|
| 1 | +# streams |
| 2 | + |
| 3 | +node.js has a handy interface for shuffling data around |
| 4 | +called streams |
| 5 | + |
| 6 | +--- |
| 7 | +# reference materials! |
| 8 | + |
| 9 | +https://github.com/substack/stream-handbook |
| 10 | +http://nodeschool.io/#stream-adventure |
| 11 | + |
| 12 | +--- |
| 13 | +# stream origins |
| 14 | + |
| 15 | +``` |
| 16 | +"We should have some ways of connecting programs like garden |
| 17 | +hose--screw in another segment when it becomes necessary to |
| 18 | +massage data in another way. This is the way of IO also." |
| 19 | +``` |
| 20 | + |
| 21 | +[Doug McIlroy. October 11, 1964]( |
| 22 | +http://cm.bell-labs.com/who/dmr/mdmpipe.html) |
| 23 | + |
| 24 | +--- |
| 25 | +# why streams? |
| 26 | + |
| 27 | +* we can compose streaming abstractions |
| 28 | +* we can operate on data chunk by chunk |
| 29 | + |
| 30 | +--- |
| 31 | +# composition |
| 32 | + |
| 33 | +Just like how in unix we can pipe commands together: |
| 34 | + |
| 35 | +``` |
| 36 | +$ <mobydick.txt sed -r 's/\s+/\n/g' | grep -i whale | wc -l |
| 37 | +1691 |
| 38 | +``` |
| 39 | + |
| 40 | +we can pipe abstractions together with streams using |
| 41 | +`.pipe()`: |
| 42 | + |
| 43 | +``` js |
| 44 | +read('mobydick.txt') |
| 45 | + .pipe(replace(/\s+/g, '\n')) |
| 46 | + .pipe(filter(/whale/i)) |
| 47 | + .pipe(linecount(function (count) { |
| 48 | + console.log(count) |
| 49 | + })) |
| 50 | +; |
| 51 | +``` |
| 52 | + |
| 53 | +--- |
| 54 | +# chunk by chunk |
| 55 | + |
| 56 | +With streams, we can operate on data chunk by chunk, without |
| 57 | +buffering everything into memory. |
| 58 | + |
| 59 | +This means we can write programs that operate on very large |
| 60 | +files! |
| 61 | + |
| 62 | +It also means we can have hundreds or thousands of |
| 63 | +concurrent streams without using much memory. |
| 64 | + |
| 65 | +--- |
| 66 | +# fs |
| 67 | + |
| 68 | +We can read a file and stream the file contents to stdout: |
| 69 | + |
| 70 | +``` js |
| 71 | +var fs = require('fs'); |
| 72 | + |
| 73 | + |
| 74 | +fs.createReadStream('greetz.txt') |
| 75 | + .pipe(process.stdout) |
| 76 | +; |
| 77 | +``` |
| 78 | + |
| 79 | +--- |
| 80 | + |
| 81 | +``` |
| 82 | +$ echo beep boop > greetz.txt |
| 83 | +$ node greetz.js |
| 84 | +beep boop |
| 85 | +``` |
| 86 | + |
| 87 | +--- |
| 88 | + |
| 89 | +now let's transform the data before we print it out! |
| 90 | + |
| 91 | +--- |
| 92 | +# fs |
| 93 | + |
| 94 | +You can chain `.pipe()` calls together just like the `|` |
| 95 | +operator in bash: |
| 96 | + |
| 97 | +``` js |
| 98 | +var fs = require('fs'); |
| 99 | + |
| 100 | + |
| 101 | +fs.createReadStream('greetz.txt') |
| 102 | + .pipe(...) |
| 103 | + .pipe(process.stdout) |
| 104 | +; |
| 105 | +``` |
| 106 | + |
| 107 | +--- |
| 108 | +# fs |
| 109 | + |
| 110 | +``` js |
| 111 | +var fs = require('fs'); |
| 112 | +var through = require('through2'); |
| 113 | + |
| 114 | +fs.createReadStream('greetz.txt') |
| 115 | + .pipe(through(toUpper)) |
| 116 | + .pipe(process.stdout) |
| 117 | +; |
| 118 | + |
| 119 | +function toUpper (buf, enc, next) { |
| 120 | + var up = buf.toString().toUpperCase(); |
| 121 | + |
| 122 | + |
| 123 | +} |
| 124 | +``` |
| 125 | + |
| 126 | +--- |
| 127 | +# fs |
| 128 | + |
| 129 | +``` js |
| 130 | +var fs = require('fs'); |
| 131 | +var through = require('through2'); |
| 132 | + |
| 133 | +fs.createReadStream('greetz.txt') |
| 134 | + .pipe(through(toUpper)) |
| 135 | + .pipe(process.stdout) |
| 136 | +; |
| 137 | + |
| 138 | +function toUpper (buf, enc, next) { |
| 139 | + var up = buf.toString().toUpperCase(); |
| 140 | + this.push(up); |
| 141 | + |
| 142 | +} |
| 143 | +``` |
| 144 | + |
| 145 | +--- |
| 146 | +# fs |
| 147 | + |
| 148 | +``` js |
| 149 | +var fs = require('fs'); |
