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Class ConnectionPool

A pool of connections which dynamically resizes, and emit events related to pool activity

Hierarchy

Index

Constructors

Events

CONNECTION_CHECKED_IN: "connectionCheckedIn" = CONNECTION_CHECKED_IN

Emitted each time a connection is successfully checked into the connection pool

CONNECTION_CHECKED_OUT: "connectionCheckedOut" = CONNECTION_CHECKED_OUT

Emitted each time a connection is successfully checked out of the connection pool

CONNECTION_CHECK_OUT_FAILED: "connectionCheckOutFailed" = CONNECTION_CHECK_OUT_FAILED

Emitted when an attempt to check out a connection fails

CONNECTION_CHECK_OUT_STARTED: "connectionCheckOutStarted" = CONNECTION_CHECK_OUT_STARTED

Emitted when an attempt to check out a connection begins

CONNECTION_CLOSED: "connectionClosed" = CONNECTION_CLOSED

Emitted when a connection is closed

CONNECTION_CREATED: "connectionCreated" = CONNECTION_CREATED

Emitted when a connection is created.

CONNECTION_POOL_CLEARED: "connectionPoolCleared" = CONNECTION_POOL_CLEARED

Emitted each time the connection pool is cleared and it's generation incremented

CONNECTION_POOL_CLOSED: "connectionPoolClosed" = CONNECTION_POOL_CLOSED

Emitted once when the connection pool is closed

CONNECTION_POOL_CREATED: "connectionPoolCreated" = CONNECTION_POOL_CREATED

Emitted when the connection pool is created.

CONNECTION_READY: "connectionReady" = CONNECTION_READY

Emitted when a connection becomes established, and is ready to use

Properties

[kCancellationToken]: CancellationToken
[kCheckedOut]: number
[kConnectionCounter]: Generator<number, any, unknown>
[kConnections]: Denque<Connection>
[kGeneration]: number

An integer representing the SDAM generation of the pool

[kLogger]: Logger
[kMinPoolSizeTimer]?: Timeout
[kPermits]: number

An integer expressing how many total connections are permitted

[kProcessingWaitQueue]: boolean
[kServiceGenerations]: Map<string, number>

A map of generations to service ids

[kWaitQueue]: Denque<WaitQueueMember>
closed: boolean
options: Readonly<ConnectionPoolOptions>
captureRejectionSymbol: typeof captureRejectionSymbol
captureRejections: boolean

Sets or gets the default captureRejection value for all emitters.

defaultMaxListeners: number
errorMonitor: typeof errorMonitor

This symbol shall be used to install a listener for only monitoring 'error' events. Listeners installed using this symbol are called before the regular 'error' listeners are called.

Installing a listener using this symbol does not change the behavior once an 'error' event is emitted, therefore the process will still crash if no regular 'error' listener is installed.

Accessors

  • get address(): string
  • get availableConnectionCount(): number
  • get currentCheckedOutCount(): number
  • get generation(): number
  • get loadBalanced(): boolean
  • get serviceGenerations(): Map<string, number>
  • get totalConnectionCount(): number
  • get waitQueueSize(): number

Methods

  • Check a connection out of this pool. The connection will continue to be tracked, but no reference to it will be held by the pool. This means that if a connection is checked out it MUST be checked back in or explicitly destroyed by the new owner.

    Parameters

    Returns void

  • Clear the pool

    Pool reset is handled by incrementing the pool's generation count. Any existing connection of a previous generation will eventually be pruned during subsequent checkouts.

    Parameters

    Returns void

  • emit<EventKey>(event: symbol | EventKey, ...args: Parameters<ConnectionPoolEvents[EventKey]>): boolean
  • Type parameters

    • EventKey: "commandStarted" | "commandSucceeded" | "commandFailed" | "clusterTimeReceived" | "pinned" | "unpinned" | "connectionPoolCreated" | "connectionPoolClosed" | "connectionPoolCleared" | "connectionCreated" | "connectionReady" | "connectionClosed" | "connectionCheckOutStarted" | "connectionCheckOutFailed" | "connectionCheckedOut" | "connectionCheckedIn"

    Parameters

    Returns boolean

  • eventNames(): string[]
  • getMaxListeners(): number
  • listenerCount<EventKey>(type: string | symbol | EventKey): number
  • Type parameters

    • EventKey: "commandStarted" | "commandSucceeded" | "commandFailed" | "clusterTimeReceived" | "pinned" | "unpinned" | "connectionPoolCreated" | "connectionPoolClosed" | "connectionPoolCleared" | "connectionCreated" | "connectionReady" | "connectionClosed" | "connectionCheckOutStarted" | "connectionCheckOutFailed" | "connectionCheckedOut" | "connectionCheckedIn"

