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 As a result of being offline, data that is modified by a client can become outdated with the server.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.
Install the conflict package inside your Node.js project.
- 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
- Add version field to GraphQL type that should support conflict resolution
- Add example mutations
- Implement resolver for mutation
Every conflict can be handled using a set of predefined steps
Resolvers can be implemented to handle conflicts on client .
Implementing Custom Conflict Resolution
ObjectState 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: