stable-memory

What this skill does

# Stable Memory & Canister Upgrades

## What This Is
Stable memory is persistent storage on Internet Computer that survives canister upgrades. Heap memory (regular variables) is wiped on every upgrade. Any data you care about MUST be in stable memory, or it will be lost the next time the canister is deployed.

## Prerequisites

- For Motoko: mops with `core = "2.0.0"` in mops.toml
- For Rust: `ic-stable-structures = "0.7"` in Cargo.toml

## Canister IDs
No external canister dependencies. Stable memory is a local canister feature.

## Mistakes That Break Your Build

1. **Using `thread_local! { RefCell<T> }` for user data (Rust)** -- This is heap memory. It is wiped on every canister upgrade. All user data, balances, settings stored this way will vanish after `icp deploy`. Use `StableBTreeMap` instead.

2. **Forgetting `#[post_upgrade]` handler (Rust)** -- Without a `post_upgrade` function, the canister may silently reset state or behave unexpectedly after upgrade. Always define both `#[init]` and `#[post_upgrade]`.

3. **Using `stable` keyword in persistent actors (Motoko)** -- In mo:core `persistent actor`, all `let` and `var` declarations are automatically stable. Writing `stable let` produces warning M0218 and `stable var` is redundant. Just use `let` and `var`.

4. **Confusing heap memory limits with stable memory limits (Rust)** -- Heap (Wasm linear) memory is limited to 4GB for wasm32 and 6GB for wasm64. Stable memory can grow up to hundreds of GB (the subnet storage limit). The real danger: if you use `pre_upgrade`/`post_upgrade` hooks to serialize heap data to stable memory and deserialize it back, you are limited by the heap memory size AND by the instruction limit for upgrade hooks. Large datasets will trap during upgrade, bricking the canister. The solution is to use stable structures (`StableBTreeMap`, `StableCell`, etc.) that read/write directly to stable memory, bypassing the heap entirely. Use `MemoryManager` to partition stable memory into virtual memories so multiple structures can coexist without overwriting each other.

5. **Changing record field types between upgrades (Motoko)** -- Altering the type of a persistent field (e.g., `Nat` to `Int`, or renaming a record field) will trap on upgrade and data is unrecoverable. Only ADD new optional fields. Never remove or rename existing ones.

6. **Serializing large data in pre_upgrade (Rust)** -- `pre_upgrade` has a fixed instruction limit. If you serialize a large HashMap to stable memory in pre_upgrade, it will hit the limit and trap, bricking the canister. Use `StableBTreeMap` which writes directly to stable memory and needs no serialization step.

7. **Using `actor { }` instead of `persistent actor { }` (Motoko)** -- Plain `actor` in mo:core requires explicit `stable` annotations and pre/post_upgrade hooks. `persistent actor` makes everything stable by default. Always use `persistent actor`.

## Implementation

### Motoko

With mo:core 2.0, `persistent actor` makes stable storage trivial. All `let` and `var` declarations inside the actor body are automatically persisted across upgrades.

```motoko
import Map "mo:core/Map";
import List "mo:core/List";
import Nat "mo:core/Nat";
import Text "mo:core/Text";
import Time "mo:core/Time";

persistent actor {

  // Types -- must be inside actor body
  type User = {
    id : Nat;
    name : Text;
    created : Int;
  };

  // These survive upgrades automatically -- no "stable" keyword needed
  let users = Map.empty<Nat, User>();
  var userCounter : Nat = 0;
  let tags = List.empty<Text>();

  // Transient data -- reset to initial value on every upgrade
  transient var requestCount : Nat = 0;

  public func addUser(name : Text) : async Nat {
    let id = userCounter;
    Map.add(users, Nat.compare, id, {
      id;
      name;
      created = Time.now();
    });
    userCounter += 1;
    requestCount += 1;
    id
  };

  public query func getUser(id : Nat) : async ?User {
    Map.get(users, Nat.compare, id)
  };

  public query func getUserCount() : async Nat {
    Map.size(users)
  };

  // requestCount resets to 0 after every upgrade
  public query func getRequestCount() : async Nat {
    requestCount
  };
}
```

