Calimero SDK

Build distributed applications with automatic CRDT synchronization and conflict-free state management.

Quick Start

use calimero_sdk::app;
use calimero_sdk::borsh::{BorshSerialize, BorshDeserialize};
use calimero_storage::collections::UnorderedMap;

#[app::state]
#[derive(BorshSerialize, BorshDeserialize)]
#[borsh(crate = "calimero_sdk::borsh")]
pub struct MyApp {
    items: UnorderedMap<String, String>,
}

#[app::logic]
impl MyApp {
    #[app::init]
    pub fn init() -> MyApp {
        MyApp {
            items: UnorderedMap::new(),
        }
    }
    
    pub fn add_item(&mut self, key: String, value: String) -> app::Result<()> {
        self.items.insert(key, value)?;
        Ok(())
    }
    
    pub fn get_item(&self, key: &str) -> app::Result<Option<String>> {
        self.items.get(key).map_err(Into::into)
    }
}

Architecture

WASM Execution Flow

sequenceDiagram
    participant Client as JSON-RPC Client
    participant Node as Node Runtime
    participant WASM as WASM Module
    participant Storage as CRDT Storage
    participant Network as P2P Network
    
    Client->>Node: call("add_item", args)
    Node->>WASM: execute(method, args)
    
    rect rgb(220, 237, 255)
        Note over WASM,Storage: Transaction Execution
        WASM->>Storage: map.insert(key, value)
        Storage->>Storage: Generate Action::Update
        Storage->>Storage: Calculate Merkle hashes
        Storage->>Storage: Collect in DELTA_CONTEXT
    end
    
    WASM-->>Node: ExecutionOutcome {<br/>root_hash, events, ...}
    
    rect rgb(255, 237, 220)
        Note over Node,Network: Delta Propagation
        Node->>Node: Create CausalDelta {<br/>parents: dag_heads,<br/>payload: actions}
        Node->>Network: Broadcast StateDelta
    end
    
    Network->>Node: Propagate to peers
    Node-->>Client: TransactionResult

Event Lifecycle

stateDiagram-v2
    [*] --> Emitted: emit event
    
    Emitted --> Collected: Add to outcome
    
    state "Author Node Check" as AuthorCheck
    Collected --> AuthorCheck
    
    AuthorCheck --> SkipHandler: If author node
    AuthorCheck --> IncludeInDelta: If not author
    
    SkipHandler --> IncludeInDelta: Include in broadcast
    
    IncludeInDelta --> Broadcast: StateDelta message
    
    Broadcast --> PeerReceive: Gossipsub propagation
    
    state "Delta Application Check" as DeltaCheck
    PeerReceive --> DeltaCheck
    
    DeltaCheck --> ExecuteHandler: If parents ready
    DeltaCheck --> Buffer: If parents missing
    
    Buffer --> ExecuteHandler: When parents arrive
    
    ExecuteHandler --> WASMExec: execute handler
    
    state "Handler Result" as HandlerResult
    WASMExec --> HandlerResult
    
    HandlerResult --> NewEvents: If emits events
    HandlerResult --> WebSocket: Emit to clients
    
    NewEvents --> Emitted: Recursive
    WebSocket --> [*]
    
    note right of SkipHandler
        Prevents infinite loops
    end note
    
    note right of Buffer
        Events lost if delta never applied
    end note

Core Concepts

CRDT Collections

All state in Calimero apps uses CRDTs (Conflict-free Replicated Data Types):

use calimero_storage::collections::{UnorderedMap, Vector, Counter};

// Key-value map
let mut map = UnorderedMap::new();
map.insert("key".to_string(), "value".to_string())?;

// Ordered list  
let mut list = Vector::new();
list.push("item".to_string())?;

// Distributed counter (G-Counter)
let mut counter = Counter::new();
counter.increment()?;  // Uses node's identity
let total = counter.value()?;  // Sum across all nodes

Why CRDTs?

