Memcached 协议 中英文对照(一)

Memcached 协议 中英文对照(一)
2011年11月25日
　　Memcached 协议中英文对照
　　协议 Protocol
　　memcached 的客户端使用TCP链接与 服务器通讯。（UDP接口也同样有效，参考后文的 “UDP协议” ）一个运行中的memcached服务器监视一些（可设置）端口。客户端连接这些端口，发送命令到服务器，读取回应，最后关闭连接。 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. 
　　结束会话不需要发送任何命令。当不再需memcached服务时，要客户端可以在任何时候关闭连接。需要注意的是，鼓励客户端缓存这些连接，而不是每次需要存取数据时都重新打开连接。这是因为memcached 被特意设计成及时开启很多连接也能够高效的工作（数百个，上千个如果需要的话）。缓存这些连接，可以消除建立连接所带来的开销（/*/相对而言，在服务器端建立一个新连接的准备工作所带来的开销，可以忽略不计。）。 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). 
　　在memcache协议中发送的数据分两种：文本行和 自由数据。 文本行被用于来自客户端的命令和服务器的回应。自由数据用于客户端从服务器端存取数据时。同样服务器会以字节流的方式传回自由数据。/*/服务器不用关心自由数据的字节顺序。自由数据的特征没有任何限制；但是通过前文提到的文本行，这项数据的接受者（服务器或客户端），便能够精确地获知所发送的数据库的长度。
　　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
　　transmitted. 
　　文本行固定以“\r\n”(回车符紧跟一个换行符)结束。 自由数据也是同样会以“\r\n”结束，但是 \r(回车符)、\n(换行符)，以及任何其他8位字符，均可出现在数据中。因此，当客户端从服务器取回数据时，必须使用数据区块的长度来确定数据区块的结束位置，而不要依据数据区块末尾的“\r\n”，即使它们固定存在于此。
　　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
　　does.    
　　键值 Keys
　　存储在memcached中的数据通过键值来标识。键值是一个文本字符串，对于需要存取这项数据的客户端而言，它必须是唯一的。键值当前的长度限制设定为250字符（当然，客户端通常不会用到这么长的键）；键值中不能使用制表符和其他空白字符（例如空格，换行等）。
　　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. 
　　命令 Commands
　　所有命令分为3种类型 There are three types of commands. 
　　存储命令（有3项：’set’、’add’、’repalce’）指示服务器储存一些由键值标识的数据。客户端发送一行命令，后面跟着数据区块；然后，客户端等待接收服务器回传的命令行，指示成功与否。
　　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. 
　　取回命令（只有一项：’get’）指示服务器返回与所给键值相符合的数据（一个请求中右一个或多个键值）。客户端发送一行命令，包括所有请求的键值；服务器每找到一项内容，都会发送回客户端一行关于这项内容的信息，紧跟着是对应的数据区块；直到服务器以一行“END”回应命令结束。
　　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.
　　/*?*/其他的命令都不能携带自由数据。在这些命令中，客户端发送一行命令，然后等待（由命令所决定）一行回应，或最终以一行“END”结束的多行命令。
　　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. 
　　一行命令固定以命令名称开始，接着是以空格隔开的参数（如果有参数的话）。命令名称大小写敏感，并且必须小写。
　　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. 
　　一些客户端发送给服务器的命令会包含一些时限（针对内容或客户端请求的操作）。这时，时限的具体内容既可以是Unix时间戳（从1970年1月1日开始的秒钟数），或当前时间开始的秒钟数。对后者而言，不能超过 60*60*24*30（30天）；如果超出，服务器将会理解为Unix时间戳，而不是从当前时间起的秒偏移。
　　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. 
　　错误字串
　　Error strings
　　每一个由客户端发送的命令，都可能收到来自服务器的错误字串回复。这些错误字串会以三种形式出现： 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" 
　　意味着客户端发送了不存在的命令名称。 means the client sent a nonexistent command name. 
　　- "CLIENT_ERROR \r\n" 
　　意味着输入的命令行里存在一些客户端错误，例如输入未遵循协议。部分是人类易于理解的错误解说…… means some sort of client error in the input line, i.e. the input
　　doesn't conform to the protocol in some way.  is a
　　human-readable error string. 
　　- "SERVER_ERROR \r\n" 
　　意味着一些服务器错误，导致命令无法执行。部分是人类易于理解的错误解说。在一些严重的情形下（通常应该不会遇到），服务器将在发送这行错误后关闭连接。这是服务器主动关闭连接的唯一情况。
　　means some sort of server error prevents the server from carrying
　　out the command.  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. 
