Version: 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 As a result of being offline, data that is modified by a client can become outdated with the server.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

  1. Add conflict package dependency to project
const { conflictHandler } = require('offix-conflicts-server');
  1. Add version field to GraphQL type that should support conflict resolution
type Greeting {
msg: String
version: Int
}
  1. Add example mutations
type Mutation {
changeGreeting(msg: String!, version: Int!): Greeting
}
  1. 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": {
//..
}
},
}
}
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