    Parameters

    • type: string | symbol | EventKey

    Returns number

  • Type parameters

    • EventKey: "commandStarted" | "commandSucceeded" | "commandFailed" | "clusterTimeReceived" | "pinned" | "unpinned" | "connectionPoolCreated" | "connectionPoolClosed" | "connectionPoolCleared" | "connectionCreated" | "connectionReady" | "connectionClosed" | "connectionCheckOutStarted" | "connectionCheckOutFailed" | "connectionCheckedOut" | "connectionCheckedIn"

    Parameters

    • event: string | symbol | EventKey

    Returns ConnectionPoolEvents[EventKey][]

  • Type parameters

    • EventKey: "commandStarted" | "commandSucceeded" | "commandFailed" | "clusterTimeReceived" | "pinned" | "unpinned" | "connectionPoolCreated" | "connectionPoolClosed" | "connectionPoolCleared" | "connectionCreated" | "connectionReady" | "connectionClosed" | "connectionCheckOutStarted" | "connectionCheckOutFailed" | "connectionCheckedOut" | "connectionCheckedIn"

    Parameters

    • event: string | symbol | EventKey

    Returns ConnectionPoolEvents[EventKey][]

  • removeAllListeners<EventKey>(event?: string | symbol | EventKey): ConnectionPool
  • Type parameters

    • EventKey: "commandStarted" | "commandSucceeded" | "commandFailed" | "clusterTimeReceived" | "pinned" | "unpinned" | "connectionPoolCreated" | "connectionPoolClosed" | "connectionPoolCleared" | "connectionCreated" | "connectionReady" | "connectionClosed" | "connectionCheckOutStarted" | "connectionCheckOutFailed" | "connectionCheckedOut" | "connectionCheckedIn"

    Parameters

    • Optional event: string | symbol | EventKey

    Returns ConnectionPool

  • waitQueueErrorMetrics(): string
  • Runs a lambda with an implicitly checked out connection, checking that connection back in when the lambda has completed by calling back.

    NOTE: please note the required signature of fn

    remarks

    When in load balancer mode, connections can be pinned to cursors or transactions. In these cases we pass the connection in to this method to ensure it is used and a new connection is not checked out.

    Parameters

    Returns void

  • getEventListeners(emitter: DOMEventTarget | EventEmitter, name: string | symbol): Function[]
  • Returns a copy of the array of listeners for the event named eventName.

    For EventEmitters this behaves exactly the same as calling .listeners on the emitter.

    For EventTargets this is the only way to get the event listeners for the event target. This is useful for debugging and diagnostic purposes.

    const { getEventListeners, EventEmitter } = require('events');

    {
    const ee = new EventEmitter();
    const listener = () => console.log('Events are fun');
    ee.on('foo', listener);
    getEventListeners(ee, 'foo'); // [listener]
    }
    {
    const et = new EventTarget();
    const listener = () => console.log('Events are fun');
    et.addEventListener('foo', listener);
    getEventListeners(et, 'foo'); // [listener]
    }
    since

    v15.2.0, v14.17.0

    Parameters

    • emitter: DOMEventTarget | EventEmitter
    • name: string | symbol

    Returns Function[]

  • listenerCount(emitter: EventEmitter, eventName: string | symbol): number
  • A class method that returns the number of listeners for the given eventNameregistered on the given emitter.

    const { EventEmitter, listenerCount } = require('events');
    const myEmitter = new EventEmitter();
    myEmitter.on('event', () => {});
    myEmitter.on('event', () => {});
    console.log(listenerCount(myEmitter, 'event'));
    // Prints: 2
    since

    v0.9.12

    deprecated

    Since v3.2.0 - Use listenerCount instead.

    Parameters

    • emitter: EventEmitter

      The emitter to query

    • eventName: string | symbol

      The event name

    Returns number

  • on(emitter: EventEmitter, eventName: string, options?: StaticEventEmitterOptions): AsyncIterableIterator<any>
  • ```js const { on, EventEmitter } = require('events');

    (async () => { const ee = new EventEmitter();

    // Emit later on process.nextTick(() => { ee.emit('foo', 'bar'); ee.emit('foo', 42); });

    for await (const event of on(ee, 'foo')) { // The execution of this inner block is synchronous and it // processes one event at a time (even with await). Do not use // if concurrent execution is required. console.log(event); // prints ['bar'] [42] } // Unreachable here })();


    Returns an `AsyncIterator` that iterates `eventName` events. It will throw
    if the `EventEmitter` emits `'error'`. It removes all listeners when
    exiting the loop. The `value` returned by each iteration is an array
    composed of the emitted event arguments.

    An `AbortSignal` can be used to cancel waiting on events:

    ```js
    const { on, EventEmitter } = require('events');
    const ac = new AbortController();

    (async () => {
    const ee = new EventEmitter();

    // Emit later on
    process.nextTick(() => {
    ee.emit('foo', 'bar');
    ee.emit('foo', 42);
    });

    for await (const event of on(ee, 'foo', { signal: ac.signal })) {
    // The execution of this inner block is synchronous and it
    // processes one event at a time (even with await). Do not use
    // if concurrent execution is required.
    console.log(event); // prints ['bar'] [42]
    }
    // Unreachable here
    })();

    process.nextTick(() => ac.abort());
    since

    v13.6.0, v12.16.0

    Parameters

    • emitter: EventEmitter
    • eventName: string

      The name of the event being listened for

    • Optional options: StaticEventEmitterOptions

    Returns AsyncIterableIterator<any>

    that iterates eventName events emitted by the emitter

  • once(emitter: NodeEventTarget, eventName: string | symbol, options?: StaticEventEmitterOptions): Promise<any[]>
  • once(emitter: DOMEventTarget, eventName: string, options?: StaticEventEmitterOptions): Promise<any[]>
  • Creates a Promise that is fulfilled when the EventEmitter emits the given event or that is rejected if the EventEmitter emits 'error' while waiting. The Promise will resolve with an array of all the arguments emitted to the given event.

    This method is intentionally generic and works with the web platform EventTarget interface, which has no special'error' event semantics and does not listen to the 'error' event.

    const { once, EventEmitter } = require('events');

    async function run() {
    const ee = new EventEmitter();

    process.nextTick(() => {
    ee.emit('myevent', 42);
    });

    const [value] = await once(ee, 'myevent');
    console.log(value);

    const err = new Error('kaboom');
    process.nextTick(() => {
    ee.emit('error', err);
    });

    try {
    await once(ee, 'myevent');
    } catch (err) {
    console.log('error happened', err);
    }
    }

    run();

    The special handling of the 'error' event is only used when events.once()is used to wait for another event. If events.once() is used to wait for the 'error' event itself, then it is treated as any other kind of event without special handling:

    const { EventEmitter, once } = require('events');

    const ee = new EventEmitter();

    once(ee, 'error')
    .then(([err]) => console.log('ok', err.message))
    .catch((err) => console.log('error', err.message));

    ee.emit('error', new Error('boom'));

    // Prints: ok boom

    An AbortSignal can be used to cancel waiting for the event:

    const { EventEmitter, once } = require('events');

    const ee = new EventEmitter();
    const ac = new AbortController();

    async function foo(emitter, event, signal) {
    try {
    await once(emitter, event, { signal });
    console.log('event emitted!');
    } catch (error) {
    if (error.name === 'AbortError') {
    console.error('Waiting for the event was canceled!');
    } else {
    console.error('There was an error', error.message);
    }
    }
    }

    foo(ee, 'foo', ac.signal);
    ac.abort(); // Abort waiting for the event
    ee.emit('foo'); // Prints: Waiting for the event was canceled!
    since

    v11.13.0, v10.16.0

    Parameters

    • emitter: NodeEventTarget
    • eventName: string | symbol
    • Optional options: StaticEventEmitterOptions

    Returns Promise<any[]>

  • Parameters

    • emitter: DOMEventTarget
    • eventName: string
    • Optional options: StaticEventEmitterOptions

    Returns Promise<any[]>

  • setMaxListeners(n?: number, ...eventTargets: (DOMEventTarget | EventEmitter)[]): void
  • By default EventEmitters will print a warning if more than 10 listeners are added for a particular event. This is a useful default that helps finding memory leaks. The EventEmitter.setMaxListeners() method allows the default limit to be modified (if eventTargets is empty) or modify the limit specified in every EventTarget | EventEmitter passed as arguments. The value can be set toInfinity (or 0) to indicate an unlimited number of listeners.

    EventEmitter.setMaxListeners(20);
    // Equivalent to
    EventEmitter.defaultMaxListeners = 20;

    const eventTarget = new EventTarget();
    // Only way to increase limit for `EventTarget` instances
    // as these doesn't expose its own `setMaxListeners` method
    EventEmitter.setMaxListeners(20, eventTarget);
    since

    v15.3.0, v14.17.0

    Parameters

    • Optional n: number
    • Rest ...eventTargets: (DOMEventTarget | EventEmitter)[]

    Returns void

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