• ✅ Automatic conflict resolution
• ✅ No coordination needed for updates
• ✅ Eventually consistent across nodes
• ✅ Works offline, syncs when reconnected

CRDT Conflict Resolution

flowchart TB
    Start([Two nodes update concurrently]) --> Fork
    
    subgraph "Node A"
        A1[map.insert 'key', 'value_A'<br/>timestamp: 1000]
        A1 --> A2[Broadcast Delta A]
    end
    
    subgraph "Node B"
        B1[map.insert 'key', 'value_B'<br/>timestamp: 1001]
        B1 --> B2[Broadcast Delta B]
    end
    
    Fork --> A1
    Fork --> B1
    
    A2 --> Merge{Both nodes receive<br/>both deltas}
    B2 --> Merge
    
    Merge --> Compare[Compare timestamps:<br/>1001 > 1000]
    
    Compare --> LWW[Last-Write-Wins:<br/>Keep 'value_B']
    
    LWW --> Converge([Both nodes:<br/>key = 'value_B'])
    
    style Fork fill:#FF6B6B,stroke:#333,stroke-width:3px,color:#000
    style Merge fill:#FFB84D,stroke:#333,stroke-width:3px,color:#000
    style LWW fill:#4ECDC4,stroke:#333,stroke-width:3px,color:#000
    style Converge fill:#51CF66,stroke:#333,stroke-width:3px,color:#000

G-Counter (Distributed Counter)

flowchart LR
    subgraph "Initial State"
        I[counter = 0<br/>storage: empty]
    end
    
    subgraph "Node A Increments"
        A1[executor_id = 'node_a']
        A2[storage['node_a'] = 1]
        A3[counter.value = 1]
    end
    
    subgraph "Node B Increments (Concurrent)"
        B1[executor_id = 'node_b']
        B2[storage['node_b'] = 1]
        B3[counter.value = 1]
    end
    
    subgraph "After Sync"
        S1[storage['node_a'] = 1<br/>storage['node_b'] = 1]
        S2[counter.value = sum<br/>= 1 + 1 = 2]
    end
    
    I --> A1
    I --> B1
    A1 --> A2 --> A3
    B1 --> B2 --> B3
    A3 --> S1
    B3 --> S1
    S1 --> S2
    
    style I fill:#4DABF7,stroke:#333,stroke-width:3px,color:#000
    style A3 fill:#FF6B6B,stroke:#333,stroke-width:3px,color:#000
    style B3 fill:#FF6B6B,stroke:#333,stroke-width:3px,color:#000
    style S2 fill:#51CF66,stroke:#333,stroke-width:3px,color:#000

Events and Handlers

Emit events to notify other parts of your app:

#[app::event]
pub enum Event {
    ItemAdded { key: String, value: String },
    ItemRemoved { key: String },
}

// Emit event without handler
app::emit!(Event::ItemAdded { 
    key: "foo".to_owned(), 
    value: "bar".to_owned() 
});

// Emit event WITH handler
app::emit!((
    Event::ItemAdded { 
        key: "foo".to_owned(), 
        value: "bar".to_owned() 
    },
    "on_item_added"  // Handler name
));

⚠️ CRITICAL: Event Handler Requirements

Handlers may execute in PARALLEL (not guaranteed sequential order).

Handler Safety Check

flowchart TD
    Start([Design handler]) --> Q1{Commutative?<br/>Order doesn't matter?}
    
    Q1 -->|No| Unsafe1[❌ UNSAFE<br/>Handler depends on<br/>execution order]
    Q1 -->|Yes| Q2{Independent?<br/>No shared state?}
    
    Q2 -->|No| Unsafe2[❌ UNSAFE<br/>Race condition on<br/>shared data]
    Q2 -->|Yes| Q3{Idempotent?<br/>Safe to retry?}
    
    Q3 -->|No| Unsafe3[❌ UNSAFE<br/>May execute multiple<br/>times]
    Q3 -->|Yes| Q4{Pure?<br/>No side effects?}
    
    Q4 -->|No| Unsafe4[❌ UNSAFE<br/>External calls not<br/>deterministic]
    Q4 -->|Yes| Safe[✅ SAFE<br/>Ready for parallel<br/>execution]
    
    Unsafe1 --> Fix1[Fix: Use CRDTs]
    Unsafe2 --> Fix2[Fix: Use unique keys]
    Unsafe3 --> Fix3[Fix: Make idempotent]
    Unsafe4 --> Fix4[Fix: Only modify CRDTs]
    
