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4 Clients of memcached communicate with server through TCP connections.
16 necessary) of open connections. Caching connections will eliminate the
18 of preparing for a new connection on the server side is insignificant
21 There are two kinds of data sent in the memcache protocol: text lines
27 unstructured data and isn't aware of them. There are no limitations on
29 of such data (either a client or a server) will always know, from a
30 preceding text line, the exact length of the data block being
36 data from a server, it must use the length of the data block (which it
38 the fact that \r\n follows the end of the data block, even though it
44 Data stored by memcached is identified with the help of a key. A key
47 length limit of a key is set at 250 characters (of course, normally
54 There are three types of commands.
59 that the client expects one line of response, which will indicate
63 retrieve data corresponding to a set of keys (one or more keys in one
70 All other commands don't involve unstructured data. In all of them,
72 command) either one line of response, or several lines of response
75 A command line always starts with the name of the command, followed by
82 Meta commands are a set of new ASCII-based commands. They follow the same
83 structure of the original commands but have a new flexible feature set.
86 large number of high level commands. These commands completely replace the
87 usage of basic Storage and Retrieval commands.
109 Where <RC> is a 2 character return code. The number of flags returned are
110 based off of the flags supplied.
115 behavior of the command. If a flag requests a response flag (ie 't' for TTL
120 behavior of a command. Flags may contain token arguments, which come after the
124 seconds of TTL remaining, would return 't20' in the response.
142 Some commands involve a client sending some kind of expiration time
145 Unix time (number of seconds since January 1, 1970, as a 32-bit
146 value), or a number of seconds starting from current time. In the
147 latter case, this number of seconds may not exceed 60*60*24*30 (number
148 of seconds in 30 days); if the number sent by a client is larger than
152 Note that a TTL of 1 will sometimes immediately expire. Time is internally
168 means some sort of client error in the input line, i.e. the input
174 means some sort of server error prevents the server from carrying
176 of severe server errors, which make it impossible to continue
182 In the descriptions of individual commands below, these error lines
196 of the username/password payload.
240 of 16, but you might want to restrict yourself to 16 bits for
251 - <bytes> is the number of bytes in the data block to follow, *not*
255 - <cas unique> is a unique 64-bit value of an existing entry.
269 - <data block> is a chunk of arbitrary 8-bit data of length <bytes>
278 because of an error. This normally means that the
298 After this command, the client expects zero or more items, each of
304 to indicate the end of response.
315 - <bytes> is the length of the data block to follow, *not* including
323 If some of the keys appearing in a retrieval request are not sent back
333 The command "delete" allows for explicit deletion of items:
337 - <key> is the key of the item the client wishes the server to delete
343 The response line to this command can be one of:
351 of all existing items.
359 treated as decimal representation of a 64-bit unsigned integer. If
374 - <key> is the key of the item the client wishes to change
377 the item. It is a decimal representation of a 64-bit unsigned integer.
383 The response will be one of:
387 - <value>\r\n , where <value> is the new value of the item's data,
402 The "touch" command is used to update the expiration time of an existing item
407 - <key> is the key of the item the client wishes the server to touch
412 expiration time of "20", the item would then expire in 20 seconds.
418 The response line to this command can be one of:
429 expiration time of an existing items.
438 After this command, the client expects zero or more items, each of
444 to indicate the end of response.
455 - <bytes> is the length of the data block to follow, *not* including
466 The meta debug command is a human readable dump of all available internal
467 metadata of an item, minus the value.
483 Each of the keys and values are the internal data for the item.
498 memcached. Based on the flags supplied, it can replace all of the commands:
505 - <flags> are a set of single character codes ended with a space or newline.
522 - <size> is the size of <data block> in bytes, minus the \r\n
572 it up. This allows storing and fetching of binary packed keys, so long as they
581 The above two flags return the value of "hit before?" and "last access time"
583 hit or always show an access time of "0" unless combined with the "u" flag.
588 pipelining of requests while saving bytes on the wire for responses. For
591 long this can generate a lot of traffic, especially if the data block is very
592 small. Instead, you can supply an "O" flag for each mg with tokens of "1" "2"
600 Noreply is a method of reducing the amount of data sent back by memcached to
601 the client for normal responses. In the case of metaget, if an item is not
606 pipeline several mg's together and read all of the responses without the
614 of the LRU. This also avoids marking an item as being hit or updating its last
632 Used to help with so called "dog piling" problems with recaching of popular
638 The automatically created item has 0 bytes of data.
647 Similar to and can be combined with 'N'. If the remaining TTL of an item is
654 Similar to "touch" and "gat" commands, updates the remaining TTL of an item if
661 ownership of a cache item. This allows a single client to be atomically
666 This is used when the act of recaching an item can cause undue load on another
696 - <datalen> is the length of the payload data.
