AppSync in real world - pitfalls, unexpected benefits & lessons learnt

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BENGALURU

AppSync in the Real World
ADITHYA REDDY | SDE @ BRANCH FINANCIAL

What is AppSync
• “Build data-driven apps with real-time and offline capabilities”
• Fully managed, serverless GraphQL service
• Client SDKs + Offline programming model

Clients
iOS
Android
JS
Managed
Datastores
RDS
DynamoDB
Neptune
Middle-Tier
K8s
Docker
VMs
Storage
Streams
Search
Logging
Auth
ETL
Analytics
Validation
Logic
You operate You don’t operate

Clients
iOS
Android
JS
Managed
Datastores
RDS
DynamoDB
Neptune
Middle-Tier
K8s
Docker
VMs
S3
Kinesis
ES Service
CloudWatch
Cognito
EMR
Athena
Validation
Logic
You operate You don’t operate

Hub for all your managed services
• Provide a single unified API layer over all your data sources.
• Fetch, shape, transform, and aggregate data into exactly
what the client asked for.
• Replace the middle layer of (most) apps.

• Validating requests and payloads
• Authentication
• Fine-grained permissions
• Access control
What AppSync can do
• Fetching/writing data to/from data sources
• Real-time updates
• Offline with optimistic UI updates
• Resolving data conflicts

GraphQL
101

A strongly typed, declarative
query language for APIs.
graphql.org

Describe your data with a schema
type Tweet {
id: ID
body: String
date: Date
Author: User
}
type User {
id: ID
username: String
tweets: [Tweet]
}

Queries and Mutations
type Query {
getTweet(id: ID): Tweet
allTweets: [Tweet]
}
type Mutation {
newTweet(body: String): Tweet
newUser(username: String): User
}

Ask for exactly the data you need
query {
getUser(id: "123") {
username
tweets {
body
date
}
}
}
{
"username": "...",
"tweets": [
{
"body": "...",
"date": "..."
},
{
"body": "...",
"date": "..."
},
]
}

Subscriptions
type Subscription {
userTweeted(id: ID): Tweet
@aws_subscribe(mutations: ["newTweet"])
}
Subscription {
userTweeted(id: "jdoe123") {
body
}
}

• DynamoDB
• Elasticsearch
• Lambda
• HTTP
• Everything else (via Lambda)
Data Sources

Resolvers
• Translate GraphQL requests to / from formats your data sources understand.
• Request and response mapping templates.
• Apache VTL (Velocity Template Language).

Request Mapping
Template
Response Mapping
Template

{
"version": "2017-02-28",
"operation": "PutItem",
"key": {
"requestId": { "S": $util.autoId() }
},
"attributeValues": {
"timestamp": { "S": $util.time.nowISO8601() },
"requesterName": { "S": $context.args.name },
"requesterPhone": { "S": $context.args.phone }
}
}
mutation {
requestCall(
name: String!,
phone: String!
): Boolean
}
DynamoDB PutItem

#set($moistures = [])
#foreach($reading in ${context.args.moisture})
$util.qr($moistures.add($util.dynamodb.toMapValues($reading)))
#end
#set($pHs = [])
#foreach($reading in ${context.args.pH})
$util.qr($pHs.add($util.dynamodb.toMapValues($reading)))
#end
{
"version": "2018-05-29",
"operation": "BatchPutItem",
"tables": {
"moistureReadings": $util.toJson($moistures),
"pHReadings": $util.toJson($pHs)
}
}
input Moisture {
timestamp: String
value: Float
}
input pH {
timestamp: String
value: Float
}
mutation {
record(
moisture: [Moisture],
pH: [pH]
)
}
DynamoDB Multi-Table Batch PutItem

Field Resolvers
type Post {
id: ID!
text: String
comments: [Comment]
}
type Comment {
postId: ID!
commentId: String!
text: String
}
Posts
id text
Comments
postId commentId text

Field Resolvers
type Post {
id: ID!
text: String
comments: [Comment]
}
type Comment {
postId: ID!
commentId: String!
text: String
}
{
"version": "2017-02-28",
"operation": "Query",
"query": {
"expression": "postId = :postId",
"expresssionValues": {
":postId": { "S": "$context.source.id" }
}
}
}

More things you can do
• Pagination & Scans (limit, nextToken)
• Multi-table Batch Get / Update / Delete
• Conditional Get / Put / Update / Delete (via Conditional Expressions)

• API Key
• AWS IAM (access key + secret key / Cognito Federated Identities)
• Cognito User Pools
Security

context.identity
{
"sub": "...uuid...",
"username": "...",
"claims": { ...x },
"cognitoIdentityId": "...",
"cognitoIdentityPoolId": "..."
}

type Mutation {
addPost(id: ID!, title: String!): Post!
@aws_auth(cognito_groups: [“Bloggers", "Admins"])
}
Using Cognito Groups

"condition": {
"expression": “contains(#author, :username)",
"expressionNames": {
"#author": "Author"
},
"expressionValues": {
“:username": { "S": "${context.identity.username}" }
}
}
Proceed only if author field matches username

• JavaScript (+ React, Angular, Vue.js)
• React Native
• iOS (Objective-C / Swift)
• Android (Java / Kotlin)
AppSync Client SDKs

.NET Clojure
GoElixir PHP Scala
RubyPython
Standards Compliant GraphQL Clients
Unity
Perl
Extras (offline support / optimistic updates) library dependent

AWS Amplify Apollo
• Auth
• Storage
• Analytics
• Notifications
• Declarative data fetching
• Normalized write-through cache
• Optimistic UI
• Deferred queries

Online
UI Component

Offline
UI Component

Back Online
UI Component

React Example
GET_POSTS = gql`
{
posts {
title
publishDate
previewText
}
}
`;
<Query query={GET_POSTS}>
{({ loading, data }) => {
if (loading) return <LoadingSpinner />;
return <PostsList data={data.posts} />;
}}
</Query>

Optimistic UI
UI Component
message {
timestamp: "2018-09-30T23:43:05+05:30"
text: "Hello",
sent: False
}

Optimistic UI
UI Component
message {
timestamp: "2018-09-30T23:43:05+05:30"
text: "Hello",
sent: True
}

Branch’s Architecture
Other
services
Customer
Apps
Staff
Staff
AppSync
Customer
AppSync
Customer Auth Staff Auth

What we love 😍
• Much much much less code!
• No VMs, no containers to orchestrate, no uptime / reliability issues.
• Tightly integrated dev/ops workflow.
• Focus on what sets your business apart, instead of re-inventing the wheel.

What isn’t so great 😕
• No first-class RDS support (have to go through Lambda).
• HTTP resolvers only work for public APIs — have to go through Lambda for endpoints
inside VPCs.
• Resolver mapping templates are hard to debug.
• Need a Lambda function to index DynamoDB into Elasticsearch (super common).
• Terraform support lagging behind.
• Subscription updates are only triggered by mutations — not by external sources.

Adithya Reddy
@TheTallpants
github.com/tallpants
TheTallpants@gmail.com

AppSync in real world - pitfalls, unexpected benefits & lessons learnt

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