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A satellite can store data using two types of memory: heap and stable. While both types are forms of random-access memory that only exist as long as the smart contract lives, they can be compared to a familiar analogy. Think of heap as similar to the RAM in a computer, and stable as more akin to ROM.

In a nutshell

Heap memory offers the best performance for accessing data, both for reading and writing. However, it has a limited capacity in terms of the space it can occupy, with a max of 2 GB.

On the other hand, stable memory has a higher memory threshold with a maximum limit of 96 GB minus the heap size, allowing it to store more data in terms of size. However, it is slightly slower.

Additionally, heap memory needs to be deserialized and serialized each time you upgrade the smart contract's code. This process becomes heavier as the heap memory size grows.

On the contrary, stable memory doesn't require processing during an upgrade. However, the data it contains needs to be deserialized and serialized each time it is accessed, which can make its usage more costly.


There are no strict rules governing the choice of memory type for your use case. Ultimately, the decision lies with you, based on what best suits your project. This is why both the datastore and storage support both memory types.

However, we generally recommend using heap for small data or data that require frequent access, and stable for large data or data accessed less often.

This is why the default options for creating new collections are heap for datastore and stable for storage.

Default usage

For the reasons mentioned above, your users, as well as your dapp's bundle and assets (including JS, HTML, images, etc., everything you deploy to your satellite using juno deploy), are stored in the heap memory.

In contrast, the analytics data is saved within stable memory.


AspectHeap MemoryStable Memory
CapacityMax 2 GBMax 96 GB (minus heap size)
PerformanceFast for read and write operationsSlightly slower
CostLower costHigher cost (~20x)
UpgradesData must be deserialized/serialized during upgradesData are not processed during upgrades
UsageSuitable for small or frequently accessed dataSuitable for large or less frequently accessed data