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Mongo Wire Protocol

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{dochub:mongowireprotocol}

{toc}{anchor:TableOfContents} {redirect:http://docs.mongodb.org/meta-driver/latest/legacy/mongodb-wire-protocol/}

h3. 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.

{info:title=Default Socket Port} The default port is 27017, but this is configurable and will vary.
{info}Clients should connect to the database with a regular TCP/IP socket. Currently, there is no connection handshake.

{info:Format Notation}To describe the message structure, a C-like {{struct}} is used. The types used in this document ({{cstring}}, {{int32}}, etc.) are the same as those defined in the [BSON specification|http://bsonspec.org/#/specification]. The standard message header is typed as {{MsgHeader}}. Integer constants are in capitals (e.g. {{ZERO}} for the integer value of 0).

In the case where more than one of something is possible (like in a [OP_INSERT|#OP_INSERT] or [OP_KILL_CURSORS|#OP_KILL_CURSORS]), we again use the notation from the [BSON specification|http://bsonspec.org/#/specification] (e.g. {{int64\*}}). This simply indicates that one or more of the specified type can be written to the socket, one after another.
{info}
{note:title=Byte Ordering}Note that like BSON documents, all data in the mongo wire protocol is little-endian.
{note}
h3. Messages Types and Formats

[#TableOfContents]

There are two types of messages, client requests and database responses, each having a slightly different structure.

h6. 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 :

{code}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
}
{code}{{{}{*}messageLength{*}}} : This is the total size of the message in bytes. This total includes the 4 bytes that holds the message length.

{{{*}requestID{*}}} : This is a client or database-generated identifier that uniquely identifies this message. For the case of client-generated messages (e.g. [OP_QUERY|#OP_QUERY] and [CONTRIB:OP_GET_MORE|#OP_GET_MORE]), it will be returned in the
{{responseTo}} field of the [CONTRIB:OP_REPLY|#OP_REPLY] message. Along with the {{reponseTo}} field in responses, clients can use this to associate query responses with the originating query.

{{{*}responseTo{*}}} : In the case of a message from the database, this will be the requestID taken from the [CONTRIB:OP_QUERY|#OP_QUERY] or [CONTRIB:OP_GET_MORE|#OP_GET_MORE] messages from the client. Along with the {{requestID}} field in queries, clients can use this to associate query responses with the originating query.

{{{*}opCode{*}}} : Type of message. See the table below in the next section.

h6. Request Opcodes

[#TableOfContents]

The following are the currently supported opcodes :

|| Opcode Name || opCode value || Comment ||
| OP_REPLY | 1 | Reply to a client request. responseTo is set |
| OP_MSG | 1000 | generic msg command followed by a string |
| OP_UPDATE | 2001 | update document |
| OP_INSERT | 2002 | insert new document |
| RESERVED | 2003 | formerly used for OP_GET_BY_OID |
| OP_QUERY | 2004 | query a collection |
| OP_GETMORE | 2005 | Get more data from a query. See Cursors |
| OP_DELETE | 2006 | Delete documents |
| OP_KILL_CURSORS | 2007 | Tell database client is done with a cursor |

h3. Client Request Messages

[#TableOfContents]

Clients can send all messages except for [CONTRIB:OP_REPLY|#OP_REPLY]. This is reserved for use by the database.

Note that only the [CONTRIB:OP_QUERY|#OP_QUERY] and [CONTRIB:OP_GET_MORE|#OP_GET_MORE] messages result in a response from the database. There will be no response sent for any other message.

You can determine if a message was successful with a getLastError command.

h6. OP_UPDATE {anchor:OP_UPDATE}

The OP_UPDATE message is used to update a document in a collection. The format of a OP_UPDATE message is

{code}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
}
{code}{{{}{*}fullCollectionName{*}}} : The full collection name. The full collection name is the concatenation of the database name with the collection name, using a "." for the concatenation. For example, for the database "foo" and the collection "bar", the full collection name is "foo.bar".

{{{*}flags{*}}} :
|| bit num || name || description ||
| 0 | Upsert | If set, the database will insert the supplied object into the collection if no matching document is found. |
| 1 | MultiUpdate | If set, the database will update all matching objects in the collection. Otherwise only updates first matching doc. |
| 2-31 | Reserved | Must be set to 0. |

{{{*}selector{*}}} : BSON document that specifies the query for selection of the document to update.

