sql considered harmful

GDC 2008

SQL Considered Harmful

Shannon PosniewskiCryptic Studios

GDC 2008

Current Relational DBMSs are Inappropriate for Low-

Latency, Controlled Working-Set Databases

and what we did trying to circumvent thatand how that didn’t work out very well

and what we’re doing now

Shannon Posniewski

GDC 2008

Database Fundamentals

Structured collection of data

“ACID” rules

GDC 2008

Atomicity

Consistency

Isolation

Durability

GDC 2008

Database Types

Relational

Object

Object-Relational

Hierarchical

etc.

GDC 2008

Relational Model

Defined mathematically

Based onset theory

predicate logic

GDC 2008

Relational Database

TablesRows

aka records

aka entities

Columnsvalues for that entity

GDC 2008

SQL

computer language for manipulating relational databases

GDC 2008

Unique and Foreign Keys

Unique key in one table refers to other rows in another table.

Used to represent hierarchical data

GDC 2008

Player ID PlayerName … AttribN

818 Warthog … foo

237 John … bar

Player ID Item Count

818 Hat 1

818 Brooch 1

818 Pteradactyl 1

237 Monkey Paw 1

GDC 2008

Player ID Name … Class ID

818 Warthog … 2

237 John … 3

Class ID Name

1 Elf

2 Orc

3 Monkey

GDC 2008

SQL DBMSs

Provide all this and more

Have object extensions, BLOBs, etc

Used by practically everyone

Safe, tested

Lots of experience

GDC 2008

Why not use it?

GDC 2008

The Good

SQL servers optimized for general case

Usually good at arbitrary queries

Will always “work”

GDC 2008

The Bad

Long query set up time

Self-optimizes on the fly

Unpredictable latency

GDC 2008

The Ugly

Inserts are SLOW

Per row overhead is high on updates

GDC 2008

Object-Relational Impedance Mismatch

Hierarchy and references are typical

Joins and normalization are often slow

GDC 2008

The Evils of Normalization

Doing it the “the right way” can backfire

GDC 2008

Player ID Stat ID Value

818 Time Online 2,123

818 Fights Won 700

818 Fights Lost 3

…

818 Stat N Value N

This was Bad

GDC 2008

Player ID Stat 0 Value Stat 1 Value … Stat N Value

818 2,123 200 … Value N

237 5,235 60 … Value N

54 675 23 … Value N

This was Good

Abandons relational DB concepts

GDC 2008

Solution: Caching

Built a write-through cache

Reads come from the cache

Writes go to cache and then to SQL

All active players in memory (fast)

GDC 2008

Not Enough

Still too slow

Cache smoothes out latency issues

Throughput still a problem

GDC 2008

Reduce Throughput Needs

Don’t save HP, location, etc

Snapshot instead

ACID is pretty much shot

GDC 2008

Data Ownership

A process “owns” an entity’s data

Vast majority of changes are local

Changes are periodically flushed to DB

GDC 2008

Cross-Owner Transactions

Flush both characters

Still a window of failure

We just did our best

GDC 2008

Optimized and played tricks on the database…

…and stopped getting the benefits of using one

GDC 2008

It’s just not the right tool

GDC 2008

Cryptic DB

GDC 2008

Requirements

Regain ACID (as much as possible)

Low, predictable latency

high read/write throughput

minimize code-database impedance

friendly to schema changes

easy backup and restore

GDC 2008

Relaxations

Offline character access can be slower

Domain integrity responsibility of programmer

General ad hoc queries can be slower

SQL does not need to be supported

GDC 2008

StructParser

Structured text definition of data

Generated from C structs

Used for serializing static data

Unifies static and DB data

GDC 2008

Example

AUTO_STRUCT AST_CONTAINER;typedef struct Entity{

EntityRef myRef; NO_AST const ContainerID myContainerID; AST(PERSIST, KEY) Vec3 pos; AST(PERSIST, SERVER_ONLY, NO_TRANSACT)

…

} Entity;

GDC 2008

Struct Metadata

Generates metadata on structsmemory layout

data types

Generic mutators are possibleCryptic DB

GDC 2008

Basic Operations

Cryptic DB supports only a few basic operations

Simple transactions built-in

More complicated things need code

GDC 2008

Data and Logic Ownership

A server “owns” the data for an entity

Knows how to modify these entities

GDC 2008

Map ServerMap Server

Architecture

Map ServerTeam Server

Cryptic DBTransaction

Server

GDC 2008

Cryptic Engine Transactions

Local if possible

Dispatched by Transaction Server if not

AUTO_TRANSACTIONS in C

Field-level locking

GDC 2008

AUTO_TRANSACTION

AUTO_TRANSACTION; enumTransactionOutcome trGiveGold(ATR_ARGS,

NOCONST(Entity) *pEnt1,NOCONST(Entity) *pEnt2,int amount)

{ if (pEnt1->gold < amount) TRANSACTION_RETURN(TRANSACTION_OUTCOME_FAILURE,

”NotEnoughGold”); pEnt1->gold -= amount; pEnt2->gold += amount;

return TRANSACTION_OUTCOME_SUCCESS;}

C code plus special macros

GDC 2008

Step 1Transaction Server

Server 1

A

Server 2

BDoTrade

Trans(“DoTrade”, A, B)

GDC 2008

Step 2Transaction Server

Server 1

Trans(“DoTrade”, A, B)

A

Server 2

BDoTrade

1234…

1234…

GDC 2008

Step 3Transaction Server

Server 1

A

Server 2

BDoTrade

1234…

B

GDC 2008

Step 4Transaction Server

Server 1

A

Server 2

B

DoTrade

1234…

B

A’

B’

GDC 2008

Step 4Transaction Server

Server 1

A

Server 2

B

1234…

B

A’

dB

B’

dB

B’

GDC 2008

Step 4Transaction Server

Server 1 Server 2

1234…

A’ B’

GDC 2008

Abstract Cryptic DB

Get transaction

Log it to journal

Apply change to in-memory DB

Occasionally write out updated DB file

Mark the journal resolved to that point

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