ExperimentalStableMemory

Byte-level access to (virtual) stable memory.

WARNING: As its name suggests, this library is experimental, subject to change and may be replaced by safer alternatives in later versions of Motoko. Use at your own risk and discretion.

This is a lightweight abstraction over IC stable memory and supports persisting raw binary data across Motoko upgrades. Use of this module is fully compatible with Motoko's use of stable variables, whose persistence mechanism also uses (real) IC stable memory internally, but does not interfere with this API.

Memory is allocated, using grow(pages), sequentially and on demand, in units of 64KiB pages, starting with 0 allocated pages. New pages are zero initialized. Growth is capped by a soft limit on page count controlled by compile-time flag --max-stable-pages <n> (the default is 65536, or 4GiB).

Each load operation loads from byte address offset in little-endian format using the natural bit-width of the type in question. The operation traps if attempting to read beyond the current stable memory size.

Each store operation stores to byte address offset in little-endian format using the natural bit-width of the type in question. The operation traps if attempting to write beyond the current stable memory size.

Text values can be handled by using Text.decodeUtf8 and Text.encodeUtf8, in conjunction with loadBlob and storeBlob.

The current page allocation and page contents is preserved across upgrades.

NB: The IC's actual stable memory size (ic0.stable_size) may exceed the page size reported by Motoko function size(). This (and the cap on growth) are to accommodate Motoko's stable variables. Applications that plan to use Motoko stable variables sparingly or not at all can increase --max-stable-pages as desired, approaching the IC maximum (initially 8GiB, then 32Gib, currently 48Gib). All applications should reserve at least one page for stable variable data, even when no stable variables are used.

Usage:

import StableMemory "mo:base/ExperimentalStableMemory";

Value size

let size : () -> (pages : Nat64)

Current size of the stable memory, in pages. Each page is 64KiB (65536 bytes). Initially 0. Preserved across upgrades, together with contents of allocated stable memory.

Example:

let beforeSize = StableMemory.size();
ignore StableMemory.grow(10);
let afterSize = StableMemory.size();
afterSize - beforeSize // => 10

Value grow

let grow : (newPages : Nat64) -> (oldPages : Nat64)

Grow current size of stable memory by the given number of pages. Each page is 64KiB (65536 bytes). Returns the previous size when able to grow. Returns 0xFFFF_FFFF_FFFF_FFFF if remaining pages insufficient. Every new page is zero-initialized, containing byte 0x00 at every offset. Function grow is capped by a soft limit on size controlled by compile-time flag --max-stable-pages <n> (the default is 65536, or 4GiB).

Example:

import Error "mo:base/Error";

let beforeSize = StableMemory.grow(10);
if (beforeSize == 0xFFFF_FFFF_FFFF_FFFF) {
  throw Error.reject("Out of memory");
};
let afterSize = StableMemory.size();
afterSize - beforeSize // => 10

Value stableVarQuery

let stableVarQuery : () -> (shared query () -> async { size : Nat64 })

Returns a query that, when called, returns the number of bytes of (real) IC stable memory that would be occupied by persisting its current stable variables before an upgrade. This function may be used to monitor or limit real stable memory usage. The query computes the estimate by running the first half of an upgrade, including any preupgrade system method. Like any other query, its state changes are discarded so no actual upgrade (or other state change) takes place. The query can only be called by the enclosing actor and will trap for other callers.

Example:

actor {
  stable var state = "";
  public func example() : async Text {
    let memoryUsage = StableMemory.stableVarQuery();
    let beforeSize = (await memoryUsage()).size;
    state #= "abcdefghijklmnopqrstuvwxyz";
    let afterSize = (await memoryUsage()).size;
    debug_show (afterSize - beforeSize)
  };
};

Value loadNat32

let loadNat32 : (offset : Nat64) -> Nat32

Loads a Nat32 value from stable memory at the given offset. Traps on an out-of-bounds access.

Example:

let offset = 0;
let value = 123;
StableMemory.storeNat32(offset, value);
StableMemory.loadNat32(offset) // => 123

Value storeNat32

let storeNat32 : (offset : Nat64, value : Nat32) -> ()

Stores a Nat32 value in stable memory at the given offset. Traps on an out-of-bounds access.

Example:

let offset = 0;
let value = 123;
StableMemory.storeNat32(offset, value);
StableMemory.loadNat32(offset) // => 123

Value loadNat8

let loadNat8 : (offset : Nat64) -> Nat8

Loads a Nat8 value from stable memory at the given offset. Traps on an out-of-bounds access.

Example:

let offset = 0;
let value = 123;
StableMemory.storeNat8(offset, value);
StableMemory.loadNat8(offset) // => 123

Value storeNat8

let storeNat8 : (offset : Nat64, value : Nat8) -> ()

Stores a Nat8 value in stable memory at the given offset. Traps on an out-of-bounds access.

Example:

let offset = 0;
let value = 123;
StableMemory.storeNat8(offset, value);
StableMemory.loadNat8(offset) // => 123

Value loadNat16

let loadNat16 : (offset : Nat64) -> Nat16

Loads a Nat16 value from stable memory at the given offset. Traps on an out-of-bounds access.