{{{*}update{*}}} : BSON document that specifies the update to be performed. For information on specifying updates see the documentation on [updating].

There is no response to an OP_UPDATE message.

h6. OP_INSERT {anchor: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

{code}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
}
{code}{{{}{*}fullCollectionName{*}}} : The full collection name. The full collection name is the concatenation of the database name with the collection name, using a "." for the concatenation. For example, for the database "foo" and the collection "bar", the full collection name is "foo.bar".

{{{*}documents{*}}} : One or more documents to insert into the collection. If there are more than one, they are written to the socket in sequence, one after another.

{{{*}flags{*}}} :
|| bit num || name || description ||
| 0 | ContinueOnError | If set, the database will not stop processing a bulk insert if one fails (eg due to duplicate IDs). This makes bulk insert behave similarly to a series of single inserts, except lastError will be set if any insert fails, not just the last one. If multiple errors occur, only the most recent will be reported by getLastError. (new in 1.9.1) |
| 1-31 | Reserved | Must be set to 0. |

There is no response to an OP_INSERT message.

h6. OP_QUERY {anchor: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 :

{code}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.
}
{code}{{{}{*}flags{*}}} :
|| bit num || name || description ||
| 0 | Reserved | Must be set to 0. |
| 1 | TailableCursor | Tailable means cursor is not closed when the last data is retrieved. Rather, the cursor marks the final object's position. You can resume using the cursor later, from where it was located, if more data were received. Like any "latent cursor", the cursor may become invalid at some point (CursorNotFound) -- for example if the final object it references were deleted. |
| 2 | SlaveOk | Allow query of replica slave. Normally these return an error except for namespace "local". |
| 3 | OplogReplay | Internal replication use only - driver should not set |
| 4 | NoCursorTimeout | The server normally times out idle cursors after an inactivity period (10 minutes) to prevent excess memory use. Set this option to prevent that. |
| 5 | AwaitData | Use with TailableCursor. If we are at the end of the data, block for a while rather than returning no data. After a timeout period, we do return as normal. |
| 6 | Exhaust | Stream the data down full blast in multiple "more" packages, on the assumption that the client will fully read all data queried. Faster when you are pulling a lot of data and know you want to pull it all down. Note: the client is not allowed to not read all the data unless it closes the connection. |
| 7 | Partial | Get partial results from a {{mongos}} if some shards are down (instead of throwing an error) |
| 8-31 | Reserved | Must be set to 0. |

{{{*}fullCollectionName{*}}} : The full collection name. The full collection name is the concatenation of the database name with the collection name, using a "." for the concatenation. For example, for the database "foo" and the collection "bar", the full collection name is "foo.bar".

{{{*}numberToSkip{*}}} : Sets the number of documents to omit - starting from the first document in the resulting dataset - when returning the result of the query.

{{{*}numberToReturn{*}}} : Limits the number of documents in the *first* [CONTRIB:OP_REPLY|#OP_REPLY] message to the query. However, the database will still establish a cursor and return the {{cursorID}} to the client if there are more results than {{numberToReturn}}. If the client driver offers 'limit' functionality (like the SQL *LIMIT* keyword), then it is up to the client driver to ensure that no more than the specified number of document are returned to the calling application. If {{numberToReturn}} is 0, the db will use the default return size. If the number is negative, then the database will return that number and close the cursor. No futher results for that query can be fetched. If {{numberToReturn}} is 1 the server will treat it as \-1 (closing the cursor automatically).

{{{*}query{*}}} : BSON document that represents the query. The query will contain one or more elements, all of which must match for a document to be included in the result set. Possible elements include {{$query}}, {{$orderby}}, {{$hint}}, {{$explain}}, and {{$snapshot}}.

{{{*}returnFieldsSelector{*}}} : OPTIONAL BSON document that limits the fields in the returned documents. The returnFieldsSelector contains one or more elements, each of which is the name of a field that should be returned, and and the integer value 1. In JSON notation, a returnFieldsSelector to limit to the fields "a", "b" and "c" would be :

{code}{ a : 1, b : 1, c : 1}
{code}The database will respond to an OP_QUERY message with an [CONTRIB:OP_REPLY|#OP_REPLY] message.

h6. OP_GETMORE {anchor: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 :

{code}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
}
{code}{{{}{*}fullCollectionName{*}}} : The full collection name. The full collection name is the concatenation of the database name with the collection name, using a "." for the concatenation. For example, for the database "foo" and the collection "bar", the full collection name is "foo.bar".

