Conflict Resolution
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Conflicts arise when two or more users make changes to the same piece of data independently. This can happen due to latency between device and server or loss of connectivity. In this event, Atlas Device Sync automatically uses conflict resolution strategies to merge the changes. Specifically, Device Sync handles conflict resolution using operational transformation, a set of rules that guarantee strong eventual consistency, meaning all clients' versions will eventually converge to identical states. This will be true even if changes were made in a different order.
You must be aware of the rules to ensure consistent results, but the upside is that by following those rules, you can have devices work entirely offline and still converge on meaningful results when they meet.
This page uses the example of a dog walking app to illustrate how Device Sync resolves conflicts. In this app, Matt and Sarah are both dog walkers who use the app to track their clients' dogs and their walking schedules.
Rules of Conflict Resolution
At a very high level, the rules are as follows:
- Deletes always win.
- If one side deletes an object it will always stay deleted, even if the other side has made changes to it later on.
- Last update wins.
- If two sides update the same property, Device Sync will keep the value from the most recent update.
- Inserts in lists are ordered by time.
- If two items are inserted at the same position, the item that was inserted first will end up before the other item. This means that if both sides append items to the end of a list, will include both items in order of insertion time.
- Primary keys designate object identity.
- If two sides both create objects of the same class with identical primary keys, they will be treated as instances of the same object.
Example
Conflict Resolution Between Two Users
Matt and Sarah are working on data for their dog walking business. Matt deletes data on one of their client's dogs, Doug, as they no longer need to walk him. While Sarah is out without internet connection, she edits Doug's required walk time data on her local, offline version, as she does not know about Matt's deletion of Doug's data.
Once Sarah regains internet connection, her change will be sent to the server. The server will send her Matt's deletion operation. As deletes always win according to Device Sync's conflict resolution rules, Matt's deletion is kept rather than Sarah's edit. The server will not send Sarah's edits to Matt's device. The data is again in agreement across Matt and Sarah's devices.
Special Considerations
Counters
Using integers for counting is a special case. The way that most
programming languages would implement an increment operation (like v
+= 1) is to read the value, increment the result, and then store it
back. This will obviously not work if you have multiple parties doing
incrementing simultaneously (they may both read 10, increment it to 11,
and when it merged you would get a result of 11 rather than the intended
12).
To support this common case, we offer a way to express whether you are incrementing (or decrementing) the value, giving enough hints so the merge can reach the correct result. You have the choice to update the entire value or edit it in a way that conveys more meaning, allowing you to get more precise control of the conflict resolution.
Nested Collections
Nested collections are treated similarly to embedded objects. That is, they are considered child objects with a specific parent object. Any updates to the parent object will always win in a conflict resolution, even if it overwrites the child object.
Updates to a nested collection are resolved as follows:
If multiple devices update the same existing nested collection, Device Sync uses normal conflict resolution rules to incorporate both changes.
If multiple devices create new unique collections nested in the same parent object, the "last update wins" rule applies, and Device Sync overwrites all other updates with the last update made.
For example, client details in Sarah and Matt's dog walking app are modeled as
arrays nested in the parent client property. Sarah - and then later, Matt -
each create a new details collection for the same client on their
devices while offline. When the devices sync, Device Sync keeps the entry created
by Matt because his update (creating a new nested details collection) was the last
one made.
Later, Sarah and Matt each make unique updates to the newly created details entry. Device Sync will merge these updates according to normal conflict resolution rules, inserting the new or modified items to the list in chronological order.
Strings
Device Sync interprets the value of a string as a whole and does not merge conflicts on a per-character basis. For example, this means that if a character or substring is inserted or deleted within a string, Device Sync will treat this as a replacement of the entire value of the string.
Dictionaries
Device Sync considers the removal of a dictionary key to be an update, rather than a deletion. As a result, the "Last update wins" rule is applied instead of the "Deletes always win" rule.
For example, the dog walking app contains an optional collection of details for
each dog, which are entered as key-values. While both users are offline, Sarah
deletes the entire favorite toy entry for a dog, and later Matt updates the
same dog's favorite toy to tennis ball. When both come back online, Device
Sync considers both as updates to the favorite toy entry and applies Matt's
updates because he made them after Sarah's deletion.
Custom Conflict Resolution
Generally speaking, the conflict resolution of Device Sync should work for most purposes, and you should not need to customize it. That said, the typical way to do custom conflict resolution is to change a property type from string to list. Each side can then add its updates to the list and apply any conflict resolution rules it wants directly in the data model. You can use this technique to implement max, min, first write wins, last write wins, or any other kind of resolution you can think of.
Summary
Device Sync implements a conflict resolution system to allow multiple offline writers to write simultaneously and still eventually converge on the same result.
- The conflict resolution system follows four rules:
Deletes always win.
The last update wins.
Inserts in lists are ordered by time.
Primary keys designate object identity.
Counters, nested collections, and strings are special cases to be aware of in your client code.