Indexes

Hash Table

This feature is introduced in TrailDB 0.6. [TODO]

Stored Procedures

Not Supported

Each TrailDB is a read-only immutable file, it does not support stored procedures.

Query Interface

Custom API

TrailDB system does not support the standard SQL query interface. It offers the query interface in several programming languages: C, Go, Python, R, Haskell, and D. TrailDB system also provides a query engine called trck, which is a domain specific language to aggregate metrics based on events of identical UUID.

System Architecture

Embedded

Foreign Keys

Not Supported

In TrailDB, each database consists of a collection of trails each of which is identified by a unique UUID. There are no multiple tables within a database and no constraints among databases. Thus, it does not support foreign keys.

Compression

Delta Encoding Run-Length Encoding Prefix Compression

First, within a trail, events are always sorted by time. Thus, it utilizes Delta Encoding to compress the 64-bit timestamps.

Second, since events are grouped by UUID, which usually represents a logical entity such as an online shopping customer, these events within a trail tend to be predictable and TrailDB only encodes every change in behavior. This is not exactly the same as the Run-Length Encoding but similar.

Third, Huffman Coding, which is a kind of Prefix Compression method, is used to encode the skewed, low-entropy distributions of values.

Data Model

Relational

TrailDB adopts a specific relational data model.

Each database is a collection of trails.

Each trail is identified by a 128-bit user-defined ID and an automatically assigned trail ID. Each trail consists a sequence of events which is ordered by time.

Each event consists of a 64-bit timestamp and several user-pre-defined fields.

Each field contains a set of values.

Concurrency Control

Not Supported

As each TrailDB is an immutable file, modifications are not allowed. There's only one process to produce a database and no one can issue read operations before the creation is finalized. Thus, there's no concurrency in TrailDB.

Logging

Not Supported

TrailDB does not support logging and there's only one process to create the database. There is no recovery handler if the process crashes during the creation of the database. Thus, users need to start from the very beginning of the producing process.

But, TrailDB system allows merging existing TrailDBs to create a new immutable database. It is suggested to do so if there's a huge number of input events.

Checkpoints

Not Supported

TrailDB does not support checkpoints as each database is immutable once produced.

Isolation Levels

Serializable

When creating a database, there's only one process to handle it and others cannot access it. Once the database is produced, it is a read-only immutable file. Thus, everyone can issue read requests to it, but cannot issue any write operations. In this view, it is equivalent to the serializable isolation level.

Views

Not Supported

TrailDB system does not support views. But, as each database is an immutable file, users can create "views" by creating another immutable database by extracting data from the existing TrailDBs.