# DDP Specification

DDP is a protocol between a client and a server that supports two operations:

 * Remote procedure calls by the client to the server.
 * The client subscribing to a set of documents, and the server keeping the
   client informed about the contents of those documents as they change over
   time.

This document specifies the version "pre2" of DDP. It's a rough description of
the protocol and not intended to be entirely definitive.

## General Message Structure:

DDP may use either SockJS or WebSockets as a lower-level message transport. (For
now, you connect via SockJS at the URL `/sockjs` and via WebSockets at the URL
`/websocket`. The latter is likely to change to be the main app URL specifying a
WebSocket subprotocol.)

DDP messages are JSON objects, with some fields specified to be EJSON.  Each one
has a `msg` field that specifies the message type, as well as other fields
depending on message type.

## Establishing a DDP Connection:

### Messages:

 * `connect` (client -> server)
   - `session`: string (if trying to reconnect to an existing DDP session)
   - `version`: string (the proposed protocol version)
   - `support`: array of strings (protocol versions supported by the client,
      in order of preference)
 * `connected` (server->client)
   - `session`: string (an identifier for the DDP session)
 * `failed` (server->client)
   - `version`: string (a suggested protocol version to connect with)

### Procedure:

The server may send an initial message which is a JSON object lacking
a `msg` key. If so, the client should ignore it. The client does not
have to wait for this message. (The message was once used to help
implement Meteor's hot code reload feature; it is now only included to
force old clients to update).

 * The client sends a `connect` message.
 * If the server is willing to speak the `version` of the protocol specified in
   the `connect` message, it sends back a `connected` message.
 * Otherwise the server sends back a `failed` message with a version of DDP it
   would rather speak, informed by the `connect` message's `support` field, and
   closes the underlying transport.
 * The client is then free to attempt to connect again speaking a different
   version of DDP. It can do that by sending another `connect` message on a new
   connection. The client may optimistically send more messages after the
   `connect` message, assuming that the server will support the proposed
   protocol version. If the server does not support that version, it must ignore
   those additional messages.

The versions in the `support` field of the client's `connect` message
are ordered according to the client's preference, most preferred
first. If, according to this ordering, the `version` proposed by the
client is not the best version that the server supports, the server
must force the client to switch to the better version by sending a
`failed` message.

When a client is connecting to a server for the first time it will
typically set `version` equal to its most preferred version. If
desired, the client can then remember the version that is ultimately
negotiated with the server and begin with that version in future
connections. The client can rely on the server sending a `failed`
message if a better version is possible as a result of the client or
the server having been upgraded.

## Heartbeats

### Messages:

 * `ping`
   - `id`: optional string (identifier used to correlate with response)
 * `pong`
   - `id`: optional string (same as received in the `ping` message)

### Procedure:

At any time after the connection is established either side may send a `ping`
message. The sender may chose to include an `id` field in the `ping`
message. When the other side receives a `ping` it must immediately respond with
a `pong` message. If the received `ping` message includes an `id` field, the
`pong` message must include the same `id` field.

## Managing Data:

### Messages:

 * `sub` (client -> server):
   - `id`: string (an arbitrary client-determined identifier for this subscription)
   - `name`: string (the name of the subscription)
   - `params`: optional array of EJSON items (parameters to the subscription)
 * `unsub` (client -> server):
   - `id`: string (the id passed to 'sub')
 * `nosub` (server -> client):
   - `id`: string (the id passed to 'sub')
   - `error`: optional Error (an error raised by the subscription as it
    concludes, or sub-not-found)
 * `added` (server -> client):
   - `collection`: string (collection name)
   - `id`: string (document ID)
   - `fields`: optional object with EJSON values
 * `changed` (server -> client):
   - `collection`: string (collection name)
   - `id`: string (document ID)
   - `fields`: optional object with EJSON values
   - `cleared`: optional array of strings (field names to delete)
 * `removed` (server -> client):
   - `collection`: string (collection name)
   - `id`: string (document ID)
 * `ready` (server -> client):
   - `subs`: array of strings (ids passed to 'sub' which have sent their
     initial batch of data)
 * `addedBefore` (server -> client):
   - `collection`: string (collection name)
   - `id`: string (document ID)
   - `fields`: optional object with EJSON values
   - `before`: string or null (the document ID to add the document before,
     or null to add at the end)
 * `movedBefore` (server -> client):
   - `collection`: string
   - `id`: string (the document ID)
   - `before`: string or null (the document ID to move the document before, or
     null to move to the end)

### Procedure:

 * The client specifies sets of information it is interested in by sending
   `sub` messages to the server.

 * At any time, but generally informed by the `sub` messages, the server can
   send data messages to the client.  Data consist of `added`, `changed`, and
   `removed` messages.  These messages model a local set of data the client
   should keep track of.

   - An `added` message indicates a document was added to the local set. The ID
     of the document is specified in the `id` field, and the fields of the
     document are specified in the `fields` field.  Minimongo interperets the
     string id field in a special way that transforms it to the _id field of
     Mongo documents.

   - A `changed` message indicates a document in the local set has new values
     for some fields or has had some fields removed. The `id` field is the ID of
     the document that has changed.  The `fields` object, if present, indicates
     fields in the document that should be replaced with new values.  The
     `cleared` field contains an array of fields that are no longer in the
     document.

   - A `removed` message indicates a document was removed from the local
     set. The `id` field is the ID of the document.