Key rules for Motoko persistent actors:
- `let` for Map, List, Set, Queue -- auto-persisted, no serialization
- `var` for simple values (Nat, Text, Bool) -- auto-persisted
- `transient var` for caches, counters that should reset on upgrade
- NO `pre_upgrade` / `post_upgrade` needed -- the runtime handles it
- NO `stable` keyword -- it is redundant and produces warnings

#### mops.toml

```toml
[package]
name = "my-project"
version = "0.1.0"

[dependencies]
core = "2.0.0"
```

### Rust

Rust canisters use `ic-stable-structures` for persistent storage. The `MemoryManager` partitions stable memory (up to hundreds of GB, limited by subnet storage) into virtual memories, each backing a different data structure.

#### Cargo.toml

```toml
[package]
name = "stable_memory_backend"
version = "0.1.0"
edition = "2021"

[lib]
crate-type = ["cdylib"]

[dependencies]
ic-cdk = "0.19"
ic-stable-structures = "0.7"
candid = "0.10"
serde = { version = "1", features = ["derive"] }
ciborium = "0.2"
```

#### Single Stable Structure (Simple Case)

```rust
use ic_stable_structures::{
    memory_manager::{MemoryId, MemoryManager, VirtualMemory},
    storable::{Bound, Storable},
    DefaultMemoryImpl, StableBTreeMap,
};
use ic_cdk::{init, post_upgrade, query, update};
use candid::CandidType;
use serde::{Deserialize, Serialize};
use std::borrow::Cow;
use std::cell::RefCell;

type Memory = VirtualMemory<DefaultMemoryImpl>;

// -- Implement Storable for custom types --
// StableBTreeMap keys need Storable + Ord, values need Storable.
// Storable defines how a type is serialized to/from bytes in stable memory.
// Use CBOR (via ciborium) for serialization -- compact binary format, faster than candid.

#[derive(CandidType, Serialize, Deserialize, Clone)]
struct User {
    id: u64,
    name: String,
    created: u64,
}

impl Storable for User {
    // Recommended: prefer Unbounded to avoid backwards compatibility issues when adding new fields.
    // Bounded requires a fixed max_size -- adding a field that increases the size will break existing data.
    const BOUND: Bound = Bound::Unbounded;

    fn to_bytes(&self) -> Cow<'_, [u8]> {
        let mut buf = vec![];
        ciborium::into_writer(self, &mut buf).expect("Failed to encode User");
        Cow::Owned(buf)
    }

    fn into_bytes(self) -> Vec<u8> {
        let mut buf = vec![];
        ciborium::into_writer(&self, &mut buf).expect("Failed to encode User");
        buf
    }

    fn from_bytes(bytes: Cow<'_, [u8]>) -> Self {
        ciborium::from_reader(bytes.as_ref()).expect("Failed to decode User")
    }
}
// Bound::Bounded { max_size, is_fixed_size: true } exists for fixed-size types but is NOT
// recommended -- adding a new field later will exceed max_size and break deserialization.

// Stable storage -- survives upgrades
thread_local! {
    static MEMORY_MANAGER: RefCell<MemoryManager<DefaultMemoryImpl>> =
        RefCell::new(MemoryManager::init(DefaultMemoryImpl::default()));

    static USERS: RefCell<StableBTreeMap<u64, User, Memory>> =
        RefCell::new(StableBTreeMap::init(
            MEMORY_MANAGER.with(|m| m.borrow().get(MemoryId::new(0)))
        ));

    // Counter stored in stable memory via StableCell
    static COUNTER: RefCell<ic_stable_structures::StableCell<u64, Memory>> =
        RefCell::new(ic_stable_structures::StableCell::init(
            MEMORY_MANAGER.with(|m| m.borrow().get(MemoryId::new(1))),
            0u64,
        ));
}

#[init]
fn init() {
    // Any one-time initialization
}

#[post_upgrade]
fn post_upgrade() {
    // Stable structures auto-restore -- no deserialization needed
    // Re-init timers or other transient state here
}

#[update]
fn add_user(name: String) -> u64 {
    let id = COUNTER.with(|c| {
        let mut cell = c.borrow_mut();
        let current