level-proxy

proxy a leveldb reference so you can swap backend instances on the fly

This is useful for modules that need to switch out references transparently, like automatically upgrading an ordinary vanilla reference into a multilevel handle. You could probably also use this module to implement even crazier things, like a transparent hash ring.

example

In this example, we'll create 2 db handles proxied by the level-proxy handle db: a and b. The handles will swap into being the active handle every 3 seconds.

var level = require('level');
var levelSwap = require('level-proxy');
 
var a = level('/tmp/db-a');
var b = level('/tmp/db-b');
var db = levelSwap(a);
 
var n = 0;
setInterval(function () { db.put('x', n++) }, 250);
 
setInterval(function () {
    a.get('x', function (err, x) { console.log('a.x=', x) });
    b.get('x', function (err, x) { console.log('b.x=', x) });
}, 1000);
 
var index = 0;
setInterval(function () {
    db.swap([ a, b ][++ index % 2]);
}, 3000);

output:

$ node example/swap.js
a.x= 2
b.x= 22
a.x= 6
b.x= 22
a.x= 10
b.x= 22
a.x= 10
b.x= 14
a.x= 10
b.x= 18
^C

You can proxy streaming methods with buffering too:

var level = require('level');
var levelProxy = require('level-proxy');
 
var a = level('/tmp/stream-a');
var b = level('/tmp/stream-b');
 
a.batch([
    { type: 'put', key: 'a', value: 3 },
    { type: 'put', key: 'b', value: 4 },
    { type: 'put', key: 'c', value: 5 },
]);
 
b.batch([
    { type: 'put', key: 'x', value: 7 },
    { type: 'put', key: 'y', value: 8 },
    { type: 'put', key: 'z', value: 9 },
]);
 
var db = levelProxy();
 
setInterval(function () {
    var s = db.createKeyStream();
    var keys = [];
    s.on('data', function (key) { keys.push(key) })
    s.on('end', function () { console.log(keys) });
}, 1000);
 
var index = 0;
setInterval(function () {
    db.swap([ a, b ][++ index % 2]);
}, 3000);

output:

$ node example/stream.js
[ 'x', 'y', 'z' ]
[ 'x', 'y', 'z' ]
[ 'x', 'y', 'z' ]
[ 'x', 'y', 'z' ]
[ 'x', 'y', 'z' ]
[ 'a', 'b', 'c' ]
[ 'a', 'b', 'c' ]
[ 'a', 'b', 'c' ]
[ 'x', 'y', 'z' ]
[ 'x', 'y', 'z' ]
[ 'x', 'y', 'z' ]
[ 'a', 'b', 'c' ]
^C

methods

var levelProxy = require('level-proxy');

var db = levelProxy(initDb)

Create a new proxied database handle db, including events. All the leveldown methods are available on the db instance and all method calls will be queued when a database handle isn't yet available so you can start using the handle immediately.

You can optionally give an initial initDb handle to use, which is the same as calling db.swap(initDb) after creating an instance.

db.swap(newDb)

Swap out the db's internal proxy for a leveldb handle newDb.

Any buffered method calls that have queued up will fire on newDb on the same tick.

This event triggers a 'swap' event with the newDb reference.

db.get(), db.put(), db.batch(), db.createReadStream(), ...

All of these methods behave the same as with an ordinary level db handle.

db.db.iterator(), ...

leveldown methods such as db.db.iterator() are also proxied.

install

With npm do:

npm install level-proxy

license

MIT