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

• icp-cli >= 0.1.0 (brew install dfinity/tap/icp-cli)
• 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. 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 4GB heap 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.

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

[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

[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"] }

Single Stable Structure (Simple Case)

use ic_stable_structures::{
    memory_manager::{MemoryId, MemoryManager, VirtualMemory},
    DefaultMemoryImpl, StableBTreeMap,
};
use ic_cdk::{init, post_upgrade, query, update};
use candid::{CandidType, Deserialize};
use std::cell::RefCell;

type Memory = VirtualMemory<DefaultMemoryImpl>;

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

    static USERS: RefCell<StableBTreeMap<u64, Vec<u8>, 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,
        ));
}

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

#[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 = *cell.get();
        cell.set(current + 1);
        current
    });

    let user = User {
        id,
        name,
        created: ic_cdk::api::time(),
    };

    let serialized = candid::encode_one(&user).expect("Failed to serialize user");

    USERS.with(|users| {
        users.borrow_mut().insert(id, serialized);
    });

    id
}

#[query]
fn get_user(id: u64) -> Option<User> {
    USERS.with(|users| {
        users.borrow().get(&id).and_then(|bytes| {
            candid::decode_one(&bytes).ok()
        })
    })
}

#[query]
fn get_user_count() -> u64 {
    USERS.with(|users| users.borrow().len())
}

Multiple Stable Structures with MemoryManager

use ic_stable_structures::{
    memory_manager::{MemoryId, MemoryManager, VirtualMemory},
    DefaultMemoryImpl, StableBTreeMap, StableCell, StableLog,
};
use std::cell::RefCell;

type Memory = VirtualMemory<DefaultMemoryImpl>;

// Each structure gets its own MemoryId -- NEVER reuse IDs
const USERS_MEM_ID: MemoryId = MemoryId::new(0);
const POSTS_MEM_ID: MemoryId = MemoryId::new(1);
const COUNTER_MEM_ID: MemoryId = MemoryId::new(2);
const LOG_INDEX_MEM_ID: MemoryId = MemoryId::new(3);
const LOG_DATA_MEM_ID: MemoryId = MemoryId::new(4);

thread_local! {
    static MEMORY_MANAGER: RefCell<MemoryManager<DefaultMemoryImpl>> =
        RefCell::new(MemoryManager::init(DefaultMemoryImpl::default()));

    static USERS: RefCell<StableBTreeMap<u64, Vec<u8>, Memory>> =
        RefCell::new(StableBTreeMap::init(
            MEMORY_MANAGER.with(|m| m.borrow().get(USERS_MEM_ID))
        ));

    static POSTS: RefCell<StableBTreeMap<u64, Vec<u8>, Memory>> =
        RefCell::new(StableBTreeMap::init(
            MEMORY_MANAGER.with(|m| m.borrow().get(POSTS_MEM_ID))
        ));

    static COUNTER: RefCell<StableCell<u64, Memory>> =
        RefCell::new(StableCell::init(
            MEMORY_MANAGER.with(|m| m.borrow().get(COUNTER_MEM_ID)),
            0u64,
        ));

    static AUDIT_LOG: RefCell<StableLog<Vec<u8>, Memory, Memory>> =
        RefCell::new(StableLog::init(
            MEMORY_MANAGER.with(|m| m.borrow().get(LOG_INDEX_MEM_ID)),
            MEMORY_MANAGER.with(|m| m.borrow().get(LOG_DATA_MEM_ID)),
        ));
}

Key rules for Rust stable structures:

• MemoryManager partitions stable memory -- each structure gets a unique MemoryId
• NEVER reuse a MemoryId for two different structures -- they will corrupt each other
• StableBTreeMap keys must implement Storable + Ord, values must implement Storable
• For complex types, serialize to Vec<u8> with candid or serde
• StableCell for single values (counters, config)
• StableLog for append-only logs (needs two memory regions: index + data)
• thread_local! { RefCell<StableBTreeMap<...>> } is the correct pattern -- the RefCell wraps the stable structure, not a heap HashMap
• No pre_upgrade/post_upgrade serialization needed -- data is already in stable memory

Deploy & Test

Motoko: Verify Persistence Across Upgrades

# Start local replica
icp network start -d

# Deploy
icp deploy backend

# Add data
icp canister call backend addUser '("Alice")'
# Expected: (0 : nat)

icp canister call backend addUser '("Bob")'
# Expected: (1 : nat)

# Verify data exists
icp canister call backend getUserCount '()'
# Expected: (2 : nat)

icp canister call backend getUser '(0)'
# Expected: (opt record { id = 0 : nat; name = "Alice"; created = ... })

# Now upgrade the canister (simulates code change + redeploy)
icp deploy backend

# Verify data survived the upgrade
icp canister call backend getUserCount '()'
# Expected: (2 : nat) -- STILL 2, not 0

icp canister call backend getUser '(1)'
# Expected: (opt record { id = 1 : nat; name = "Bob"; created = ... })

Rust: Verify Persistence Across Upgrades

icp network start -d

icp deploy backend

icp canister call backend add_user '("Alice")'
# Expected: (0 : nat64)

icp canister call backend get_user_count '()'
# Expected: (1 : nat64)

# Upgrade
icp deploy backend

# Verify persistence
icp canister call backend get_user_count '()'
# Expected: (1 : nat64) -- data survived

icp canister call backend get_user '(0)'
# Expected: (opt record { id = 0 : nat64; name = "Alice"; created = ... })

Verify It Works

The definitive test for stable memory: data survives upgrade.

# 1. Deploy and add data
icp deploy backend
icp canister call backend addUser '("TestUser")'

# 2. Record the count
icp canister call backend getUserCount '()'
# Note the number

# 3. Upgrade (redeploy)
icp deploy backend

# 4. Check count again -- must be identical
icp canister call backend getUserCount '()'
# Must match step 2

# 5. Verify transient data DID reset
icp canister call backend getRequestCount '()'
# Expected: (0 : nat) -- transient var resets on upgrade

If the count drops to 0 after step 3, your data is NOT in stable memory. Review your storage declarations.