    Fix1 --> Start
    Fix2 --> Start
    Fix3 --> Start
    Fix4 --> Start
    
    style Start fill:#4DABF7,stroke:#333,stroke-width:3px,color:#000
    style Q1 fill:#FFB84D,stroke:#333,stroke-width:3px,color:#000
    style Q2 fill:#FFB84D,stroke:#333,stroke-width:3px,color:#000
    style Q3 fill:#FFB84D,stroke:#333,stroke-width:3px,color:#000
    style Q4 fill:#FFB84D,stroke:#333,stroke-width:3px,color:#000
    style Safe fill:#51CF66,stroke:#333,stroke-width:3px,color:#000
    style Unsafe1 fill:#FF6B6B,stroke:#333,stroke-width:3px,color:#000
    style Unsafe2 fill:#FF6B6B,stroke:#333,stroke-width:3px,color:#000
    style Unsafe3 fill:#FF6B6B,stroke:#333,stroke-width:3px,color:#000
    style Unsafe4 fill:#FF6B6B,stroke:#333,stroke-width:3px,color:#000
    style Fix1 fill:#4ECDC4,stroke:#333,stroke-width:3px,color:#000
    style Fix2 fill:#4ECDC4,stroke:#333,stroke-width:3px,color:#000
    style Fix3 fill:#4ECDC4,stroke:#333,stroke-width:3px,color:#000
    style Fix4 fill:#4ECDC4,stroke:#333,stroke-width:3px,color:#000

Your handlers MUST be:

1. Commutative (Order-Independent)

// ✅ SAFE - Counter increment is commutative
pub fn handler_a(&mut self) { self.counter.increment(); }
pub fn handler_b(&mut self) { self.counter.increment(); }
// Result: counter = 2, regardless of order

// ❌ UNSAFE - Operations depend on order
pub fn create(&mut self, id: &str) { self.items.insert(id, "new"); }
pub fn update(&mut self, id: &str) {
    let item = self.items.get(id).expect("must exist");  // BREAKS if create() not run first!
    self.items.insert(id, format!("{} updated", item));
}

2. Independent (No Shared State)

// ✅ SAFE - Each handler uses unique key
pub fn handler_a(&mut self, user: &str) {
    self.counters.insert(format!("a_{}", user), 1);
}
pub fn handler_b(&mut self, user: &str) {
    self.counters.insert(format!("b_{}", user), 1);
}

// ❌ UNSAFE - Both modify same key
pub fn handler_a(&mut self) {
    self.shared.insert("count", "1");  // RACE CONDITION!
}
pub fn handler_b(&mut self) {
    self.shared.insert("count", "2");  // RACE CONDITION!
}

3. Idempotent (Safe to Retry)

// ✅ SAFE - CRDT operations are naturally idempotent
pub fn handler(&mut self) {
    self.counter.increment();  // Safe to call multiple times
}

// ❌ UNSAFE - External calls are not idempotent
pub fn handler(&mut self, amount: u64) {
    external_payment_api::charge(amount);  // DANGER: May charge twice!
}

4. Pure (No External Side Effects)

// ✅ SAFE - Only modifies CRDT state
pub fn handler(&mut self, item: &str) {
    self.items.insert(item.to_owned(), "processed".to_owned());
    app::log!("Handler called");  // Logging is fine
}

// ❌ UNSAFE - External side effects
pub fn handler(&mut self, email: &str) {
    http_client::post("/notify", email);  // DANGER: Not deterministic!
    write_to_file(email);  // DANGER: Not WASM-compatible!
}

Best Practices

Use CRDTs for Everything

// ❌ BAD - Simple counter (loses concurrent updates)
struct App {
    counter: u64,
}
impl App {
    pub fn increment(&mut self) {
        self.counter += 1;  // Concurrent updates = lost count!
    }
}

// ✅ GOOD - G-Counter (handles concurrent updates)
struct App {
    counter: Counter,
}
impl App {
    pub fn increment(&mut self) {
        self.counter.increment();  // Each node tracks separately
    }
    pub fn total(&self) -> u64 {
        self.counter.value()  // Sum across all nodes
    }
}

Keep Handlers Simple

// ✅ GOOD - Simple CRDT operation
pub fn on_user_registered(&mut self, user_id: &str) {
    self.registration_count.increment();
}

// ❌ BAD - Complex logic with dependencies
pub fn on_user_registered(&mut self, user_id: &str) {
    // Creates ordering dependency!
    self.create_user_profile(user_id);
    self.send_welcome_email(user_id);
    self.update_analytics();
}

Use Events for Workflow

// Instead of chaining in handlers, emit events:
pub fn register_user(&mut self, user: User) -> app::Result<()> {
    self.users.insert(user.id.clone(), user.clone())?;
    
    // Each event has its own independent handler
    app::emit!((Event::UserRegistered { id: user.id.clone() }, "count_registration"));
    app::emit!((Event::UserRegistered { id: user.id.clone() }, "log_registration"));
    app::emit!((Event::UserRegistered { id: user.id }, "notify_admins"));
    
    Ok(())
}

// Handlers can run in parallel safely (each modifies different CRDT)
pub fn count_registration(&mut self, id: &str) {
    self.registrations.increment();
}
pub fn log_registration(&mut self, id: &str) {
    self.logs.push(format!("User {} registered", id));
}
pub fn notify_admins(&mut self, id: &str) {
    self.notifications.insert(id.to_owned(), "new_user".to_owned());
}

