Identity/AttachedServices/StorageServerProtocol

Status: Obsolete

This document describe a protocol for the Delta-Sync storage model, which we're hoping to replace with the new Queue-Sync storage model. It is being kept around for reference, but will not see implementation. Eventually this document will be replaced with one describing the Queue-Sync protocol.

Summary

This is a working proposal for the PiCL Storage API, to implement the concepts described in Identity/CryptoIdeas/04-Delta-Sync. It's a work in progress that will eventually obsolete Identity/AttachedServices/StorageProtocolZero.

Delta-Sync Data Model

The storage server hosts a number of independent named collections for each user. Each collection is a key-value store whose contents can be atomically modified by the client.

Each modification of a collection creates a new version with corresponding version identifier in the format <seqnum>:<hash>:<hmac>, giving a signed hash of the contents of the collection at that version. The server ensures that versions can only be created with monotonically-increasing sequence numbers.

A collection can be marked as obsolete. This will cause any further attempts to access it to return an error code. Obsolete collections may be garbage-collected by the storage server after 24 hours.

More details at Identity/CryptoIdeas/04-Delta-Sync.

Authentication

To access the storage service, a client device must authenticate by providing a BrowserID assertion and a Device ID. It will receive in exchange:

• the current version number of each collection
• a short-lived id/key pair that can be used to authenticate subsequent requests using the Hawk request-signing scheme
• a URL to which further requests should be directed

You can think of this as establishing a "login session" with the server, although we're also tunneling some basic metadata in order to reduce the number of round-trips.

Example:

   >  POST <server-url>
   >  {
   >   "assertion": <browserid assertion>,
   >   "device": <device UUID>
   >  }
   .
   <  200 OK
   <  Content-Type: application/json
   <  {
   <   "base_url": <user-specific access url>,
   <   "id": <hawk auth id>,
   <   "key": <hawk auth secret key>,
   <   "collections": {
   <     "XXXXX": <version id for this collection>,
   <     "YYYYY": <version id for this collection>,
   <     <...etc...>
   <   }
   <  }

The user and device identity information is encoded in the hawk auth id, to avoid re-sending it on each request. The server may also include additional state in this value, depending on the implementation. It's opaque to the client.

The base_url may include a unique identifier for the user, in order to improve RESTful-icity of the API. Or it might point the client to a specific data-center which houses their write master. It's opaque to the client.

Data Access

The client now makes Hawk-authenticated requests to the storage API under its assigned base_url. The following operations are available.

GET <base-url>

Get the current version id for all collections. This is the same data as returned in the session-establishment call above, but it may be useful if the client wants to refresh its view. Example:

   >  GET <base-url>
   >  Authorization:  <hawk auth parameters>
   .
   <  200 OK
   <  Content-Type: application/json
   <  {
   <   "collections": {
   <     "XXXXX": <version id for this collection>,
   <     "YYYYY": <version id for this collection>,
   <     <...etc...>
   <   }
   <  }

GET <base-url>/<collection>

Get the current metadata for a specific collection: its version id, obsolete status, and last read/write times. Example:

   >  GET <base-url>/<collection>
   >  Authorization:  <hawk auth parameters>
   .
   <  200 OK
   <  Content-Type: application/json
   <  {
   <   "version": <version id for this collection>,
   <   "obsolete": false,
   <   "atime": <last-accessed timestamp for this collection>,
   <   "mtime": <last-modified timestamp for this collection>
   <  }

POST <base-url>/<collection>

Update writeable metadata for a specific collection. Currently the only piece of metadata that can be updated is the "obsolete" flag, which can be flipped to true:

   >  POST <base-url>/<collection>
   >  Authorization:  <hawk auth parameters>
   >  {
   >    "obsolete": true
   >  }
   .
   <  200 OK
   <  Content-Type: application/json
   <  {
   <   "version": <version id for this collection>,
   <   "obsolete": true,
   <   "atime": <last-accessed timestamp for this collection>,
   <   "mtime": <last-modified timestamp for this collection>
   <  }

GET <base-url>/<collection>/<version>

Get the contents of a specific version of a specific collection. In the simplest case, we GET the full contents like so:

   >  GET <base-url>/<collection>/<version>
   >  Authorization:  <hawk auth parameters>
   .
   <  200 OK
   <  Content-Type: application/json
   <  {
   <   "items": {
   <    "key1": "value1",
   <    "key2": "value2",
   <    <..etc..>
   <   }
   <  }

However, clients will usually want to request a delta from a previous version. They can do this by specifying the "from" parameter. New or updated keys are represented with their value, while deleted keys are represented with a value of null. Like so:

   >  GET <base-url>/<collection>/<version>?from=<previous version>
   >  Authorization:  <hawk auth parameters>
   .
   <  200 OK
   <  Content-Type: application/json
   <  {
   <   "items": {
   <     "key1": "value1",  // a key that was updated
   <     "key2": null       // a key that was deleted
   <   }
   <  }

To allow reliable transfer of a large number of items, both client and server may choose to paginate responses to this query.

