Offline ServiceWorker

Provide offline syncing functionality via a service worker.

This unit works by reading package declarations from the config, syncing those when internet is available and providing its own micro-ReST server when not.

It works by using the new Service Worker browser technology which sits between the requesting browser and the server which can mediate requests.

Syncing

The syncing process is made up of the following steps. FE is the front end, BE the backend and SW the service worker

1. [FE] Send a message to the Service Worker to sync
2. [SW] Setup a cache
3. [SW] Fetch resources from the server
4. [BE] Compute a list of resources - this is the flattened version of app.config.offline.packages[] specific to the active user with all dependencies resolved
5. [SW] Compose a list of all resources to check
6. [SW] Request each resource as a list, if the resource is a collection try to optimize this by fetching the collection IDs + version only then compare, if the resource is a URL just pull it

Config

As Service Workers are self contained JS files injecting the offline configuration is somewhat complex.

The easiest method is simply to use the supplied gulp build process while specifying a source file from where the config can be retrieved.

OFFLINE_CONFIG_FILE=./my-config.js
gulp build

In the above ./my-config.js is expected to export a JavaScript object which will be stringified and used as the offline config.

Config options:

Property	Type	Description
enabled	boolean	Whether the service worker is enabled
cleanInterval	number	Time in milliseconds between cleans
debug	boolean	Whether the service worker should output debugging messages
debugFailures	boolean	Whether all failures should be cached in caches/failures
debugPrefix	string	The prefix to use when debugging
fetchOptions	object	Additional options to pass into fetch()
fetchAttempts	number	The number of retry attempts before failing sync on network errors
fetchConcurrent	number	Number of concurrent requests allowed by Promise.allLimit()
forceOffline	boolean	If true, treat all incoming connections as offline (inherited from caches/meta/flags/forceOffline{enabled: true} if its present)
autoSyncFirst	number	Time in milliseconds until first sync (falsy to disable)
autoSyncPollingOnline	number	Time in milliseconds between polls after the first sync when we are online
autoSyncPollingOffline	number	Time in milliseconds between polls after the first sync when we are offline
ignore	array <string>	List of RegExps to skip caching for (these have to be strings to survive JSON encoding)
maxPostCycles	number	Maximum number of recursive post attempts before giving up, set this to the ideal depth of A creates B creates C style loops that are possible
packages	array <object>	Package specification, see below

The base config file is used to set the above to appropriate defaults. The object resolved by the file at OFFLINE_CONFIG_FILE override those defaults.

Config Package declaration

The offline.packages object is composed as a collection with the following properties:

Property	Type	Description
id	string	The unique ID to identify the package
title	string	The human friendly string to refer to the package
description	string	A longer description of the package
payload	array	A list of resources that should be pulled within this package

Payload us an array of the following objects:

Property	Type	Default	Description
type	string		How the resource is mediated. Enum of: "url", "collection"
url	string		The URL to pull the resource from, use {{id}} as a specifier when iterating over a collection
version	boolean	Calculated	Look for versioning information when pulling the resource, set to falsy to force 304 validation instead of using __v values. This is assumed true for type == 'collection' and false otherwise
create	boolean	false	(If type==collection) specifies that posting to the URL will be handled as a document creation
count	boolean	true	(If type==collection) specifies that the count of documents matching should be returned
query	boolean	true	(If type==collection) specifies that ReST / Mongo / Sift style queries should provide a list of matching documents
post	boolean	false	(If type==collection) specifies that posting to the URL will be handled as a document update