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* Alternative SessionPool.
*
* Design notes (diverges from session-pool.ts):
* - Single source of truth: #all (Set). Idle is a derived LIFO stack.
* - Idle is LIFO for cache warmth (server-side plan cache).
* - Busy has no explicit marker — it's "in #all but not in #available".
* - No background sweeper. Dead sessions are dropped lazily on acquire.
* - Waiters have a hard cap (maxSize * 8) — caller gets fast failure
* under thundering-herd rather than unbounded queue growth.
* - A dropped session opens a slot; we proactively refill for waiters.
* - close() is atomic w.r.t. in-flight creates: we own their abort
* controllers, cancel them on close, and allSettle before returning.
*
* Memory-leak hygiene (important on Node <22.6 and Bun):
* - No AbortSignal.any anywhere. Every `abort`-listener we add is also
* explicitly removed on success paths (see #mergeSignals / detach()).
* - SessionLease mirrors session.signal into a *lease-scoped* controller
* so each acquire/release cycle attaches exactly one listener on the
* long-lived session signal and removes it on dispose. Otherwise a
* hot session serving N queries would accumulate N listeners.
*/
import { channel as dc, tracingChannel } from 'node:diagnostics_channel'
import { linkSignals } from '@ydbjs/abortable'
import type { Driver, DriverIdentity } from '@ydbjs/core'
import { loggers } from '@ydbjs/debug'
import { Session, type SessionCloseReason } from './session.js'
type SessionAcquireCtx = { driver: DriverIdentity }
type SessionCreateCtx = { driver: DriverIdentity }
let sessionCreateCh = tracingChannel<SessionCreateCtx, SessionCreateCtx>(
'tracing:ydb:query.session.create'
)
export let sessionAcquireCh = tracingChannel<SessionAcquireCtx, SessionAcquireCtx>(
'tracing:ydb:query.session.acquire'
)
let dbg = loggers.query.extend('pool2')
export class SessionPoolClosedError extends Error {
constructor() {
super('session pool is closed')
this.name = 'SessionPoolClosedError'
}
}
export class SessionPoolFullError extends Error {
constructor(limit: number) {
super(`session pool wait queue is full (limit=${limit})`)
this.name = 'SessionPoolFullError'
}
}
export type SessionPoolOptions = {
/** Hard cap on simultaneously-live sessions (idle + busy + creating). */
maxSize?: number
/**
* Soft floor reported as `ydb.query.session.min` for dashboard parity
* with other YDB SDKs. The JS pool does not eagerly warm sessions up to
* this number. Default 0.
*/
minSize?: number
/** Multiplier over maxSize for the wait queue. Default 8. */
waitQueueFactor?: number
}
let DEFAULTS = {
maxSize: 50,
minSize: 0,
waitQueueFactor: 8,
} satisfies Required<SessionPoolOptions>
type Waiter = {
resolve: (s: Session) => void
reject: (e: Error) => void
/** Detaches the external AbortSignal listener if any. */
detach: () => void
}
/**
* A lease's signal is NOT session.signal directly — it's a private
* controller that mirrors session.signal for the lifetime of the lease.
* When the lease is disposed, we explicitly detach from session.signal so
* a hot session serving thousands of queries doesn't accrue thousands of
* listeners. For merging further signals with `lease.signal` downstream,
* use `linkSignals` (@ydbjs/abortable) — it cleans up on dispose, unlike
* `AbortSignal.any` which leaks on Node <22.6 / Bun.