698 - <flags> are a set of single character codes ended with a space or newline.
705 - <data block> is a chunk of arbitrary 8-bit data of length supplied by an 'S'
710 the reply, which is of the format:
714 Where CD is one of:
719 because of an error.
738 - s: return the size of the stored item on success (ie; new size on append)
752 matches the current CAS value of the item. When combined with the 'I' flag, a
770 Noreply is a method of reducing the amount of data sent back by memcached to
771 the client for normal responses. In the case of metaset, a response that
794 TTL from N instead of T to be consistent with the usage of N in other
800 The meta delete command allows for explicit deletion of items, as well as
807 - <flags> are a set of single character codes ended with a space or newline.
814 Where CD is one of:
843 Instead of removing an item, this will give the item a new CAS value and mark
853 See description under 'Meta Set'. In the case of meta delete, this will hide
859 TTL as well; limiting the amount of time an item will live while stale and
864 This deletes the value off of an item (by replacing it with an empty value
866 tombstone of a previous value.
880 - <flags> are a set of single character codes ended with a space or newline.
887 Where CD is one of:
970 "mn\r\n" command can be tagged to the end of the chain, which will return an
977 NOTE: This command is subject to change as of this writing.
987 A source class id of -1 means "pick from any valid class"
991 The response line could be one of:
1011 NOTE: This command is subject to change as of this writing.
1029 worth of free chunks in a slab class. Pages are then re-assigned back into
1051 This would cap HOT_LRU at 10% of the cache, or tail is idle longer than
1052 10% of COLD_LRU. WARM_LRU is up to 25% of cache, or tail is idle longer
1060 - "temp_ttl" <ttl>: If TTL is less than zero, disable usage of TEMP_LRU. If
1065 The response line could be one of:
1067 - "OK" to indicate a successful update of the settings.
1074 NOTE: This command (and related commands) are subject to change as of this
1078 toward the head of requested slab classes, actively freeing memory for expired
1079 items. This is useful if you have a mix of items with both long and short
1081 usage, and can add small amounts of latency and increase CPU usage.
1087 The response line could be one of:
1095 - The number of microseconds to sleep in between each item checked for
1097 A value of "0" disables the sleep, "1000000" (one second) is the max.
1099 The response line could be one of:
1107 - The maximum number of items to inspect in a slab class per run request. This
1108 allows you to avoid scanning all of very large slabs when it is unlikely to
1111 The response line could be one of:
1119 - Takes a single, or a list of, numeric classids (ie: 1,3,10). This instructs
1120 the crawler to start at the tail of each of these classids and run to the
1127 The response line could be one of:
1142 if "hash" is specified instead of a classid or "all", the crawler will dump
1143 items by directly walking the hash table instead of the LRU's. This makes it
1155 The response line could be one of:
1168 If "hash" is specified instead of a classid or "all", the crawler will dump
1169 items by directly walking the hash table instead of the LRU's. This makes it
1178 The response line could be one of:
1210 The response line could be one of:
1218 number of the log line. Given how logs are collected internally they may be
1219 printed out of order. If this is important the GID may be used to put log
1222 The value of keys (and potentially other things) are "URI encoded". Since most
1273 kinds of arguments and the data sent are not documented in this version
1274 of the protocol, and are subject to change for the convenience of
1282 a number of lines which look like this:
1290 In each line of statistics, <name> is the name of this statistic, and
1291 <value> is the data. The following is the list of all names sent in
1292 response to the "stats" command, together with the type of the value
1293 sent for this name, and the meaning of the value.
