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

const RandomAccessStorage = require('random-access-storage')
const fs = require('fs')

const 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) {
  let fd = 0
  return new RandomAccessStorage({
    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) {
      const 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

const storage = new RandomAccessStorage([options])

Make a new instance. Options include:

{
  createAlways: false, // always create storage on first open
  open: fn, // sets ._open
  read: fn, // sets ._read
  write: fn, // sets ._write
  del: fn, // sets ._del
  truncate: fn, // sets ._truncate
  stat: fn, // sets ._stat
  suspend: fn, // sets ._suspend
  close: fn, // sets ._close
  destroy: fn // sets ._destroy
}

storage.readable

True if the storage implements ._read.

storage.writable

True if the storage implements ._write.

storage.deletable

True if the storage implements ._del.

storage.truncatable

True if the storage implements ._truncate.

storage.statable

True if the storage implements ._stat.

storage.opened

True if the storage has been fully opened.

storage.suspendable

True if the storage implements ._suspend.

storage.closed

True if the storage has been fully closed.

storage.destroyed

True is the destorage has been fully destroyed.

storage.on('open')

Emitted when the storage is fully open.

storage.on('close')

Emitted when the storage is fully closed.

storage.on('destroy')

Emitted when the storage is fully destroyed.

storage.on('suspend')

Emitted when the storage is fully suspended

storage.on('unsuspend')

Emitted when the storage comes out of suspension.

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.

storage._open(req)

Implement storage open.

• req.create is true if the storage should be created.

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 at 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.truncate(offset, [callback])

Truncate the storage at the specified offset. Optionally pass a callback that is called with (err) when the truncate has completed.

storage._truncate(req)

Implement storage truncate. Defaults to storage._del(req).

• req.offset contains the byte offset to truncate at.

Call req.callback(err) when the truncate 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.suspend([callback])

Suspend (temporarily close) the storage instance.

storage._suspend(req)

Implement storage suspend. Defaults to calling _close.

Optionally implement this to add a way for your storage instance to temporarily free resources.

Call req.callback(err) when the storage has been fully suspended.

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.

storage.destroy([callback])

Destroy the storage instance, removing all underlying data.

storage._destroy(req)

Implement storage destroy.

Call req.callback(err) when the storage has been fully destroyed.

Note this is guaranteed to run after .close() has been called and no methods will be run after.

License

MIT