{{{*}numberToReturn{*}}} : Limits the number of documents in the *first* [CONTRIB:OP_REPLY|#OP_REPLY] message to the query. However, the database will still establish a cursor and return the {{cursorID}} to the client if there are more results than {{numberToReturn}}. If the client driver offers 'limit' functionality (like the SQL *LIMIT* keyword), then it is up to the client driver to ensure that no more than the specified number of document are returned to the calling application. If {{numberToReturn}} is 0, the db will used the default return size.

{{{*}cursorID{*}}} : Cursor identifier that came in the [CONTRIB:OP_REPLY|#OP_REPLY]. This must be the value that came from the database.

The database will respond to an OP_GETMORE message with an [CONTRIB:OP_REPLY|#OP_REPLY] message.

h6. OP_DELETE {anchor:OP_DELETE}

The OP_DELETE message is used to remove one or more documents from a collection. The format of the OP_DELETE message is :

{code}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.
}
{code}{{{}{*}fullCollectionName{*}}} : The full collection name. The full collection name is the concatenation of the database name with the collection name, using a "." for the concatenation. For example, for the database "foo" and the collection "bar", the full collection name is "foo.bar".

{{{*}flags{*}}} :
|| bit num || name || description ||
| 0 | SingleRemove | If set, the database will remove only the first matching document in the collection. Otherwise all matching documents will be removed. |
| 1-31 | Reserved | Must be set to 0. |

{{{*}selector{*}}} : BSON document that represent the query used to select the documents to be removed. The selector will contain one or more elements, all of which must match for a document to be removed from the collection. Please see $$$ TODO QUERY for more information.

There is no reponse to an OP_DELETE message.

h6. OP_KILL_CURSORS {anchor:OP_KILL_CURSORS}

The OP_KILL_CURSORS message is used to close an active cursor in the database. This is necessary to ensure that database resources are reclaimed at the end of the query. The format of the OP_KILL_CURSORS message is :

{code}struct {
MsgHeader header; // standard message header
int32 ZERO; // 0 - reserved for future use
int32 numberOfCursorIDs; // number of cursorIDs in message
int64* cursorIDs; // sequence of cursorIDs to close
}
{code}{{{}{*}numberOfCursorIDs{*}}} : The number of cursors that are in the message.

{{{*}cursorIDs{*}}} : "array" of cursor IDs to be closed. If there are more than one, they are written to the socket in sequence, one after another.

Note that if a cursor is read until exhausted (read until OP_QUERY or OP_GETMORE returns zero for the cursor id), there is no need to kill the cursor.

h6. OP_MSG {anchor:OP_MSG}

Deprecated. OP_MSG sends a diagnostic message to the database.  The database sends back a fixed resonse.  The format is

{code}struct {
MsgHeader header; // standard message header
cstring message; // message for the database
}
{code}Drivers do not need to implement OP_MSG.

h3. Database Response Messages

[#TableOfContents]

h6. OP_REPLY {anchor:OP_REPLY}

The OP_REPLY message is sent by the database in response to an [CONTRIB:OP_QUERY|#OP_QUERY] or [CONTRIB:OP_GET_MORE|#OP_GET_MORE]
message. The format of an OP_REPLY message is:

{code}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
}
{code}{{{}{*}responseFlags{*}}} :
|| bit num || name || description ||
| 0 | CursorNotFound | Set when getMore is called but the cursor id is not valid at the server. Returned with zero results. |
| 1 | QueryFailure | Set when query failed. Results consist of one document containing an "$err" field describing the failure. |
| 2 | ShardConfigStale | Drivers should ignore this. Only mongos will ever see this set, in which case, it needs to update config from the server. |
| 3 | AwaitCapable | Set when the server supports the AwaitData Query option. If it doesn't, a client should sleep a little between getMore's of a Tailable cursor. Mongod version 1.6 supports AwaitData and thus always sets AwaitCapable. |
| 4-31 | Reserved | Ignore |

{{{*}cursorID{*}}} : The cursorID that this OP_REPLY is a part of. In the event that the result set of the query fits into one OP_REPLY message, {{cursorID}} will be 0. This {{cursorID}} must be used in any [CONTRIB:OP_GET_MORE|#OP_GET_MORE] messages used to get more data, and also must be closed by the client when no longer needed via a [CONTRIB:OP_KILL_CURSORS|#OP_KILL_CURSORS] message.