Example:

let offset = 0;
let value = 123;
StableMemory.storeNat16(offset, value);
StableMemory.loadNat16(offset) // => 123

Value storeNat16

let storeNat16 : (offset : Nat64, value : Nat16) -> ()

Stores a Nat16 value in stable memory at the given offset. Traps on an out-of-bounds access.

Example:

let offset = 0;
let value = 123;
StableMemory.storeNat16(offset, value);
StableMemory.loadNat16(offset) // => 123

Value loadNat64

let loadNat64 : (offset : Nat64) -> Nat64

Loads a Nat64 value from stable memory at the given offset. Traps on an out-of-bounds access.

Example:

let offset = 0;
let value = 123;
StableMemory.storeNat64(offset, value);
StableMemory.loadNat64(offset) // => 123

Value storeNat64

let storeNat64 : (offset : Nat64, value : Nat64) -> ()

Stores a Nat64 value in stable memory at the given offset. Traps on an out-of-bounds access.

Example:

let offset = 0;
let value = 123;
StableMemory.storeNat64(offset, value);
StableMemory.loadNat64(offset) // => 123

Value loadInt32

let loadInt32 : (offset : Nat64) -> Int32

Loads an Int32 value from stable memory at the given offset. Traps on an out-of-bounds access.

Example:

let offset = 0;
let value = 123;
StableMemory.storeInt32(offset, value);
StableMemory.loadInt32(offset) // => 123

Value storeInt32

let storeInt32 : (offset : Nat64, value : Int32) -> ()

Stores an Int32 value in stable memory at the given offset. Traps on an out-of-bounds access.

Example:

let offset = 0;
let value = 123;
StableMemory.storeInt32(offset, value);
StableMemory.loadInt32(offset) // => 123

Value loadInt8

let loadInt8 : (offset : Nat64) -> Int8

Loads an Int8 value from stable memory at the given offset. Traps on an out-of-bounds access.

Example:

let offset = 0;
let value = 123;
StableMemory.storeInt8(offset, value);
StableMemory.loadInt8(offset) // => 123

Value storeInt8

let storeInt8 : (offset : Nat64, value : Int8) -> ()

Stores an Int8 value in stable memory at the given offset. Traps on an out-of-bounds access.

Example:

let offset = 0;
let value = 123;
StableMemory.storeInt8(offset, value);
StableMemory.loadInt8(offset) // => 123

Value loadInt16

let loadInt16 : (offset : Nat64) -> Int16

Loads an Int16 value from stable memory at the given offset. Traps on an out-of-bounds access.

Example:

let offset = 0;
let value = 123;
StableMemory.storeInt16(offset, value);
StableMemory.loadInt16(offset) // => 123

Value storeInt16

let storeInt16 : (offset : Nat64, value : Int16) -> ()

Stores an Int16 value in stable memory at the given offset. Traps on an out-of-bounds access.

Example:

let offset = 0;
let value = 123;
StableMemory.storeInt16(offset, value);
StableMemory.loadInt16(offset) // => 123

Value loadInt64

let loadInt64 : (offset : Nat64) -> Int64

Loads an Int64 value from stable memory at the given offset. Traps on an out-of-bounds access.

Example:

let offset = 0;
let value = 123;
StableMemory.storeInt64(offset, value);
StableMemory.loadInt64(offset) // => 123

Value storeInt64

let storeInt64 : (offset : Nat64, value : Int64) -> ()

Stores an Int64 value in stable memory at the given offset. Traps on an out-of-bounds access.

Example:

let offset = 0;
let value = 123;
StableMemory.storeInt64(offset, value);
StableMemory.loadInt64(offset) // => 123

Value loadFloat

let loadFloat : (offset : Nat64) -> Float

Loads a Float value from stable memory at the given offset. Traps on an out-of-bounds access.

Example:

let offset = 0;
let value = 1.25;
StableMemory.storeFloat(offset, value);
StableMemory.loadFloat(offset) // => 1.25

Value storeFloat

let storeFloat : (offset : Nat64, value : Float) -> ()

Stores a Float value in stable memory at the given offset. Traps on an out-of-bounds access.

Example:

let offset = 0;
let value = 1.25;
StableMemory.storeFloat(offset, value);
StableMemory.loadFloat(offset) // => 1.25

Value loadBlob

let loadBlob : (offset : Nat64, size : Nat) -> Blob

Load size bytes starting from offset as a Blob. Traps on an out-of-bounds access.

Example:

import Blob "mo:base/Blob";

let offset = 0;
let value = Blob.fromArray([1, 2, 3]);
let size = value.size();
StableMemory.storeBlob(offset, value);
Blob.toArray(StableMemory.loadBlob(offset, size)) // => [1, 2, 3]

Value storeBlob

let storeBlob : (offset : Nat64, value : Blob) -> ()

Write bytes of blob beginning at offset. Traps on an out-of-bounds access.

Example:

import Blob "mo:base/Blob";

let offset = 0;
let value = Blob.fromArray([1, 2, 3]);
let size = value.size();
StableMemory.storeBlob(offset, value);
Blob.toArray(StableMemory.loadBlob(offset, size)) // => [1, 2, 3]