Question 1

Is DynamoDB worth learning in 2026 if I already know MongoDB?

Accepted Answer

Yes, if you work or want to work on AWS. They overlap on document modelling but the mental model is different — DynamoDB forces you to design access patterns up front, which is uncomfortable coming from MongoDB but is a transferable skill (it's basically applied data-modelling discipline). Many serverless-first Indian startups specifically ask for DynamoDB experience, and it commands a small premium over generic NoSQL on the salary axis.

Question 2

How much does a DynamoDB-experienced engineer earn in India?

Accepted Answer

₹8-26 LPA in 2026 for backend engineers with DynamoDB as a meaningful part of their stack. Companies hiring: Amazon (obviously), Razorpay, CRED, Zerodha's high-throughput teams, Postman, Lyft India, and most serverless-first early-stage startups. Specialised areas (Lambda + DynamoDB at scale, single-table design experts) pay at the upper end.

Question 3

Should I always use single-table design?

Accepted Answer

Not always. Single-table design pays off when (a) you have multiple related entities that you fetch together in the same access patterns, and (b) you can invest the upfront design time. For very small apps, isolated microservices owning one entity each, or quick prototypes, multi-table is fine and easier to reason about. Many real production systems are pragmatic — a 'mostly single-table' design with a few specialised tables for things that don't fit (analytics rollups, lookup tables).

Question 4

How does DynamoDB compare to Cassandra or ScyllaDB?

Accepted Answer

All three are partitioned NoSQL stores with similar access patterns (key-based, secondary indexes, no joins). DynamoDB wins on operations — zero ops, autoscaling, fully managed. Cassandra and ScyllaDB win on flexibility — they run anywhere, no vendor lock-in, and Scylla in particular often outperforms DynamoDB on per-node throughput. Pick DynamoDB when you're on AWS and want to ship fast. Pick Cassandra/Scylla when you need multi-cloud or have a team that wants to own the database operationally.

Question 5

When should I NOT use DynamoDB?

Accepted Answer

When you need rich ad-hoc queries (use Postgres or MongoDB), when your access patterns will change unpredictably and you can't refactor easily (use a relational DB), when you need complex aggregations on every request (use OLAP — Redshift, BigQuery, ClickHouse), when items routinely exceed 400 KB (use S3 + DynamoDB as a metadata index), or when you're not on AWS and don't want to be (DynamoDB is AWS-only — there's no self-hosted version beyond DynamoDB Local for dev).

Question 6

What is DynamoDB and what problems does it solve?

Accepted Answer

DynamoDB is AWS's fully-managed, serverless NoSQL database. It was built (and famously described in a 2007 SOSP paper) to solve scaling pain points Amazon hit with relational databases during peak shopping events: unpredictable latency under load, expensive vertical scaling, and operational overhead of running clustered RDBMS. DynamoDB takes a different shape — it's a key-value + document store with O(1) lookups on a hash-partitioned key, automatic horizontal partitioning, and no minimum cluster size to pay for. You give it a primary key, you get back single-digit-millisecond reads/writes at any scale from 1 request/sec to millions. There are no servers to provision, no patching, no replica failover to manage. The trade-off is that you lose SQL's flexibility — you must design your access patterns up front, because joins, complex aggregations, and ad-hoc queries are either impossible or expensive (Scans).

Question 7

What's the difference between a partition key and a sort key?

Accepted Answer

The partition key (PK, also called hash key) determines which physical partition the item lives on — DynamoDB hashes it and routes to a node. A table with only a PK is a pure key-value store: you can only fetch one item by its exact key. The sort key (SK, also called range key) is the second part of a composite primary key. Items with the same PK are stored together on the same partition, sorted by SK. This lets you do range queries: 'all orders for user 42 between Jan and Mar', 'the last 10 messages in chat room X'. The combination (PK, SK) must be unique. Choosing the right PK is the single most important DynamoDB modelling decision — get it wrong and you create hot partitions; get it right and your queries scale linearly.

Question 8

What is the document model in DynamoDB?

Accepted Answer

DynamoDB items are JSON-like documents — you can store nested maps, lists, sets, strings, numbers, binary, booleans, and null. The maximum item size is 400 KB (including the key names). Unlike strict relational tables, two items with the same primary key can have entirely different attributes — DynamoDB is schemaless beyond the key. In practice you do impose a schema in your code (often via a type system or a library like ElectroDB), but the database itself doesn't enforce it. The document model is what lets single-table design work: User items, Order items, and Product items can all sit in the same table with completely different attribute shapes, distinguished only by their PK/SK pattern.

Question 9

What are the data types DynamoDB supports?

Accepted Answer

DynamoDB has three categories: scalars (String, Number, Binary, Boolean, Null), documents (Map, List), and sets (String Set, Number Set, Binary Set). Numbers are stored as variable-length decimals with up to 38 digits of precision — no separate int/float distinction. Strings are UTF-8 and can be empty (since 2020). Sets are unordered collections of unique values of a single type — handy for tags or member lists, and they support atomic ADD/DELETE operations. Lists preserve order and can mix types. Maps are nested objects, also mixed-type. There's no native date type — store ISO 8601 strings (good for sorting) or Unix epoch numbers (good for TTL).

Question 10

How do you create a basic DynamoDB table?

Accepted Answer

You define the table name, the primary key schema (PK only, or PK + SK), and the billing mode (PAY_PER_REQUEST for on-demand, PROVISIONED for fixed capacity). You only declare the key attributes — every other attribute is added implicitly when you write an item with it. This is core to DynamoDB's flexibility: you never run an ALTER TABLE for a new field.

