Replace unbounded index data array with fixed-size ring buffer

src/Index/ReadOnlyIndex.js

@@ -24,9 +24,9 @@ class ReadOnlyIndex extends watchesFile(ReadableIndex) {
         if (!this.fd) {
             return;
         }
-        const prevLength = this.data.length;
+        const prevLength = this._cache.length;
         const newLength = this.readFileLength();
-        this.data.length = newLength;
+        this._cache.truncate(newLength);
         if (newLength > prevLength) {
             this.emit('append', prevLength, newLength);
         }

src/Index/ReadableIndex.js

@@ -3,6 +3,7 @@ const path = require('path');
 const events = require('events');
 const Entry = require('../IndexEntry');
 const { assert, wrapAndCheck, binarySearch } = require('../util');
+const RingBuffer = require('./RingBuffer');
 
 // node-event-store-index V01
 const HEADER_MAGIC = "nesidx01";
@@ -38,6 +39,7 @@ class ReadableIndex extends events.EventEmitter {
      * @param {object} [options] An object with additional index options.
      * @param {typeof EntryInterface} [options.EntryClass] The entry class to use for index items. Must implement the EntryInterface methods.
      * @param {string} [options.dataDirectory] The directory to store the index file in. Default '.'.
+     * @param {number} [options.cacheSize] The number of most-recent index entries to keep in memory. Default 1024.
      * @param {number} [options.writeBufferSize] The number of bytes to use for the write buffer. Default 4096.
      * @param {number} [options.flushDelay] How many ms to delay the write buffer flush to optimize throughput. Default 100.
      * @param {object} [options.metadata] An object containing the metadata information for this index. Will be written on initial creation and checked on subsequent openings.
@@ -65,8 +67,8 @@ class ReadableIndex extends events.EventEmitter {
      * @param {object} options
      */
     initialize(options) {
-        /* @type Array<Entry> */
-        this.data = [];
+        const cacheSize = options.cacheSize !== undefined ? options.cacheSize : 1024;
+        this._cache = new RingBuffer(cacheSize);
         this.fd = null;
         this.fileMode = 'r';
         this.EntryClass = options.EntryClass;
@@ -100,7 +102,7 @@ class ReadableIndex extends events.EventEmitter {
      * @returns {number}
      */
     get length() {
-        return this.data.length;
+        return this._cache.length;
     }
 
     /**
@@ -155,7 +157,7 @@ class ReadableIndex extends events.EventEmitter {
 
         const length = this.readFileLength();
         if (length > 0) {
-            this.data = new Array(length);
+            this._cache.truncate(length);
             // Read last item to get the index started
             this.read(length);
         }
@@ -232,7 +234,7 @@ class ReadableIndex extends events.EventEmitter {
      * @api
      */
     close() {
-        this.data = [];
+        this._cache.reset();
         this.readUntil = -1;
         this.readBuffer.fill(0);
         if (this.fd) {
@@ -250,21 +252,24 @@ class ReadableIndex extends events.EventEmitter {
      * @returns {Entry} The index entry at the given position.
      */
     read(index) {
-        index = Number(index) - 1;
+        const i = Number(index) - 1; // 0-based
 
-        fs.readSync(this.fd, this.readBuffer, 0, this.EntryClass.size, this.headerSize + index * this.EntryClass.size);
-        if (index === this.readUntil + 1) {
+        fs.readSync(this.fd, this.readBuffer, 0, this.EntryClass.size, this.headerSize + i * this.EntryClass.size);
+        if (i === this.readUntil + 1) {
             this.readUntil++;
         }
-        this.data[index] = this.EntryClass.fromBuffer(this.readBuffer);
-
-        return this.data[index];
+        const entry = this.EntryClass.fromBuffer(this.readBuffer);
+        this._cache.set(i, entry);
+        return entry;
     }
 
     /**
      * Read a range of entries from disk. This method will not do any range checks.
      * It will however optimize to prevent reading entries that have already been read sequentially from start.
      *
+     * Entries within the cache window are stored in the cache; entries outside the window
+     * (older than cacheSize) are read from disk for the return value but not cached.
+     *
      * @private
      * @param {number} from The 1-based index position from where to read from (inclusive).
      * @param {number} until The 1-based index position until which to read to (inclusive).
@@ -275,25 +280,58 @@ class ReadableIndex extends events.EventEmitter {
             return [this.read(from)];
         }
 
