Blaze.ByteString.Builder.Internal

Implementation of the Builder monoid.

A standard library user must never import this module directly. Instead, he should import Blaze.ByteString.Builder, which re-exports the Builder type and its associated public functions defined in this module.

Developers of other libraries may import this module to gain access to the internal representation of builders. For example, in some cases, creating a Builder with a custom low-level BuildStep may improve performance considerably compared to the creating it using the public Builder combinators (e.g., fromWrite1List in Blaze.ByteString.Builder.Write). Another example, is the use of ModifyChunks to efficiently wire the Builder type with another library that generates lazy bytestrings.

In any case, whenever you import this module you must reference the full version of the 'blaze-builder' package in your cabal file, as the implementation and the guarantees given in this file may change in any version! The release notes will tell, if this was the case.

The Builder type

newtype Builder

Intuitively, a builder denotes the construction of a lazy bytestring.

Builders can be created from primitive buffer manipulations using the Write abstraction provided by in Blaze.ByteString.Builder.Write. However for many Haskell values, there exist predefined functions doing that already. For example, UTF-8 encoding Char and String values is provided by the functions in Blaze.ByteString.Builder.Char.Utf8. Concatenating builders is done using their Monoid instance.

Semantically, builders are nothing special. They just denote a sequence of bytes. However, their representation is chosen such that this sequence of bytes can be efficiently (in terms of CPU cycles) computed in an incremental, chunk-wise fashion such that the average chunk-size is large. Note that the large average chunk size allows to make good use of cache prefetching in later processing steps (e.g. compression) or to reduce the sytem call overhead when writing the resulting lazy bytestring to a file or sending it over the network.

For precisely understanding the performance of a specific Builder, benchmarking is unavoidable. Moreover, it also helps to understand the implementation of builders and the predefined combinators. This should be amenable to the average Haskell programmer by reading the source code of Blaze.ByteString.Builder.Internal and the other modules of this library.

The guiding implementation principle was to reduce the abstraction cost per output byte. We use continuation passing to achieve a constant time append. The output buffer is filled by the individual builders as long as possible. They call each other directly when they are done and control is returned to the driver (e.g., toLazyByteString) only when the buffer is full, a bytestring needs to be inserted directly, or no more bytes can be written. We also try to take the pressure off the cache by moving variables as far out of loops as possible. This leads to some duplication of code, but results in sometimes dramatic increases in performance. For example, see the fromWord8s function in Blaze.ByteString.Builder.Word.

Constructors

Builder (BuildStep -> BuildStep)

Instances

Monoid Builder

type BuildStep

Arguments

 = Ptr Word8

Pointer to the next free byte in the buffer. A BuildStep must start writing its data from this address.

-> Ptr Word8

Pointer to the first byte after the buffer. A BuildStep must never write data at or after this address.

-> IO BuildSignal

Signal to the driver about the next step to be taken.

A BuildStep fills a buffer from the given start pointer as long as possible and returns control to the caller using a BuildSignal, once it is required.

data BuildSignal

A BuildSignal signals to the driver of the Builder execution the next step to be taken.

Constructors

Done !(Ptr Word8)

Done pf signals that the BuildStep is finished and data has been written up to the next free byte pf.

BufferFull !Int !(Ptr Word8) !BuildStep

BufferFull newSize pf nextStep signals that the buffer is full and data has been written up to the next free byte pf. Moreover, the next build step to be executed nextStep requires a buffer of at least size newSize to execute successfully.

A driver must guarantee that the buffer used to call nextStep is at least of size newSize.

ModifyChunks !(Ptr Word8) !(ByteString -> ByteString) !BuildStep

ModifyChunks pf fbs nextStep signals that the data written up to the next free byte pf must be output and the remaining lazy bytestring that is produced by executing nextStep must be modified using the function fbs.

This signal is used to insert bytestrings directly into the output stream. It can also be used to efficiently hand over control to another library for generating streams of strict bytestrings.

Flushing the buffer

flush :: Builder

Output all data written in the current buffer and start a new chunk.

The use uf this function depends on how the resulting bytestrings are consumed. flush is possibly not very useful in non-interactive scenarios. However, it is kept for compatibility with the builder provided by Data.Binary.Builder.

When using toLazyByteString to extract a lazy ByteString from a Builder, this means that a new chunk will be started in the resulting lazy ByteString. The remaining part of the buffer is spilled, if the reamining free space is smaller than the minimal desired buffer size.

Executing builders

toLazyByteStringWith

Arguments

:: Int

Buffer size (upper-bounds the resulting chunk size).

