Handle backpressure

package.json

@@ -23,7 +23,7 @@
     },
     "scripts": {
     "test": "NODE_PATH=src node node_modules/.bin/ava 'tests/*.spec.ts' -e",
-    "test:debug": "NODE_PATH=src node inspect node_modules/ava/profile.ts",
+    "test:debug": "NODE_PATH=src node inspect node_modules/ava/profile.js",
     "test:all": "NODE_PATH=src node node_modules/.bin/ava",
     "lint": "tslint -p tsconfig.json",
     "validate:tslint": "tslint-config-prettier-check ./tslint.json",

src/functions/demux.ts

@@ -14,10 +14,15 @@ export function demux(
 }
 
 class Demux extends Writable {
-    private keyMap: {
+    private streamsByKey: {
-        [key: string]: NodeJS.WritableStream | NodeJS.ReadWriteStream;
+        [key: string]: {
+            stream: NodeJS.WritableStream | NodeJS.ReadWriteStream;
+            writable: boolean;
+        };
     };
     private demuxer: (chunk: any) => string;
+    private isWritable: boolean;
+    private nonWritableStreams: Array<string>;
     private construct: (
         destKey?: string,
     ) => NodeJS.WritableStream | NodeJS.ReadWriteStream;
@@ -36,16 +41,51 @@ class Demux extends Writable {
         }
         this.demuxer = demuxBy.keyBy || ((chunk: any) => chunk[demuxBy.key!]);
         this.construct = construct;
-        this.keyMap = {};
+        this.streamsByKey = {};
+        this.isWritable = true;
     }
 
     public _write(chunk: any, encoding: string, cb: any) {
         const destKey = this.demuxer(chunk);
-        if (this.keyMap[destKey] === undefined) {
+        if (this.streamsByKey[destKey] === undefined) {
-            this.keyMap[destKey] = this.construct(destKey).on("error", e => {
+            this.streamsByKey[destKey] = {
-                this.emit("error", e);
+                stream: this.construct(destKey),
+                writable: true,
+            };
+        }
+        // Throttle when one stream is not writable anymore
+        // Set writable to false
+        // keep state of all the streams, if one is not writable demux shouldnt be writable
+        // Small optimization is to keep writing until you get a following event to the unwritable destination
+
+        let res = false;
+        if (this.isWritable && this.streamsByKey[destKey].writable) {
+            res = this.streamsByKey[destKey].stream.write(chunk, encoding, cb);
+        } else if (this.isWritable) {
+            this.isWritable = false;
+            // Buffer chunk?
+            return this.isWritable;
+        }
+
+        /* If write above returns false and the stream written to was writable previously, we need to make demux
+         * non-writable and update state to know the stream is nonWritable.
+         * If write returns true and the stream was previously not writable, we need to update which streams
+         * are non writable and determine if it is safe for demux to become writable (all streams are writable)
+         */
+        if (!res) {
+            this.streamsByKey[destKey].writable = false;
+            this.nonWritableStreams.push(destKey);
+            this.isWritable = false;
+            this.streamsByKey[destKey].stream.once("drain", () => {
+                this.streamsByKey[destKey].writable = true;
+                this.nonWritableStreams = this.nonWritableStreams.filter(
+                    key => key !== destKey,
+                );
+
+                this.isWritable = this.nonWritableStreams.length === 0;
             });
         }
-        return this.keyMap[destKey].write(chunk, encoding, cb);
+
+        return this.writable;
     }
 }

src/functions/map.ts

@@ -14,6 +14,7 @@ export function map<T, R>(
         writableObjectMode: true,
     },
 ): Transform {
+    // remove try catch
     return new Transform({
         ...options,
         async transform(chunk: T, encoding, callback) {
@@ -22,6 +23,7 @@ export function map<T, R>(
                 this.push(mapped);
                 callback();
             } catch (err) {
+                console.log("caught error", err.message);
                 callback(err);
             }
         },

tests/demux.spec.ts

@@ -107,3 +107,58 @@ test.cb("should spread per key using keyBy", t => {
     input.forEach(event => demuxed.write(event));
     demuxed.end();
 });
+
+test.cb("should emit errors", t => {
+    t.plan(2);
+    const input = [
+        { key: "a", val: 1 },
+        { key: "b", val: 2 },
+        { key: "a", val: 3 },
+        { key: "a", val: 4 },
+    ];
+    const results = [
+        { key: "a", val: 2 },
+        { key: "b", val: 3 },
+        { key: "a", val: 4 },
+        { key: "a", val: 5 },
+    ];
+    const destinationStreamKeys = [];
+    const sink = new Writable({
+        objectMode: true,
+        write(chunk, enc, cb) {
+            expect(results).to.deep.include(chunk);
+            expect(input).to.not.deep.include(chunk);
+            t.pass();
+            cb();
+        },
+    }).on("unpipe", e => console.log("sink err"));
+
+    const construct = (destKey: string) => {
+        destinationStreamKeys.push(destKey);
+        const dest = map((chunk: Test) => {
+            if (chunk.key === "b") {
+                throw new Error("Caught object with key 'b'");
+            }
+            return {
+                ...chunk,
+                val: chunk.val + 1,
+            };
+        }).on("error", e => console.log("got err"));
+
+        dest.pipe(sink);
+        return dest;
+    };
+
+    const demuxed = demux(
+        construct,
+        { keyBy: (chunk: any) => chunk.key },
+        { objectMode: true },
+    );
+    demuxed.on("error", e => {
+        expect(e.message).to.equal("Caught object with key 'b'");
+        t.pass();
+        t.end();
+    });
+    input.forEach(event => demuxed.write(event));
+    demuxed.end();
+});