Architect.Identities

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Architect.Identities

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Reliable unique ID generation for distributed applications.

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# Architect.Identities

Reliable unique ID generation for distributed applications.

This package provides highly tuned tools for ID generation and management.

- [TLDR](#tldr)
  * [What ID should I use?](#what-id-should-i-use)
- [Introduction](#introduction)
  * [UUID version 7](#uuid-version-7)
- [Distributed IDs](#distributed-ids)
  * [Example Value](#example-value)
  * [Example Usage](#example-usage)
  * [Benefits](#benefits)
  * [Trade-offs](#trade-offs)
  * [Structure](#structure)
  * [Collision Resistance](#collision-resistance)
    + [The degenerate worst case](#the-degenerate-worst-case)
    + [Absolute certainty](#absolute-certainty)
  * [Guessability](#guessability)
  * [Attack Surface](#attack-surface)
  * [Entity Framework](#entity-framework)
  * [Alternatives](#alternatives)
- [DistributedId128](#distributedid128)
  * [Additional Advantages](#additional-advantages)
  * [128-bit Structure](#128-bit-structure)
- [Public Identities](#public-identities)
  * [Example Value](#example-value-1)
  * [Example Usage](#example-usage-1)
  * [Implementation](#implementation)
  * [Forgery Resistance](#forgery-resistance)

## TLDR

The **[DistributedId](#distributed-ids)** is a single, compact ID that combines the advantages of auto-increment IDs and UUIDs.

The **[DistributedId128](#distributedid128)** is a UUID version 7 variant with the advantages of the DistributedId and practically no rate limits or collisions.

For sensitive scenarios where zero metadata must be leaked from an ID, **[Public Identities](#public-identities)** can transform any ID into a public representation that reveals nothing, without ever introducing an unrelated secondary ID.

### What ID should I use?

If you just want to decide what kind of ID to use, follow these steps:

- Avoid `Guid.CreateVersion7()`, `Guid.NewGuid()`, and auto-increment _as default picks_.
- If an ID's creation timestamp is considered safe to expose:
  - If you need to deal with pre-existing UUIDs, choose *[DistributedId128](#distributedid128)*.
  - If you need to scale to billions of new IDs per day, choose *[DistributedId128](#distributedid128)*.
    - _This is probably not you._
  - Otherwise, choose the more compact *[DistributedId](#distributed-ids)*.
    - _Note that the numeric representation is more pleasant to store, but the string representations is shorter to humans and plays nice with SQL Server's columnstores._
- If an ID's creation timestamp is considered sensitive (e.g. "when was this bank account opened"):
  - If you need to deal with pre-existing UUIDs, choose `Guid.NewGuid()`.
  - If the table size will never be significant, choose `Guid.NewGuid()`.
  - If a secret key can be managed easily enough, choose *[DistributedId](#distributed-ids)* or auto-increment, and use *[Public Identities](#public-identities)* to expose a deterministic, indistinguishable-from-random public representation.
  - If the above is infeasible, concede to `Guid.NewGuid()`.

## Introduction

Should entity IDs use UUIDs or auto-increment?

Auto-increment IDs are ill-suited for public exposure: they leak hints about the row count and are easy to guess. Moreover, they are generated very late, on insertion, posing challenges to the creation of entities that point to one another by ID.

Traditional random UUIDs, on the other hand, cause poor performance as database/storage keys due to random access patterns.

Using both types of ID on a single entity is cumbersome and may leak a technical workaround into the domain model.

Fortunately, we can do better.

### UUID version 7

This package predates the official specification of UUID version 7 (UUIDv7) and its implementation in .NET 9.

UUIDv7 is an excellent answer to the problem posed above.
As great as it is to have a solution built into .NET, however, that implementation leaves something to be desired.