　　在后面每项命令的描述中，这些错误行不会再特别提到，但是客户端必须考虑到这些它们存在的可能性。 In the descriptions of individual commands below, these error lines
　　are not again specifically mentioned, but clients must allow for their
　　possibility. 
　　存储命令
　　Storage commands
　　首先，客户端会发送一行像这样的命令： First, the client sends a command line which looks like this: 
　　    \r\n 
　　-  是 set, add, 或者 repalce -  is "set", "add" or "replace" 
　　set 意思是 “储存此数据” 
　　add 意思是 “储存此数据，只在服务器*未*保留此键值的数据时” 
　　replace意思是 “储存此数据，只在服务器*曾*保留此键值的数据时”
　　"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".
　　-  是接下来的客户端所要求储存的数据的键值 -  is the key under which the client asks to store the data 
　　-  是在取回内容时，与数据和发送块一同保存服务器上的任意16位无符号整形（用十进制来书写）。客户端可以用它作为“位域”来存储一些特定的信息；它对服务器是不透明的。
　　-  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. 
　　-  是终止时间。如果为0，该项永不过期(虽然它可能被删除，以便为其他缓存项目腾出位置)。如果非0（Unix时间戳或当前时刻的秒偏移），到达终止时间后，客户端无法再获得这项内容。
　　-  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). 
　　-  是随后的数据区块的字节长度，不包括用于分野的“\r\n”。它可以是0（这时后面跟随一个空的数据区块）。 -  is the number of bytes in the data block to follow, *not*
　　including the delimiting \r\n.  may be zero (in which case
　　it's followed by an empty data block). 
　　在这一行以后，客户端发送数据区块。 After this line, the client sends the data block: 
　　\r\n 
　　-  是大段的8位数据，其长度由前面的命令行中的指定。 -  is a chunk of arbitrary 8-bit data of length 
　　from the previous line. 
　　发送命令行和数据区块以后，客户端等待回复，可能的回复如下： After sending the command line and the data blockm the client awaits
　　the reply, which may be: 
　　- "STORED\r\n" 
　　表明成功. to indicate success. 
　　- "NOT_STORED\r\n" 
　　表明数据没有被存储，但不是因为发生错误。这通常意味着add 或 replace命令的条件不成立，或者，项目已经位列删除队列（参考后文的“delete”命令）。
　　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). 
　　取回命令
　　Retrieval command
　　一行取回命令如下： The retrieval command looks like this: 
　　get *\r\n 
　　- * 表示一个或多个键值，由空格隔开的字串 - * means one or more key strings separated by whitespace. 
　　这行命令以后，客户端的等待0个或多个项目，每项都会收到一行文本，然后跟着数据区块。所有项目传送完毕后，服务器发送以下字串：
　　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 
　　"END\r\n" 
　　来指示回应完毕。 to indicate the end of response. 
　　服务器用以下形式发送每项内容： Each item sent by the server looks like this: 
　　VALUE   \r\n
　　\r\n 
　　-  是所发送的键名 -  is the key for the item being sent 
　　-  是存储命令所设置的记号 
　　-  is the flags value set by the storage command -  是随后数据块的长度，*不包括* 它的界定符“\r\n” -  is the length of the data block to follow, *not* including
　　its delimiting \r\n 
　　-  是发送的数据 -  is the data for this item. 
　　如果在取回请求中发送了一些键名，而服务器没有送回项目列表，这意味着服务器没这些键名（可能因为它们从未被存储，或者为给其他内容腾出空间而被删除，或者到期，或者被已客户端删除）。
　　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). 
　　删除 Deletion
　　命令“delete”允许从外部删除内容： The command "delete" allows for explicit deletion of items: 
　　delete  \r\n 
　　-  是客户端希望服务器删除的内容的键名 -  is the key of the item the client wishes the server to delete 
　　-  是一个单位为秒的时间（或代表直到某一刻的Unix时间），在该时间内服务器会拒绝对于此键名的“add”和“replace”命令。此时内容被放入delete队列，无法再通过“get”得到该内容，也无法是用“add”和“replace”命令（但是“set”命令可用）。直到指定时间，这些内容被最终从服务器的内存中彻底清除。
　　-  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. 
　　参数 是可选的，缺省为0（表示内容会立刻清除，并且随后的存储命令均可用）。
　　The parameter  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). 
　　此命令有一行回应： The response line to this command can be one of: 
　　- "DELETED\r\n" 
　　表示执行成功 to indicate success 
　　- "NOT_FOUND\r\n" 
　　表示没有找到这项内容 to indicate that the item with this key was not found. 
　　参考随后的“flush_all”命令使所有内容无效
　　See the "flush_all" command below for immediate invalidation
　　of all existing items. 
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