asker

asker is a wrapper for http.request, which incorporates:

• response deflating using gzip,
• requests retrying,
• connection pools tuning.

If you are looking for a module to fetch 3rd-party web content (pages, RSS, files or something else), don't waste your time and look at the request module, as asker doesn't support cookies and redirects out of the box.

The main goal of asker is to communicate between frontends and backends that use some kind of SLA.

Quick start

var ask = require('asker');

ask({ host: 'yandex.com' }, function(error, response) {
    if (error) {
        return error.log();
    }

    console.log('Response retrieved in %s ms', response.meta.time.total);
    console.log('Response data', response.data);
});

Options

All parameters are optional.

• {String} host="localhost"
• {String} hostname – The same as the host, used for compatibility with results of the url.parse(). Has higher priority than the host.
• {Number} port=80
• {String} path="/"
• {String} protocol – Default is "http:" if port is set to 80, and "https:" if port is 443.
• {String} url – Shorthand alternative for protocol, hostname, port and path options.
• {String} method="GET"
• {Object} headers – HTTP headers
• {Object} query – Query parameters.
• {String} requestId="" – Request identifier which is used in log messages.
• {*} body – Request body. Encoding method is set by bodyEncoding option.
• {String} bodyEncoding="string" – Body encoding method (string, json, urlencoded, multipart or the one implemented by user). See "Body encoding" for details.
• {Number} queueTimeout=50 – Timeout from the moment, when asker initiated the request. Useful if pool manager failed to provide a socket for any reason.
• {Number} timeout=500 – Timeout from the moment when a socket was given by the pool manager.
• {Function} isNetworkError – A function that checks whether the given status code should be treated as network error. See "Response status codes processing" for details.
• {Number} maxRetries=0 – Maximum number of retries allowed for the request.
• {Number} minRetriesTimeout=300 – The number of milliseconds before starting the first retry if maxRetries is greater than 0.
• {Number} maxRetriesTimeout=Infinity – The maximum number of milliseconds between two retries.
• {Function} isRetryAllowed – A function that determines if retry is allowed for the given reason.
• {Boolean} allowGzip=true – Allows response compression with gzip.
• {Object|false} agent – http.Agent options, see Connection pools tuning section for details.

Response format

Successful requests will return data and additional information in the following format:

{Object} response

• {Number} statusCode http status code
• {Object} headers returned http headers (names are lowercased)
• {Object} meta meta information
  • {Object} time request timers
    • {Number} network the time from socket was opened and until the request was completed
    • {Number} total total execution time
  • {Object} options options which was provided for request creation
  • {Object} retries
    • {Number} used number of retries used
    • {Number} limit retries limit for a given request
• {*} data received data. If response body wasn't provided, null is returned.

Response status codes processing

isNetworkError(statusCode)

When response status code is received, asker passes status code through the filter function isNetworkError(statusCode), which determines whether this response code is acceptable:

• {Number} statusCode – A response status code provided by asker.

The function must return true if particular statusCode should be treated as network error. By default all status codes less than 500 are allowed.

Note, that result must be returned ASAP, because execution time of the filter WILL affect request timeouts.

Example

Suppose, we want to accept only responses with 200, 201 and 304 status codes.

var ask = require('asker');

function isNetworkError(statusCode) {
    return [200, 201, 304].indexOf(statusCode) === -1;
}

ask({ host: 'yandex.com', isNetworkError: isNetworkError }, function(error, response) {
    // @see http://npm.im/terror for details about error codes
    if (error.code === ask.Error.CODES.UNEXPECTED_STATUS_CODE) {
        console.error('Response status code is not 200, 201 or 304');
    }

    // ...
});

isRetryAllowed(retryReason)

In addition to status code filtering, asker can check whether particular failed response is allowed for retrying. isRetryAllowed(retryReason) filter function is used for that:

• {AskerError} retryReason – An error to check, if retry is allowed.

Example

Let's assume that the backend server we ask is quite laggy, so we want to retry our requests in case of temporary errors:

var ask = require('asker');

function isRetryAllowed(retryReason) {
    if (retryReason.code === ask.Error.CODES.UNEXPECTED_STATUS_CODE) {
        // in case of network errors, retry only if backend returns "Server Unavailable"
        return retryReason.data.statusCode === 503;
    }
    return true;
}

ask({ host: 'yandex.com', maxRetries: 5, isRetryAllowed: isRetryAllowed }, function(error, response) {
    if ( ! error) {
        console.log('Retries used: %d', response.meta.retries.used);
    }
});

Body encoding

Body encoder converts body to corresponding format and sets Content-type header.