The **[DistributedId128](#distributedid128)** is a UUIDv7 variant with several advantages.
Most prominently, it remains incremental even when multiple IDs are generated at the same millisecond, so that items created in sequence (as with batches) stay in that order.
They also stay incremental when the time synchronization protocol makes small adjustments to the system clock.
The absence of these properties in .NET's own implementation is a source of [astonishment](https://stackoverflow.com/q/79559842/543814) and [disappointment](https://www.reddit.com/r/dotnet/comments/1ea5mgt/comment/lekva4j/?utm_source=share&utm_medium=web3x&utm_name=web3xcss&utm_term=1&utm_content=share_button).

As an alternative, the **[DistributedId](#distributed-ids)** is a shorter equivalent that can be stored numerically as just 13 bytes, or alphanumerically as a mere 16 characters.

## Distributed IDs

The DistributedId is an alternative to UUIDs that is generated on-the-fly (without orchestration), unique, hard to guess, easy to store and sort, and highly efficient as a database key.

A DistributedId is created as a 93-bit decimal value of 28 digits, but can also be represented as a (case-sensitive) 16-char alphanumeric value or even as a `Guid`.

Distributed applications can create unique DistributedIds with no synchronization mechanism between them. This holds true under almost any load. Even under extreme conditions, [collisions](#collision-resistance) (i.e. duplicates) tend to be far under 1 collision per 350 billion IDs generated.

DistributedIds are designed to be unique within a logical context, such as a database table, a Bounded Context, or even a whole company. These form the most common boundaries within which uniqueness is required. Any number of distributed applications may generate new IDs within such a context.

Note that a DistributedId **reveals its creation timestamp**, which may be considered sensitive data in certain contexts.

### Example Value

- `decimal` value: `1088824355131185736905670087` (28 digits)
- Alphanumeric encoding: `3zfAkCP7ZtzfeQYp` (16 alphanumeric characters)

### Example Usage

```cs
decimal id = DistributedId.CreateId(); // 1088824355131185736905670087

// Alternatively, for a more compact representation, IDs can be encoded in alphanumeric
string compactId = id.ToAlphanumeric(); // "3zfAkCP7ZtzfeQYp"
decimal originalId = AlphanumericIdEncoder.DecodeDecimalOrDefault(compactId)
	?? throw new ArgumentException("Not a valid encoded ID.");
```

For SQL databases, the recommended column type is `DECIMAL(28, 0)`. Alternatively, a DistributedId can be stored as 16 _case-sensitive_ ASCII characters, or even as a UUID. (The latter is discouraged, as storage engines differ in how they sort UUIDs.)

The ID generation can be controlled from the outside, such as in unit tests that require constant IDs:

```cs
[Fact]
public void ShowInversionOfControl()
{
	// A custom generator is included in the package
	const decimal fixedId = 1m;
	using (new DistributedIdGeneratorScope(new CustomDistributedIdGenerator(() => fixedId)))
	{
		var entity = new Entity(); // Constructor implementation uses DistributedId.CreateId()
		Assert.Equal(fixedId, entity.Id); // True
		
		// A simple incremental generator is included as well
		using (new DistributedIdGeneratorScope(new IncrementalDistributedIdGenerator(fixedId)))
		{
			Assert.Equal(1m, DistributedId.CreateId()); // True
			Assert.Equal(2m, DistributedId.CreateId()); // True
			Assert.Equal(3m, DistributedId.CreateId()); // True
		}
		
		Assert.Equal(fixedId, DistributedId.CreateId()); // True
	}
}
```

### Benefits

- Is incremental (even intra-millisecond), making it _drastically_ more efficient as a primary key than a UUID.
- Remains incremental even if clock synchronization adjusts the clock (for adjustments of up to 1 second).
- Is shorter than a UUID, making it more efficient as a primary key.
- Like a UUID, can be generated on-the-fly, with no registration or synchronization whatsoever.
- Like a UUID, makes collisions extremely unlikely.
- Like a UUID, is hard to guess due to a significant random component.
- Like a UUID, does not require database insertion for determining the ID, nor reading the ID back in after insertion (as with auto-increment).
- Consists of digits only.
- Can be encoded as 16 alphanumeric characters, for a shorter representation.
- Uses the common `decimal` type, which is intuitively represented, sorted, and manipulated in .NET and databases (which cannot be said for UUIDs).
- Supports comparison operators (unlike UUIDs, which make comparisons notoriously hard to write using the Entity Framework).
- Is suitable for use in URLs.
- Can by selected in UIs (such as for copying) by double-clicking, as it consists of only word characters in both its numeric and alphanumeric form.

