Version: offix-0.15.0

Server Side Conflict Resolution

When performing data synchronization between multiple clients it is common for remote devices to become offline for a certain amount of time. As a result of being offline, data that is modified by a client can become outdated with the server. Further operations on that record can cause a conflict (often called a collision).

Offix provides out of the box conflict resolution for any Node.JS platform. For other languages, you can follow the conflict specification chapter to implement a custom detection mechanism for your resolvers.

Usage#

Install the conflict package inside your Node.js project.

npm install offix-conflicts-server

Conflict Resolution#

Prerequisites:#

GraphQL server with resolvers
Database or any other form of data storage that can cause data conflicts

Enabling Conflict Resolution#

To enable conflict resolution developers need to use one of the pluggable conflict implementations in each individual resolver.

Version Based Conflict Resolution#

Add conflict package dependency to project

const { conflictHandler } = require('offix-conflicts-server');

Add version field to GraphQL type that should support conflict resolution

type Greeting {
  msg: String
  version: Int
}

Add example mutations

type Mutation {
  changeGreeting(msg: String!, version: Int!): Greeting
}

Implement resolver for mutation

Every conflict can be handled using a set of predefined steps

// 1. Read data from data source
// 2. Check for conflicts
const conflict = conflictHandler.checkForConflicts(serverData, clientData);
if (conflict) {
  throw conflict;
}
// 5. Save object to data source

Resolvers can be implemented to handle conflicts on client .

Implementing Custom Conflict Resolution#

TheObjectState interface is a complete conflict resolution implementation that provides a set of rules to detect and handle conflicts. The interface will allow developers to handle conflicts on the client. Client-side applications will need to match the server side implementation. Currently, we support the following implementations:

VersionObjectState - allows to operate based on version field in schema
HashObjectState - allows to operate based on object hashes

Conflict Resolution Specification for other languages#

The conflict resolution package can be replicated to any language by implementing conflict detection logic in resolvers.

Conflict detection should return a specific format of error when a conflict has been detected. This error will be processed by offix-client and sent back again as a new mutation. To detect conflicts on the server you can use one of many strategies, for example:

Calculate hashes of the data
Use specialized field like version or lastChangedDate
Have separate storage with history of the changes

Structure of the conflict error#

The server needs to return a specific error when a conflict is detected containing both the server and client states:

 "extensions": {
        "code": "INTERNAL_SERVER_ERROR",
        "exception": {
          "conflictInfo": {
            "serverState": {
                 //..
            },
            "clientState": {
              //..
            }
          },
        }
 }