| 150 | +var through = require('through2'); |
| 151 | + |
| 152 | +fs.createReadStream('greetz.txt') |
| 153 | + .pipe(through(toUpper)) |
| 154 | + .pipe(process.stdout) |
| 155 | +; |
| 156 | + |
| 157 | +function toUpper (buf, enc, next) { |
| 158 | + var up = buf.toString().toUpperCase(); |
| 159 | + this.push(up); |
| 160 | + next(); |
| 161 | +} |
| 162 | +``` |
| 163 | + |
| 164 | +--- |
| 165 | +# fs |
| 166 | + |
| 167 | +``` |
| 168 | +$ node greetz.js |
| 169 | +BEEP BOOP |
| 170 | +``` |
| 171 | + |
| 172 | +--- |
| 173 | +# stdin |
| 174 | + |
| 175 | +What if we want to read from stdin instead of a file? |
| 176 | +Just pipe from `process.stdin` instead of |
| 177 | +`fs.createReadStream()`. |
| 178 | + |
| 179 | +--- |
| 180 | + |
| 181 | +before: |
| 182 | + |
| 183 | +``` js |
| 184 | +var fs = require('fs'); |
| 185 | +var through = require('through2'); |
| 186 | + |
| 187 | +fs.createReadStream('greetz.txt') |
| 188 | + .pipe(through(toUpper)) |
| 189 | + .pipe(process.stdout) |
| 190 | +; |
| 191 | + |
| 192 | +function toUpper (buf, enc, next) { |
| 193 | + var up = buf.toString().toUpperCase(); |
| 194 | + this.push(up); |
| 195 | + next(); |
| 196 | +} |
| 197 | +``` |
| 198 | + |
| 199 | +--- |
| 200 | + |
| 201 | +after: |
| 202 | + |
| 203 | +``` js |
| 204 | + |
| 205 | +var through = require('through2'); |
| 206 | + |
| 207 | +process.stdin |
| 208 | + .pipe(through(toUpper)) |
| 209 | + .pipe(process.stdout) |
| 210 | +; |
| 211 | + |
| 212 | +function toUpper (buf, enc, next) { |
| 213 | + var up = buf.toString().toUpperCase(); |
| 214 | + this.push(up); |
| 215 | + next(); |
| 216 | +} |
| 217 | +``` |
| 218 | + |
| 219 | +--- |
| 220 | +# through2 |
| 221 | + |
| 222 | +through2 is a module you can install with npm: |
| 223 | + |
| 224 | +``` |
| 225 | +$ npm install through2 |
| 226 | +``` |
| 227 | + |
| 228 | +It makes setting up a transform stream less verbose than |
| 229 | +using the core methods. |
| 230 | + |
| 231 | +--- |
| 232 | + |
| 233 | +a version of our program using core streams: |
| 234 | + |
| 235 | +``` js |
| 236 | +var Transform = require('stream').Transform; |
| 237 | +var toUpper = new Transform; |
| 238 | +toUpper._transform = function (buf, enc, next) { |
| 239 | + var up = buf.toString().toUpperCase(); |
| 240 | + this.push(up); |
| 241 | + next(); |
| 242 | +}; |
| 243 | + |
| 244 | +process.stdin |
| 245 | + .pipe(toUpper) |
| 246 | + .pipe(process.stdout) |
| 247 | +; |
| 248 | +``` |
| 249 | + |
| 250 | +--- |
| 251 | +# through2 vs stream.Transform |
| 252 | + |
| 253 | +rules of thumb: |
| 254 | + |
| 255 | +* use through when you only want to transform some data |
| 256 | +* use core Transform when you want to use inheritance |
| 257 | + |
| 258 | +--- |
| 259 | +# through(write, end) |
| 260 | + |
| 261 | +With through there are 2 parameters: `write` and `end`. |
| 262 | +Both are optional. |
| 263 | + |
| 264 | +* `function write (buf, enc, next) {}` |
| 265 | +* `function end () {}` |
| 266 | + |
| 267 | +Call `next()` when you're ready for the next chunk. |
| 268 | +If you don't call `next()`, your stream will hang! |
| 269 | + |
| 270 | +Call `this.push(VALUE)` inside the callback to put VALUE |
| 271 | +into the stream's output. |
| 272 | + |
| 273 | +Use a `VALUE` of `null` to end the stream. |
| 274 | + |
| 275 | +--- |
| 276 | + |
| 277 | +--- |
| 278 | +# concat-stream |
| 279 | + |
| 280 | + |
| 281 | + |
| 282 | +--- |
| 283 | +# http |
| 284 | + |
| 285 | +--- |
| 286 | +# readable |
| 287 | + |
| 288 | +--- |
| 289 | +# writable |
| 290 | + |
| 291 | +--- |
| 292 | +# through |
| 293 | + |
| 294 | +``` |
| 295 | +process.stdin.pipe(process.stdout) |
| 296 | +``` |
| 297 | + |
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