Comparing DynamoDB and MongoDB

Quick Comparison Table

DynamoDBMongoDB
Deployment OptionsAs a service on Amazon Web Services
  • MongoDB can be deployed anywhere
  • MongoDB can be deployed as a service with MongoDB Atlas
Data Model
  • Key-value with JSON support
  • Up to 400 kb record size
  • Limited data type support
  • JSON-like document
  • Up to 16 mb record size
QueryingKey-value queriesQuery & analyze data in multiple ways — by single keys, ranges, faceted search, graph traversals, and geospatial queries through to complex aggregations
Indexing
  • Indexes are sized & provisioned separately from data
  • Hash or hash-range indexes only
  • Global secondary indexes (GSIs) are inconsistent with underlying data
  • Local secondary indexes (LSIs) must be defined when a table is created
  • Up to 5 GSIs & LSIs per table
  • Add secondary indexes on any field
  • Indexes are consistent with data
  • Indexing strategies include: compound, unique, array, partial, TTL, geospatial, sparse, hash, and text
Data integrity
  • Data is eventually consistent by default
  • No data validation
  • Data is strongly consistent by default
  • Native document validation included
MonitoringLimited visibility into real-time database behaviorMongoDB Atlas database as a service tracks 100+ metrics that could impact performance
BackupOn-demand and continuous backups are available at different costs. Backups and restores are an additional charge.MongoDB Atlas includes continuous, queryable backups with point-in-time recovery
Throughput LimitsUp to 10k ops / sec. Additional throughput can be requested with a web form.None
PricingThroughput-based pricing. A wide range of inputs may affect price. See Pricing and Commercial Considerations.MongoDB Atlas pricing is based on RAM, I/O, and storage.

What is DynamoDB?

DynamoDB is a proprietary NoSQL database service built by Amazon and offered as part of the Amazon Web Services (AWS) portfolio.

The name comes from Dynamo, a highly available key-value store developed in response to holiday outages on the Amazon e-commerce platform in 2004. Initially, however, few teams within Amazon adopted Dynamo due to its high operational complexity and the trade-offs that needed to be made between performance, reliability, query flexibility, and data consistency.

Around the same time, Amazon found that its developers enjoyed using SimpleDB, its primary NoSQL database service at the time which allowed users to offload database administration work. But SimpleDB, which is no longer being updated by Amazon, had severe limitations when it came to scale; its strict storage limitation of 10 GB and the limited number of operations it could support per second made it only viable for small workloads.

DynamoDB, which was launched as a database service on AWS in 2012, was built to address the limitations of both SimpleDB and Dynamo.

What is MongoDB?

MongoDB is an open source NoSQL database built by MongoDB, Inc. The company was established in 2007 by former executives and engineers from DoubleClick, which Google acquired and now uses as the backbone of its advertising products. The founders originally focused on building a platform as a service using entirely open source components, but when they struggled to find an existing database that could meet their requirements for building a service in the cloud, they began work on their own database system. After realizing the potential of the database software on its own, the team shifted their focus to what is now MongoDB. The company released MongoDB as an open source project in 2009.

MongoDB was designed with developer productivity, continuous availability, geo-distribution and scalability in mind, and includes out-of-the-box replication and auto-sharding.

MongoDB stores data in flexible, JSON-like documents, meaning fields can vary from document to document and data structure can be changed over time. This model maps to the objects in application code, making data easy to work with for developers. Related information is typically stored together for fast query access through the MongoDB query language. MongoDB uses dynamic schemas, allowing users to create records without first defining the structure, such as the fields or the types of their values. Users can change the structure of records (which we call documents) simply by adding new fields or deleting existing ones. This flexible data model makes it easy for developers to represent hierarchical relationships and other more complex structures. Documents in a collection need not have an identical set of fields and denormalization of data is common.

In summer of 2016, MongoDB Atlas, a fully managed service for MongoDB, was announced. MongoDB Atlas allows users to offload operational tasks and features built-in best practices for running the database.

Terminology and Concepts

Many concepts in DynamoDB have close analogs in MongoDB. The table below outlines some of the common concepts across DynamoDB and MongoDB.

DynamoDBMongoDB
TableCollection
ItemDocument
AttributeField
Secondary IndexSecondary Index

Deployment Environments

MongoDB is an open source database that can be run anywhere – from a developer’s laptop to an on-prem data center to within any of the public cloud platforms. As mentioned above, MongoDB is also available as a fully managed service with MongoDB Atlas; this model is most similar to how DynamoDB is delivered.

In contrast, DynamoDB is a proprietary database only available on Amazon Web Services. While a downloadable version of the database is available for prototyping on a local machine, the database can only be run in production in AWS. As such, organizations looking into DynamoDB should consider the implications of building on a data layer inextricably tied to a single cloud vendor.

