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Dashbird

Graph DB in Serverless Flavor: Hacking DynamoDB

byrro profile image Renato Byrro ・7 min read

In this article we'll cover:

  • 3️⃣ 3 steps to grasp DynamoDB working as a Graph database
  • 📺 Work with examples and illustrative data
  • 💪 Increasing visibility of the serverless stack

DynamoDB is the NoSQL, managed database by AWS. It is designed more as a key-value store. But it can be hacked to work as a graph database.

Which is pretty cool 😉

How so?

There is a data access pattern called Adjacency List. It's a way to describe nodes and edges. We can use it in DynamoDB to leverage graph relationships.

Building the concept in three steps:

Step 1: Basic Keys

Each entry in DynamoDB has two basic keys: primary and sort.

The first (primary-key) allows efficient access to the entries in the DB. Think of it as the entries' IDs.

The second (sort-key) allows... well, for sorting results, obviously.

As an example, we could model e-commerce purchases as:

  • primary-key: order-ID
  • sort-key: timestamp
Primary-Key Sort-Key Products
order-1 1570147200 ['Tesla Roadster', 'Sunglasses']
order-2 1569976212 ['Chocolate', 'Gummies']
. . . . . . . . .

This allows us to query by Order ID, as well as sort orders by their timestamp.

Step 2: Many-to-Many Relationships

One could say a graph (node-edge model) is a form of a many-to-many relationship. As such, it can be represented by an adjacency list. The primary-key represents the top-level item, while the sort-key is used for associations. Consider:

  • A blog has two posts: "Post-1" and "Post-2"
  • There are three possible tags: "cool", "awesome", "neat"
Primary Key Sort Key Title Text
Post-1 Post-1 Hello World! I'm a Post inside Dynamo!
Post-2 Post-2 Foo Bar Can't wait for the graph
Tag-1 Tag-1 cool null
Tag-1 Tag-1 awesome null
Tag-1 Tag-1 neat null

Access Patterns:

  1. Retrieve Post-1:
    primary-key == 'Post-1' && sort-key == 'Post-1'

  2. Retrieve Tag-2:
    primary-key == 'Tag-2' && sort-key == 'Tag-2'

Now, say Post-1 has one tag: cool.

Primary Key Sort Key Primary Title Sort Title
Post-1 Tag-1 Hello World! cool
Tag-1 Post-1 cool Hello World!

The connection is represented twice, which is important from an access pattern standpoint, as you can see below:

Access patterns:

  1. Get tags assigned to Post-1:
    primary-key == 'Post-1' & sort-key BEGINS_WITH 'Tag'

  2. Get posts tagged with Tag-1:
    primary-key == 'Tag-1' & sort-key BEGINS_WITH 'Post'

The BEGINS_WITH operator allows matching multiple Tags associated with a Post, or vice versa.

By having two entries in the DB for each connection, we can reach the other node regardless of our starting point. If we have a tag, we can find all posts associated. If we have a post, we can find all tags assigned.

Step 3: The Graph

We'll take the tripestore model for this example. It consists of subject-predicate-object, such as:

Subject Predicate Object
John Likes Music
Ross Friends With Chandler

Now, how do we model this data structure in DynamoDB? We leverage the adjacency pattern!

Say we have a Friendly social media site. Here's how we could model it in DynamoDB.

3.1 Nodes

We start by describing the nodes. In our example, we only have users. These will be the subjects and objects of the triplestore model. A name property is also added to each user:

Primary Key Sort Key Name
User-1 User-1 Ross
User-2 User-2 Rachel
User-3 User-3 Monica

3.2 Predicates

Primary Key Sort Key
Pred-Sibling Pred-Sibling
Pred-Friend Pred-Friend
Pred-Married Pred-Married

In the case of predicates, there are no properties associated.

3.3 Connections

Finally, we model the relationships between nodes using predicates to connect them. We also add a property indicating when the relationship started:

Primary Key Sort Key Relationship Start
User-1 Pred-Sibling-User-3 1994-09-24
User-3 Pred-Sibling-User-1 1994-09-24
User-2 Pred-Friend-User-3 1994-09-24
User-3 Pred-Friend-User-2 1994-09-24
User-1 Pred-Married-User-2 1999-05-20
User-2 Pred-Married-User-1 1999-05-20

Again, we have two entries for each connection, also for access patterns reasons, as explained above in the post <> tag example.

