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Concept
A database on object storage keeps its data in an object store (a durable service for files) and uses server memory and disks as cache. It works like a library with an off-site warehouse: every book is stored safely there, and each reading room keeps copies of the popular ones on its shelves. Because the servers do not own the data, you can grow storage without adding servers, add servers when traffic grows, and lose a server without losing saved data. The price is that reading something not yet in the cache, or confirming a write, takes longer.

Learning objectives

After reading this article you will be able to:
  • Explain how a database keeps its data in object storage and uses disks as cache
  • Compare local-disk and object-storage databases on durability, scaling, and speed
  • Describe the trade-offs in cold reads, write latency, and caching
  • Recognize when object storage lowers cost and when it is a poor fit

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How do traditional databases store data?

Traditionally, each database server keeps its own data on its own disks, whether the database is relational or a graph, and some systems keep the whole dataset in memory. Durability (surviving a crash) comes from writing to that disk and copying it to other servers. This ties storage and compute together:
  • Storing more data means bigger disks or more memory per machine, or splitting the data into shards across more machines.
  • Serving more reads means adding replicas, and each replica keeps its own copy of the data (or of its shard).
  • Replacing or adding a server means copying data to it before it can serve traffic.
This works well while the dataset fits comfortably on a few machines. It gets expensive when data grows faster than query load, because you pay for compute and fast storage to hold rarely read data, such as years of listening history in a music app when most queries touch recent weeks. Graph databases often feel this sharply: many keep a large graph in memory or on local SSD for fast traversal, so the cost of fast storage grows with the whole graph, and some limit its size to what one machine can hold. Vector indexes such as HNSW, used by many vector databases, are also often kept in memory.

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How does a database on object storage work?

The object store, often a service that implements the S3 API, holds the permanent copy of everything. Query servers keep the data they use most in memory and on local disk.
  • Object storage is the source of truth. Data files and index files, such as the structures behind vector search and full-text search, live in the object store. The metadata that says which files make up the current version is typically stored there too, or in a separate strongly consistent metadata service.
  • Compute nodes are close to stateless. They hold caches in memory and on local NVMe or SSD (fast local drives), and can be added, removed, or replaced without moving data.
  • Reads go through the cache hierarchy: memory first, then local disk, then object storage on a miss.
  • Writes are batched and published atomically. A writer typically uploads new immutable files (files never changed once written), then updates the metadata to make them part of the current version. Object stores typically do not support updating part of an object in place; an object is written or replaced as a whole, so many designs use log-structured or immutable-file layouts.
Under the hood, correctness depends on a few storage guarantees. Newly written objects must be readable immediately (read-after-write consistency), so a reader that follows new metadata finds the files it points to. The system also needs an atomic way to advance the current version, such as a conditional write (“create only if absent” or “replace only if unchanged”) or a separate coordination service, so two writers cannot both believe they committed the same version.

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How does an object-storage database compare with a local-disk database?

The difference is where the permanent copy lives. Because query servers do not own it, storage and compute are separated, and each side can be sized, scaled, and replaced on its own.

Try HelixDB

Keep your graph, embeddings, and text in open-source HelixDB, which uses object storage as its source of truth and scales storage independently of compute.

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What are the tradeoffs of building a database on object storage?

In short, you give up some speed in exchange for storage that is typically cheaper per byte and scales on its own.
  • Cold-read latency. A cache miss pays an object-store round trip, much slower than local NVMe or memory, so latency depends on how much of the working set (the data queries touch most) the caches hold.
  • A write latency floor. When the object store is the only durable tier, a commit is durable only after the object store acknowledges it. Batching many writes into one upload keeps throughput high, but a single write cannot finish faster than that round trip. Some designs acknowledge a commit once it reaches a write-ahead log (a durable record of changes) replicated across several servers, and upload it later; losing one server then still loses no committed data.
  • Caching becomes central. Cache sizing, warming, and eviction policy decide latency, so many systems tune caching to the workload and warm caches after a restart.
  • Request-oriented pricing and access. Object stores work best with fewer, larger requests, so data layout and batching matter more than on local disk.

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When does a database on object storage cost less at scale?

It usually costs less when the bulk of a large dataset is cold, meaning rarely read, and a smaller working set serves most queries. Think of a chatbot that answers from years of archived documents with RAG, or an agent’s long-term memory of text and embeddings that keeps growing while each question touches a small part. An object-storage design matches cost to that shape:
  • Bulk data is priced at object-storage rates, which are typically far lower per byte than provisioned SSD or memory, and the object store provides durability.
  • Compute is sized for the working set and the query load, not for the total dataset.
  • Compute scales with demand without copying data. New compute nodes share the same stored data and fill their caches as they serve queries, and nodes can be removed after a traffic spike because no data lives only on them.
Workloads with many small writes or frequent cache misses pay more in per-request charges, which can offset the per-byte savings.

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When is object storage a poor fit?

A local-disk or in-memory database may be the better choice when every read, including the first, needs guaranteed sub-millisecond latency; when individual writes need the lowest possible commit latency; or when the dataset is small and stable enough that one well-provisioned machine holds it comfortably.

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How does HelixDB use object storage?

HelixDB keeps a property graph, vectors, and text in one database, built on object storage:
  • Object storage is the source of truth. NVMe or SSD and memory are caches, and storage scales independently of compute.
  • The system can recover from full cache loss by reading object storage. Cache misses fall through to object storage, so caching affects latency, not results.
  • Helix Cloud runs a gateway, a single writer process, and readers that scale automatically. The writer uses MVCC (multiversion concurrency control, which can keep several versions of a record), runs write transactions concurrently, and resolves conflicts at commit.
  • Readers see new commits after a snapshot refresh. Reads served by the writer alone give read-after-write consistency.
Cold reads pay object-storage latency. See Architecture for the read and write paths and Tradeoffs for where the design fits and where another system may fit better.

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Frequently asked questions

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What is the cheapest way to store a large graph?

At scale, it is usually to keep the full graph, such as a large knowledge graph, in object storage and cache only the frequently traversed part. Keeping the whole graph in memory or on provisioned SSD means paying fast-storage prices for data that is rarely read. The object-storage approach trades that cost for slower traversals that touch uncached data.

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Is a database on object storage slower?

It can be. Cache hits perform like a traditional database, but a miss pays object-storage latency, and a commit typically waits for a durable object-store write. Whether that matters depends on how well your working set fits in cache and how latency-sensitive your writes are.

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Is this the same as a serverless database?

Not exactly, but they are related. Separating storage from compute is one of the main things that lets serverless databases add and remove compute quickly, because no data lives only on a server. A database can use object storage without being offered as a serverless service.

Graph, vector, and text in one database

The hidden costs of stitching separate stores together.

What is a graph database?

Nodes, edges, traversals, and when a graph is the right model.

What is a vector database?

Storage and indexing for embeddings, and where vector-only storage falls short.

Helix Cloud architecture

The gateway, writer, readers, caches, and object storage.

Helix Cloud tradeoffs

What the architecture is optimized for, and where it is not.