Available Macros

#[app::state]           // Mark application state struct
#[app::state(emits = Event)]  // With events

#[app::logic]           // Mark implementation block
#[app::init]            // Mark constructor
#[app::event]           // Mark event enum

app::emit!(event)       // Emit event
app::emit!((event, "handler"))  // Emit with handler
app::log!("msg")        // Logging
app::bail!(Error::X)    // Early return with error

Building

# Add WASM target
rustup target add wasm32-unknown-unknown

# Build application
cargo build --target wasm32-unknown-unknown --release

# Output: target/wasm32-unknown-unknown/release/my_app.wasm

Testing

Unit Tests (Application Logic)

#[cfg(test)]
mod tests {
    use super::*;
    
    #[test]
    fn test_add_item() {
        let mut app = MyApp::init();
        app.add_item("key".to_owned(), "value".to_owned()).unwrap();
        assert_eq!(app.get_item("key").unwrap(), Some("value".to_owned()));
    }
    
    #[test]
    fn test_counter_increments() {
        let mut app = MyApp::init();
        app.counter.increment().unwrap();
        app.counter.increment().unwrap();
        assert_eq!(app.counter.value().unwrap(), 2);
    }
}

E2E Tests (Real Network Scenarios)

E2E tests validate multi-node scenarios in e2e-tests/ directory.

E2E Test: kv-store-with-handlers

sequenceDiagram
    participant Test as Test Runner
    participant NodeInv as Node Inviter
    participant NodeInv2 as Node Invitee
    participant RPC as JSON-RPC
    
    rect rgb(220, 237, 255)
        Note over Test,RPC: Setup: Start 2 nodes, create context
        Test->>NodeInv: Start merod (inviter)
        Test->>NodeInv2: Start merod (invitee)
        Test->>RPC: create_context(kv-store-with-handlers)
        RPC-->>Test: context_id
    end
    
    rect rgb(255, 237, 220)
        Note over Test,NodeInv2: Test 1: set_value (inviter)
        Test->>RPC: call("set_value", {key: "test", value: "hello"})
        RPC->>NodeInv: Execute WASM
        NodeInv->>NodeInv: map.insert("test", "hello")
        NodeInv->>NodeInv: Emit event: ValueSet
        NodeInv->>NodeInv: Broadcast delta
        NodeInv->>NodeInv2: Propagate via Gossipsub
        NodeInv2->>NodeInv2: Apply delta
        NodeInv2->>NodeInv2: Execute handler: log_handler_call
        NodeInv2->>NodeInv2: counter.increment()
    end
    
    rect rgb(220, 255, 237)
        Note over Test,NodeInv2: Test 2: Verify handler execution count
        Test->>RPC: call("get_handler_execution_count")
        
        RPC->>NodeInv: Query counter
        NodeInv-->>RPC: counter.value() = 1<br/>(only invitee executed)
        
        RPC->>NodeInv2: Query counter
        NodeInv2-->>RPC: counter.value() = 1<br/>(invitee executed)
        
        Note over Test: ✅ PASS: Global count = 1<br/>(only receiving node executed)
    end
    
    rect rgb(237, 220, 255)
        Note over Test,NodeInv2: Test 3: Multiple operations
        Test->>RPC: call("set_value", {key: "k2", value: "v2"})
        Test->>RPC: call("set_value", {key: "k3", value: "v3"})
        
        NodeInv->>NodeInv2: Broadcast 2 deltas
        NodeInv2->>NodeInv2: Execute 2 handlers
        
        Test->>RPC: get_handler_execution_count
        RPC-->>Test: 3 total executions ✅
    end

What it validates:

• Event handlers execute on receiving nodes only
• Author node skips its own handlers
• G-Counter correctly sums across nodes
• Real network propagation via Gossipsub
• CRDT consistency in multi-node setup

Test file: e2e-tests/config/protocols/near/kv-store-with-handlers-test.json

E2E Test Flow Diagram

flowchart TB
    Start([Test Start]) --> Build[Build WASM apps<br/>cargo build --release]
    Build --> StartNodes[Start merod processes<br/>2-3 nodes]
    
    StartNodes --> CreateCtx[Create context<br/>Install app]
    
    CreateCtx --> Test1[Test 1: Basic operation]
    Test1 --> Call1[JSON-RPC call]
    Call1 --> Wait1[Wait for sync<br/>3-5 seconds]
    Wait1 --> Verify1{Verify result<br/>on all nodes?}
    