Here's how we read the node connections:

  • Ross and Monica are siblings
  • Monica and Rachel are friends
  • Rachel and Ross are married

Access patterns to retrieve our data:

  1. Who is married to Ross?
    primary-key == 'User-1' & sort-key BEGINS_WITH 'Pred-Married-User'

  2. Who are Monica's siblings?
    primary-key == 'User-3' & sort-key BEGINS_WITH 'Pred-Sibling-User'

  3. Who are Rachels's friends?
    primary-key == 'User-2' & sort-key BEGINS_WITH 'Pred-Friend-User'

Going deeper

The example above is very simplified, but it illustrates the idea.

It's possible to add other types of nodes to the graph. For instance:

Primary Key Sort Key Title
Location-1 Location-1 Cafeteria

One advantage is that links can have their own properties as well. For instance: in the Ross-Married-Rachel link, we could have properties such as where the couple met:

Primary Key Sort Key Meeting Place
User-1 Pred-Married-User-2 Location-1

Or we could model the "Meeting Place" as a node in the graph:

Primary Key Sort Key
Pred-MeetingPlace Pred-MeetingPlace
User-1 Pred-MeetingPlace-Location-1-User-2
User-1 Pred-MeetingPlace-User-2-Location-1

Access patterns:

  1. Get everyone Ross met at the Cafeteria:
    primary-key == 'User-1' & sort-key BEGINS_WITH 'Pred-MeetingPlace-Location-1'

  2. Where did Ross meet Rachel?
    primary-key == 'User-1' & sort-key BEGINS_WITH 'Pred-MeetingPlace-User-2'

This is actually kind of a hack of the triplestore model. The predicate Pred-MeetingPlace-Location-1 is actually a combination of a real predicate Pred-MeetingPlace and a node Location-1. This allows for flexible queries. Think of it like the node attached modifying, or characterizing the predicate.

The full table combined is available down at the end of the article.

Heads up

It is important to think about your access patterns before modeling your data. It may be difficult to add support for different access patterns down the road.

Check the DynamoDB documentation to learn more.

Why to Graph On DynamoDB?

DynamoDB is easy to get started with and keeps infrastructure hurdles to a minimum. Especially for startups and small teams that can't afford a DevOps team to maintain healthy DB servers/instances, it can be a great ally.

In summary, Dynamo will give us:

  • Fully managed & serverless: minimal infrastructure overhead
  • Hyper scalability: in both IO and storage, Dynamo can scale to tens of thousands of concurrent requests and exabytes of data
  • Integration with Lambda
  • Microsecond latency
  • Built-in global replication

Will I lose control of my DB by using serverless?

That's a common concern about serverless.

We really need to ask ourselves: what and why do I want to control?

Usually, it boils down to monitoring, making sure everything is running smoothly. Serverless indeed requires a different approach to observability.

There are professional services, that can take care of that. Dashbird, especially, was created from the ground up with the goal to provide full visibility over entire serverless stacks, so you might want to check them out.


Full Dynamo Tables

Post + Tag Example

Primary Key Sort Key Primary Title Sort Title Text
Post-1 Post-1 Hello World null I'm a Post in Dynamo!
Post-2 Post-2 Foo Bar null Graph is cool!
Tag-1 Tag-1 cool null null
Tag-1 Tag-1 awesome null null
Tag-1 Tag-1 neat null null
Post-1 Tag-1 Hello World cool null
Tag-1 Post-1 cool Hello World null

Friends Social Media

Primary Key Sort Key Title Rel. Start
User-1 User-1 Ross null
User-2 User-2 Rachel null
User-3 User-3 Monica null
Location-1 Location-1 Cafeteria null
Pred-Sibling Pred-Sibling null null
Pred-Friend Pred-Friend null null
Pred-Married Pred-Married null null
Pred-MeetingPlace Pred-MeetingPlace null null
User-1 Pred-Sibling-User-3 null 1994-09-24
User-3 Pred-Sibling-User-1 null 1994-09-24
User-2 Pred-Friend-User-3 null 1994-09-24
User-3 Pred-Friend-User-2 null 1994-09-24
User-1 Pred-Married-User-2 null 1999-05-20
User-2 Pred-Married-User-1 null 1999-05-20
User-1 Pred-MeetingPlace-Location-1-User-2 null null
User-1 Pred-MeetingPlace-User-2-Location-1 null null
User-2 Pred-MeetingPlace-Location-1-User-1 null null
User-2 Pred-MeetingPlace-User-1-Location-1 null null

Photo Credits:

Full Disclosure

I work as a Developer Advocate at Dashbird.


