447, 0xD20D85FD, 0xA50AB56B, 0x35B5A8FA, 0x42B2986C,
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0xDBBBC9D6, 0xACBCF940, 0x32D86CE3, 0x45DF5C75, 0xDCD60DCF, 0xABD13D59,
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0x26D930AC, 0x51DE003A, 0xC8D75180, 0xBFD06116, 0x21B4F4B5, 0x56B3C423,
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0xCFBA9599, 0xB8BDA50F, 0x2802B89E, 0x5F058808, 0xC60CD9B2, 0xB10BE924,
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0x2F6F7C87, 0x58684C11, 0xC1611DAB, 0xB6662D3D, 0x76DC4190, 0x01DB7106,
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0x98D220BC, 0xEFD5102A, 0x71B18589, 0x06B6B51F, 0x9FBFE4A5, 0xE8B8D433,
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0x7807C9A2, 0x0F00F934, 0x9609A88E, 0xE10E9818, 0x7F6A0DBB, 0x086D3D2D,
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0x91646C97, 0xE6635C01, 0x6B6B51F4, 0x1C6C6162, 0x856530D8, 0xF262004E,
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0x6C0695ED, 0x1B01A57B, 0x8208F4C1, 0xF50FC457, 0x65B0D9C6, 0x12B7E950,
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0x8BBEB8EA, 0xFCB9887C, 0x62DD1DDF, 0x15DA2D49, 0x8CD37CF3, 0xFBD44C65,
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0x4DB26158, 0x3AB551CE, 0xA3BC0074, 0xD4BB30E2, 0x4ADFA541, 0x3DD895D7,
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0xA4D1C46D, 0xD3D6F4FB, 0x4369E96A, 0x346ED9FC, 0xAD678846, 0xDA60B8D0,
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0x44042D73, 0x33031DE5, 0xAA0A4C5F, 0xDD0D7CC9, 0x5005713C, 0x270241AA,
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0xBE0B1010, 0xC90C2086, 0x5768B525, 0x206F85B3, 0xB966D409, 0xCE61E49F,
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0x5EDEF90E, 0x29D9C998, 0xB0D09822, 0xC7D7A8B4, 0x59B33D17, 0x2EB40D81,
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0xB7BD5C3B, 0xC0BA6CAD, 0xEDB88320, 0x9ABFB3B6, 0x03B6E20C, 0x74B1D29A,
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0xEAD54739, 0x9DD277AF, 0x04DB2615, 0x73DC1683, 0xE3630B12, 0x94643B84,
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0x0D6D6A3E, 0x7A6A5AA8, 0xE40ECF0B, 0x9309FF9D, 0x0A00AE27, 0x7D079EB1,
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0xF00F9344, 0x8708A3D2, 0x1E01F268, 0x6906C2FE, 0xF762575D, 0x806567CB,
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0x196C3671, 0x6E6B06E7, 0xFED41B76, 0x89D32BE0, 0x10DA7A5A, 0x67DD4ACC,
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0xF9B9DF6F, 0x8EBEEFF9, 0x17B7BE43, 0x60B08ED5, 0xD6D6A3E8, 0xA1D1937E,
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0x38D8C2C4, 0x4FDFF252, 0xD1BB67F1, 0xA6BC5767, 0x3FB506DD, 0x48B2364B,
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0xD80D2BDA, 0xAF0A1B4C, 0x36034AF6, 0x41047A60, 0xDF60EFC3, 0xA867DF55,
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0x316E8EEF, 0x4669BE79, 0xCB61B38C, 0xBC66831A, 0x256FD2A0, 0x5268E236,
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0xCC0C7795, 0xBB0B4703, 0x220216B9, 0x5505262F, 0xC5BA3BBE, 0xB2BD0B28,
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0x2BB45A92, 0x5CB36A04, 0xC2D7FFA7, 0xB5D0CF31, 0x2CD99E8B, 0x5BDEAE1D,
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0x9B64C2B0, 0xEC63F226, 0x756AA39C, 0x026D930A, 0x9C0906A9, 0xEB0E363F,
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0x72076785, 0x05005713, 0x95BF4A82, 0xE2B87A14, 0x7BB12BAE, 0x0CB61B38,
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0x92D28E9B, 0xE5D5BE0D, 0x7CDCEFB7, 0x0BDBDF21, 0x86D3D2D4, 0xF1D4E242,
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0x68DDB3F8, 0x1FDA836E, 0x81BE16CD, 0xF6B9265B, 0x6FB077E1, 0x18B74777,
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0x88085AE6, 0xFF0F6A70, 0x66063BCA, 0x11010B5C, 0x8F659EFF, 0xF862AE69,
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0x616BFFD3, 0x166CCF45, 0xA00AE278, 0xD70DD2EE, 0x4E048354, 0x3903B3C2,
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0xA7672661, 0xD06016F7, 0x4969474D, 0x3E6E77DB, 0xAED16A4A, 0xD9D65ADC,
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0x40DF0B66, 0x37D83BF0, 0xA9BCAE53, 0xDEBB9EC5, 0x47B2CF7F, 0x30B5FFE9,
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0xBDBDF21C, 0xCABAC28A, 0x53B39330, 0x24B4A3A6, 0xBAD03605, 0xCDD70693,
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0x54DE5729, 0x23D967BF, 0xB3667A2E, 0xC4614AB8, 0x5D681B02, 0x2A6F2B94,
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0xB40BBE37, 0xC30C8EA1, 0x5A05DF1B, 0x2D02EF8D });
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local setmetatable = setmetatable
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local tonumber = tonumber
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local tostring = tostring
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local type = type
