TeaBag

Centrally Orchestrated Multi Node Log-Based Storage with Distributed Conflict Resolution

TeaBag is a log-based multi node storage based on the principle of operation propagation and oplog pruning. Operations are pushed to the storage and conflict resolution is delegated to the client. Conflict resolution is done by retrieving the different operation logs and replaying them in a merged version.

Read are always first attempted locally (delayed if the data was never retrieved yet). A central authority server serves as rendez-vous to retrieve the mount table, that is, the different nodes for each root path, called channels.

The client library only stores data in memory. Tokens have an expiry date. The entire system operates on the following asumptions:

• Small amount of data. Holds in memory.
• Log based with history replay and distributed reconciliation
• Client nodes store data in memory
• Writes get commited on network but optimistically accepted. Disconnected OK.
• Notifications when pathes are modified (local and remote)
• Temporary keys { node, end_time, signature }

All calls to the nodes are trackable thanks to the token attached to them.

OPLOG Structure:

The oplog is an array of operations with the following structure:

{ 
  date: 12312...
  sha: a2ef...
  payload: { ... }
}

{ 
  date: 12312...
  sha: a2ef...
  value: { ... }
}

The payload is defined by the user and an operation is always associated with a type. The user provides reducers per type she uses.

Operations with a value field (called values) are used to allow the pruning of the oplog. Whenever a value is encoutered, history before that value is ignored by the conflict resolution algorithm.

An oplog is initialized with a value equal to null.

The oplog is automatically pruned by inserting values into it whenever a client considers that all the stores it knows of are in sync. Inserting a value may fail on some stores, but it will eventually get propagated later on by other clients to all of the stores upon reconnect.

Finally, stores stream operations they accept so that clients can stay up to date, so that most oplog read happen locally.

Session Tokens:

• Sessions are created on the table with a timeout
• store_token are sent to the stores and are generated by the table
• store_token are checked initially by the the store with table and inherit the parent session_token timeout
• Revoking the session on the table will revoke the store_tokens on each store as well.

Conflict Resolution:

READ PATH:

On each store, Read oplog if not already in memory
When first oplog received
Reduce current value and return
Store in memory all other oplogs and register to stream
Attempt syncing and pruning

WRITE PATH:

On each store, read and then insert op in oplog locally, recompute value
push op to store
Attempt syncing and pruning

SYNCING & PRUNING:

On each store, read oplog
Compare each store oplogs
If discrepancies, push ops to oplogs unaware of them
Otherwise push a new value to all oplogs for pruning

TeaBag Table

user_id -> { 
  master,                                              // hash(user_id, pwd)
  channel -> [ { id, store_url } ],     
  [ tokens ]
};


/* ADMIN */

// master
PUT  /user/:user_id/master/:master                     // revoke all sessions

/* PUBLIC */

// sessions
GET  /user/:user_id/session/new
     master, timeout, description
GET  /user/:user_id/session/all
     master
DEL  /user/:user_id/session/:session_token              
GET  /user/:user_id/session/check/:session_token

// table
POST /user/:user_id/table/:channel/store
     master
     { store_url, code }
DEL  /user/:user_id/table/:channel/store/:store_id
     master
DEL  /user/:user_id/table/:channel
     master
GET  /user/:user_id/table
     master | session_token
GET  /user/:user_id/table/:channel
     master | session_token
GET  /user/:user_id/table/check/:store_token

Storage:
- user's master: $TEABAG_DATA/:salt/:user/user.json
- user's tokens: $TEABAG_DATA/:salt/:user/sessions.json
- user's table: $TEABAG_DATA/:salt/:user/table.json

TeaBag Store

user_id -> {
  table: { id, table_url }, 
  { path, type } -> { [ op ] },
}
op := { date, sha, payload }

BASE_URL = /user/:user_id

/* ADMIN */

// confirmation
PUT  /admin/user/:user_id
GET  /admin/user/:user_id/code

/* PUBLIC */

// table confirmation
POST {BASE_URL}/confirm
     { table_url, code }

// store_token
DEL  {BASE_URL}/session/:store_token

// oplog
POST {BASE_URL}/oplog
     store_token, path, type
     { date, payload|value, sha }
GET  {BASE_URL}/oplog
     store_token, path, type
GET  {BASE_URL}/oplog/stream
     store_token, [reg_id]

Storage:
- user's meta:   $TEABAG_DATA/:salt/:user/user.json
- user's data:   $TEABAG_DATA/:salt/:user/root/:type/...[path]...

TODO: BLOB Storage

TeaBag Client

var cli = require('teabag').teabag({
  session_token: '...',
  url: '...'
});

cli.init(cb_());
cli.register(type, reduce_fun);

cli.get(channel, type, path, cb_(err, value));
cli.push(channel, type, path, op, cb_(err, value));

cli.on(channel, type, path, cb_(type, value, [op]));