### Trade-offs

- Reveals its creation timestamp in milliseconds.
- Throttles when the _sustained_ generation rate exceeds 128K IDs per second, per application replica. (Note that cloud applications tend to scale _out_ rather than up.)
- Is designed to be unique within a chosen context rather than globally. (For most companies, the context could easily by the entire company's landscape.)
- Is slightly less efficient than a 64-bit integer.

### Structure

- Is represented as a positive `decimal` of exactly 28 digits, with 0 decimal places.
- Occcupies 16 bytes in memory.
- Is represented as `DECIMAL(28, 0)` in SQL databases.
- Requires 13 bytes of storage in many SQL databases, including SQL Server and MySQL. (This is more compact than a UUID, which takes up 16 bytes.)
- Can be represented in a natural, workable form by most SQL databases, being a simple `decimal`. (By contrast, not all databases have a UUID type, requiring the use of binary types, making manual queries cumbersome.)
- Is ordered intuitively and consistently in .NET and databases. (By contrast, SQL Server orders UUIDs by some of the _middle_ bytes, making it very hard to implement an ordered UUID type.)
- Can be represented numerically, as exactly 28 digits.
- Can be represented alphanumerically, as exactly 16 alphanumeric characters.
- Can be represented hexadecimally, as exactly 26 hex characters.
- Contains 93 bits worth of data.
- Contains the number of milliseconds since the start of the year 1900 in its first 45 bits.
- Contains a cryptographically-secure pseudorandom sequence in its last 48 bits.
- Uses 41-bit cryptographically-secure pseudorandom increments intra-millisecond to remain incremental on the same millisecond.
- Can represent timestamps beyond the year 3000.

### Rate Limits

Per application replica, the maximum sustained ID generation rate is roughly 128 IDs per millisecond, or 128K per second. The rate limit makes it possible to have incremental IDs even intra-millisecond, without sacrificing the other benefits.

To reduce the impact of the rate limit, each replica can burst generate up to 128K IDs instantly. During reduced activity, consumed burst capacity is regained according to the unused portion of the normal maximum rate. For example, after one second of not generating any IDs, the burst capacity is back up to its full 128K IDs. The same is true after two seconds of generating at half capacity.

Note that, in practice, cloud applications tend to scale _out_ rather than up. Few applications require any single replica to generate over 128K IDs per second.

### Collision Resistance

DistributedIds have strong collision resistance. The probability of generating the same ID twice is neglible for almost all contexts.

Most notably, collisions across different timestamps are impossible, since the millisecond values differ.

Within a single application replica, collisions during a particular millisecond are avoided (while maintaining the incremental nature) by reusing the previous random value (48 bits) and incrementing it by a smaller random value (41 bits). This _guarantees_ unique IDs within the application replica, provided that the system clock is not adjusted backwards by more than 1 second. For larger backwards adjustments, the scenario is comparable to having an additional replica (addressed below) during the repeated time span.

The scenario where collisions can occur is when multiple application replicas are generating IDs at the same millisecond. It is detailed below and should be negligible.

#### The degenerate worst case

The chances of a collision occurring have been measured. Under the worst possible circumstances, they are as follows:

- On average, with 2 application replicas at maximum throughput, there is **1 collision per 3500 billion IDs**. (That is 3,500,000,000,000. As a frame of reference, it takes 2 billion IDs to exhaust an `int` primary key.)
- On average, with 10 application replicas at maximum throughput, there is **1 collision 350 billion IDs**.
- On average, with 100 application replicas at maximum throughput, there is **1 collision per 35 billion IDs**.

It is important to note that **the above is only in the degenerate scenario** where _all replicas_ are generating IDs _at the maximum rate per millisecond_, and always _on the exact same millisecond_. In practice, far fewer IDs tend to be generated per millisecond, thus spreading IDs out over more timestamps. This significantly reduces the realistic probability of a collision, to 1 per many trillions, a negligible number for the intended purposes.

