# 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
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).

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

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
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