*/
export class SessionLease implements Disposable {
#pool: SessionPool
readonly session: Session
#mirror = new AbortController()
#detach: (() => void) | undefined
constructor(pool: SessionPool, session: Session) {
this.#pool = pool
this.session = session
Iif (session.signal.aborted) {
this.#mirror.abort(session.signal.reason)
return
}
let handler = () => this.#mirror.abort(session.signal.reason)
session.signal.addEventListener('abort', handler, { once: true })
this.#detach = () => session.signal.removeEventListener('abort', handler)
}
get id(): string {
return this.session.id
}
get nodeId(): bigint {
return this.session.nodeId
}
/** Aborts when the session dies. Lease-scoped: safe to chain further. */
get signal(): AbortSignal {
return this.#mirror.signal
}
[Symbol.dispose](): void {
Eif (this.#detach) {
this.#detach()
this.#detach = undefined
}
this.#pool.release(this.session)
}
}
export class SessionPool implements AsyncDisposable {
#driver: Driver
#maxSize: number
#minSize: number
#maxWaiters: number
#all = new Set<Session>()
#available: Session[] = [] // LIFO: pop from end, push to end
#waiters: Waiter[] = [] // FIFO
#creates = new Set<Promise<unknown>>()
#close = new AbortController()
#evictionHandlers = new WeakMap<Session, () => void>()
constructor(driver: Driver, options: SessionPoolOptions = {}) {
this.#driver = driver
this.#maxSize = options.maxSize ?? DEFAULTS.maxSize
this.#minSize = options.minSize ?? DEFAULTS.minSize
this.#maxWaiters = this.#maxSize * (options.waitQueueFactor ?? DEFAULTS.waitQueueFactor)
if (this.#minSize > this.#maxSize) {
throw new RangeError(
`minSize (${this.#minSize}) cannot exceed maxSize (${this.#maxSize})`
)
}
// Single config snapshot for this pool. Metrics subscribers anchor
// their per-pool ObservableGauges here; per-session events deliberately
// do not repeat config.
dc('ydb:query.session.pool.opened').publish({
driver: this.#driver.identity,
maxSize: this.#maxSize,
minSize: this.#minSize,
maxWaiters: this.#maxWaiters,
})
this.#refillToMinSize()
}
get closed(): boolean {
return this.#close.signal.aborted
}
get stats() {
return {
total: this.#all.size,
idle: this.#available.length,
busy: this.#all.size - this.#available.length,
creating: this.#creates.size,
waiting: this.#waiters.length,
maxSize: this.#maxSize,
minSize: this.#minSize,
}
}
async acquire(signal?: AbortSignal): Promise<SessionLease> {
try {
let session = await this.#acquireSession(signal)
dc('ydb:query.session.acquired').publish({
driver: this.#driver.identity,
nodeId: session.nodeId,
sessionId: session.id,
})
return new SessionLease(this, session)
} catch (error) {
// Caller cancellation isn't a pool-side failure — the request was
// withdrawn before we could serve it. Skip the failed event so the
// `acquire.failures` counter stays a pool-health signal rather than
// a count of upstream-cancelled requests. Pool-internal aborts
// (pool.close, full queue, timeout we picked) still publish.
Eif (!signal?.aborted) {
dc('ydb:query.session.acquire.failed').publish({
driver: this.#driver.identity,
error,
})
}
throw error
}
}
release(session: Session): void {
// Pairs with `ydb:query.session.acquired`: exactly one release per
// lease, whatever path the session takes afterwards.
dc('ydb:query.session.released').publish({
driver: this.#driver.identity,
nodeId: session.nodeId,
sessionId: session.id,
})
// Session died while someone was using it. The eviction listener
// already popped it from #all and published `closed{stream_*}`. The
// server has either killed the session itself (stream_closed) or the
// stream errored — either way the server-side TTL reaps it, so no
// extra DeleteSession RPC is required from us.
if (!session.alive || !this.#all.has(session)) {
return
}
if (this.closed) {
// Pool was closed between acquire() and release(); already-tracked
// sessions get a `pool_close` event so subscribers see one event
// per session lifecycle regardless of timing. Detach the eviction
// listener first — session.close() aborts the signal, which would
// otherwise re-fire `closed{stream_*}` for the same session.
this.#detachEviction(session)
this.#publishClosed(session, 'pool_close')
this.#all.delete(session)
session.close('pool_close')
return
}
session.lastUsedAt = Date.now()
// Direct handoff: no bounce through #available.