Comparethemarket.com, the UK’s leading price comparison service, recently completed a transition from on-prem deployments with Microsoft SQL Server to AWS and MongoDB. When asked why they hadn’t selected DynamoDB, a company representative was quoted as saying "DynamoDB was eschewed to help avoid AWS vendor lock-in."

Data Model

MongoDB stores data in a JSON-like format called BSON, which allows the database to support a wide spectrum of data types including dates, 64-bit integers, & Decimal128. MongoDB documents can be up to 16 MB in size; with GridFS, even larger assets can be natively stored within the database.

Unlike some NoSQL databases that push enforcement of data quality controls back into the application code, MongoDB provides built-in document validation. Users can enforce checks on document structure, data types, data ranges and the presence of mandatory fields. As a result, DBAs can apply data governance standards, while developers maintain the benefits of a flexible document model.

DynamoDB is a key-value store with added support for JSON to provide document-like data structures that better match with objects in application code. An item or record cannot exceed 400KB. Compared to MongoDB, DynamoDB also has limited support for different data types. For example, it supports only one numeric type and does not support dates. As a result, developers must preserve data types on the client, which adds application complexity and reduces data re-use across different applications. DynamoDB does not have native data validation capabilities.

Queries and Indexes

MongoDB's query language enables developers to build applications that can query and analyze their data in multiple ways – by single keys, ranges, faceted search, graph traversals, and geospatial queries through to complex aggregations, returning responses in milliseconds. Complex queries are executed natively in the database without having to use additional analytics frameworks or tools. This helps users avoid the latency that comes from syncing data between operational and analytical engines.

MongoDB ensures fast access to data by any field with full support for secondary indexes. Indexes can be applied to any field in a document, down to individual values in arrays.

MongoDB 4.0, scheduled for Summer 2018*, will add support for multi-document transactions, making it the only database to combine the ACID guarantees of traditional relational databases, the speed, flexibility, and power of the document model, with the intelligent distributed systems design to scale-out and place data where you need it.

Multi-document transactions will feel just like the transactions developers are familiar with from relational databases – multi-statement, similar syntax, and easy to add to any application. Through snapshot isolation, transactions provide a globally consistent view of data, enforce all-or-nothing execution, and they will not impact performance for workloads that do not require them. The addition of multi-document transactions makes it even easier for developers to address more use-cases with MongoDB. Sign up for the beta program (http://www.mongodb.com/transactions).

Supported indexing strategies such as compound, unique, array, partial, TTL, geospatial, sparse, hash, and text ensure optimal performance for multiple query patterns, data types, and application requirements. Indexes are strongly consistent with the underlying data.

DynamoDB supports key-value queries. For queries requiring aggregations, graph traversals, or search, data must be copied into additional AWS technologies, such as Elastic MapReduce or Redshift, increasing latency, cost, and complexity. The database supports two types of indexes: Global secondary indexes (GSIs) and local secondary indexes (LSIs). Users can define up to 5 GSIs and 5 LSIs per table. Indexes can be defined as hash or hash-range indexes; more advanced indexing strategies are not supported.

GSIs, which are eventually consistent with the underlying data, do not support ad-hoc queries and usage requires knowledge of data access patterns in advance. LSIs, which can be queried to return strongly consistent data, must be defined when the table is created. They cannot be added to existing tables and they cannot be removed without dropping the table.

DynamoDB indexes are sized and provisioned separately from the underlying tables, which may result in unforeseen issues at runtime. The DynamoDB documentation explains, "Because some or all writes to a DynamoDB table result in writes to related GSIs, it is possible that a GSI’s provisioned throughput can be exhausted. In such a scenario, subsequent writes to the table will be throttled. This can occur even if the table has available write capacity units."

Consistency

MongoDB is strongly consistent by default as all read/writes go to the primary in a MongoDB replica set. If desired, consistency requirements for read operations can be relaxed. Through secondary consistency controls, read queries can be routed only to secondary replicas that fall within acceptable consistency limits with the primary server.

DynamoDB is eventually consistent by default. Users can configure read operations to return only strongly consistent data, but this doubles the cost of the read (see Pricing and Commercial Considerations) and adds latency. There is also no way to guarantee read consistency when querying against DynamoDB’s global secondary indexes (GSIs); any operation performed against a GSI will be eventually consistent, returning potentially stale or deleted data, and therefore increasing application complexity.

Operational Maturity

MongoDB Atlas allows users to deploy, manage, and scale their MongoDB deployments using built in operational and security best practices, such as end-to-end encryption, network isolation, role-based access control, VPC peering, and more. With this “MongoDB as a service,” database deployments are guaranteed to be available and durable with distributed and auto-healing replica set members and continuous backups with point in time recovery to protect against data corruption. MongoDB Atlas is fully elastic with zero downtime configuration changes that can all be triggered by the user. Atlas also grants organizations deep insights into how their databases are performing with a comprehensive monitoring dashboard, a real-time performance panel, and customizable alerting.