The client may specify "first" as the key at which to (lexicographically) start the listing, and "upto" as the key at which to stop the listing. It may also specify an integer "limit" to restrict the total number of keys sent at once. The server may enforce a default value and/or upper-bound on "limit".

If the set of items is truncated, the server will send the response argument "next" to give the next available key in iteration order. The client should make another request setting "first" equal to the provided value of "next" in order to fetch additional items.

As an example, suppose that the client requests at most two items per response, and the collection contains items "key1", "key2" and "key3". It would would need to fetch them in two batches like so:

   >  GET <base-url>/<collection>/<version>?limit=2
   >  Authorization:  <hawk auth parameters>
   .
   <  200 OK
   <  Content-Type: application/json
   <  {
   <   "next": "key3",
   <   "items": {
   <     "key1": "value1",
   <     "key2": "value2"
   <   }
   <  }
   .
   .
   >  GET <base-url>/<collection>/<version>?first=key3&limit=2
   >  Authorization:  <hawk auth parameters>
   .
   <  200 OK
   <  Content-Type: application/json
   <  {
   <   "items": {
   <     "key3": "value3"
   <   }
   <  }

XXX TODO: There are several error cases that need to be distinguished, possibly by HTTP status code or possibly by some information in the error response body:

• The requested version is not known or no longer present on the server
• We can't generate a delta from the specified "from" version to the request version
• The specified "from" version is invalid (e.g. due to lost writes during a rollback)

POST <base-url>/<collection>/<version>

Creates a new version of a specific collection. In the simplest case, we POST up the full contents of the new version like so:

   >  POST <base-url>/<collection>/<version>
   >  Authorization:  <hawk auth parameters>
   >  {
   >   "items": {
   >     "key1": "value1",
   >     "key2": "value2",
   >     <..etc..>
   >   }
   >  } 
   .
   <  201 Created

However, clients will usually want to send just the changes from a previous version. They can do this by specifying the "from" parameter. New or updated keys are represented with their value, while deleted keys are represented with a value of null. Like so:

   >  POST <base-url>/<collection>/<version>?from=<previous version>
   >  Authorization:  <hawk auth parameters>
   >  {
   >   "items": {
   >     "key1": "value1",  // a key to be updated
   >     "key2": null       // a key to be deleted
   >   }
   >  } 
   .
   <  201 Created

To guard against intermittent or unreliable connections, the client can also send data in batches. It can specify the argument "first" to indicate a key offset at which this batch begins, and the argument "upto" to specify a key offset at which this batch ends. The server will spool all the incoming items until it sees a batch with no "upto" argument, then create the new version as an atomic unit.

As an example, here is how the client might create a new version by sending items one at a time:

   >  POST <base-url>/<collection>/<version>?upto=key2
   >  Authorization:  <hawk auth parameters>
   >  {
   >   "items": {
   >    "key1": value1"
   >   }
   >  }
   .
   <  202 Accepted
   .
   .
   >  POST <base-url>/<collection>/<version>?start=key2&upto=key3
   >  Authorization:  <hawk auth parameters>
   >  {
   >   "items": {
   >    "key2": "value2"
   >   }
   >  }
   .
   <  202 Accepted
   .
   .
   >  POST <base-url>/<collection>/<version>?start=key3
   >  Authorization:  <hawk auth parameters>
   >  {
   >   "items": {
   >    "key3": "value3"
   >   }
   >  }
   .
   <  201 Created

XXX TODO: There are several error cases that need to be distinguished, possibly by HTTP status code or possibly by some information in the error response body:

• There was a conflicting write, so you can no longer create the requested version
• The requested version is invalid, e.g. wrong sequence number
• The specified "from" version is too old, so we can't use it as the start point of a delta
• The specified "from" version is invalid (e.g. due to lost writes during a rollback)
• The provided batches had holes, or were otherwise invalid
• The server forgot a previous batch and you'll have to start again

Things To Think About

• Currently there's no explicit way for the server to track the current version held by each client. We could add this in the initial handshake, or intuit it based on their activity.
• Is json the best format for this transfer, or could we come up with a more efficient representation?
• Should we add a way to retrieve specific keys, for real-time updating of just the important bits?