let waiter = this.#waiters.shift()
if (waiter) {
waiter.detach()
dc('ydb:query.session.waiter.dequeued').publish({
driver: this.#driver.identity,
})
waiter.resolve(session)
return
}
this.#available.push(session)
}
async close(): Promise<void> {
if (this.closed) return
this.#close.abort(new SessionPoolClosedError())
// Reject every waiter with a clear cause.
let waiters = this.#waiters.splice(0)
for (let w of waiters) {
w.detach()
dc('ydb:query.session.waiter.dequeued').publish({
driver: this.#driver.identity,
})
w.reject(new SessionPoolClosedError())
}
// Only thing worth awaiting: in-flight Session.open calls. The
// pool-close signal already propagated into them, so they'll bail
// quickly. Once settled, nothing new can land in the pool.
await Promise.allSettled([...this.#creates])
let sessions = [...this.#all]
this.#all.clear()
this.#available.length = 0
// close() is sync + fire-and-forget — no need to await each RPC.
// Detach the eviction listener BEFORE close() so we don't double-fire
// `closed{stream_*}` for sessions that the pool itself is tearing down.
for (let s of sessions) {
this.#detachEviction(s)
this.#publishClosed(s, 'pool_close')
s.close('pool_close')
}
// Pairs with `pool.opened`; tells subscribers to drop per-pool state.
dc('ydb:query.session.pool.closed').publish({ driver: this.#driver.identity })
dbg.log('pool closed (%d sessions)', sessions.length)
}
async [Symbol.asyncDispose](): Promise<void> {
await this.close()
}
// --- internals -------------------------------------------------------
async #acquireSession(signal?: AbortSignal): Promise<Session> {
Iif (this.closed) throw new SessionPoolClosedError()
signal?.throwIfAborted()
// 1. Fast path: pop a warm idle session (LIFO). Skip dead ones lazily.
while (this.#available.length > 0) {
let candidate = this.#available.pop()!
Eif (candidate.alive) return candidate
this.#all.delete(candidate) // already reaped by eviction, belt & braces
}
// 2. Grow the pool if there's capacity.
if (this.#all.size + this.#creates.size < this.#maxSize) {
return this.#grow(signal)
}
// 3. Queue behind existing waiters.
return this.#enqueue(signal)
}
async #grow(signal?: AbortSignal): Promise<Session> {
// `using` auto-detaches listeners from every input (external + pool-
// close) when the block exits — no accumulation on #close.signal as
// many concurrent creates fan out.
using linked = linkSignals(signal, this.#close.signal)
let createCtx: SessionCreateCtx = { driver: this.#driver.identity }
let promise = sessionCreateCh.tracePromise(
() => Session.open(this.#driver, linked.signal),
createCtx
)
this.#creates.add(promise)
try {
let session = await promise
// The pool closed while we were creating. Dispose it, don't leak.
Iif (this.closed) {
session.close('pool_close')
throw new SessionPoolClosedError()
}
this.#all.add(session)
this.#bindEviction(session)
dc('ydb:query.session.created').publish({
driver: this.#driver.identity,
sessionId: session.id,
nodeId: session.nodeId,
})
return session
} finally {
this.#creates.delete(promise)
}
}
#bindEviction(session: Session): void {
let handler = () => {
this.#evictionHandlers.delete(session)
this.#all.delete(session)
let idx = this.#available.indexOf(session)
if (idx !== -1) this.#available.splice(idx, 1)
// Session.#markBroken set closeReason before aborting the signal;
// fall back to stream_closed only as a defensive default if a
// future code path aborts the signal without going through
// markBroken (today nothing does).
let reason = session.closeReason ?? 'stream_closed'
dbg.log('session %s left the pool (%s)', session.id, reason)
this.#publishClosed(session, reason)
// Order matters: waiters first (the freed capacity belongs to
// whoever was already queued), then top up to minSize if the
// slot is still empty.
this.#pumpWaitersAfterEviction()
this.#refillToMinSize()
}
this.#evictionHandlers.set(session, handler)
session.signal.addEventListener('abort', handler, { once: true })
}
/**
* Detach the eviction listener bound by #bindEviction. Used by close()
* to avoid a double `closed` event when the pool itself tears down a
* session (which abort() would otherwise reflect back through the
* eviction listener with reason=stream_*).