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local brshift = bit.rshift
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local bxor = bit.bxor
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local band = bit.band
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local new_tab
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do
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local ok
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ok, new_tab = pcall(require, "table.new")
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if not ok then
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new_tab = function(narr, nrec) return {} end
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end
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end
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-- queue data types
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--
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-- this queue is a double-ended queue and the first node
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-- is reserved for the queue itself.
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-- the implementation is mostly borrowed from nginx's ngx_queue_t data
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-- structure.
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ffi.cdef[[
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/* A lrucache_pureffi_queue_s node hook together three data structures:
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* o. the key/value store as embodied by the "id" (which is in essence the
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* indentifier of key/pair pair) and the "expire" (which is a metadata
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* of the corresponding key/pair pair).
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* o. The LRU queue via the prev/next fields.
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* o. The hash-tabble as embodied by the "conflict" field.
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*/
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typedef struct lrucache_pureffi_queue_s lrucache_pureffi_queue_t;
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struct lrucache_pureffi_queue_s {
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/* Each node is assigned a unique ID at construction time, and the
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* ID remain immutatble, regardless the node is in active-list or
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* free-list. The queue header is assigned ID 0. Since queue-header
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* is a sentinel node, 0 denodes "invalid ID".
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*
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* Intuitively, we can view the "id" as the identifier of key/value
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* pair.
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*/
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int id;
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/* The bucket of the hash-table is implemented as a singly-linked list.
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* The "conflict" refers to the ID of the next node in the bucket.
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*/
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int conflict;
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uint32_t user_flags;
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double expire; /* in seconds */
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lrucache_pureffi_queue_t *prev;
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lrucache_pureffi_queue_t *next;
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};
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]]
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local queue_arr_type = ffi.typeof("lrucache_pureffi_queue_t[?]")
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--local queue_ptr_type = ffi.typeof("lrucache_pureffi_queue_t*")
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local queue_type = ffi.typeof("lrucache_pureffi_queue_t")
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local NULL = ffi.null
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--========================================================================
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--
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-- Queue utility functions
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--
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--========================================================================
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-- Append the element "x" to the given queue "h".