#### Absolute Certainty

Fortunately, we can protect ourselves even against the extremely unlikely event of a collision.

For contexts where even a single collision could be catastrophic, such as in certain financial domains, it is advisable to avoid "upserts", and always explicitly separate inserts from updates. This way, even if a collision did occur, it would merely cause one single transaction to fail (out of billions or trillions), rather than overwriting an existing record. This is good practice in general.

Alternatively, the [DistributedId128](#distributedid128) offers far greater collision resistance at the cost of an unwieldy format.

### Guessability

Presupposing knowledge of the millisecond timestamp on which an ID was generated, the probability of guessing that ID is between 1/2^41 and 1/2^48, thanks to the 48-bit cryptographically-secure pseudorandom sequence. In practice, the timestamp component tends to reduce the guessability, since for most milliseconds no IDs at will will have been generated.

The difference between the two probabilities (given knowledge of the timestamp) stems from the way the incremental property is achieved. If only one ID was generated on a timestamp, as tends to be common, the probability is 1/2^48. If the maximum number of IDs were generated on that timestamp, or if another ID from the same timestamp is known, an educated guess has a 1/2^41 probability of being correct.

To reduce the guessability to 1/2^128, see [Public Identities](#public-identities).

### Attack Surface

A DistributedId reveals its creation timestamp. Otherwise, it consists of cryptographically-secure pseudorandom data.

### Entity Framework

When DistributedIds (or any decimal IDs) are used in Entity Framework, the column type needs to be configured. Although this can be done manually, there are more convenient ways.

If the [Architect.DomainModeling](https://github.com/TheArchitectDev/Architect.DomainModeling) package is used, its [Entity Framework conventions](https://github.com/TheArchitectDev/Architect.DomainModeling?tab=readme-ov-file#entity-framework-conventions) can take care of all identity mappings, as well as make it easy to customize them.

Alternatively, the [Architect.Identities.EntityFramework](https://www.nuget.org/packages/Architect.Identities.EntityFramework) companion package facilitates mapping plain DistributedIds:

```cs
protected override void ConfigureConventions(ModelConfigurationBuilder configurationBuilder)
{
	base.ConfigureConventions(configurationBuilder);

configurationBuilder.ConfigureDecimalIdTypes(modelAssemblies: typeof(SomeEntity).Assembly);
}
```

`ConfigureDecimalIdTypes()` uses precision 28, scale 0, and conversions to and from the decimal type (where necessary).

The conventions are applied to any entity properties named "*Id" or "*ID" whose type is either decimal or a decimal-convertible type, including nullable wrappers.

Optionally, the extension method takes any number of assemblies as input. From those assemblies, it finds all types named "*Id" or "*ID" that are decimal-convertible, and configures a `DefaultTypeMapping` for them using the same conventions.

A `DefaultTypeMapping` applies when the type appears in EF-generated queries where the context of a column is lost, such as when EF generates a call to `CAST()`. Without such a mapping, EF may choose to convert a decimal to some default precision, which is generally too low.

### Alternatives

There exist [various alternatives](https://www.ietf.org/archive/id/draft-peabody-dispatch-new-uuid-format-04.html#name-informative-references-2) to the DistributedId. The most common feature they lack is the combination of being incremental and unpredictable, particularly for IDs generated on the same timestamp.

To highlight a few examples:

- [`Guid.CreationVersion7()`](https://learn.microsoft.com/en-us/dotnet/api/system.guid.createversion7)
  - Stored as 16 bytes or 36 base-64 characters.
  - Next ID on the same timestamp is [potentially our of order](https://github.com/dotnet/runtime/blob/9d5a6a9aa463d6d10b0b0ba6d5982cc82f363dc3/src/libraries/System.Private.CoreLib/src/System/Guid.cs#L311).
- [Universally Unique Lexicographically Sortable Identifier (ULID)](https://github.com/ulid/spec)
  - Stored as 16 bytes or 26 base-32 characters.
  - Next ID on the same timestamp is either [predictable](https://github.com/ulid/spec#monotonicity) or [potentially out of order](https://github.com/ulid/spec#sorting).
- [MongoDB's ObjectId](https://www.mongodb.com/docs/manual/reference/method/ObjectId/)
  - Stored as 12 bytes or 24 hexadecimal characters.
  - Next ID of the same second is predictable.
  - Uniquefier based on replica: once two replicas collide, multiple collisions can be expected, until redeployment.