 * A collection is either ordered, or not. If a collection is ordered,
   the `added` message is replaced by `addedBefore`, which
   additionally contains the ID of the document after the one being
   added in the `before` field.  If the document is being added at the
   end, `before` is set to null. For a given collection, the server
   should only send `added` messages or `addedBefore` messages, not a
   mixture of both, and should only send `movedBefore` messages for a
   collection with `addedBefore` messages.

   NOTE: The ordered collection DDP messages are not currently used by Meteor.
   They will likely be used by Meteor in the future.

 * The client maintains one set of data per collection.  Each subscription does
   not get its own datastore, but rather overlapping subscriptions cause the
   server to send the union of facts about the one collection's data.  For
   example, if subscription A says document `x` has fields `{foo: 1, bar: 2}`
   and subscription B says document `x` has fields `{foo: 1, baz:3}`, then the
   client will be informed that document `x` has fields `{foo: 1, bar: 2, baz:
   3}`.  If field values from different subscriptions conflict with each other,
   the server should send one of the possible field values.

 * When one or more subscriptions have finished sending their initial batch of
   data, the server will send a `ready` message with their IDs.

## Remote Procedure Calls:

### Messages:

 * `method` (client -> server):
   - `method`: string (method name)
   - `params`: optional array of EJSON items (parameters to the method)
   - `id`: string (an arbitrary client-determined identifier for this method call)
   - `randomSeed`: optional JSON value (an arbitrary client-determined seed
     for pseudo-random generators)
 * `result` (server -> client):
   - `id`: string (the id passed to 'method')
   - `error`: optional Error (an error thrown by the method (or method-not-found)
   - `result`: optional EJSON item (the return value of the method, if any)
 * `updated` (server -> client):
   - `methods`: array of strings (ids passed to 'method', all of whose writes
     have been reflected in data messages)

### Procedure:

 * The client sends a `method` message to the server

 * The server responds with a `result` message to the client, carrying either
   the result of the method call, or an appropriate error.

 * Method calls can affect data that the client is subscribed to.  Once the
   server has finished sending the client all the relevant data messages based
   on this procedure call, the server should send an `updated` message to the
   client with this method's ID.

 * There is no particular required ordering between `result` and `updated`
   messages for a method call.

 * The client may provide a randomSeed JSON value.  If provided, this value is
   used to seed pseudo-random number generation.  By using the same seed with
   the same algorithm, the same pseudo-random values can be generated on the
   client and the server.  In particular, this is used for generating ids for
   newly created documents.  If randomSeed is not provided, then values
   generated on the server and the client will not be identical.

 * Currently randomSeed is expected to be a string, and the algorithm by which
   values are produced from this is not yet documented.  It will likely be
   formally specified in future when we are confident that the complete
   requirements are known, or when a compatible implementation requires this
   to be specified.

## Errors:

Errors appear in `result` and `nosub` messages in an optional error field. An
error is an Object with the following fields:

 * `error`: number
 * `reason`: optional string
 * `details`: optional string

Such an Error is used to represent errors raised by the method or subscription,
as well as an attempt to subscribe to an unknown subscription or call an unknown
method.

Other erroneous messages sent from the client to the server can result in
receiving a top-level `msg: 'error'` message in response. These conditions
include:

 * sending messages which are not valid JSON objects
 * unknown `msg` type
 * other malformed client requests (not including required fields)
 * sending anything other than `connect` as the first message, or sending
   `connect` as a non-initial message

The error message contains the following fields:

 * `reason`: string describing the error
 * `offendingMessage`: if the original message parsed properly, it is included
   here

## Appendix: EJSON

EJSON is a way of embedding more than the built-in JSON types in JSON.  It
supports all types built into JSON as plain JSON, plus the following:

**Dates:**

    {"$date": MILLISECONDS_SINCE_EPOCH}

**Binary data:**

    {"$binary": BASE_64_STRING}

(The base 64 string has `+` and `/` as characters 62 and 63, and has no maximum line length)

**Escaped things** that might otherwise look like EJSON types:

    {"$escape": THING}

For example, here is the JSON value `{$date: 10000}` stored in EJSON:

    {"$escape": {"$date": 10000}}

Note that escaping only causes keys to be literal for one level down; you can
have further EJSON inside.  For example, the following is the key `$date` mapped
to a Date object:

    {"$escape": {"$date": {"$date": 32491}}}

**User-specified types:**

    {"$type": TYPENAME, "$value": VALUE}

Implementations of EJSON should try to preserve key order where they can.  Users
of EJSON should not rely on key order, if possible.

> MongoDB relies on key order.  When using EJSON with MongoDB, the
> implementation of EJSON must preserve key order.


## Appendix 2: randomSeed backward/forward compatibility

randomSeed was added into DDP pre2, but it does not break backward or forward
compatibility.

If the client stub and the server produce documents that are different in any
way, Meteor will reconcile this difference. This may cause 'flicker' in the UI
as the values change on the client to reflect what happened on the server, but
the final result will be correct: the server and the client will agree.

Consistent id generation / randomSeed does not alter the syncing process, and
thus will (at worst) be the same:

* If neither the server nor the client support randomSeed, we will get the
classical/flicker behavior.

* If the client supports randomSeed, but the server does not, the server will
ignore randomSeed, as it ignores any unknown properties in a DDP method call.
Different ids will be generated, but this will be fixed by syncing.

* If the server supports randomSeed, but the client does not, the server will
generate unseeded random values (providing a randomSeed is optional); different
ids will be generated; and again this will be fixed by syncing.

* If both client and server support randomSeed, but different ids are
generated, either because the generation procedure is buggy, or the stub
behaves differently to the server, then syncing will fix this.

* If both client and server support randomSeed, in the normal case the ids
generated will be the same, and syncing will be a no-op.