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local function queue_insert_tail(h, x)
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local last = h[0].prev
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x.prev = last
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last.next = x
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x.next = h
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h[0].prev = x
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end
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--[[
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Allocate a queue with size + 1 elements. Elements are linked together in a
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circular way, i.e. the last element's "next" points to the first element,
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while the first element's "prev" element points to the last element.
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]]
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local function queue_init(size)
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if not size then
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size = 0
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end
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local q = ffi_new(queue_arr_type, size + 1)
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ffi_fill(q, ffi_sizeof(queue_type, size + 1), 0)
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if size == 0 then
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q[0].prev = q
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q[0].next = q
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else
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local prev = q[0]
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for i = 1, size do
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local e = q[i]
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e.id = i
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e.user_flags = 0
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prev.next = e
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e.prev = prev
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prev = e
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end
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local last = q[size]
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last.next = q
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q[0].prev = last
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end
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return q
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end
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local function queue_is_empty(q)
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-- print("q: ", tostring(q), "q.prev: ", tostring(q), ": ", q == q.prev)
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return q == q[0].prev
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end
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local function queue_remove(x)
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local prev = x.prev
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local next = x.next
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next.prev = prev
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prev.next = next
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-- for debugging purpose only:
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x.prev = NULL
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x.next = NULL
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end
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-- Insert the element "x" the to the given queue "h"
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local function queue_insert_head(h, x)
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x.next = h[0].next
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x.next.prev = x
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x.prev = h
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h[0].next = x
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end
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local function queue_last(h)
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return h[0].prev
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end
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local function queue_head(h)
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return h[0].next
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end
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--========================================================================
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--
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-- Miscellaneous Utility Functions
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--
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--========================================================================
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local function ptr2num(ptr)
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return tonumber(ffi_cast(uintptr_t, ptr))
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end
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local function crc32_ptr(ptr)
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local p = brshift(ptr2num(ptr), 3)
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local b = band(p, 255)
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local crc32 = crc_tab[b]
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b = band(brshift(p, 8), 255)
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crc32 = bxor(brshift(crc32, 8), crc_tab[band(bxor(crc32, b), 255)])
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b = band(brshift(p, 16), 255)
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crc32 = bxor(brshift(crc32, 8), crc_tab[band(bxor(crc32, b), 255)])
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--b = band(brshift(p, 24), 255)
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--crc32 = bxor(brshift(crc32, 8), crc_tab[band(bxor(crc32, b), 255)])
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return crc32
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end
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--========================================================================
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--
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-- Implementation of "export" functions
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--
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--========================================================================
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local _M = {
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_VERSION = '0.09'
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}
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local mt = { __index = _M }
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-- "size" specifies the maximum number of entries in the LRU queue, and the
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-- "load_factor" designates the 'load factor' of the hash-table we are using
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-- internally. The default value of load-factor is 0.5 (i.e. 50%); if the
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-- load-factor is specified, it will be clamped to the range of [0.1, 1](i.e.
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-- if load-factor is greater than 1, it will be saturated to 1, likewise,
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-- if load-factor is smaller than 0.1, it will be clamped to 0.1).
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function _M.new(size, load_factor)
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if size < 1 then
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return nil, "size too small"
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end
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-- Determine bucket size, which must be power of two.
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local load_f = load_factor
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if not load_factor then
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load_f = 0.5
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elseif load_factor > 1 then
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load_f = 1
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elseif load_factor < 0.1 then
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load_f = 0.1
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end
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local bs_min = size / load_f
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-- The bucket_sz *MUST* be a power-of-two. See the hash_string().
