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Clients of memcached communicate with server through TCP connections.
(A UDP interface is also available; details are below under "UDP
protocol.") A given running memcached server listens on some
(configurable) port; clients connect to that port, send commands to
the server, read responses, and eventually close the connection.
There is no need to send any command to end the session. A client may
just close the connection at any moment it no longer needs it. Note,
however, that clients are encouraged to cache their connections rather
than reopen them every time they need to store or retrieve data. This
is because memcached is especially designed to work very efficiently
with a very large number (many hundreds, more than a thousand if
necessary) of open connections. Caching connections will eliminate the
overhead associated with establishing a TCP connection (the overhead
of preparing for a new connection on the server side is insignificant
compared to this).
There are two kinds of data sent in the memcache protocol: text lines
and unstructured data. Text lines are used for commands from clients
and responses from servers. Unstructured data is sent when a client
wants to store or retrieve data. The server will transmit back
unstructured data in exactly the same way it received it, as a byte
stream. The server doesn't care about byte order issues in
unstructured data and isn't aware of them. There are no limitations on
characters that may appear in unstructured data; however, the reader
of such data (either a client or a server) will always know, from a
preceding text line, the exact length of the data block being
Text lines are always terminated by \r\n. Unstructured data is _also_
terminated by \r\n, even though \r, \n or any other 8-bit characters
may also appear inside the data. Therefore, when a client retrieves
data from a server, it must use the length of the data block (which it
will be provided with) to determine where the data block ends, and not
the fact that \r\n follows the end of the data block, even though it
Data stored by memcached is identified with the help of a key. A key
is a text string which should uniquely identify the data for clients
that are interested in storing and retrieving it. Currently the
length limit of a key is set at 250 characters (of course, normally
clients wouldn't need to use such long keys); the key must not include
control characters or whitespace.
There are three types of commands.
Storage commands (there are three: "set", "add" and "replace") ask the
server to store some data identified by a key. The client sends a
command line, and then a data block; after that the client expects one
line of response, which will indicate success or faulure.
Retrieval commands (there is only one: "get") ask the server to
retrieve data corresponding to a set of keys (one or more keys in one
request). The client sends a command line, which includes all the
requested keys; after that for each item the server finds it sends to
the client one response line with information about the item, and one
data block with the item's data; this continues until the server
finished with the "END" response line.
All other commands don't involve unstructured data. In all of them,
the client sends one command line, and expects (depending on the
command) either one line of response, or several lines of response
ending with "END" on the last line.
A command line always starts with the name of the command, followed by
parameters (if any) delimited by whitespace. Command names are
lower-case and are case-sensitive.
Expiration times
Some commands involve a client sending some kind of expiration time
(relative to an item or to an operation requested by the client) to
the server. In all such cases, the actual value sent may either be
Unix time (number of seconds since January 1, 1970, as a 32-bit
value), or a number of seconds starting from current time. In the
latter case, this number of seconds may not exceed 60*60*24*30 (number
of seconds in 30 days); if the number sent by a client is larger than
that, the server will consider it to be real Unix time value rather
than an offset from current time.
Error strings
Each command sent by a client may be answered with an error string
from the server. These error strings come in three types:
- "ERROR\r\n"
means the client sent a nonexistent command name.
- "CLIENT_ERROR <error>\r\n"
means some sort of client error in the input line, i.e. the input
doesn't conform to the protocol in some way. <error> is a
human-readable error string.
- "SERVER_ERROR <error>\r\n"
means some sort of server error prevents the server from carrying
out the command. <error> is a human-readable error string. In cases
of severe server errors, which make it impossible to continue
serving the client (this shouldn't normally happen), the server will
close the connection after sending the error line. This is the only
case in which the server closes a connection to a client.
In the descriptions of individual commands below, these error lines
are not again specifically mentioned, but clients must allow for their
Storage commands
First, the client sends a command line which looks like this:
<command name> <key> <flags> <exptime> <bytes>\r\n
- <command name> is "set", "add" or "replace"
"set" means "store this data".
"add" means "store this data, but only if the server *doesn't* already
hold data for this key".
"replace" means "store this data, but only if the server *does*
already hold data for this key".
- <key> is the key under which the client asks to store the data
- <flags> is an arbitrary 16-bit unsigned integer (written out in
decimal) that the server stores along with the data and sends back
when the item is retrieved. Clients may use this as a bit field to
store data-specific information; this field is opaque to the server.
Note that in memcached 1.2.1 and higher, flags may be 32-bits, instead
of 16, but you might want to restrict yourself to 16 bits for
compatibility with older versions.
