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A data store that syncs with a server.

Download using npm:

npm install stepmuel/deltalib
cd node_modules/deltasync
node server

Since npm does not update modules from git repos automatically, npm install stepmuel/deltalib has to be called again to fetch new versions. When using Deltalib in your own project, it is recommended to use a particular revision (like github:stepmuel/deltalib#1ecbec2) since updates might still change the public interface from time to time.

Clone with git:

git clone
cd deltalib
npm install
node server


The DeltaSync protocol relies heavily on JSON, hence the terms object, array and value will be used according to their definition in the JSON specification:

  • object: unordered map from string to value
  • array: ordered collection of values
  • value: string, number, object, array, true, false or null
  • string: sequence of zero or more Unicode characters (DeltaSync uses UTF-8)
  • number: integer or float. Special values like INF or NAN are not allowed.

The functions diff and patch work similarly to the UNIX programs with the same name, but instead of files, they work on objects.

d = diff(a, b) generates the delta between the two objects a and b. (A delta might also be called a diff or patch.)

c = patch(a, d) applies the delta to a (or patches a), resulting in object c.

c will be equal to b. Two values are equal, if they represent the same data.

The store is a single object which contains data that is synced between server and client. A store can not contain the value null since null represents a deleted value in diff and patch.

A revision is a particular state of a store with a unique revision id. If two revisions have the same id, their stores are equal.

A key path is an arrays of member names, describing the position of a value within an object hierarchy. [] points to the object itself, ['a', 'b'] points to the member b of member a of the object.


delta.utils includes several helper functions used to implement the delta diff protocol.

const du = require('deltalib').utils;

Imports the utils object into the current scope.

b = du.clone(a)

Creates a copy b of value a. Any patches or other changes to objects or arrays inside b will no longer affect a.


Returns whether value obj is an object (in the JSON sense). Many functions like merge, patch and diff only work on objects.


Returns whether object obj is an empty object {}.

du.merge(data[, obj... ])

Deeply merges obj into data. All members from obj will be inserted into data, replacing existing non-object values. If both values are objects, they will be merged recursively. Values of obj will be cloned if necessary, so changes to data will never affect obj.

If the function is called with more than two parameters, the additional objects will also be merged into data. Later objects will overwrite members set by earlier objects.

Returns data. To clone data before applying a patch, use dataClone = du.patch({}, data, obj).

du.patch(data[, obj... ])

Same as du.merge, but if a member of obj has a value of null, the corresponding member in data will be deleted.

d = du.diff(a, b)

Creates the (minimal) delta from a to b. Result is can be used with du.patch.

v = du.pathGet(data, path, def)

Returns the value at the key path path from object data. If the key path does not exist, it will return def, or undefined if the argument is omitted.

du.pathSet(data, path, value)

Will set the value at key path path to value. If the key path does not exist, new object members will be inserted. Returns data.

pathSet can be used to quickly create new patches: patch = du.pathSet({}, ['a', 'b'], "c").


delta.Observer provides basic key-value observing. It can notify a subscriber when a value at a certain key path changes.

var obs = new delta.Observer()

Creates a new observer.

obs.update(newObj, oldObj)

Has to be called if the object to be observed might have changed. newObj is the object after and oldObj before the suspected change. This call will notify all observers when needed.

obs.on(path, func, thisArg)

Adds an observer to path. Will call, path, newObj, diff) if the value at path changes. newObj will be the object after the observed change, and diff = du.diff(oldObj, newObj). To get the observed value, use value = du.pathGet(path, newObj)., func)

Removes all observers with the given path and callback.


TODO: finish this section

Members / Optioins

sync.url = options.url || null;
sync.backoff = options.backoff || backoff;


emit('retry', delay)
emit('poll', data)
emit('server_response', data)
emit('request_error', status, err, xhr)
emit('server_error', d)
emit('data', data, oldData)


sync.get(path, def)
sync.set(path, value)
sync.rpc(method, params, callback, thisArg, patch)
sync.subscribe(path, func, thisArg)
sync.unsubscribe(path, func)


Simple DeltaSync server. Start with node server.


  • -p <port> Port to listen to (default 8888).
  • -l <path> JSON file to use as initial store.
  • -s <path> Save revisions to (and resume from) this file.

Unless -s is provided, the server will start with a new, empty store. -l replaces the current store at launch (even if -s is provided). The server listens on the given port for HTTP POST requests, and expects an object with the following (optional) members:

  • patch: Patch to be applied to the current store.
  • base: Instead of sending the whole store, send a patch from the base revision.
  • wait: Unless patch is set, will not send empty patches, but delay the request until a non-empty patch becomes available. (data responses will still be sent immediately.)
  • rpc: Remote procedure calls as specified by the JSON-RPC 2.0 specification.

The response will contain the following members:

  • revision: Revision id of current revision.
  • patch or data: Patch from base to current revision, or the whole store (as data) if no patch is available.
  • ans: JSON-RPC response. (only available if rpc was set in request.)

RPC Requests

server supports two RPC methods:

  • echo will return its params.
  • revision will return the current store revision id. If the request changes the revision (e.g. contains a patch), the returned revision id might be before or after those patches have been applied.

Other requests will be answered with the appropriate error object.

Known Limitations

  • Keeps a clone of all past revisions in memory


DeltaSync client for development. Use with node client.


  • -h <host> server host (default localhost)
  • -p <port> server port (default 8888)
  • -j <value> patch value in JSON
  • -k <keypath> key path for the patch, separated by '.'.
  • -b <rev_id> base revision
  • -w set wait = true for first request
  • -f will keep requesting new patches (follow)
  • -q will not show first response (quiet)


node client

Request current revision.

node client -q -f

Show all non-empty patches, ignoring the current revision sent initially. This simulates actual client behavior.

node client -k a.b -j '"c"'

Will set the value at key path ['a', 'b'] to "c" and show the current revision including the new change.

node client -j '{"d": "e"}'

Will send a raw patch.

node client -q -k foo -j 1

Will send a patch without displaying the current revision.


A data store that syncs with a server.







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