Version: 0.15.x

Soft Deletes with delta queries

This is the simplest strategy for Data Synchronization that is also quite straightforward to set up.

Setup#

Start off with the official Graphback template for MongoDB here to follow along. Adding this strategy is relatively simple, consisting of the following two steps:

Annotate the required models#

Add the @datasync annotation to your model(s) in your GraphQL SDL found in the model folder:

"""
@model
@datasync
"""
type Comment {
id: ID!
text: String
description: String
}

The @datasync annotation ensures consistency of your data and gives you delta queries.

note

The @datasync annotation is a shorthand for two annotations @versioned and @delta. Refer to the last section for details.

@datasync transforms your model by adding two fields: updatedAt and createdAt

"""
@model
@datasync
"""
type Comment {
id: ID!
text: String
description: String
createdAt: String
updatedAt: String
}

The @datasync annotation adds a sync query or a delta query:

type Query {
syncComments(lastSync: String!, filter: CommentFilter): CommentDeltaList!
}

This allows you to get all the changed documents (updated and deleted) in a collection since the lastSync timestamp. Internally this uses the updatedAt database field to check if any documents in the database have been modified, by comparing client provided lastSync timestamp value.

note

The sync query also accepts a filter argument for filtering

The @datasync annotation also adds a Delta type and a DeltaList type:

type CommentDelta {
id: ID!
text: String
description: String
createdAt: String
updatedAt: String
_deleted: Boolean
}
type CommentDeltaList {
items: [CommentDelta]!
lastSync: String
}

The Delta type for a model consists of all of the model's transformed properties, as well as a _deleted field which is used internally to mark documents as deleted in the database. Thus delete mutations only mark documents with _deleted: true instead of actually removing them.

The DeltaList is a container for Delta type, which also returns a lastSync timestamp, that can be used in a subsequent sync query.

note

Soft Deletes strategy can only get you the latest version of changed documents ignoring any in-between states that may have transpired between lastSync and now.

Modify the template to support Data Synchronization#

In the src/index.ts file of the template, use DataSyncPlugin and compliant data sources in buildGraphbackAPI:

import { createDataSyncMongoDbProvider, createDataSyncCRUDService, DataSyncPlugin } from '@graphback/datasync'
const { typeDefs, resolvers, contextCreator } = buildGraphbackAPI(modelDefs, {
dataProviderCreator: createDataSyncMongoDbProvider(db),
serviceCreator: createDataSyncCRUDService({ pubSub: new PubSub() }),
plugins: [
new DataSyncPlugin()
]
});

The data sources provided by @graphback/datasync ensure that the documents are only marked with _deleted: true instead of being removed from the database, meaning that a compatible client can issue a sync query to obtain information about this deletion.

Issuing Delta/Sync queries from client#

As an example consider the usecase when your application has stayed offline for a while. You can then use the sync<Type>s query to retrieve only the changed documents rather than having to re-fetch all of the documents.

query {
syncComments(lastSync: "1590679886048") {
id
text
description
createdAt
updatedAt
_deleted
}
}

As an example response you might get:

{
"data": {
"syncComments": {
"items": [
{
"id": "5ee0a1da7f1f39313744185a",
"text": "First!",
"description": null,
"createdAt": "1591779802551",
"updatedAt": "1591852693075",
"_deleted": true
},
{
"id": "5ee0a67345beff3862220be4",
"text": "Second!",
"description": null,
"createdAt": "1591780979988",
"updatedAt": "1591780979988",
"_deleted": false
}
],
"lastSync": "1591852700920"
}
}
}

The response is a list of the latest versions of the changed documents along with a _deleted flag that is set to true if the document has been deleted since lastSync and false otherwise. The response also contains a lastSync timestamp which can be used in the next syncX query.

In the example response, we get that the "First!" comment has been deleted, while a new comment "Second!" has been created.

note

There is no support for querying relationships through a delta query, all relationship fields are removed when constructing a delta Type.

Taking an example model definition with a oneToMany relationship from the Note type to the Comment type:

""" @model """
type Note {
id: ID!
title: String!
"""
@oneToMany(field: 'note')
"""
comments: [Comment]!
}
"""
@model
@datasync
"""
type Comment {
id: ID!
text: String
}

Since the Comment type has a @datasync annotation, Graphback will construct a CommentDelta type as follows:

type CommentDelta {
id: ID!
text: String
description: String
createdAt: String
updatedAt: String
_deleted: Boolean
}

Notice that there are is no relationship field in the CommentDelta type.

Advanced topics#

@delta annotation#

The @datasync annotation provided by @graphback/datasync is a shorthand for two separate annotations: @delta and @versioned. Vanilla Graphback supports @versioned out of the box, see this page.

The @delta annotation is only provided by the @graphback/datasync package. However, we do not support using this annotation on it's own without @versioned.

Example#

Replacing @datasync in any the above examples with @versioned and @delta yields the exact same results, for example:

"""
@model
@versioned
@delta
"""
type Comment {
id: ID!
text: String
description: String
}

This model exactly yields all of the type-definitions outlined above:

"""
@model
@versioned
@delta
"""
type Comment {
id: ID!
text: String
description: String
createdAt: String
updatedAt: String
}

Similarly, this will also yield a Delta type, a DeltaList type and a sync query.

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