server-link

Manage links between client and servers to allow client to wait until linked servers are ready.
Very useful for installations involving multiple servers that should start in a particular sequence.
Linked Docker containers is a very typical use case for this.

• Installation
• Usage
• Server link status
• Server link path
• Server link retry options
• Link to external servers
• Server link errors
• API reference

Installation

$ npm install server-link

Usage

For internal servers, simply setup link support on server that client depends on:

// == Server application ==
var express = require('express');
var app = express();
...
// Setup link support
require('server-link')(app);
 
// Perform normal server init and startup
app.route(...), app.use(...) etc
app.listen(....)

Now client can wait until linked server is ready:

// == Client application ==
require('server-link').wait('http://localhost:3000').then(function() {
  // Linked server is ready
})
.catch(function(err) {
  // Connection with linked server failed 
});

Provide a custom linker function to link with external servers you do not control. Server-link will handle all retries and timeouts.

// == Client application ==
require('server-link').wait('http://localhost:3000', function(host) {
  // Handle your own server connection
  // The linker function and return values are explained below
  return my_server_connection(host);
}).then(function() {
  // Linked server is ready
})
.catch(function(err) {
  // Connection with linked server failed 
});

Client can wait for multiple servers. The returned promise isn't resolved until all linked servers are ready:

// == Client application ==
require('server-link').wait([
  'http://www.someserver.com:3000', 
  'http://www.anotherserver.com:4000'
])
.then(function() {
  // All linked servers are ready
})
.catch(function(err) {
  // Connection with one or more linked servers failed
});

Server link status

By default, server is assumed to be ready as soon as it starts listening on a socket and routing incoming requests. This behavior can be changed by providing a link status during setup as follows:

// == Server application ==
var express = require('express');
var app = express();
var link = require('server-link')(app, 'starting');

Now the server won't report itself as ready until the link status is manually changed from 'starting' to 'online' based on your server specific conditions:

// == Server application ==
link.status = 'online';

Server-link supports the following link status values:

• 'offline': The linked server is not responding to link requests
• 'starting': The linked server is starting up but not yet ready
• 'online': The linked server is ready for incoming requests
• 'error': The linked server is not ready due to an internal error
• 'invalid': The linked server sends invalid response to link requests

Assigning any other value to link.status will result in an error exception. The client will wait until the server reports status 'online'. As long as status 'offline' or 'starting' is reported, the client will continue to poll the server periodically until max number of retries is reached, based on the specified retry options (see below). When status 'error' or 'invalid' is reported, the client will abort the wait immediately and report a link failure.

The client can poll the current link status at any time:

// == Client application ==
require('server-link').get('http://someserver.com:300').then(function(status) {
  console.log('link status=' + status);
});

Server link path

By default, link status requests are routed through path '/serverlink' on the server. A custom path can be used as long as both the client and server are setup with the same path as follows:

// == Server application ==
require('server-link')(app, '', '/custom/link/path');

// == Client application ==
require('server-link').wait('http://someserver.com:3000', '/custom/link/path');

Please note that server URL should not contain any path or trailing '/' and custom path should start with '/'.

Server link retry options

Server-link uses NPM package retry to perform retries during waits by the client until the linked server is ready. Retry options can be specified in the wait call and are passed on directly to this package:

// == Client application ==
require('server-link').wait('http://someserver.com:3000', {
  retries: 2,       // Max nbr of times to retry (default 10)
  factor: 1,        // Exponential factor (default 2)
  minTimeout: 500,  // Time to wait before first retry (default 1000ms)
  maxTimeout: 500,  // Max time to wait between retries (default infinity)
  randomize: true   // Randomize wait time with factor between 1-2 (default false)
);

Please see the retry package for option details.

Link to external servers

If you don't own the server that you're linking to, you're most likely not able to install link support on it as described above. Instead, a custom linker function can be provided in the wait call to handle your own server connection and translate it to a status value that server-link will understand and process accordingly, just like links with internal servers:

// == Client application ==
require('server-link').wait([
  'http://www.internal.com:3000', 
  'http://www.external.com:4000'
], function(host, index) {
    // Return nothing for internal server (index 0) to have
    // server-link handle the default connection instead
    if (index == 1) {
      // Perform custom connection and return a server-link status
      return connect_my_db(host).then(function(ready) {
        return ready ? 'online' : 'starting';
      })
      .catch(function() {
        return 'error';
      });
    }
})
.then(function() {
  // All linked servers are ready
})
.catch(function(err) {
  // Connection with one or more linked servers failed
});

The linker function is called for each host specified in the list. If the function returns nothing, server-link assumes it is an internal server with link support enabled and it will handle the connection instead. If the function returns a promise or a link status, server-link will succeed, fail or retry based on the status just as with any internal servers.

Server link errors

Attempts by the client to wait for a linked server could result in the returned promise to be rejected with various errors. In addition to the typical network related errors, server-link my also report one of the following error instances:

• err.code = 'LINKNOTREADY' - Max wait time/retries reached before server reported status 'online'
  err.number = Number of times client tried to poll the server before failure

• err.code = 'LINKINVALID' - Server sent invalid response to link status requests
  err.number = Number of times client tried to poll the server before failure

• err.code = 'LINKERROR' - Server reported an internal error
  err.number = Number of times client tried to poll the server before failure

• err.code = 'LINKSNOTREADY' - Wait for multiple servers failed for one or more servers
  err.links = Array with an entry for each server the client is waiting for. The entry will contain an error instance as specified above if a link with the corresponding server failed.

API reference

var serverLink = require('server-link');

serverLink(app, [status], [path])
Enables link support on the server
app specifies the express() application for the server
status indicates the initial server status (default 'online')
path is path on the server where link status requests are routed to (default '/serverlink')
Returns a serverLink instance for this app. Use property instance.status to get & set current server status

serverLink.wait(hosts, [path], [options], [linker])
Waits until specified linked server(s) is/are ready.
hosts is either a string containing the host URL for a single linked server or an array of strings to wait for multiple servers. A host URL should only contain the base server URL with optional protocol and port but without path or trailing '/'.
path is path on the server(s) where link status request are routed to (default '/serverlink')
options are server retry options while waiting until ready. See server link retry options above for details.
linker is a function (see below) that will be called for each host in the list to handle the connection with external servers Returns a promise that is resolved when all specified servers are ready or rejected as soon as one ore more server links failed.

linker(host, index, number)
Called for each host specified in the call to serverlink.wait to handle the server connection
host is the URL of the host to connect to
index in the index in the list of hosts specified in serverlink.wait
number is the retry count for this connection
Returns a link status string or a promise that will resolve/reject a link status.
Use this.hosts, this.path and this.options in the linker function to access the arguments passed to the serverlink.wait call.

serverLink.get(host, [path])
host specifies the host URL for the linked server. The URLshould only contain the base server URL with optional protocol and port but without path or trailing '/'.
path is path on the server where link status requests are routed to (default '/serverlink')
Returns a promise that is resolved with the current link status for specified server.