1302 | pid | 32u | Process id of this server process |
1303 | uptime | 32u | Number of secs since the server started |
1305 | version | string | Version string of this server |
1306 | pointer_size | 32 | Default size of pointers on the host OS |
1312 | curr_items | 64u | Current number of items stored |
1313 | total_items | 64u | Total number of items stored since |
1315 | bytes | 64u | Current number of bytes used |
1317 | max_connections | 32u | Max number of simultaneous connections |
1318 | curr_connections | 32u | Number of open connections |
1319 | total_connections | 32u | Total number of connections opened since |
1322 | connection_structures | 32u | Number of connection structures allocated |
1324 | response_obj_oom | 64u | Connections closed by lack of memory |
1327 | | | subset of bytes from read_buf_bytes. |
1331 | read_buf_oom | 64u | Connections closed by lack of memory |
1332 | reserved_fds | 32u | Number of misc fds used internally |
1333 | proxy_conn_requests | 64u | Number of requests received by the proxy |
1334 | proxy_conn_errors | 64u | Number of internal errors from proxy |
1335 | proxy_conn_oom | 64u | Number of out of memory errors while |
1337 | proxy_req_active | 64u | Number of in-flight proxy requests |
1338 | proxy_req_await | 64u | Number of in-flight proxy async requests |
1339 | cmd_get | 64u | Cumulative number of retrieval reqs |
1340 | cmd_set | 64u | Cumulative number of storage reqs |
1341 | cmd_flush | 64u | Cumulative number of flush reqs |
1342 | cmd_touch | 64u | Cumulative number of touch reqs |
1343 | get_hits | 64u | Number of keys that have been requested |
1345 | get_misses | 64u | Number of items that have been requested |
1347 | get_expired | 64u | Number of items that have been requested |
1349 | get_flushed | 64u | Number of items that have been requested |
1351 | delete_misses | 64u | Number of deletions reqs for missing keys |
1352 | delete_hits | 64u | Number of deletion reqs resulting in |
1354 | incr_misses | 64u | Number of incr reqs against missing keys. |
1355 | incr_hits | 64u | Number of successful incr reqs. |
1356 | decr_misses | 64u | Number of decr reqs against missing keys. |
1357 | decr_hits | 64u | Number of successful decr reqs. |
1358 | cas_misses | 64u | Number of CAS reqs against missing keys. |
1359 | cas_hits | 64u | Number of successful CAS reqs. |
1360 | cas_badval | 64u | Number of CAS reqs for which a key was |
1362 | touch_hits | 64u | Number of keys that have been touched |
1364 | touch_misses | 64u | Number of items that have been touched |
1366 | store_too_large | 64u | Number of rejected storage requests |
1369 | store_no_memory | 64u | Number of rejected storage requests |
1370 | | | caused by exhaustion of the -m memory |
1372 | auth_cmds | 64u | Number of authentication commands |
1374 | auth_errors | 64u | Number of failed authentications. |
1375 | idle_kicks | 64u | Number of connections closed due to |
1377 | evictions | 64u | Number of valid items removed from cache |
1379 | reclaimed | 64u | Number of times an entry was stored using |
1381 | bytes_read | 64u | Total number of bytes read by this server |
1383 | bytes_written | 64u | Total number of bytes sent by this server |
1385 | limit_maxbytes | size_t | Number of bytes this server is allowed to |
1388 | listen_disabled_num | 64u | Number of times server has stopped |
1391 | | 64u | Number of microseconds in maxconns. |
1392 | threads | 32u | Number of worker threads requested. |
1394 | conn_yields | 64u | Number of times any connection yielded to |
1406 | | | recently but did not jump to top of LRU |
1420 | | 64u | Number of times the LRU bg thread woke up |
1428 | | 64u | Individual sections of an item rescued |
1444 | log_watchers | 64u | Number of currently active watchers. |
1445 | unexpected_napi_ids | 64u | Number of times an unexpected napi id is |
1447 | round_robin_fallback | 64u | Number of times napi id of 0 is received |
1457 The "stats" command with the argument of "settings" returns details of
1458 the settings of the running memcached. This is primarily made up of
1459 the results of processing commandline options.
1468 | maxbytes | size_t | Maximum number of bytes allowed in the cache |
1469 | maxconns | 32 | Maximum number of clients allowed. |
1474 | oldest | 32u | Age of the oldest honored object. |
1477 | umask | 32 (oct) | umask for the creation of the domain socket. |
1481 | num_threads | 32 | Number of threads (including dispatch). |
1505 | hot_lru_pct | 32 | Pct of slab memory reserved for HOT LRU |
1506 | warm_lru_pct | 32 | Pct of slab memory reserved for WARM LRU |
1507 | hot_max_factor | float | Set idle age of HOT LRU to COLD age * this |
1508 | warm_max_factor | float | Set idle age of WARM LRU to COLD age * this |
1514 | | 32u | Size of internal (not socket) write buffer |
1516 | worker_logbuf_size| 32u | Size of internal per-worker-thread buffer |
1522 | | bool | Does nothing as of 1.5.15 |
1530 | client_flags_size | 32u | Size in bytes of client flags |
1539 The "stats" command with the argument of "items" returns information about
1552 The following item values are defined as of writing.
1556 number Number of items presently stored in this class. Expired
1558 number_hot Number of items presently stored in the HOT LRU.
1559 number_warm Number of items presently stored in the WARM LRU.
1560 number_cold Number of items presently stored in the COLD LRU.
