exp-fetch

fetch

A small and pluggable lib to fetch a resource and cache the result.

Usage

By default fetch will treat all response codes expect 200 and 404 as errors. 404 will yield null and 200 the body

Caching

Fetch will parse the cache-control header. If fetch encounters private, no-cache, max-age=0 or must-revalidate it wont cache. Otherwise it will respect the max-age header.

Callback usage:

var behavior = {};
var fetch = fetchBuilder(behavior);
fetch("http://example.com/resource.json", function (err, content) {
    // Do something with the result
});

Promise usage:

var behavior = {};
var fetch = fetchBuilder(behavior);
fetch("http://example.com/resource.json").then(function (content) {
    // Do something with the result
});

Custom request headers

var behavior = {};
var fetch = fetchBuilder(behavior);
var options = {
  url: "http://example.com/resource.json",
  headers: {
    "User-agent": "exp-fetch"
  }
}
fetch(options, function (err, content) {
    // Do something with the result
});

Issuing POST requests:

var behavior = { httpMethod: "POST"};
var poster = fetchBuilder(behavior);
var body = {
    "query": "some string" 
};

poster("http://example.com/query", body, function (err, content) {
    // Do something with the result
    // The result will be cached by `url` + ` ` + `sha of body`
});

Allowed behavior options

• freeze: (default:true). When this option is set to false it will not freeze the response so it can be modified. ("use strict" is needed)
• cache: (default: an instance of AsyncCache) (https://github.com/ExpressenAB/exp-asynccache). To disable caching set {cache: null}
• cacheKeyFn: (default: caches on the url + sha1 of the body) An optional formatting function for finding the cache-key. One might, for example, want to cache on an url with the get params stripped.
• cacheValueFn: (default: caches the response body) An optional function for change what will be returned and cached from fetch.
• maxAgeFn: (default: respects the cache-control header)
• onRequestInit: If given a function, it will be called before the actual request is made, see Hooks for signature
• onNotFound: If given a function, it will be called each time fetch encounters a 404
• onError: If given a function, it will be called each time fetch encounters a non 200 nor 404 response
• onSuccess: If given a function, it will be called each time fetch encounters a 200
• requestTimeFn: (default log with level debug) If given a function, it will be called when the request returned and processed from remote end.
• logger: A logger object implementing error, warning, info, debug for example https://github.com/tj/log.js
• cacheNotFound: (default: false). If set it will cache 404s, if given a number it will cache the 404 for that time. If the maxAgeFn is given, it will get this time as the first parameter.
• errorOnRemoteError: (default: true). If set it will treat a remote > 200 statusCode as an error.
• contentType: (default: json), expected content type. Fetch will try to parse the given content type. (supported: xml|json)
• agentOptions: (default: {}), options passed to the keepAliveAgent.
• followRedirect: (default: true), should fetch follow redirects (and cache the redirect chain)
• clone: (default: true), should fetch clone objects before handing them from the cache.
• httpMethod: (default: "GET"), the HTTP-method that should be used to make requests.

CacheKeyFn

var keyFinder = function (url) {
    return url.replace(/\//g, "");
}
var fetch = fetchBuilder({cacheKeyFn: keyFinder});
Promise.all([
   fetch("http://example.com/foo/bar")
   fetch("http://example.com/foobar")
]).then(function (result) {
   result[0] === result[1];
});

CacheValueFn

var valueFn  = function (body, headers, statusCode) {
    return {
        body: body,
        headers: headers,
        statusCode: statusCode
    };
}
var fetch = fetchBuilder({cacheValueFn: valueFn});
fetch("http://example.com/resource.json", function (err, value) {
  // value will be something like:
  // { statusCode: 200, headers: { "content-type": "application/json" }, body: { "resource": "body" } }
})

maxAgeFn

function cacheNothing(maxAge, key, res, content) {
    return -1;
}
var fetch = fetchBuilder({maxAgeFn: cacheNothing});

Hooks

They are: onError, onNotFound and onSuccess. Signature:

function onError(url, cacheKey, res, content) {
    //
}

And onRequestInit with signature:

function onRequestInit(requestOptions, cacheKey) {
    //
}

The function will be called once before the actual request is made, i.e. not found in cache. Subsequent redirect requests does not call the function. The requestOptions argument is a copy of the request options and will not alter the request.

Useful when mocking requests, e.g:

var url = require("url");
var nock = require("nock");

function onRequestInit(requestOptions, cacheKey) {
    var callUrl = url.parse(requestOptions.url);
    var path = callUrl.path;
    var host = callUrl.protocol + "//" + callUrl.host;

    nock(host).get(path).reply(200, {mock: true});
}

var fetch = fetchBuilder({onRequestInit: onRequestInit});

And requestTimeFn with signature:

function requestTimeFn(requestOptions, took) {
    console.log("REQUEST", requestOption.method, ":", requestOption.url, "took", took,  "ms"); 
}

Init cache function

The fetch lib provides a convenient initLRUCache-method which sets up a cache purging it's expired content.

var initLRUCache = require("exp-fetch").initLRUCache;
var cache = new AsyncCache(initLRUCache({ age: 60, size: 2000});

Allowed params:

• size or max: the max allowed size, the unit is set by the length method. Default is value.length. Default: 2000000
• length: the length function, default is v && v.length || 1
• age or maxAge: the maximum number of seconds a key will be kept in the cache. Default 60

Stats

Get statistics for number calls and cache hit ratio:

var behavior = {};
var fetch = fetchBuilder(behavior);
var stats = fetch.stats();
console.log("Hit ration", stats.cacheHitRatio);