## DistributedId128

The DistributedId128 is a UUIDv7 variant, and the 128-bit counterpart to the DistributedId, offering additional benefits at the cost of extra space and being more unwieldy than a simple `decimal`.
It should be used if the requirements on generation rate or collision resistance are drastic (extreme scale) or unpredictable (class libraries).
Apart from leaking the generation timestamp, this ID can serve as a drop-in replacement for the common version-4 random UUID.

Class libraries are a good example of products that should prefer the DistributedId128. They can make fewer assumptions, as their usage patterns often depend on the applications using them.

### Additional Advantages

- Has practically no generation rate limit (over 100K IDs per millisecond, or 100 million per second, per application replica).
- Risks practically no collisions even globally (at 75 bits of randomness _per millisecond_, incremented with 58-bit values intra-millisecond).
  - **OTP:** 1 million servers each generating 1 ID at the same millisecond, repeatedly, expect less than 1 collision per 75 quadrillion (75,000,000,000,000,000) IDs.
  - **Batch processing:** 100K servers each generating 100K IDs at the same millisecond, repeatedly, expect less than 1 collision per 5 trillion (5,000,000,000,000) IDs.
- Is a [version-7 UUID](https://www.ietf.org/archive/id/draft-peabody-dispatch-new-uuid-format-04.html#name-uuid-version-7) (_with the [variant](https://www.rfc-editor.org/rfc/rfc4122.html#section-4.1.1) set to 0xx/"backward compatibility", to keep the 64th bit 0_), but with the rare property of being incremental _and_ hard to guess even for multiple IDs at the same millisecond.

### 128-bit Structure

- Is returned as either a `Guid` or a `UInt128` and can be freely transcoded between the two.
- Can be represented as a 32-char or 36-char UUID string anywhere (e.g. `094954a8-622c-76ad-1b93-cdefcbdf0888`).
- Can be represented as `DECIMAL(38, 0)` in SQL databases (until beyond the year 4000, at which point a 39th digit would appear).
- Occcupies 16 bytes in memory.
- Requires 17 bytes of storage as `DECIMAL(38, 0)` or 22-24 bytes as alphanumeric ASCII in many SQL databases, including SQL Server and MySQL.
- Can be represented as two (always positive) `long` values, suitable for storing in two `BIGINT` database columns, as a composite key.
- Can be represented in a natural, workable form by most SQL databases, being a simple `DECIMAL` or `VARCHAR` or pair of `BIGINT` values. (By contrast, not all databases have a UUID type, requiring the use of binary types, making manual queries cumbersome.)
- Is ordered intuitively and consistently in .NET, in both `Guid`, `Guid.ToString()`, `UInt128`, `UInt128.ToString()`, alphanumeric, and hexadecimal formats.
- Is ordered intuitively and consistently in .NET _and databases_ in all formats except `Guid` (due to varying UUID availability and ordering in databases).
- Can be represented numerically, as exactly 38 digits (until beyond the year 4000, at which point a 39th digit would appear).
- Can be represented alphanumerically, as exactly 22 alphanumeric characters.
- Can be represented hexadecimally, as exactly 32 hex characters.
- Contains the number of milliseconds since the start of the year 1700 in its first 48 bits.
- Contains the UUID version marker "7" in bits 48 through 51.
- Contains a 75-bit cryptographically-secure pseudorandom sequence in its last 76 bits, with the exception of the 64th bit, the variant indicator, which is always 0.
- Uses 58-bit cryptographically-secure pseudorandom increments intra-millisecond to remain incremental on the same millisecond.
- Can represent timestamps beyond the year 9999.