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local bucket_sz = 1
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repeat
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bucket_sz = bucket_sz * 2
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until bucket_sz >= bs_min
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local self = {
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size = size,
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bucket_sz = bucket_sz,
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free_queue = queue_init(size),
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cache_queue = queue_init(0),
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node_v = nil,
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key_v = new_tab(size, 0),
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val_v = new_tab(size, 0),
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bucket_v = ffi_new(int_array_t, bucket_sz),
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num_items = 0,
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}
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-- "note_v" is an array of all the nodes used in the LRU queue. Exprpession
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-- node_v[i] evaluates to the element of ID "i".
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self.node_v = self.free_queue
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-- Allocate the array-part of the key_v, val_v, bucket_v.
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--local key_v = self.key_v
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--local val_v = self.val_v
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--local bucket_v = self.bucket_v
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ffi_fill(self.bucket_v, ffi_sizeof(int_t, bucket_sz), 0)
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return setmetatable(self, mt)
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end
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function _M.count(self)
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return self.num_items
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end
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function _M.capacity(self)
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return self.size
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end
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local function hash_string(self, str)
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local c_str = ffi_cast(c_str_t, str)
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local hv = crc32_ptr(c_str)
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hv = band(hv, self.bucket_sz - 1)
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-- Hint: bucket is 0-based
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return hv
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end
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-- Search the node associated with the key in the bucket, if found returns
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-- the the id of the node, and the id of its previous node in the conflict list.
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-- The "bucket_hdr_id" is the ID of the first node in the bucket
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local function _find_node_in_bucket(key, key_v, node_v, bucket_hdr_id)
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if bucket_hdr_id ~= 0 then
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local prev = 0
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local cur = bucket_hdr_id
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while cur ~= 0 and key_v[cur] ~= key do
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prev = cur
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cur = node_v[cur].conflict
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end
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if cur ~= 0 then
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return cur, prev
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end
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end
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end
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-- Return the node corresponding to the key/val.
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local function find_key(self, key)
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local key_hash = hash_string(self, key)
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return _find_node_in_bucket(key, self.key_v, self.node_v,
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self.bucket_v[key_hash])
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end
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--[[ This function tries to
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1. Remove the given key and the associated value from the key/value store,
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2. Remove the entry associated with the key from the hash-table.
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NOTE: all queues remain intact.
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If there was a node bound to the key/val, return that node; otherwise,
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nil is returned.
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]]
|
|
local function remove_key(self, key)
|
|
local key_v = self.key_v
|
|
local val_v = self.val_v
|
|
local node_v = self.node_v
|
|
local bucket_v = self.bucket_v
|
|
|
|
local key_hash = hash_string(self, key)
|
|
local cur, prev =
|
|
_find_node_in_bucket(key, key_v, node_v, bucket_v[key_hash])
|
|
|
|
if cur then
|
|
-- In an attempt to make key and val dead.
|
|
key_v[cur] = nil
|
|
val_v[cur] = nil
|
|
self.num_items = self.num_items - 1
|
|
|
|
-- Remove the node from the hash table
|
|
local next_node = node_v[cur].conflict
|
|
if prev ~= 0 then
|
|
node_v[prev].conflict = next_node
|
|
else
|
|
bucket_v[key_hash] = next_node
|
|
end
|
|
node_v[cur].conflict = 0
|
|
|
|
return cur
|
|
end
|
|
end
|
|
|
|
|
|
--[[ Bind the key/val with the given node, and insert the node into the Hashtab.
