Mongo Wire Protocol（写MongoDB驱动必备）
目前MongoDB针对各种语言都已经有了相应的驱动，但是写好一个驱动的关键是理解MongoDB提供的协议，下面的内容主要是来自MongoDB的官方的Protocol描述：
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Introduction
The Mongo Wire Protocol is a simple socket-based, request-response style protocol. Clients communicate with the database server through a regular TCP/IP socket.
- Standard Message Header
In general, each message consists of a standard message header followed by request-specific data. The standard message header is structured as follows :
struct MsgHeader { int32 messageLength; // total message size, including this int32 requestID; // identifier for this message int32 responseTo; // requestID from the original request // (used in reponses from db) int32 opCode; // request type - see table below }
MsgHeader一个关键的field就是opCode，它决定了Message是干什么用的，比如OP_QUERY肯定是客户端向MongoDB服务器发送的查询请求。
- Request Opcodes
Client能够使用除OP_REPLY以外的所有Opcode，而OP_REPLY是留给Mongodb用的；
Client发送的Opcode中，只有OP_QUERY和OP_GET_MORE会得到MongoDB的response，对于其他的Opcode一概不给予答复（如果你真的需要看你发送的opcode是否执行成功，可以调用getLastError查看）；
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Client Request Messages
OP_UPDATE
The OP_UPDATE message is used to update a document in a collection. The format of a OP_UPDATE message is:
struct OP_UPDATE { MsgHeader header; // standard message header int32 ZERO; // 0 - reserved for future use cstring fullCollectionName; // "dbname.collectionname" int32 flags; // bit vector. see below document selector; // the query to select the document document update; // specification of the update to perform }
MsgHeader是每一个Message必须有的；fullCollectionName就是db + collection name；flags就是一个位描述符，每一位代表的意思如下：
Upsert就是当要更新的document不存在的时候，是否将其插入到collection中去，MultiUpdate则表示当selector找到了多个符合更新条件的document时，是否将它们全部更新; selector决定了选择哪些document去做更新操作，它也是经常用在find中的；update则表示要将找到的document更新成什么模样，也能是一部分field，或者全部field。
OP_INSERT
The OP_INSERT message is used to insert one or more documents into a collection. The format of the OP_INSERT message is：
struct { MsgHeader header; // standard message header int32 flags; // bit vector - see below cstring fullCollectionName; // "dbname.collectionname" document* documents; // one or more documents to insert into the collection }
需要注意的就是flags每位代表的意思改变了，具体查看：http://www.mongodb.org/display/DOCS/Mongo+Wire+Protocol
OP_QUERY
The OP_QUERY message is used to query the database for documents in a collection. The format of the OP_QUERY message is :
struct OP_QUERY { MsgHeader header; // standard message header int32 flags; // bit vector of query options. See below for details. cstring fullCollectionName; // "dbname.collectionname" int32 numberToSkip; // number of documents to skip int32 numberToReturn; // number of documents to return // in the first OP_REPLY batch document query; // query object. See below for details. [ document returnFieldSelector; ] // Optional. Selector indicating the fields // to return. See below for details. }
numberToSkip有skip参数决定了，表示要对查找到的document跳过多少个；numberToReturn则由limit参数决定了，表示要返回多少个document给client；query就是个查询条件；returnFieldSelector则表示返回哪些docuemnt的fields回来，因为我们也有是需要document的部分field的情况，而不是所有的fields。
OP_GETMORE
The OP_GETMORE message is used to query the database for documents in a collection. The format of the OP_GETMORE message is :
struct { MsgHeader header; // standard message header int32 ZERO; // 0 - reserved for future use cstring fullCollectionName; // "dbname.collectionname" int32 numberToReturn; // number of documents to return int64 cursorID; // cursorID from the OP_REPLY }
一般mongoDB做查询的时候返回的是Cursor对象，它并不包含实际的数据，而只是数据的一些标识，当我们需要具体的document的时候可以调用Cursor的方法来向mongoDB请求数据，这个时候发送的Opcode就是OP_GETMORE了，所以cursorID就是告诉MongoDB返回从哪个位置开始的数据罢了。
OP_DELETE
The OP_DELETE message is used to remove one or more messages from a collection. The format of the OP_DELETE message is :
struct { MsgHeader header; // standard message header int32 ZERO; // 0 - reserved for future use cstring fullCollectionName; // "dbname.collectionname" int32 flags; // bit vector - see below for details. document selector; // query object. See below for details. }
- Database Response Messages
OP_REPLY
The OP_REPLY message is sent by the database in response to an CONTRIB:OP_QUERY or CONTRIB:OP_GET_MORE
message. The format of an OP_REPLY message is:
struct { MsgHeader header; // standard message header int32 responseFlags; // bit vector - see details below int64 cursorID; // cursor id if client needs to do get more's int32 startingFrom; // where in the cursor this reply is starting int32 numberReturned; // number of documents in the reply document* documents; // documents }
更多的Opcode请参考：http://www.mongodb.org/display/DOCS/Mongo+Wire+Protocol