1 files changed, 190 insertions, 0 deletions
diff --git a/lib/balanced-pool.js b/lib/balanced-pool.js
new file mode 100644
index 0000000..10bc6a4
--- /dev/null
+++ b/lib/balanced-pool.js
@@ -0,0 +1,190 @@
+'use strict'
+
+const {
+  BalancedPoolMissingUpstreamError,
+  InvalidArgumentError
+} = require('./core/errors')
+const {
+  PoolBase,
+  kClients,
+  kNeedDrain,
+  kAddClient,
+  kRemoveClient,
+  kGetDispatcher
+} = require('./pool-base')
+const Pool = require('./pool')
+const { kUrl, kInterceptors } = require('./core/symbols')
+const { parseOrigin } = require('./core/util')
+const kFactory = Symbol('factory')
+
+const kOptions = Symbol('options')
+const kGreatestCommonDivisor = Symbol('kGreatestCommonDivisor')
+const kCurrentWeight = Symbol('kCurrentWeight')
+const kIndex = Symbol('kIndex')
+const kWeight = Symbol('kWeight')
+const kMaxWeightPerServer = Symbol('kMaxWeightPerServer')
+const kErrorPenalty = Symbol('kErrorPenalty')
+
+function getGreatestCommonDivisor (a, b) {
+  if (b === 0) return a
+  return getGreatestCommonDivisor(b, a % b)
+}
+
+function defaultFactory (origin, opts) {
+  return new Pool(origin, opts)
+}
+
+class BalancedPool extends PoolBase {
+  constructor (upstreams = [], { factory = defaultFactory, ...opts } = {}) {
+    super()
+
+    this[kOptions] = opts
+    this[kIndex] = -1
+    this[kCurrentWeight] = 0
+
+    this[kMaxWeightPerServer] = this[kOptions].maxWeightPerServer || 100
+    this[kErrorPenalty] = this[kOptions].errorPenalty || 15
+
+    if (!Array.isArray(upstreams)) {
+      upstreams = [upstreams]
+    }
+
+    if (typeof factory !== 'function') {
+      throw new InvalidArgumentError('factory must be a function.')
+    }
+
+    this[kInterceptors] = opts.interceptors && opts.interceptors.BalancedPool && Array.isArray(opts.interceptors.BalancedPool)
+      ? opts.interceptors.BalancedPool
+      : []
+    this[kFactory] = factory
+
+    for (const upstream of upstreams) {
+      this.addUpstream(upstream)
+    }
+    this._updateBalancedPoolStats()
+  }
+
+  addUpstream (upstream) {
+    const upstreamOrigin = parseOrigin(upstream).origin
+
+    if (this[kClients].find((pool) => (
+      pool[kUrl].origin === upstreamOrigin &&
+      pool.closed !== true &&
+      pool.destroyed !== true
+    ))) {
+      return this
+    }
+    const pool = this[kFactory](upstreamOrigin, Object.assign({}, this[kOptions]))
+
+    this[kAddClient](pool)
+    pool.on('connect', () => {
+      pool[kWeight] = Math.min(this[kMaxWeightPerServer], pool[kWeight] + this[kErrorPenalty])
+    })
+
+    pool.on('connectionError', () => {
+      pool[kWeight] = Math.max(1, pool[kWeight] - this[kErrorPenalty])
+      this._updateBalancedPoolStats()
+    })
+
+    pool.on('disconnect', (...args) => {
+      const err = args[2]
+      if (err && err.code === 'UND_ERR_SOCKET') {
+        // decrease the weight of the pool.
+        pool[kWeight] = Math.max(1, pool[kWeight] - this[kErrorPenalty])
+        this._updateBalancedPoolStats()
+      }
+    })
+
+    for (const client of this[kClients]) {
+      client[kWeight] = this[kMaxWeightPerServer]
+    }
+
+    this._updateBalancedPoolStats()
+
+    return this
+  }
+
+  _updateBalancedPoolStats () {
+    this[kGreatestCommonDivisor] = this[kClients].map(p => p[kWeight]).reduce(getGreatestCommonDivisor, 0)
+  }
+
+  removeUpstream (upstream) {
+    const upstreamOrigin = parseOrigin(upstream).origin
+
+    const pool = this[kClients].find((pool) => (
+      pool[kUrl].origin === upstreamOrigin &&
+      pool.closed !== true &&
+      pool.destroyed !== true
+    ))
+
+    if (pool) {
+      this[kRemoveClient](pool)
+    }
+
+    return this
+  }
+
+  get upstreams () {
+    return this[kClients]
+      .filter(dispatcher => dispatcher.closed !== true && dispatcher.destroyed !== true)
+      .map((p) => p[kUrl].origin)
+  }
+
+  [kGetDispatcher] () {
+    // We validate that pools is greater than 0,
+    // otherwise we would have to wait until an upstream
+    // is added, which might never happen.
+    if (this[kClients].length === 0) {
+      throw new BalancedPoolMissingUpstreamError()
+    }
+
+    const dispatcher = this[kClients].find(dispatcher => (
+      !dispatcher[kNeedDrain] &&
+      dispatcher.closed !== true &&
+      dispatcher.destroyed !== true
+    ))
+
+    if (!dispatcher) {
+      return
+    }
+
+    const allClientsBusy = this[kClients].map(pool => pool[kNeedDrain]).reduce((a, b) => a && b, true)
+
+    if (allClientsBusy) {
+      return
+    }
+
+    let counter = 0
+
+    let maxWeightIndex = this[kClients].findIndex(pool => !pool[kNeedDrain])
+
+    while (counter++ < this[kClients].length) {
+      this[kIndex] = (this[kIndex] + 1) % this[kClients].length
+      const pool = this[kClients][this[kIndex]]
+
+      // find pool index with the largest weight
+      if (pool[kWeight] > this[kClients][maxWeightIndex][kWeight] && !pool[kNeedDrain]) {
+        maxWeightIndex = this[kIndex]
+      }
+
+      // decrease the current weight every `this[kClients].length`.
+      if (this[kIndex] === 0) {
+        // Set the current weight to the next lower weight.
+        this[kCurrentWeight] = this[kCurrentWeight] - this[kGreatestCommonDivisor]
+
+        if (this[kCurrentWeight] <= 0) {
+          this[kCurrentWeight] = this[kMaxWeightPerServer]
+        }
+      }
+      if (pool[kWeight] >= this[kCurrentWeight] && (!pool[kNeedDrain])) {
+        return pool
+      }
+    }
+
+    this[kCurrentWeight] = this[kClients][maxWeightIndex][kWeight]
+    this[kIndex] = maxWeightIndex
+    return this[kClients][maxWeightIndex]
+  }
+}
+
+module.exports = BalancedPool