## Public Identities

Sometimes, revealing even a creation timestamp is too much. For example, an ID might represent a bank account.

Still, it is desirable to have only a single ID, and one that is efficient as a primary key, at that. To achieve that, we can create a public representation of that ID, one that reveals nothing.

### Example Value

- The regular ID can be any `long`, `ulong`, `decimal`, `UInt128`, or `Guid`, such as an auto-increment ID, a DistributedId128, or a DistributedId: `1088824355131185736905670087m`
- Resulting public `Guid` value: `30322474-a954-ffa9-941c-6f038afe4ff1` (16 bytes)
- Alphanumeric encoding: `48XoooHHCe1CiOHrghM7Dl` (22 alphanumeric characters)

### Example Usage

```cs
// Startup.cs
public void ConfigureServices(IServiceCollection services)
{
	const string key = "k7fBDJcQam02hsByaOWPeP2CqeDeGXvrPUkEAQBtAFc="; // Strong 256-bit key, encoded in base-64
	
	services.AddPublicIdentities(publicIdentities => publicIdentities.Key(key));
}
```

```cs
public void ExampleUse(IPublicIdentityConverter publicIdConverter)
{
	decimal id = DistributedId.CreateId(); // 1088824355131185736905670087
	
	// Convert to public ID
	Guid publicId = publicIdConverter.GetPublicRepresentation(id); // 30322474-a954-ffa9-941c-6f038afe4ff1
	string publicIdString = publicId.ToAlphanumeric(); // "48XoooHHCe1CiOHrghM7Dl" (22 chars)
	
	// Convert back to internal ID
	decimal originalId = publicIdConverter.GetDecimalOrDefault(AlphanumericIdEncoder.DecodeGuidOrDefault(publicIdString) ?? Guid.Empty)
		?? throw new ArgumentException("Not a valid ID.");
}
```

```cs
public void ExampleHexadecimalEncoding(IPublicIdentityConverter publicIdConverter)
{
	decimal id = DistributedId.CreateId(); // 1088824355131185736905670087
	
	// We can use Guid's own methods to get a hexadecimal representation
	Guid publicId = publicIdConverter.GetPublicRepresentation(id); // 30322474-a954-ffa9-941c-6f038afe4ff1
	string publicIdString = publicId.ToString("N").ToUpperInvariant(); // "32F0EDAC80632C685C43C889B058556E" (32 chars)
	
	// Convert back to internal ID
	decimal originalId = publicIdConverter.GetDecimalOrDefault(new Guid(publicIdString))
		?? throw new ArgumentException("Not a valid ID.");
}
```

### Implementation

Public identities are implemented by using AES encryption under a secret key. With the key, the original ID can be retrieved. _Without the key_, the data is indistinguishable from random noise. In fact, it is exactly the size of a UUID, and it can be formatted to look just like one, for that familiar feel.

Obviously, it is important that the secret key is kept safe. Moreover, the key must not be changed. Doing so would render any previously provided public identities invalid.

### Forgery Resistance

Without possession of the key, it is extremely hard to forge a valid public identity.

When a public identity is converted back into the original ID, its structure is validated. If it is invalid, `null` or `false` is returned, depending on the method used.

For `long` and `ulong` IDs, the chance to forge some valid ID is 1/2^64. For `decimal` IDs, the chance is 1/2^32. Most importantly, even if a valid value were to be forged, the resulting internal ID would be a random one and would be extremely unlikely to match an existing one. This property makes even `Guid` and `UInt128` IDs usable, despite the fact that they do not have spare space to detect forgeries.

Generally, when an ID is taken as client input, something is loaded based on that ID. As such, it is often best to simply turn an invalid public identity into a nonexistent local ID, such as 0:

```cs
// Any invalid input will result in id=0
long id = publicIdConverter.GetLongOrDefault(AlphanumericIdEncoder.DecodeGuidOrDefault(publicIdString) ?? Guid.Empty)
	?? 0L;

var entity = this.Repository.GetEntityById(id);
if (entity is null) return this.NotFound();
```

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