Question 11

What's the difference between Query and Scan?

Accepted Answer

Query reads items with a specific partition key — it's O(1) to find the partition and then scans only the items inside it (optionally filtered by sort key range). It's the workhorse: efficient, predictable, scales to billions of items. Scan reads the ENTIRE table, item by item, across every partition. It's expensive in both money (RCU charges per item read, even if filtered out) and time (consumes table-wide capacity, can throttle other workloads). The rule for production: never Scan a large table on the hot path. Acceptable Scan uses: small lookup tables (config, feature flags), one-time data migrations, exporting to S3 via parallel scan. If you find yourself wanting to Scan with a Filter, that's a sign your access pattern needs a GSI.

Question 12

What's the difference between eventually-consistent and strongly-consistent reads?

Accepted Answer

DynamoDB replicates every write to three storage nodes across Availability Zones. A write returns success after two of the three acknowledge. By default, reads are eventually-consistent — they may hit a replica that hasn't received the latest write yet, so you might see slightly stale data (typically within a second). Strongly-consistent reads always go to the leader replica and return the latest acknowledged write — they cost twice as many RCUs and have slightly higher latency. Use eventually-consistent reads for most workloads (feeds, browsing, analytics). Use strongly-consistent reads only when you immediately read-after-write something and absolutely need the latest value — for example, after creating a resource and immediately rendering it in a UI. Global Secondary Indexes (GSIs) are ALWAYS eventually-consistent — you cannot force strong consistency on a GSI read.

Question 13

What are RCUs and WCUs?

Accepted Answer

Read Capacity Units (RCU) and Write Capacity Units (WCU) are DynamoDB's unit of throughput in provisioned mode. 1 WCU = 1 write per second for items up to 1 KB. Larger items round up: a 1.5 KB write costs 2 WCUs. Transactional writes cost 2 WCUs. 1 RCU = 1 strongly-consistent read per second for items up to 4 KB, OR 2 eventually-consistent reads per second of the same size, OR 0.5 transactional reads per second. Quick math you'll be expected to do in interviews: 'How many WCUs to write 1,000 items per second, each 3 KB?' → 3 WCUs per item × 1,000 = 3,000 WCUs. 'How many RCUs for 500 eventually-consistent reads/sec on 8 KB items?' → 8 KB = 2 RCUs strong, 1 RCU eventual; 500 × 1 = 500 RCUs.

Question 14

What's the difference between on-demand and provisioned billing modes?

Accepted Answer

On-demand (PAY_PER_REQUEST) bills per request — no capacity to set, no auto-scaling to tune, scales instantly to any traffic. Costs ~7× more per unit than provisioned at steady-state but you pay $0 when idle. Provisioned bills for the RCU/WCU you reserve, regardless of usage. Cheaper at steady-state, requires auto-scaling to handle bursts, and can throttle requests if traffic exceeds capacity faster than scaling reacts. Default in 2026: start with on-demand for unpredictable workloads (new products, side-projects, internal tools). Switch to provisioned + auto-scaling when traffic is steady and predictable enough to justify the price cut — typically when monthly bill > ~$200 on a table. You can switch billing modes once every 24 hours.

Question 15

How do you write and read a single item?

Accepted Answer

PutItem inserts or replaces an item entirely. UpdateItem modifies specific attributes without overwriting the rest. GetItem fetches a single item by its full primary key. All three are O(1) operations. PutItem is destructive — if you only want to insert when no item exists, add `ConditionExpression: 'attribute_not_exists(PK)'`. This is the standard idempotent-create pattern.

DynamoDB Interview Questions

What is DynamoDB and what problems does it solve?

What's the difference between a partition key and a sort key?

What is the document model in DynamoDB?

What are the data types DynamoDB supports?

How do you create a basic DynamoDB table?

What's the difference between Query and Scan?

What's the difference between eventually-consistent and strongly-consistent reads?

What are RCUs and WCUs?

What's the difference between on-demand and provisioned billing modes?

How do you write and read a single item?

What is DynamoDB vs MongoDB — when would you pick one over the other?

What is TTL in DynamoDB?

What is single-table design and why is it the recommended DynamoDB pattern?

What's the difference between an LSI and a GSI?

How do you avoid hot partitions in DynamoDB?

What are DynamoDB Streams and how do you process them with Lambda?

What is DAX and when should you use it?

How do you implement transactions in DynamoDB?

How do you handle pagination in DynamoDB?

What is the BatchGetItem and BatchWriteItem API?

How does auto-scaling work in provisioned mode?

What is adaptive capacity?

How do you query by attributes that aren't part of the key?

How do you do an atomic counter in DynamoDB?

What is PartiQL in DynamoDB?

How do you design a single-table schema for a complex app — walk through Twitter as an example.

How do Global Tables provide multi-region replication, and what are the trade-offs?

How would you migrate a write-heavy 100 TB MongoDB collection to DynamoDB without downtime?

How do you implement leader election / distributed locks on DynamoDB?

Walk through how you'd design a high-throughput event-tracking system on DynamoDB.

Companies Hiring DynamoDB

Salary Insights

Frequently Asked Questions

Is DynamoDB worth learning in 2026 if I already know MongoDB?

How much does a DynamoDB-experienced engineer earn in India?

Should I always use single-table design?

How does DynamoDB compare to Cassandra or ScyllaDB?

When should I NOT use DynamoDB?

Introduction