-        from--;
-        until--;
-
-        const readFrom = Math.max(this.readUntil + 1, from);
-        const amount = (until - readFrom + 1);
-
-        const readBuffer = Buffer.allocUnsafe(amount * this.EntryClass.size);
-        let readSize = fs.readSync(this.fd, readBuffer, 0, readBuffer.byteLength, this.headerSize + readFrom * this.EntryClass.size);
-        let index = 0;
-        while (index < amount && readSize > 0) {
-            this.data[index + readFrom] = this.EntryClass.fromBuffer(readBuffer, index * this.EntryClass.size);
-            readSize -= this.EntryClass.size;
-            index++;
+        const f = from - 1; // 0-based
+        const u = until - 1; // 0-based
+        const cacheStart = this._cache.windowStart;
+
+        // Build the result array up front
+        const result = new Array(u - f + 1);
+
+        // Part 1: Out-of-window entries [f, min(cacheStart-1, u)] — read from disk, do not cache
+        const outEnd = Math.min(cacheStart - 1, u);
+        if (f < cacheStart && outEnd >= f) {
+            const count = outEnd - f + 1;
+            const outBuf = Buffer.allocUnsafe(count * this.EntryClass.size);
+            const bytesRead = fs.readSync(this.fd, outBuf, 0, outBuf.byteLength, this.headerSize + f * this.EntryClass.size);
+            const entries = Math.floor(bytesRead / this.EntryClass.size);
+            for (let idx = 0; idx < entries; idx++) {
+                result[idx] = this.EntryClass.fromBuffer(outBuf, idx * this.EntryClass.size);
+            }
         }
-        if (from <= this.readUntil + 1) {
-            this.readUntil = Math.max(this.readUntil, until);
+
+        // Part 2: In-window entries [max(cacheStart, f), u] — use cache + disk for uncached ones
+        const inStart = Math.max(cacheStart, f);
+        if (inStart <= u) {
+            // Optimisation: skip entries already loaded sequentially into the cache
+            const readFrom = Math.max(this.readUntil + 1, inStart);
+
+            // Trim trailing entries already present in the cache
+            let readUntil = u;
+            while (readUntil >= readFrom && this._cache.get(readUntil)) {
+                readUntil--;
+            }
+
+            if (readFrom <= readUntil) {
+                const count = readUntil - readFrom + 1;
+                const inBuf = Buffer.allocUnsafe(count * this.EntryClass.size);
+                const bytesRead = fs.readSync(this.fd, inBuf, 0, inBuf.byteLength, this.headerSize + readFrom * this.EntryClass.size);
+                const entries = Math.floor(bytesRead / this.EntryClass.size);
+                for (let idx = 0; idx < entries; idx++) {
+                    const i = readFrom + idx;
+                    this._cache.set(i, this.EntryClass.fromBuffer(inBuf, idx * this.EntryClass.size));
+                }
+                if (inStart <= this.readUntil + 1) {
+                    this.readUntil = Math.max(this.readUntil, readUntil);
+                }
+            }
+
+            // Fill the result from the cache for the in-window portion
+            for (let i = inStart; i <= u; i++) {
+                result[i - f] = this._cache.get(i);
+            }
         }
 
-        return this.data.slice(from, until + 1);
+        return result;
     }
 
     /**
@@ -318,14 +356,14 @@ class ReadableIndex extends events.EventEmitter {
      * @returns {Entry|boolean} The entry at the given index position or false if out of bounds.
      */
     get(index) {
-        index = wrapAndCheck(index, this.length);
+        index = wrapAndCheck(index, this._cache.length);
         if (index <= 0) {
             return false;
         }
 
-        if (this.data[index - 1]) {
-            return this.data[index - 1];
-        }
+        const i = index - 1; // 0-based
+        const cached = this._cache.get(i);
+        if (cached) return cached;
 
         return this.read(index);
     }
@@ -354,24 +392,41 @@ class ReadableIndex extends events.EventEmitter {
      * @returns {Array<Entry>|boolean} An array of entries for the given range or false on error.
      */
     range(from, until = -1) {
-        from = wrapAndCheck(from, this.length);
-        until = wrapAndCheck(until, this.length);
+        from = wrapAndCheck(from, this._cache.length);
+        until = wrapAndCheck(until, this._cache.length);
 
         if (from <= 0 || until < from) {
             return false;
         }
 
-        const readFrom = Math.max(this.readUntil + 1, from);
-        let readUntil = until;
-        while (readUntil >= readFrom && this.data[readUntil - 1]) {
-            readUntil--;
+        const f = from - 1; // 0-based
+        const u = until - 1; // 0-based
+        const cacheStart = this._cache.windowStart;
+
+        // Determine if any disk reads are required
+        const hasOutOfWindow = f < cacheStart;
+        const inStart = Math.max(cacheStart, f);
+        const readFrom = Math.max(this.readUntil + 1, inStart);
+        let needsDiskRead = hasOutOfWindow;
+        if (!needsDiskRead && inStart <= u) {
+            // Scan backwards for uncached in-window tail entries
+            let scanUntil = u;
+            while (scanUntil >= readFrom && this._cache.get(scanUntil)) {
+                scanUntil--;
+            }
+            needsDiskRead = readFrom <= scanUntil;
         }
 
