snowbank-slices-and-buffers

Slice, SliceReader, SliceWriter & friends

Slice is the workhorse for binary data in this codebase. It is a readonly struct (in namespace System) that wraps a segment of a byte[] — its three fields are Array (the backing array, possibly null), Offset, and Count. It predates Span<T> and is the logical equivalent of ReadOnlyMemory<byte>, but with a large library of helpers for turning bytes into and out of real-world types. Keys and values in the FoundationDB binding are Slices.

Two things to internalize first: (1) a Slice is a view, not a copy — it shares the backing array. (2) Slice.Nil (no array) and Slice.Empty (zero-length array) are different and the distinction is load-bearing. Both are covered below.

For the Span-first equivalents (SpanReader/SpanWriter, ISpanEncodable) read references/span-readers-writers.md; for pooled buffer-building (ISliceBufferWriter, SlicePool, ValueBuffer<T>, allocators) read references/buffers-and-pooling.md.

1. Nil vs Empty — the #1 gotcha

	Slice.Nil	Slice.Empty
backing array	none (null-like)	a zero-length array
IsNull	true	false
IsEmpty	false	true
IsNullOrEmpty	true	true
IsPresent	false	true
GetBytes()	returns null	returns an empty array
ToStringUtf8()	returns null	returns ""
==	Nil != Empty	distinct
CompareTo	Nil and Empty compare equal (both sort first)

tr.GetAsync(key) returns Slice.Nil for a missing key, so the canonical "does it exist?" check is value.IsNull (or IsNullOrEmpty if an empty value also counts as absent). Use Nil to mean absent and Empty to mean present but zero-length.

var v = await tr.GetAsync(key);
if (v.IsNull) { /* key does not exist */ }

2. Slice is a view — copy when you must own it

Constructing a Slice from a byte[] does not copy; the Slice references the array, so mutations to the array are visible through the slice (and its .Span). When you need an independent owner, copy:

byte[] buf = ...;
var view = buf.AsSlice();        // shares buf — buf[i] = x is visible through view
byte[] mine = view.ToArray();    // defensive copy
buf[0] = 0xFF;                   // changes `view`, not `mine`

3. Constructing a Slice

// from arrays / spans
byte[] b = ...;
b.AsSlice();                 b.AsSlice(offset, count);
new ArraySegment<byte>(b, o, n).AsSlice();
Slice.FromBytes("abc"u8);    // copies a ReadOnlySpan<byte>

// from text
Slice.FromStringUtf8("héllo");   Slice.FromString("héllo");   // UTF-8
Slice.FromStringAscii("ABC");    // ASCII only — lossy/throws on chars > 0x7F

// well-known
Slice.Empty;   Slice.Nil;   Slice.Zero(16);   // 16 zero bytes

// guids / uuids / hex
Slice.FromGuid(g);   Slice.FromUuid128(u);   Slice.FromHexString("00ff1234");

Three integer encodings — pick deliberately

This is a classic source of bugs. They are not interchangeable:

Factory	Encoding	Size (int32)	Read back with
Slice.FromInt32(v)	minimal little-endian (leading zero bytes dropped)	1–4 bytes	slice.ToInt32()
Slice.FromFixed32(v)	fixed little-endian	always 4 bytes	slice.ToInt32()
Slice.FromVarint32(v)	7-bit LEB128 varint	1–5 bytes	(via SliceReader.ReadVarInt32)

Every variant has a big-endian twin (FromInt32BE, FromFixed32BE, …) and 16/24/64/128-bit widths, plus floats (FromSingle/FromDouble) and FromDecimal. Big-endian fixed encodings are what you want when a number must sort correctly as a key. The minimal FromInt32 is for standalone values you read whole with ToInt32() — it is not self-delimiting, so don't use it mid-stream (in a SliceWriter, use the fixed-width WriteInt32/WriteInt64 or WriteVarInt* there; see §6).

⚠️ Naming differs between Slice and the writer/reader. On Slice (standalone), FromFixed32 = 4 bytes and FromInt32 = minimal. On SliceWriter/SliceReader (streams), the fixed-width method is plain WriteInt32/ReadInt32 (4 bytes LE; *BE for big-endian), and the varint is WriteVarInt32/ReadVarInt32. (WriteFixed32/ReadFixed32 exist but are [Obsolete] — use WriteInt32/ReadInt32.)

4. Reading values back

slice.ToInt64();   slice.ToInt32BE();   slice.ToGuid();   slice.ToUuid128();
slice.ToStringUtf8();    // Nil -> null, Empty -> ""
slice.ToArray();         // defensive copy to byte[]
slice.ToHexString();

// zero-copy access to the bytes
ReadOnlySpan<byte> span = slice.Span;
ReadOnlyMemory<byte> mem = slice.Memory;

// slicing (negative indices count from the end)
slice.Substring(7, 6);   slice[2..5];   slice[^1..];

5. Comparison & equality

Slice compares lexicographically by raw bytes (the same order FoundationDB sorts keys), is offset/array-independent (equal content compares equal regardless of backing array or offset), and supports ==, <, >, CompareTo, StartsWith, EndsWith, IndexOf. For dictionaries/sorted sets, use Slice.Comparer.Default (an IComparer<Slice> + IEqualityComparer<Slice>).

