This page is based on 'Conflict detection and resolution - research'.

Introduction

Conflict scenarios:

• Update-update
  Occurs when the same object was updated at more than one node.
• Update-delete
  Occurs if a row was updated at one node, but the same row was deleted at another node.
• Delete-delete
  Occurs when a row was deleted from more than one node.

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Conflict Detection

Conflict detection is based on hash codes. Every object has its own Sync hash code which can be generated basing on that object. During pull or push operation a child instance compares hash codes of a local object, parent's object and recently saved parent's object. The hash code of recently saved parent's object is persisted in the database. We have to establish some shortcuts before we explain when the conflict is detected:

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The conflict is detected when (#lastP != NULL && #lastP != #P ) && (#C #lastP != #P)#C.

Note! We have added a new row in a database sync_parent_object_hashcode table for each object pulled  from its Parent instance. We keep only the most current hash code of each object.

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Conflict Resolution

PULL:

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• these two objects have to be merged**, then
• saved on Child and Parent instance;
• hash codes on both Child and Parent instance have to be updated.

* REMEMBER: This is a hash code of an object that was calculated and saved on the last pull of this object from Parent. This hash code is stored in a separate table than pulled object.

** See Merge

PUSH:

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• these two objects have to be merged**, then
• saved on Child and Parent instance;
• hash codes on both Child and Parent instance have to be updated.

* REMEMBER: This is a hash code of an object that was calculated and saved on the last pull of this object from Parent. This hash code is stored in a separate table than pulled object.

** See Merge

Merge

Once a conflict was detected two objects will have to be merged. To do this, a new database table "Conflicts" will have to be created.
This table will have columns: openmrs_class, first_object, second_object and more if needed.

It will be possible to insert any type of object into child_object and parent_object due to its type: it will be either JSON, (TINY)BLOB, TEXT or else.

All conflicted objects will be put into this table and queued just like AuditLog does this.
However, there should be only the most recent merge conflict for any object. The old ones should be voided.

Then either chosen strategy or UI will be used to:

• determine if child_object, parent_object or mixed object will be preserved,
• determine what operation should be taken after merging (push / pull / ...)

When the conflict is resolved we should save parent's object as voided in child's database.

UI for merging will use openmrs_class field to determine what class are child_object and parent_object.
Fields of determined class will be displayed as text-boxes or buttons for each of two objects.
User will be able to choose which value he wants to keep.

Please read Merging extension points if you want to implement your own merge behaviourConflict resolution can be handled automatically or manually. After the conflict is detected it is passed to object/service which implements MergeBehaviour interface. The MergeBehaviour class defines a strategy of dealing with conflicts automatically. (In fact, you can add your own implementation. All hints can be found here.) All unresolved conflicts will be saved in order to be solved by the users. 

If the same object causes another conflict, then an old one will be replaced with the new one. So there is only one, the most recent, conflict regarding any object.

The used MergeBehaviour is determined by global property "sync2.mergeBehavior". For instance when it's set to "sync2.newIsTheBestMergeBehaviour" then the NewIsTheBestMergeBehaviour will be used.

With Sync 2 version 1.4.0 was provided two example MergeBehaviour:

1) RestrictConflictMergeBehaviourImpl (link) - "sync2.restrictConflictMergeBehaviour"

This is the default strategy which using hash codes check if the both objects were changed then create unresolved conflict in order to be solved by the users. If only one object changed then it will be choose as a newest version.

2) NewIsTheBestMergeBehaviourImpl (link) - "sync2.newIsTheBestMergeBehaviour"

In this strategy used the date of changed to determine the newest version. If the object hasn't the date of change then the strategy will chose the source object.