Built-in encoders

• string – Used by default. Converts body to String. Accepts all types.
• json – Applies JSON.stringify to the body. Accepts all types.
• urlencoded – Converts body to query string. Accepts Object.
• multipart – Formats body according to multipart/form-data spec. Accepts Object (or Buffer object).
• raw – Use body as is. Accepts instance of Buffer. Remember to set content-type header manually if required.

Content-type

If you pass Buffer as property value, mime-type application/octet-stream will be applied. And property name will be used as file name.

Otherwise, you can pass additional info (mime-type and filename) in description of the parameter:

ask({
    bodyEncoding: 'multipart', // encoder name
    body: {
        'sample.mp3': buffer, // an instance of Buffer, "sample.mp3" will be used as file name
        image: {
            filename: 'image.jpg',
            mime: 'image/jpeg',
            data: image_buffer // an instance of Buffer
        }
    }
}, function(error, response) {
    /* ... */
});

Wrap the fields in an array if you want to send multiple files:

ask({
    bodyEncoding: 'multipart',
    body: {
        images: [
            'sample.mp3': buffer,
            {
                filename: 'pic.jpg',
                mime: 'image/jpeg',
                data: pic_buffer
            }
        ]
    }
}, function(error, response) {
    /* ... */
});

Exceptions

asker may throw the following errors, if you use body encoders:

• BODY_ENCODER_NOT_EXISTS – unknown bodyEncoder has been passed;
• BODY_INCORRECT_TYPE – body's type is not allowed by the encoder.

Custom encoders

To implement you own body encoder, you must add an encoding function as the asker.bodyEncoders property. Property name will be used as the encoder name.

Example:

var ask = require('asker');

var AskerError = ask.Error;

// encoder name is 'trimText'
ask.bodyEncoders.trimText = function(body, setContentType) {
    // throw error if passed body format is not acceptable for your encoder
    if (['number', 'string', 'boolean'].indexOf(typeof body) === -1) {
        throw AskerError.createError(AskerError.CODES.UNEXPECTED_BODY_TYPE, {
            type: typeof body,
            expectedTypes: 'Object'
        });
    }

    // 'content-type' header will be set to 'text/plain'
    setContentType('text/plain');

    return String(data).trim();
};

Note: setContentType sets Content-Type header only if header was not set before. You can force overriding by passing true as second argument:

ask.bodyEncoders.trimText = function(body, setContentType) {
    setContentType('nyan/colorful', true);

    return 'Colorful nyan cat';
};

Connection pools tuning

The problem

Node.js provides an internal socket pool manager which works as follow:

• by default globalAgent is used for all outgoing http requests;
• each Agent instance, including globalAgent, has a maxSockets property, which you can change;
• socket limit is set for each unique host:port pair, that is served by this particular Agent.

The pool may behave unexpectedly in some cases.

For example, if you have two backend servers backend:3000 and backend:4000 and first server is indispensable for the application, but the second one is complementary (e.g. it makes an http call for advertisements you show later).

Under the heavy load this secondary backend (which is usually less fault-tolerant) may occupy all sockets that OS provides for the whole Node.js process, because the default pool manager cares only about host:port pairs and defaultAgent does not correct socket limit for each server according to the process limits.

In the scenario above both servers will be unavailable, even if first indispensable backend works fine.

Solution

How asker can help you manage this problem? You can create a custom instance of http.Agent for any given backend. And you can set a maxSockets property by calculating each backend priority. asker provides an API to help with that.

API

{Object} agent http.Agent options:

• {String} name='globalAgent' – Unique name for the asked server.
• {Number} maxSockets – Socket limit for the server. Node.js 0.10 and below sets it to 5 by default.

Example

var ask = require('asker');

var server1 = {
    host: 'backend',
    port: 3000,
    agent: { name: 'backend1', maxSockets: 1024 }
};

var server2 = {
    host: 'backend',
    port: 4000,
    agent: { name: 'backend2', maxSockets: 100 }  // keep maxSockets for server2 small as we don't care much about it's availability
};

ask(server1, function() {});
ask(server1, function() {});

Error handling

asker produces errors using Terror, so you can setup your own logger or use error.log() method for logging.

If you already use Terror and had created a logger for Terror itself, you shouldn't setup it again for AskerError.

AskerError class is available via request('asker').Error property. So you can, localize error messages or customize it in your own way.