    Verify1 -->|Pass| Test2[Test 2: Handler execution]
    Verify1 -->|Fail| Fail([Test Failed])
    
    Test2 --> Call2[JSON-RPC call with event]
    Call2 --> Wait2[Wait for handler<br/>3-5 seconds]
    Wait2 --> Verify2{Verify handler<br/>count?}
    
    Verify2 -->|Pass| Test3[Test 3: Multiple nodes]
    Verify2 -->|Fail| Fail
    
    Test3 --> Join[Node joins context]
    Join --> Sync[Wait for sync]
    Sync --> Verify3{All nodes<br/>consistent?}
    
    Verify3 -->|Pass| Cleanup[Cleanup:<br/>Stop nodes]
    Verify3 -->|Fail| Fail
    
    Cleanup --> Success([✅ Test Passed])
    
    style Start fill:#4DABF7,stroke:#333,stroke-width:3px,color:#000
    style Success fill:#51CF66,stroke:#333,stroke-width:3px,color:#000
    style Fail fill:#FF6B6B,stroke:#333,stroke-width:3px,color:#fff
    style Wait1 fill:#FFB84D,stroke:#333,stroke-width:3px,color:#000
    style Wait2 fill:#FFB84D,stroke:#333,stroke-width:3px,color:#000
    style Verify1 fill:#FFB84D,stroke:#333,stroke-width:3px,color:#000
    style Verify2 fill:#FFB84D,stroke:#333,stroke-width:3px,color:#000
    style Verify3 fill:#FFB84D,stroke:#333,stroke-width:3px,color:#000

E2E Test Configuration:

{
  "protocol": "near",
  "steps": [
    {
      "action": "call",
      "node": "inviter",
      "method": "set_value",
      "args": {"key": "test", "value": "hello"},
      "expectedResultJson": null
    },
    {
      "action": "wait",
      "durationMs": 3000
    },
    {
      "action": "call",
      "node": "inviter",
      "method": "get_handler_execution_count",
      "args": {},
      "expectedResultJson": 1
    }
  ]
}

Test Commands

# Unit tests (fast, no network)
cargo test -p my-app

# Integration tests (WASM execution)
cargo test -p my-app --test integration

# E2E tests (full node network)
cd e2e-tests
cargo run -- \
  --merod-binary ../target/release/merod \
  --meroctl-binary ../target/release/meroctl \
  --protocols near

Test Pyramid:

• Unit tests: Application logic, fast (<1s), no network
• Integration tests: WASM execution, medium (~5s), mocked storage
• E2E tests: Full stack, slow (~30s), real network

Common Patterns

Counter Pattern (Distributed Counting)

use calimero_storage::collections::Counter;

pub struct Analytics {
    page_views: Counter,
    user_signups: Counter,
}

impl Analytics {
    pub fn track_page_view(&mut self) {
        self.page_views.increment();
    }
    
    pub fn get_total_views(&self) -> u64 {
        self.page_views.value()  // Sum across all nodes
    }
}

Map Pattern (Key-Value Storage)

use calimero_storage::collections::UnorderedMap;

pub struct UserStore {
    users: UnorderedMap<String, User>,
}

impl UserStore {
    pub fn upsert_user(&mut self, id: String, user: User) {
        self.users.insert(id, user);  // LWW on conflict
    }
}

List Pattern (Ordered Items)

use calimero_storage::collections::Vector;

pub struct Timeline {
    posts: Vector<Post>,
}

impl Timeline {
    pub fn add_post(&mut self, post: Post) {
        self.posts.push(post);
    }
    
    pub fn get_posts(&self) -> Vec<Post> {
        self.posts.iter().cloned().collect()
    }
}

Error Handling

use calimero_sdk::app;
use thiserror::Error;

#[derive(Debug, Error)]
#[error("Not found: {0}")]
pub struct NotFoundError(String);

pub fn get_user(&self, id: &str) -> app::Result<User> {
    let Some(user) = self.users.get(id)? else {
        app::bail!(NotFoundError(id.to_owned()));
    };
    Ok(user)
}

Environment Functions

use calimero_sdk::env;

// Get current executor
let executor = env::executor_id();  // [u8; 32]

// Get context
let context = env::context_id();  // [u8; 32]

// Logging
env::log("Hello from WASM");

// Time
let now = env::time_now();  // u64 nanoseconds

See Also

• calimero-storage - CRDT collections and storage
• calimero-node - Node runtime and synchronization
• Example apps - Reference implementations

License

See COPYRIGHT and LICENSE.md in the repository root.