block-file

A library to read/write blocks from a file

npm install block-file
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Block File Libary

Apology for API changes

I offer my applogies to anyone actually using this library between 1.0.x and 1.1.x . I've come to the belief that I should not use the promise based API. I asthetically like promises, but promises come with the problem of process.nextTick versus setImmediate. I even considered a hybrid approach with a queue of tasks executing a fixed number of tasks each process.nextTick. But that is for another day (and debugging head aches). So block-file with promises via setImmediate turns out to be much slower that of block-file with async (just running time npm test is 33 seconds with promises and 23 seconds with async). Additionally the async API can be easily converted to a promise API via Y.promisify aka Y.nfbind or Y.denodify (or simmilar functions in other promise libraries).

Apology for file format change

This is getting stupid, apologizing and such, but in order to store a small amount of data in the metadata area, like an initial handle of some data structure layered over block-file, I had to modify the metadata structure. Also the API bf.setAppData() & bf.getAppData() is a dumb name, but I can't thing of anything better.

So version 1.2.0 file format is incompatable with version 1.1.1.

Future?

I have plans for a 2.0 version which will make this library more "transaction-capable". What that means I haven't quite figured out.

Purpose

A library to allocate, free, read, and write fixed size blocks in a file. Blocks are addressed by 32bit or 64bit "handles". The blocks can be any power of 2 size, but is fixed when you create a BlockFile.

Handles can indicate a contiguous set of blocks. Again, how many blocks exist it a span is a power of 2 number.

All the async functions return promises from the ya-promise npm module (a Promise/A+ library).

API

BlockFile

Blockfile.open(filename, [props], cb)`

Where cb has the signature cb(err, bf). and bf is the block-file obect.

BlockFile.open(filename, function(err, bf){
  ...
})

bf.close(cb)

Where cb has the signature cb(err).

BlockFile.open(filename, function(err, bf){
  if (err) throw err
  return bf.close(function(err){
    if (err) throw err
    console.log("%s closed", filename)
  })
})

bf.flush(cb)

Where cb has the signature cb(err).

This writes out the file header and each dirty segment free space map.

bf.store(buffer, [handle], cb)

Where cb has the signature cb(err, handle). If node handle is provided as an argument to bf.store() then a new handle is allocated. The callback contains to handle of where the buffer was stored.

bf.store(buffer, function(err, handle){
  ...
})
bf.store(buffer, handle, function(err, handle){
  ...
})

bf.load(handle, cb)

bf.load(handle, function(err, buffer){
  ...
})

boolean = bf.release(handle, cb)

Where cb has the signature cb(err).

boolean reflects whether the handle was reserved already or not.

bf.setAppData(buffer)

Possably throws Error objects if buffer is to big to fit in the remaining amount of metadata buffer. Lets say ~3.5KB .

buffer = bf.getAppData()

That is about it. Could throw an Error if the metadata values of metaPropsLen or appDataLen are corrupted and to large. That would mean a big bug is found.

Props

The parameters varying the size of segment numbers, block number, span numbers, sizes of the Free Space Maps, etcetra.

MetaProps

numHandleBits

default: 32

I haven't figured out how to get 64 to be encoded yet. Just stick with 32 ok.

Number of bit a handle value is contained as: 32 or 64. Only encoding into a 32bit value is supported with 32. While 64 can not currently be encoded into a 64bit value, ALL other constraints relating to bit lengths are supported.

I am thinking of not having a 64bit handle option. :) solved that problem...

blockSzBits

default: 12

4k blocks are default. pow(2,12) == 4096

fsmSzBits

default: blockSzBits

number of blocks per segment minus a checksum. Basically the free space map is a bit field pow(fsmSzBits) bytes long. However for safety we checksum the bit field. That checksum (either 16 or 32 bits consumes 2 or 4 bytes of the bit field. So the number of blocks in a segment is the same as the number of bits in the Free Space Map minus the checksum bits.

If you make your blockSzBits smaller, feel free to keep the fsmSzBits large by explicitly setting the value.

spanNumBits

default: 4

original block + spanNumBits as an unsigned integer blocks

  • 0 => no span blocks (just the original block)
  • 1 => 1 or 2 blocks (aka original block + one more)
  • 2 => 1, 2, 3, or 4 blocks
  • 3 => 1, 2, 3, 4, 5, 6, 7, or 8 blocks
  • 4 => 1, 2, 3, ..., or 16 blocks
  • 5 => 1, 2, 3, ..., or 32 blocks (you get the picture)

checkSumBits

default: 16

supported: 16 and 32 CRCs

checkSumOffset

default: 0

Really never used. Don't touch!

Basic Use

var async = require('async')
  , fs = require('fs')
  , BlockFile = require('block-file')
  , hdls
  , data_fn = "my-data.bf"
  , hdls_fn = "my-data-hdls.json"
  , str = "lorem ipsum ..."

BlockFile.open(data_fn, function(err, bf){
  if (err) throw err
  var strLen = Buffer.byteLength(str)
    , buf = new Buffer(strLen+2)
    , promises = []

  buf.writeUInt16BE(strLen, 0)
  buf.write(str, 2, strLen)

  async.eachSeries(
    [ buf, buf ]
  , function(b, next){ bf.store(b, next) }
  , function(err, res){
      if (err) throw err
      hdls = res
      bf.close(function(err){
        if (err) throw err
        var a = hdls.map(function(hdl){ return hdl.toString() })
        console.log(a)
        fs.writeFileSync(hdls_fn, JSON.stringify(a))
      })
  })
var async = require('async')
  , fs = require('fs')
  , BlockFile = require('..')
  , Handle = BlockFile.Handle
  , data_fn = "my-data.bf"
  , hdls_fn = "my-data-hdls.json"
  , hdls = []

var hdlStrs = JSON.parse( fs.readFileSync(hdls_fn) )
hdlStrs.forEach(function(hdlStr,i){
  var hdl = Handle.fromString( hdlStr )
  hdls.push(hdl)
})


BlockFile.open(data_fn, function(err, bf){
  if (err) throw err

  var data = []
  async.eachSeries(
    hdls
  , function(hdl, next){
      bf.load(hdl, function(err, buf, hdl_){
        if (err) { next(err); return }
        data.push([buf, hdl_])
        next()
      })
    }
  , function(err){
      if (err) throw err
      for (var i=0; i<data.length; i+=1) {
        var buf = data[i][0]
          , hdl = data[i][1]
          , len = buf.readUInt16BE(0)
          , str = buf.toString('utf8', 2, len+2)

        console.log("\nhdl = %s", hdl)
        console.log("content = %s", str)
      }
    }
  )

})
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