Clarified the sync document

docs/sync.markdown

@@ -1,57 +1,59 b''
 # INTRO #
 This document discribes the server sync protocol.
 
 # PURPOSE #
 This protocol will be used to share the models (currently imageboard posts)
 across multiple servers. The main differnce of this protocol is that the node
 can specify what models it wants to get and from whom. The nodes can get
 models from a specific server, or from all except some specific servers. Also
 the models can be filtered by timestamps or tags.
 
 # DRAFT PROTOCOL DESCRIPTION #
 The node requests other node's changes list since some time (since epoch if
 this is the start). The other node sends a list of post ids or posts in the
 XML or JSON format.
 
 Protocol version is the version of the sync api. Model version is the version
 of data models. If at least one of them is different, the sync cannot be
 performed.
 
-The node signs the data with its key. The receiving node saves the key at the
+The node signs the data with its keys. The receiving node saves the key at the
 first sync and checks it every time. If the key has changed, the info won't be
-saved from the node (or the node id must be changed).
+saved from the node (or the node id must be changed). A model can be signed 
+with several keys but at least one of them must be the same as in the global 
+ID to verify the sender.
 
 Each node can have several keys. Nodes can have shared keys to serve as a pool
 (several nodes with the same key).
 
-Each post has an ID in the unique format: node-id/post-id
+Each post has an ID in the unique format: [key-type][key][local-id]
 
-All requests pass a request type, protocol and model versions and a list of
+All requests pass a request type, protocol and model versions, and a list of
 optional arguments used for filtering.
 
-Each protocol has its own version. Version consists of 2 numbers: first is
+Each request has its own version. Version consists of 2 numbers: first is
 incompatible version (1.3 and 2.0 are not compatible and must not be in sync)
 and the second one is minor and compatible (for example, new optional field
 is added which will be igroned by those who don't support it yet).
 
 Post edits and reflinks are not saved to the sync model. The replied post ID 
 can be got from the post text, and reflinks can be computed when loading 
 posts. The edit time is not saved because a foreign post can be 'edited' (new
 replies are added) but the signature must not change (so we can't update the
 content). The inner posts can be edited, and the signature will change then
 but the local-id won't, so the other node can detect that and replace the post
 instead of adding a new one.
 
 # REQUESTS #
 Request types:
 * pull - pull the desired model ids
 * get - get models by ids
 * put - give a model to the given node (you have no guarantee the node takes
 it, consider you are just advising the node to take your post. This request
 type is useful in pool where all the nodes try to duplicate all of their data
 across the pool.
 
 # RESPONSES #
 * not supported - request is not supported
 * success - request was successfull
 * error - unexpected error