Additional resources:


Posted on by:

byrro profile

Renato Byrro

@byrro

The Quick Hello World Jumped Over The Lazy FooBar

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Discussion

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Is there really a good reason for all the duplication? It just looks like a manual index to me.

Why not just a GSI with the sort key as primary key reversed, and, depending on your needs, a few projected attributes, like the names? This also looks like the approach Amazon are using in their documentation for sort-key design for adjacency lists.

So, for getting all tags for Post-1, query the main table for "PK = Post-1 AND begins_with(SK,Tag-)".

For getting all posts for Tag-1, query the GSI for "SK = Tag-1 AND begins_with(PK, Post-)". If you want the name in the same query, and you're not tagging anything but posts (or mostly want tags for everything), you can even avoid duplicating the title column by not using begins_with (okay, probably not, you'll use them in the Post->Tag queries).

Wouldn't that both be faster and less error-prone?

 

Hi Dennis, that's a good question!

GSI is a viable solution, yes. It would simplify the implementation, but also comes with its shortcomings.

GSIs can't offer consistency. There will always be a delay between changes to the table and reflections in the secondary index. For some use cases, this is unbearable. With the approach I suggested, it's possible to wrap many item operations into one transaction, what gives you full control and consistency.

Another issue with GSI is that the developer now has two sources for the same dataset and needs to discover on which to read data from.

Consider the Friendly example. If you want to find all Ross' siblings, do you need to query the table, the GSI or both? If your data structure is really really simple, this might not be a problem. Otherwise, it will complicate your life when implementing new features that need to read data. And can easily end up in a mess. You could start missing data that exists by querying the wrong source, and showing incomplete information to your users. And you may never find out...

In order to minimize errors when writing data using the approach I outlined, you could have a single internal service for entering nodes and edges in the DB. You take care of this single entry point and make sure it's handing all relationships correctly. Then all other parts of your application will only invoke this single service to write in the DB.

Does that make sense?

 

Hmmm, not sure I'd use DDB at all if eventual consistency was a problem, as I he transaction operations throws a lot of performance away (and are somewhat limited), but fair enough.

As we're getting to the connections between equals, like the siblings example, you'd need somewhat arbitrary rules (e.g. eldest has the relation), or do the copying anyway, and then we'd both have the extra redundancy AND the GSI to worry about, so the triplestore model makes a lot of sense there.

I'm not sure that I'm convinced for data with a clearer parent-child relationship, I'll have to let it simmer for a while. :-)

By the way, you're storing the predicates in the DB, but doesn't really use them for anything. I guess that's just for discovery (e.g. when using it as storage for clients that doesn't necessarily know what relation types that exist), but are there other uses for those in a less generalised database? They seem a little bit useless to me unless you add an attribute to make a sparse index from (otherwise you'd have to do a full table scan to find them), but I've probably missed something.

Thanks for the clarification!

 

Great examples, thanks!

 

Glad it was helpful! I struggled with examples in the AWS official docs, wanted to share something easier to grasp.

 

Yes, you have to read them a few times before it actually sets in. I especially like the graph example, don't think I have seen that before.

 

This is a great concept but the implementation within the article is frustratingly non-intuitive. It’s amazing we’re in the year 2020 (13 years since the dynamo paper was written) and there’s only 1 Serverless Graph Database, Microsoft Azure CosmosDB Autopilot ... which sucks because we’re using AWS. There are so many graph databases out there yet only one serverless option

 

Hi, are you familiar with Cloud Directory? It's not precisely a graph, but follows essential concepts of a graph database. And fully serverless! 😉