|
|
NOTE: this function does not touch any queue
|
|
]]
|
|
local function insert_key(self, key, val, node)
|
|
-- Bind the key/val with the node
|
|
local node_id = node.id
|
|
self.key_v[node_id] = key
|
|
self.val_v[node_id] = val
|
|
|
|
-- Insert the node into the hash-table
|
|
local key_hash = hash_string(self, key)
|
|
local bucket_v = self.bucket_v
|
|
node.conflict = bucket_v[key_hash]
|
|
bucket_v[key_hash] = node_id
|
|
self.num_items = self.num_items + 1
|
|
end
|
|
|
|
|
|
function _M.get(self, key)
|
|
if type(key) ~= "string" then
|
|
key = tostring(key)
|
|
end
|
|
|
|
local node_id = find_key(self, key)
|
|
if not node_id then
|
|
return nil
|
|
end
|
|
|
|
-- print(key, ": moving node ", tostring(node), " to cache queue head")
|
|
local cache_queue = self.cache_queue
|
|
local node = self.node_v + node_id
|
|
queue_remove(node)
|
|
queue_insert_head(cache_queue, node)
|
|
|
|
local expire = node.expire
|
|
if expire >= 0 and expire < ngx_now() then
|
|
-- print("expired: ", node.expire, " > ", ngx_now())
|
|
return nil, self.val_v[node_id], node.user_flags
|
|
end
|
|
|
|
return self.val_v[node_id], nil, node.user_flags
|
|
end
|
|
|
|
|
|
function _M.delete(self, key)
|
|
if type(key) ~= "string" then
|
|
key = tostring(key)
|
|
end
|
|
|
|
local node_id = remove_key(self, key);
|
|
if not node_id then
|
|
return false
|
|
end
|
|
|
|
local node = self.node_v + node_id
|
|
queue_remove(node)
|
|
queue_insert_tail(self.free_queue, node)
|
|
return true
|
|
end
|
|
|
|
|
|
function _M.set(self, key, value, ttl, flags)
|
|
if type(key) ~= "string" then
|
|
key = tostring(key)
|
|
end
|
|
|
|
local node_id = find_key(self, key)
|
|
local node
|
|
if not node_id then
|
|
local free_queue = self.free_queue
|
|
if queue_is_empty(free_queue) then
|
|
-- evict the least recently used key
|
|
-- assert(not queue_is_empty(self.cache_queue))
|
|
node = queue_last(self.cache_queue)
|
|
remove_key(self, self.key_v[node.id])
|
|
else
|
|
-- take a free queue node
|
|
node = queue_head(free_queue)
|
|
-- print(key, ": get a new free node: ", tostring(node))
|
|
end
|
|
|
|
-- insert the key
|
|
insert_key(self, key, value, node)
|
|
else
|
|
node = self.node_v + node_id
|
|
self.val_v[node_id] = value
|
|
end
|
|
|
|
queue_remove(node)
|
|
queue_insert_head(self.cache_queue, node)
|
|
|
|
if ttl then
|
|
node.expire = ngx_now() + ttl
|
|
else
|
|
node.expire = -1
|
|
end
|
|
|
|
if type(flags) == "number" and flags >= 0 then
|
|
node.user_flags = flags
|
|
|
|
else
|
|
node.user_flags = 0
|
|
end
|
|
end
|
|
|
|
|
|
function _M.get_keys(self, max_count, res)
|
|
if not max_count or max_count == 0 then
|
|
max_count = self.num_items
|
|
end
|
|
|
|
if not res then
|
|
res = new_tab(max_count + 1, 0) -- + 1 for trailing hole
|
|
end
|
|
|
|
local cache_queue = self.cache_queue
|
|
local key_v = self.key_v
|
|
|
|
local i = 0
|
|
local node = queue_head(cache_queue)
|
|
|
|
while node ~= cache_queue do
|
|
if i >= max_count then
|
|
break
|
|
end
|
|
|
|
i = i + 1
|
|
res[i] = key_v[node.id]
|
|
node = node.next
|
|
end
|
|
|
|
res[i + 1] = nil
|
|
|
|
return res
|
|
end
|
|
|
|
|
|
function _M.flush_all(self)
|
|
local cache_queue = self.cache_queue
|
|
local key_v = self.key_v
|
|
|
|
local node = queue_head(cache_queue)
|
|
|
|
while node ~= cache_queue do
|
|
local key = key_v[node.id]
|
|
node = node.next
|
|
_M.delete(self, key)
|
|
end
|
|
end
|
|
|
|
|
|
return _M
|