- <exptime> is expiration time. If it's 0, the item never expires
(although it may be deleted from the cache to make place for other
items). If it's non-zero (either Unix time or offset in seconds from
current time), it is guaranteed that clients will not be able to
retrieve this item after the expiration time arrives (measured by
server time).
- <bytes> is the number of bytes in the data block to follow, *not*
including the delimiting \r\n. <bytes> may be zero (in which case
it's followed by an empty data block).
After this line, the client sends the data block:
<data block>\r\n
- <data block> is a chunk of arbitrary 8-bit data of length <bytes>
from the previous line.
After sending the command line and the data blockm the client awaits
the reply, which may be:
- "STORED\r\n", to indicate success.
- "NOT_STORED\r\n" to indicate the data was not stored, but not
because of an error. This normally means that either that the
condition for an "add" or a "replace" command wasn't met, or that the
item is in a delete queue (see the "delete" command below).
Retrieval command:
The retrieval command looks like this:
get <key>*\r\n
- <key>* means one or more key strings separated by whitespace.
After this command, the client expects zero or more items, each of
which is received as a text line followed by a data block. After all
the items have been transmitted, the server sends the string
to indicate the end of response.
Each item sent by the server looks like this:
VALUE <key> <flags> <bytes>\r\n
<data block>\r\n
- <key> is the key for the item being sent
- <flags> is the flags value set by the storage command
- <bytes> is the length of the data block to follow, *not* including
its delimiting \r\n
- <data block> is the data for this item.
If some of the keys appearing in a retrieval request are not sent back
by the server in the item list this means that the server does not
hold items with such keys (because they were never stored, or stored
but deleted to make space for more items, or expired, or explicitly
deleted by a client).
The command "delete" allows for explicit deletion of items:
delete <key> <time>\r\n
- <key> is the key of the item the client wishes the server to delete
- <time> is the amount of time in seconds (or Unix time until which)
the client wishes the server to refuse "add" and "replace" commands
with this key. For this amount of item, the item is put into a
delete queue, which means that it won't possible to retrieve it by
the "get" command, but "add" and "replace" command with this key
will also fail (the "set" command will succeed, however). After the
time passes, the item is finally deleted from server memory.
The parameter <time> is optional, and, if absent, defaults to 0
(which means that the item will be deleted immediately and further
storage commands with this key will succeed).
The response line to this command can be one of:
- "DELETED\r\n" to indicate success
- "NOT_FOUND\r\n" to indicate that the item with this key was not
See the "flush_all" command below for immediate invalidation
of all existing items.
Commands "incr" and "decr" are used to change data for some item
in-place, incrementing or decrementing it. The data for the item is
treated as decimal representation of a 32-bit unsigned integer. If the
current data value does not conform to such a representation, the
commands behave as if the value were 0. Also, the item must already
exist for incr/decr to work; these commands won't pretend that a
non-existent key exists with value 0; instead, they will fail.
The client sends the command line:
incr <key> <value>\r\n
decr <key> <value>\r\n
- <key> is the key of the item the client wishes to change
- <value> is the amount by which the client wants to increase/decrease
the item. It is a decimal representation of a 32-bit unsigned integer.
The response will be one of:
- "NOT_FOUND\r\n" to indicate the item with this value was not found
- <value>\r\n , where <value> is the new value of the item's data,
after the increment/decrement operation was carried out.
Note that underflow in the "decr" command is caught: if a client tries
to decrease the value below 0, the new value will be 0. Overflow in
the "incr" command is not checked.
Note also that decrementing a number such that it loses length isn't
guaranteed to decrement its returned length. The number MAY be
space-padded at the end, but this is purely an implementation
optimization, so you also shouldn't rely on that.
The command "stats" is used to query the server about statistics it
maintains and other internal data. It has two forms. Without
it causes the server to output general-purpose statistics and
settings, documented below. In the other form it has some arguments:
stats <args>\r\n
Depending on <args>, various internal data is sent by the server. The
kinds of arguments and the data sent are not documented in this vesion
of the protocol, and are subject to change for the convenience of
memcache developers.
General-purpose statistics
Upon receiving the "stats" command without arguments, the server sents
a number of lines which look like this:
STAT <name> <value>\r\n
The server terminates this list with the line
In each line of statistics, <name> is the name of this statistic, and
<value> is the data. The following is the list of all names sent in
response to the "stats" command, together with the type of the value
sent for this name, and the meaning of the value.
