#denmark-dawa-replicate

Replicate the DAWA database.

Installation

npm install denmark-dawa-replicate

Example

This module just provided the necessary logic for fetching the data. The actual insert, update and delete logic is a bit more complicated, a full postgresql example exists inside the example/ directory.

Documentation

const replicate = require('denmark-dawa-replicate')

DAWA (Danmarks Adressers Web API) is a service provided by the danish government, which exposes multiply APIs for getting address related information. The service supports replication of its tables, this module helps getting that data and keeping it up to date.

First create a new DAWAReplicate object.

const replicator = new DAWAReplicate(version);

If there is no data in the database, set version to 0 or just don't specify it. The replicator object is a Readable object stream, where each item has the following structure.

replicator.on('data', function (item) {
	item = {
		table: String // A tableName, e.g. postnumre,
		operation: String // Can be [insert, update, delete],
		data: Object // Object containing all properties for each table row
	}
});

To get all possible table names and associated information, see the denmark-dawa-schema module.

Using a parallel writeable stream

Since there is a lot of data, it is recommended to implement a Writeable stream that supports parallel writes. This can be done by implementing the Writeable._writev method, that takes a chunk of items.

function InsertStream() {
  stream.Writable.call(this, {
		objectMode: true,
		highWaterMark: 16 // Controls the max amount of parallel writes
	});
  this.db = new DBAbstraction();
}
util.inherits(InsertStream, stream.Writable);

InsertStream.prototype._write = function (item, encoding, callback) {
  this.db[item.operation](item.table, item.data, callback);
};

InsertStream.prototype._writev = function (chunks, callback) {
  const self = this;
  async.each(chunks, function (item, done) {
    self._write(item.chunk, item.encoding, done);
  }, callback);
};

You then just pipe the replicator as usual.

replicator.pipe(new InsertStream());

replicator.update([version], callback)

Updates the current version to version. If no version is specified the module will fetch the latest version for you. After the new version has been determined, all new data (events) from DAWA will be piped to the replicator object.

If a newer version exists a new-version event will be emitted with the new version number. As each table starts being fetched the update-table will emit, with the corresponding table name.

The callback is called when all data has been fetched. Before this you are not allowed to execute .update again.

// Standard synchronization pattern
(function recursive() {
	replicate.update(function (err, newVersion) {
		if (err) throw err;
		console.log('updated to version ' + newVersion);
		setTimeout(recursive, 1000 * 60 * 60 * 24);
	});
})();

replicator.getLatestVersion(callback)

In case you want to be more in control, this method just fetches the latest version number, without any side effects.

replicator.getLatestVersion(function (err, remoteVersion) {
	if (err) throw err;

	console.log('remote now has version ' + remoteVersion);
});

Source

DAWA documents the replication protocol at http://dawa.aws.dk/replikeringdok.