*/
#detachEviction(session: Session): void {
let handler = this.#evictionHandlers.get(session)
Iif (!handler) return
this.#evictionHandlers.delete(session)
session.signal.removeEventListener('abort', handler)
}
#publishClosed(session: Session, reason: SessionCloseReason): void {
dc('ydb:query.session.closed').publish({
driver: this.#driver.identity,
sessionId: session.id,
nodeId: session.nodeId,
reason,
uptime: Date.now() - session.createdAt,
})
}
/**
* An eviction just opened a slot. If any waiters are queued, spin up a
* replacement session for the oldest one. We only replace one session
* per eviction — if the caller's request fails, the next acquire will
* try again or another eviction will trigger another replacement.
*/
#pumpWaitersAfterEviction(): void {
Iif (this.closed) return
if (this.#waiters.length === 0) return
Iif (this.#all.size + this.#creates.size >= this.#maxSize) return
let waiter = this.#waiters.shift()!
waiter.detach()
dc('ydb:query.session.waiter.dequeued').publish({ driver: this.#driver.identity })
this.#grow().then(
(s) => waiter.resolve(s),
(e) => waiter.reject(e instanceof Error ? e : new Error(String(e)))
)
}
/**
* Brings (live + in-flight creates) up to minSize. Called from the
* constructor and after every eviction. Idempotent: computes the deficit
* each time, so over-calling is harmless. Each grow runs concurrently
* via a detached `#growAndPark`; the outer call returns immediately.
*/
#refillToMinSize(): void {
Iif (this.closed) return
let deficit = this.#minSize - (this.#all.size + this.#creates.size)
for (let i = 0; i < deficit; i++) {
void this.#growAndPark()
}
}
/**
* Grow once, then park the result: hand to a queued waiter if any,
* otherwise drop into `#available` for the next acquire().
*
* Errors are swallowed by design — warm-up is best-effort. A failed
* background create does not crash the app; the next acquire() or the
* next eviction-triggered refill will try again. `SessionPoolClosedError`
* is expected on shutdown (the linked close signal aborts pending
* creates).
*/
async #growAndPark(): Promise<void> {
let session: Session
try {
session = await this.#grow()
} catch (err) {
dbg.log('warm-up grow failed: %O', err)
return
}
Iif (this.closed) return // #grow already disposed it under this.closed
let waiter = this.#waiters.shift()
if (waiter) {
waiter.detach()
dc('ydb:query.session.waiter.dequeued').publish({
driver: this.#driver.identity,
})
waiter.resolve(session)
return
}
this.#available.push(session)
}
#enqueue(signal?: AbortSignal): Promise<Session> {
if (this.#waiters.length >= this.#maxWaiters) {
return Promise.reject(new SessionPoolFullError(this.#maxWaiters))
}
let { promise, resolve, reject } = Promise.withResolvers<Session>()
let driver = this.#driver.identity
let onAbort = () => {
let idx = this.#waiters.findIndex((w) => w.resolve === resolve)
if (idx !== -1) {
this.#waiters.splice(idx, 1)
dc('ydb:query.session.waiter.dequeued').publish({ driver })
}
reject(signal!.reason ?? new Error('aborted'))
}
let detach = signal ? () => signal.removeEventListener('abort', onAbort) : () => {}
if (signal) signal.addEventListener('abort', onAbort, { once: true })
this.#waiters.push({ resolve, reject, detach })
dc('ydb:query.session.waiter.enqueued').publish({ driver })
return promise
}
}
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