In the type column below, "32u" means a 32-bit unsigned integer, "64u"
means a 64-bit unsigner integer. '32u:32u' means two 32-but unsigned
integers separated by a colon.
Name Type Meaning
pid 32u Process id of this server process
uptime 32u Number of seconds this server has been running
time 32u current UNIX time according to the server
version string Version string of this server
rusage_user 32u:32u Accumulated user time for this process
rusage_system 32u:32u Accumulated system time for this process
curr_items 32u Current number of items stored by the server
total_items 32u Total number of items stored by this server
ever since it started
bytes 64u Current number of bytes used by this server
to store items
curr_connections 32u Number of open connections
total_connections 32u Total number of connections opened since
the server started running
connection_structures 32u Number of connection structures allocated
by the server
cmd_get 64u Cumulative number of retrieval requests
cmd_set 64u Cumulative number of storage requests
get_hits 64u Number of keys that have been requested and
found present
get_misses 64u Number of items that have been requested
and not found
evictions 64u Number of valid items removed from cache
to free memory for new items
bytes_read 64u Total number of bytes read by this server
from network
bytes_written 64u Total number of bytes sent by this server to
limit_maxbytes 32u Number of bytes this server is allowed to
use for storage.
Other commands
"flush_all" is a command with an optional numeric argument. It always
succeeds, and the server sends "OK\r\n" in response. Its effect is to
invalidate all existing items immediately (by default) or after the
expiration specified. After invalidation none of the items will be returned
in response to a retrieval command (unless it's stored again under the
same key *after* flush_all has invalidated the items). flush_all doesn't
actually free all the memory taken up by existing items; that will
happen gradually as new items are stored. The most precise definition
of what flush_all does is the following: it causes all items whose
update time is earlier than the time at which flush_all was set to be
executed to be ignored for retrieval purposes.
The intent of flush_all with a delay, was that in a setting where you
have a pool of memcached servers, and you need to flush all content,
you have the option of not resetting all memcached servers at the
same time (which could e.g. cause a spike in database load with all
clients suddenly needing to recreate content that would otherwise
have been found in the memcached daemon).
The delay option allows you to have them reset in e.g. 10 second
intervals (by passing 0 to the first, 10 to the second, 20 to the
third, etc. etc.).
"flush_regex" is a command with an optional string argument. It will
expire all items whose keys match the given regular expression. THIS
will lock up the server while it runs, preventing any other client requests
from being serviced. It is intended for debugging and disaster recovery
purposes, not as a general-purpose deletion mechanism. Note that if you
have more than one memcached server, you will need to run this command on
each of them, since there may be keys matching a regular expression on any
host in a memcached cluster.
"version" is a command with no arguments:
In response, the server sends
"VERSION <version>\r\n", where <version> is the version string for the
"verbosity" is a command with a numeric argument. It always
succeeds, and the server sends "OK\r\n" in response. Its effect is to
set the verbosity level of the logging output.
"quit" is a command with no arguments:
Upon receiving this command, the server closes the
connection. However, the client may also simply close the connection
when it no longer needs it, without issuing this command.
UDP protocol
For very large installations where the number of clients is high enough
that the number of TCP connections causes scaling difficulties, there is
also a UDP-based interface. The UDP interface does not provide guaranteed
delivery, so should only be used for operations that aren't required to
succeed; typically it is used for "get" requests where a missing or
incomplete response can simply be treated as a cache miss.
Each UDP datagram contains a simple frame header, followed by data in the
same format as the TCP protocol described above. In the current
implementation, requests must be contained in a single UDP datagram, but
responses may span several datagrams. (The only common requests that would
span multiple datagrams are huge multi-key "get" requests and "set"
requests, both of which are more suitable to TCP transport for reliability
reasons anyway.)
The frame header is 8 bytes long, as follows (all values are 16-bit integers
in network byte order, high byte first):
0-1 Request ID
2-3 Sequence number
4-5 Total number of datagrams in this message
6-7 Reserved for future use; must be 0
The request ID is supplied by the client. Typically it will be a
monotonically increasing value starting from a random seed, but the client
is free to use whatever request IDs it likes. The server's response will
contain the same ID as the incoming request. The client uses the request ID
to differentiate between responses to outstanding requests if there are
several pending from the same server; any datagrams with an unknown request
ID are probably delayed responses to an earlier request and should be
The sequence number ranges from 0 to n-1, where n is the total number of
datagrams in the message. The client should concatenate the payloads of the
datagrams for a given response in sequence number order; the resulting byte
stream will contain a complete response in the same format as the TCP
protocol (including terminating \r\n sequences).
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