Memory
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 1 GB.
On the other hand, stable memory has a higher memory threshold with a maximum limit of 400 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.
Recommendations
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.
In practice, heap memory can be recommended for small datasets or data that require quick or frequent access, while stable memory is preferred for large data or data accessed less often.
This is why, for example, your dapp's bundle assets (including JS, HTML, images, etc.) are stored within the heap memory of satellites.
However, this decision, along with the memory limitations, results in a significant portion of the heap memory being allocated. Although stable memory is slightly slower and comes at a higher cost, it is well-suited for storing data and ensuring smooth smart contract upgrades. This is particularly important for the operation and lifecycle of a project.
That's why the default option for creating new collections is set to stable for both datastore and storage.
Default usage
As mentioned in the previous chapter, your dapp's bundle and assets (everything you deploy to your satellite using juno deploy), are stored in the heap memory.
In contrast, your users (as of Satellite version 0.0.16) and the analytics data are saved within stable memory.
Summary
| Aspect | Heap Memory | Stable Memory |
|---|---|---|
| Capacity | Max 1 GB | Max 400 GB (minus heap size) |
| Performance | Fast for read and write operations | Slightly slower |
| Cost | Lower cost | Higher cost (~20x) |
| Upgrades | Data must be deserialized/serialized during upgrades | Data are not processed during upgrades |
| Usage | Suitable for small or frequently accessed data | Suitable for large or less frequently accessed data |