For organizations that would prefer to run MongoDB on their own infrastructure, MongoDB, Inc. offers advanced operational tooling to handle the automation of the entire database lifecycle, comprehensive monitoring (tracking 100+ metrics that could impact performance), and continuous backup. Product packages like MongoDB Enterprise Advanced bundle operational tooling and visualization and performance optimization platforms with end-to-end security controls for applications managing sensitive data.

Finally, MongoDB’s deployment flexibility allows single clusters to span racks, data centers and continents. With replica sets supporting up to 50 members and zone sharding across regions, administrators can provision clusters that support active/active data center deployments, with write local/read global access patterns and data locality.

Offered only as a managed service on AWS, DynamoDB abstracts away its underlying partitioning and replication schemes. And while provisioning is simple, other key operational tasks are lacking when compared to MongoDB:

  • Only 15 database metrics are reported by AWS Cloudwatch, which limits visibility into real-time database behavior

  • Limited toolset to allow developers and/or DBAs to optimize performance by visualizing schema or graphically profiling query performance

  • Limited support for encryption of data at rest. This feature can only be configured for new tables, when they are created. It cannot be applied to existing tables. It is not possible to enable encryption at rest for DynamoDB Streams, which are used for change data capture and global replication. Partition and sort keys are not encrypted, leaving a potential security gap for sensitive data. Users are also charged separately for all calls to the AWS Key Management Service, increasing costs and pricing complexity.

  • Throughput rates are capped at 10k ops/sec. Any requirements to expand capacity must be made by a web form, and only granted once an AWS representative responds to the request. Users can increase capacity multiple times per day, but resources can take up to several hours to become available. As a result, DynamoDB may not provision capacity sufficiently quickly to handle sudden spikes in load. In addition, only four downsizing events are supported per day

Pricing & Commercial Considerations

In this section we will again compare DynamoDB with its closest analog from MongoDB, Inc., MongoDB Atlas.

DynamoDB's pricing model is based on throughput. Users pay for a certain capacity on a given table and AWS automatically throttles any reads or writes that exceed that capacity.

This sounds simple in theory, but the reality is that correctly provisioning throughput and estimating pricing is far more nuanced.

Below is a list of all the factors that could impact the cost of running DynamoDB:

  • Size of the data set per month

  • Size of each item

  • Number of reads per second (pricing is based on “read capacity units”, which are equivalent to reading a 4KB object) and whether those reads need to be strongly consistent or eventually consistent (the former is twice as expensive)

    • If accessing a JSON object, the entire document must be retrieved, even if the application needs to read only a single element

  • Number of writes per second (pricing is based on “write capacity units”, which are the equivalent of writing a 1KB object)

  • Size and throughput requirements for each index created against the table

  • Data transferred by Dynamo streams per month

  • Data transfers both in and out of the database per month

  • Cross-regional data transfers, EC2 instances, and SQS queues needed for cross-regional deployments

  • The use of additional AWS services to address what is missing from DynamoDB’s limited key value query model

  • Use of on-demand or reserved instances

  • Number of metrics pushed into CloudWatch for monitoring

  • Number of events pushed into CloudTrail for database auditing

Key things to point out from the list above are that indexes affect pricing and consistent reads are twice as expensive. Another important fact to keep in mind with DynamoDB is that throughput pricing actually dictates the number of partitions (individual partitions are limited to 10 GB with 3,000 read capacity units or 1,000 write capacity units), not total throughput. Since users don’t have any control over partitioning, if any individual partition is saturated, one would have to double capacity to split partitions rather than scale linearly. Very careful design of the data model is essential to ensure that provisioned throughput can be realized.

Compared to DynamoDB, pricing for MongoDB Atlas is relatively straightforward:

  • Select the instance size with enough RAM to accommodate the portion of your data (included indexes) that clients access most often

  • Determine your IOPS requirement

  • Add storage as necessary

  • Adjust your capacity on demand

When to use DynamoDB vs. MongoDB

DynamoDB may work for organizations that are:

  • Looking for a database to support relatively simple key-value workloads

  • Heavily invested in AWS with no plans to change their deployment environment in the future

For organizations that need their database to support a wider range of use cases with more deployment flexibility and no platform lock-in, MongoDB would likely be a better fit. Biotechnology giant Thermo Fisher migrated from DynamoDB to MongoDB for their Instrument Connect IoT app, citing that while both databases were easy to deploy, MongoDB Atlas allowed for richer queries and much simpler schema evolution.

Fully managed MongoDB: Spin up a free cluster in minutes

MongoDB Atlas is the easiest way to deploy, manage, and scale MongoDB in the cloud. Get started with the free tier, which includes 512 MB of storage for learning and prototyping your application.

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*Safe Harbour Statement: The development, release, and timing of any features or functionality described for our products remains at our sole discretion. This information is merely intended to outline our general product direction and it should not be relied on in making a purchasing decision nor is this a commitment, promise or legal obligation to deliver any material, code, or functionality.