1561 number_temp Number of items presently stored in the TEMPORARY LRU.
1562 age_hot Age of the oldest item in HOT LRU.
1563 age_warm Age of the oldest item in WARM LRU.
1564 age Age of the oldest item in the LRU.
1565 mem_requested Number of bytes requested to be stored in this LRU[*]
1566 evicted Number of times an item had to be evicted from the LRU
1568 evicted_nonzero Number of times an item which had an explicit expire
1574 outofmemory Number of times the underlying slab class was unable to
1577 tailrepairs Number of times we self-healed a slab with a refcount
1580 reclaimed Number of times an entry was stored using memory from
1582 expired_unfetched Number of expired items reclaimed from the LRU which
1584 evicted_unfetched Number of valid items evicted from the LRU which were
1586 evicted_active Number of valid items evicted from the LRU which were
1588 the top of the LRU again.
1589 crawler_reclaimed Number of items freed by the LRU Crawler.
1590 lrutail_reflocked Number of items found to be refcount locked in the
1592 moves_to_cold Number of items moved from HOT or WARM into COLD.
1593 moves_to_warm Number of items moved from COLD to WARM.
1594 moves_within_lru Number of times active items were bumped within
1596 direct_reclaims Number of times worker threads had to directly pull LRU
1601 hits_to_temp Number of get_hits to each sub-LRU.
1607 item. mem_requested shows the size of all items within a
1609 wasted in a slab class. If you see a lot of waste, consider tuning
1618 The "stats" command with the argument of "sizes" returns information about the
1619 general size and count of all items stored in the cache.
1631 'size' is an approximate size of the item, within 32 bytes.
1632 'count' is the approximate amount of items that exist within that 32-byte
1635 This is essentially a display of all of your items if there was a slab class
1639 most of your items are less than 200 bytes in size.
1662 If you want to make a histogram of item sizes for a memcached instance that is
1671 The "stats" command with the argument of "slabs" returns information about
1672 each of the slabs created by memcached during runtime. This includes per-slab
1685 | chunk_size | The amount of space each chunk uses. One item will use |
1686 | | one chunk of the appropriate size. |
1690 | total_pages | Total number of pages allocated to the slab class. |
1691 | total_chunks | Total number of chunks allocated to the slab class. |
1692 | get_hits | Total number of get requests serviced by this class. |
1693 | cmd_set | Total number of set requests storing data in this class. |
1694 | delete_hits | Total number of successful deletes from this class. |
1695 | incr_hits | Total number of incrs modifying this class. |
1696 | decr_hits | Total number of decrs modifying this class. |
1697 | cas_hits | Total number of CAS commands modifying this class. |
1698 | cas_badval | Total number of CAS commands that failed to modify a |
1700 | touch_hits | Total number of touches serviced by this class. |
1703 | free_chunks_end | Number of free chunks at the end of the last allocated |
1705 | active_slabs | Total number of slab classes allocated. |
1706 | total_malloced | Total amount of memory allocated to slab pages. |
1712 The "stats" command with the argument of "conns" returns information
1727 | addr | The address of the remote side. For listening |
1731 | listen_addr | The address of the server. This field is absent |
1733 | state | The current state of the connection. See below. |
1734 | secs_since_last_cmd | The number of seconds since the most recently |
1736 | | the time since the start of the command, so if |
1738 | | this will be the number of seconds the current |
1742 The value of the "state" stat may be one of the following:
1754 | | of parsing it or executing it. |
1759 | | waiting for the rest of it to arrive (note the difference |
1762 | | sending back multiple lines of response data). |
1775 | ssl_handshake_errors | 64u | Number of times the server has |
1778 | ssl_proto_errors | 64u | Number of times a client has |
1783 | | | enabled, the number of newly |
1787 | time_since_server_cert_refresh | 32u | Number of seconds that have |
1800 specified. After invalidation none of the items will be returned in
1805 definition of what flush_all does is the following: it causes all
1809 The intent of flush_all with a delay, was that in a setting where you
1810 have a pool of memcached servers, and you need to flush all content,
1811 you have the option of not resetting all memcached servers at the
1821 adjustments of the cache memory limit. It returns "OK\r\n" or an error (unless
1847 as the last parameter). Its effect is to set the verbosity level of
1877 For very large installations where the number of clients is high enough
1878 that the number of TCP connections causes scaling difficulties, there is
1889 requests, both of which are more suitable to TCP transport for reliability
1897 4-5 Total number of datagrams in this message
1909 The sequence number ranges from 0 to n-1, where n is the total number of
1910 datagrams in the message. The client should concatenate the payloads of the