a.CompareTo(b) < 0;            // a sorts before b
key.StartsWith(prefix);        // prefix match
var set = new SortedSet<Slice>(Slice.Comparer.Default);

6. SliceWriter — build a buffer

SliceWriter is a mutable, growable builder (struct, IBufferWriter<byte>, IDisposable). Start from default(SliceWriter) (heap-backed, grows as needed) or new SliceWriter(pool) (rents from an ArrayPool<byte>):

var w = new SliceWriter();
w.WriteInt32(42);                    // fixed 4 bytes LE  (self-delimiting)
w.WriteVarInt32(1000);               // LEB128            (self-delimiting)
w.WriteVarString("hello");           // length-prefixed UTF-8
w.WriteStringUtf8("raw");            // raw UTF-8, NO length prefix
w.WriteBytes(payload);               // append bytes
Slice result = w.ToSlice();          // the written region (a view into the writer's buffer)

• Use self-delimiting writes (fixed-width WriteInt32/WriteInt64/…, WriteVarInt*, WriteVarString) for anything you'll parse back sequentially. A raw WriteStringUtf8/WriteBytes has no length, so the reader must already know the length.
• Position, Reset(), Rewind(), Skip(n), Allocate(n)/AllocateSpan(n) (reserve space to fill in place).
• Pooling caveat: if you pass an ArrayPool<byte>, you must either Dispose() the writer or hand the buffer off with ToSliceOwner() — otherwise the rented array is never returned. ToSlice() returns a view into the writer's buffer; if the writer (or its pooled buffer) is disposed/reused, that view becomes invalid — ToArray() or ToSliceOwner() it to keep it.

7. SliceReader — parse a buffer

SliceReader is a forward cursor over a Slice. Pair each read with the matching write:

var r = result.ToSliceReader();
int n     = r.ReadInt32();           // <-> WriteInt32  (fixed 4 bytes)
uint k    = r.ReadVarInt32();        // <-> WriteVarInt32
string s  = r.ReadVarString();       // <-> WriteVarString
// raw / fixed-length string written without a prefix: read the known number of bytes
string raw = r.ReadBytes(3).ToStringUtf8();
Slice rest = r.ReadToEnd();

Remaining, HasMore, Head (bytes already read), Tail (bytes not yet read), and non-advancing PeekByte()/PeekBytes(n) round out the API. There is no ReadStringUtf8(n) — use ReadBytes(n).ToStringUtf8().

8. SliceOwner — pooled, disposable Slices

SliceOwner is a rented Slice that returns its buffer to an ArrayPool<byte> on Dispose — the allocation-free analogue of IMemoryOwner<byte>. The contract: you MUST Dispose it, and MUST NOT use its data afterward.

using (var owner = Slice.FromBytes(payload, ArrayPool<byte>.Shared))
{
    Slice data = owner.Data;     // valid only inside the using
    Use(data.Span);
}   // buffer returned to the pool here

owner.IsValid, owner.Count, owner.Span, owner.Pool; SliceOwner.Wrap/Create/Copy and writer.ToSliceOwner() produce them. Don't let an owner's Data escape the using.

9. Span / Memory interop & ISpanEncodable

Slice interops freely with the modern primitives: slice.Span (ReadOnlySpan<byte>), slice.Memory (ReadOnlyMemory<byte>), byte[].AsSlice(). Many hot types (keys, values, the writers) implement ISpanEncodable so they can be rendered into a caller's buffer with no intermediate Slice allocation — TryGetSpan(out span) / TryGetSizeHint(out size) / TryEncode(dest, out written). That interface is how subspace.Key(...)/FdbValue.* write themselves into pooled buffers at the last moment.

For working directly over Span<byte> (a caller-owned, fixed buffer) instead of Slice, use SpanReader/SpanWriter — see references/span-readers-writers.md.

10. Round-trip example

// build
var w = new SliceWriter();
w.WriteInt32(order.Id);
w.WriteVarString(order.Customer);
w.WriteVarInt64(order.Total);
Slice packed = w.ToSlice();

// parse
var r = packed.ToSliceReader();
int id        = r.ReadInt32();
string cust   = r.ReadVarString();
long total    = (long) r.ReadVarInt64();

11. Self-check

• Did I use IsNull/IsNullOrEmpty (not == Slice.Empty) to test for a missing value?
• Am I treating Slice as a view — copying with ToArray()/ToSliceOwner() before mutating shared arrays or outliving a pooled buffer?
• Did I pick the right integer encoding (Fixed*/*BE for sortable keys; VarInt*/Fixed* for self-delimiting stream fields; FromInt32 only for standalone whole-slice values)?
• Do my SliceWriter writes and SliceReader reads pair up (WriteInt32↔ReadInt32, VarInt↔VarInt, VarString↔VarString)?
• If I rented from an ArrayPool (SliceWriter(pool) / SliceOwner), did I Dispose/ToSliceOwner() so the buffer returns to the pool — and not use the data after disposal?