-> Int

Minimal free buffer space for continuing filling the same buffer after a flush or a direct bytestring insertion. This corresponds to the minimal desired chunk size.

-> Int

Size of the first buffer to be used and copied for larger resulting sequences

-> Builder

Builder to run.

-> ByteString

Lazy bytestring to output after the builder is finished.

-> ByteString

Resulting lazy bytestring

Run a Builder with the given buffer sizes.

Use this function for integrating the Builder type with other libraries that generate lazy bytestrings.

Note that the builders should guarantee that on average the desired chunk size is attained. Builders may decide to start a new buffer and not completely fill the existing buffer, if this is faster. However, they should not spill too much of the buffer, if they cannot compensate for it.

A call toLazyByteStringWith bufSize minBufSize firstBufSize will generate a lazy bytestring according to the following strategy. First, we allocate a buffer of size firstBufSize and start filling it. If it overflows, we allocate a buffer of size minBufSize and copy the first buffer to it in order to avoid generating a too small chunk. Finally, every next buffer will be of size bufSize. This, slow startup strategy is required to achieve good speed for short (<200 bytes) resulting bytestrings, as for them the allocation cost is of a large buffer cannot be compensated. Moreover, this strategy also allows us to avoid spilling too much memory for short resulting bytestrings.

Note that setting firstBufSize >= minBufSize implies that the first buffer is no longer copied but allocated and filled directly. Hence, setting firstBufSize = bufSize means that all chunks will use an underlying buffer of size bufSize. This is recommended, if you know that you always output more than minBufSize bytes.

toLazyByteString :: Builder -> ByteString

Extract the lazy ByteString from the builder by running it with default buffer sizes. Use this function, if you do not have any special considerations with respect to buffer sizes.

 toLazyByteString b = toLazyByteStringWith defaultBufferSize defaultMinimalBufferSize defaultFirstBufferSize b L.empty

Note that toLazyByteString is a Monoid homomorphism.

 toLazyByteString mempty          == mempty
 toLazyByteString (x `mappend` y) == toLazyByteString x `mappend` toLazyByteString y

However, in the second equation, the left-hand-side is generally faster to execute.

toByteString :: Builder -> ByteString

Run the builder to construct a strict bytestring containing the sequence of bytes denoted by the builder. This is done by first serializing to a lazy bytestring and then packing its chunks to a appropriately sized strict bytestring.

 toByteString = packChunks . toLazyByteString

Note that toByteString is a Monoid homomorphism.

 toByteString mempty          == mempty
 toByteString (x `mappend` y) == toByteString x `mappend` toByteString y

However, in the second equation, the left-hand-side is generally faster to execute.

toByteStringIOWith

Arguments

:: Int

Buffer size (upper bounds the number of bytes forced per call to the IO action).

-> (ByteString -> IO ())

IO action to execute per full buffer, which is referenced by a strict ByteString.

-> Builder

Builder to run.

-> IO ()

Resulting IO action.

toByteStringIOWith bufSize io b runs the builder b with a buffer of at least the size bufSize and executes the IO action io whenever the buffer is full.

Compared to toLazyByteStringWith this function requires less allocation, as the output buffer is only allocated once at the start of the serialization and whenever something bigger than the current buffer size has to be copied into the buffer, which should happen very seldomly for the default buffer size of 32kb. Hence, the pressure on the garbage collector is reduced, which can be an advantage when building long sequences of bytes.

toByteStringIO :: (ByteString -> IO ()) -> Builder -> IO ()

Run the builder with a defaultBufferSized buffer and execute the given IO action whenever the buffer is full or gets flushed.

 toByteStringIO = toByteStringIOWith defaultBufferSize

This is a Monoid homomorphism in the following sense.

 toByteStringIO io mempty          == return ()
 toByteStringIO io (x `mappend` y) == toByteStringIO io x >> toByteStringIO io y

Default sizes

defaultBufferSize :: Int

Default size (~32kb) for the buffer that becomes a chunk of the output stream once it is filled.

defaultMinimalBufferSize :: Int

The minimal length (~4kb) a buffer must have before filling it and outputting it as a chunk of the output stream.

This size determines when a buffer is spilled after a flush or a direct bytestring insertion. It is also the size of the first chunk generated by toLazyByteString.

defaultMaximalCopySize :: Int

The maximal number of bytes for that copying is cheaper than direct insertion into the output stream. This takes into account the fragmentation that may occur in the output buffer due to the early flush implied by the direct bytestring insertion.

defaultMaximalCopySize = 2 * defaultMinimalBufferSize