sidebar_position	title	description	sidebar_custom_props
100	Events	Monitor off-chain smart contract changes.	migration href label https://developers.stellar.org/docs/learn/smart-contract-internals/events Events

<title>Monitor off-chain smart contract changes.</title>

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This page has been migrated to the Stellar Developers documentation. Please click here for the most up-to-date information

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Events are the mechanism that applications off-chain can use to monitor changes and events in contracts on-chain.

How are events emitted?

ContractEvents are emitted in Stellar Core's TransactionMeta. You can see in the TransactionMetaV3 XDR below that there is a list of OperationEvents called events. Each OperationEvent corresponds to an operation in a transaction, and itself contains a list of ContractEvents. Note that events will only be populated if the transaction succeeds. Take a look at this example to learn more about how to emit an event in your contract.

ContractEvent

struct ContractEvent
{
    // We can use this to add more fields, or because it
    // is first, to change ContractEvent into a union.
    ExtensionPoint ext;

    Hash* contractID;
    ContractEventType type;

    union switch (int v)
    {
    case 0:
        struct
        {
            SCVec topics;
            SCVal data;
        } v0;
    }
    body;
};

OperationEvents

struct OperationEvents
{
    ContractEvent events<>;
};

TransactionMetaV3

struct TransactionMetaV3
{
    LedgerEntryChanges txChangesBefore; // tx level changes before operations
                                        // are applied if any
    OperationMeta operations<>;         // meta for each operation
    LedgerEntryChanges txChangesAfter;  // tx level changes after operations are
                                        // applied if any
    OperationEvents events<>;           // custom events populated by the
                                        // contracts themselves. One list per operation.
    TransactionResult txResult;

    Hash hashes[3];                     // stores sha256(txChangesBefore, operations, txChangesAfter),
                                        // sha256(events), and sha256(txResult)

    // Diagnostics events that are not hashed. One list per operation.
    // This will contain all contract and diagnostic events. Even ones
    // that were emitted in a failed contract call.
    OperationDiagnosticEvents diagnosticEvents<>;
};

Link to the XDR above.

Event types

There are three ContractEventType's -

1. CONTRACT events are events emitted by contracts that use the contract_event host function to convey state changes.
2. SYSTEM events are events emitted by the host. At the moment, there's only one system event emitted by the host. It is emitted when the update_current_contract_wasm host function is called, where topics = ["executable_update", old_executable: ContractExecutable, old_executable: ContractExecutable] and data = [].
3. DIAGNOSTIC events are meant for debugging and will not be emitted unless the host instance explictly enables it. You can read more about this below.

What are diagnosticEvents?

While looking at the TransactionMetaV3 XDR struct above, you may have noticed the diagnosticEvents field. This list will be empty by default unless your stellar-core instance has ENABLE_SOROBAN_DIAGNOSTIC_EVENTS=true in its config file. If diagnostic events are enabled, this list will not only include all ContractEvents in events, but will also include events from failed contract calls, errors from the host, events to trace the contract call stack, and logs from the log_from_linear_memory host function. These events can be identified by type == DIAGNOSTIC. The diagnostic events emitted by the host to track the call stack are defined below.

fn_call

The fn_call diagnostic event is emitted when a contract is called and contains -

• Topics
  1. The symbol "fn_call".
  2. The contract id of the contract about to be called.
  3. A symbol containing the name of the function being called.
• Data
  1. A vector of the arguments passed to the function being called.

fn_return

The fn_return diagnostic event is emitted when a contract call completes and contains -

• Topics
  1. The symbol "fn_return".
  2. A symbol containing the name of the function that is about to return.
• Data
  1. The value returned by the contract function.

When should diagnostic events be enabled?

events contain ContractEvents that should convey information about state changes. diagnosticEvents on the other hand contain events that are not useful for most users, but may be helpful in debugging issues or building the contract call stack. Because they won't be used by most users, they can be optionally enabled because they are not hashed into the ledger, and therefore are not part of the protocol. This is done so a stellar-core node can stay in sync with the network while emitting these events that normally would not be useful for most users.

Due to the fact that a node with diagnostic events enabled will be executing code paths that diverge from a regular node, we highly encourage only using this feature on watcher node (nodes where NODE_IS_VALIDATOR=false is set).