-        if (readFrom <= readUntil) {
-            this.readRange(readFrom, readUntil);
+        if (needsDiskRead) {
+            return this.readRange(from, until);
         }
 
-        return this.data.slice(from - 1, until);
+        // All required entries are already in the cache — build result directly
+        const result = new Array(u - f + 1);
+        for (let i = f; i <= u; i++) {
+            result[i - f] = this._cache.get(i);
+        }
+        return result;
     }
 
     /**

src/Index/RingBuffer.js

@@ -0,0 +1,129 @@
+/**
+ * A fixed-capacity ring buffer used as an index entry cache.
+ *
+ * Only the most-recent `capacity` entries are kept in memory.  The buffer also
+ * tracks the total number of entries ever added (`length`) so callers can tell
+ * whether a slot is within the live in-memory window.
+ *
+ * API contract
+ * ------------
+ * - `get(index)`        — cached item at 0-based `index`, or `null` when the
+ *                         index is outside the in-memory window or the slot has
+ *                         not been written yet.
+ * - `set(index, item)`  — stores `item` at 0-based `index` if it falls inside
+ *                         the current window; silently ignores out-of-window
+ *                         writes.
+ * - `add(item)`         — appends `item` at position `length`, advances
+ *                         `length`, and returns the new length.
+ * - `truncate(newLength)` — discards entries from `newLength` onwards (nulls
+ *                         their slots) and sets `length = newLength`.  Safe to
+ *                         call with `newLength >= length` (grow-only update).
+ * - `reset()`           — clears all slots and resets `length` to 0.
+ */
+class RingBuffer {
+
+    /**
+     * @param {number} capacity  Maximum number of entries held in memory.
+     */
+    constructor(capacity) {
+        this._capacity = Math.max(1, capacity >>> 0); // jshint ignore:line
+        this._buffer = new Array(this._capacity);
+        this._length = 0;
+    }
+
+    /**
+     * Total number of items ever appended (not capped at capacity).
+     * @type {number}
+     */
+    get length() {
+        return this._length;
+    }
+
+    /**
+     * Maximum number of items kept in memory.
+     * @type {number}
+     */
+    get capacity() {
+        return this._capacity;
+    }
+
+    /**
+     * The smallest 0-based index that is currently inside the in-memory window.
+     * Indices below this value are not cached and require a disk read.
+     * @type {number}
+     */
+    get windowStart() {
+        return Math.max(0, this._length - this._capacity);
+    }
+
+    /**
+     * Return the cached item at the given 0-based index, or `null` if the
+     * index is outside the in-memory window or the slot has not been populated.
+     *
+     * @param {number} index  0-based position.
+     * @returns {*|null}
+     */
+    get(index) {
+        if (index < this.windowStart) {
+            return null;
+        }
+        const item = this._buffer[index % this._capacity];
+        return item !== undefined ? item : null;
+    }
+
+    /**
+     * Store `item` at the given 0-based `index`.
+     * Writes outside the current in-memory window are silently ignored.
+     *
+     * @param {number} index  0-based position.
+     * @param {*}      item
+     */
+    set(index, item) {
+        if (index < this.windowStart) {
+            return;
+        }
+        this._buffer[index % this._capacity] = item;
+    }
+
+    /**
+     * Append `item` at position `length` and advance `length`.
+     *
+     * @param {*} item
+     * @returns {number} The new length (1-based position of the appended item).
+     */
+    add(item) {
+        this._buffer[this._length % this._capacity] = item;
+        this._length++;
+        return this._length;
+    }
+
+    /**
+     * Discard entries from `newLength` onwards by nulling their cache slots,
+     * then set `length = newLength`.
+     *
+     * When `newLength >= length` no eviction is performed and only `length` is
+     * updated (useful when the underlying file has grown and the caller just
+     * needs to advance the length counter without populating new slots).
+     *
+     * @param {number} newLength  The new total length.
+     */
+    truncate(newLength) {
+        if (newLength < this._length) {
+            const cacheStart = this.windowStart;
+            for (let i = Math.max(cacheStart, newLength); i < this._length; i++) {
+                this._buffer[i % this._capacity] = null;
+            }
+        }
+        this._length = newLength;
+    }
+
+    /**
+     * Clear all cached slots and reset `length` to 0.
+     */
+    reset() {
+        this._buffer.fill(null);
+        this._length = 0;
+    }
+}
+
+module.exports = RingBuffer;