random-access-storage

random-access-storage

Easily make random-access-storage instances

npm install random-access-storage

A random-access-storage instance is a common interface for a storage abstraction, that provides the following core api.

• read(offset, size) - Read a buffer at a custom offset.
• write(offset, data) - Write a buffer to a custom offset.
• del(offset, size) - Delete data at a custom offset.

This module exposes a base class that implements most of the plumbing and flow you'd usually want to implement when making one.

Usage

var randomAccess = require('random-access-storage')
var fs = require('fs')

var file = fileReader('index.js')

file.read(0, 10, (err, buf) => console.log('0-10: ' + buf.toString()))
file.read(40, 15, (err, buf) => console.log('40-55: ' + buf.toString()))
file.close()

function fileReader (name) {
  var fd = 0
  return randomAccess({
    open: function (req) {
      // called once automatically before the first read call
      fs.open(name, 'r', function (err, res) {
        if (err) return req.callback(err)
        fd = res
        req.callback(null)
      })
    },
    read: function (req) {
      var buf = Buffer.allocUnsafe(req.size)
      fs.read(fd, buf, 0, buf.length, req.offset, function (err, read) {
        if (err) return req.callback(err)
        if (read < buf.length) return req.callback(new Error('Could not read'))
        req.callback(null, buf)
      })
    },
    close: function (req) {
      if (!fd) return req.callback(null)
      fs.close(fd, err => req.callback(err))
    }
  })
}

API

var storage = randomAccessStorage([options])

Make a new instance. Options include:

{
  open: fn, // sets ._open
  openReadonly: fn, // sets ._openReadonly
  read: fn, // sets ._read
  write: fn, // sets ._write
  del: fn, // sets ._del
  stat: fn, // sets ._stat
  close: fn, // sets ._close
}

storage.readable

True is the storage implements ._read.

storage.writable

True is the storage implements ._write.

storage.deletable

True is the storage implements ._del.

storage.statable

True is the storage implements ._stat.

storage.on('open')

Emitted when the storage is fully open.

`storage.on('close')

Emitted when the storage is fully closed.

storage.open(cb)

Explicitly open the storage. If you do not call this yourself, it will automatically called before any read/write/del/stat operation.

It is safe to call this more than once.

Triggers one call to _open if you implement that.

If you implement _openReadonly and the operation that triggers the open is not a write/del then _openReadable will be called instead.

If a write is later performed a _open call will be triggered as well, expecting you to open the storage in read/write mode.

storage._open(req)

Implement storage open.

Call req.callback when it is fully opened.

storage._openReadonly(req)

Implement storage readonly open.

Useful if you prefer to open the storage in readonly mode unless a write is performed. Call req.callback when it is fully opened.

storage.read(offset, size, callback)

Read the specified bytes from the storage at the specified byte offset. Calls the callback with a (err, buffer).

storage._read(req)

Implement storage read.

• req.offset contains the byte offset you should read at.
• req.size contains the amount of bytes you should read.

Call req.callback(err, buffer) when the read is completed.

Note that this is guaranteed to run after the storage has been opened and not after it has been closed.

storage.write(offset, buffer, [callback])

Write the specified buffer to the specified byte offset. Optionally pass a callback that is called with (err) when the write has completed.

storage._write(req)

Implement storage write.

• req.offset contains the byte offset you should write at.
• req.data contains the buffer you should write.

Call req.callback(err) when the write is completed.

Note that this is guaranteed to run after the storage has been opened and not after it has been closed.

storage.del(offset, size, [callback])

Delete the specified amount of bytes as the specified offset. Optionally pass a callback that is called with (err) when the delete has completed.

storage._del(req)

Implement storage delete.

• req.offset contains the byte offset to delete at.
• req.size contains the amount of bytes to delete.

Call req.callback(err) when the delete has completed.

Note that this is guaranteed to run after the storage has been opened and not after it has been closed.

storage.stat(callback)

Stat the storage. Should return an object with useful information about the underlying storage, including:

{
  size: number // how many bytes of data is stored?
}

storage._stat(req)

Implement storage stat.

Call req.callback(err, statObject) when the stat has completed.

Note that this is guaranteed to run after the storage has been opened and not after it has been closed.

storage.close([callback])

Close the storage instance.

storage._close(req)

Implement storage close.

Call req.callback(err) when the storage is fully closed.

Note this is guaranteed to run after all pending read/write/stat/del operations has finished and no methods will run after.

License

MIT