mirror of https://github.com/apache/druid.git
57 lines
2.4 KiB
Markdown
57 lines
2.4 KiB
Markdown
|
---
|
||
|
|
layout: doc_page
|
||
|
|
---
|
||
|
|
|
||
|
|
# Developing on Druid
|
||
|
|
|
||
|
|
Druid's codebase consists of several major components. For developers interested in learning the code, this document provides
|
||
|
|
a high level overview of the main components that make up Druid and the relevant classes to start from to learn the code.
|
||
|
|
|
||
|
|
## Storage Format
|
||
|
|
|
||
|
|
Data in Druid is stored in a custom column format known as a [segment](../design/segments.html). Segments are composed of
|
||
|
|
different types of columns. `Column.java` and the classes that extend it is a great place to looking into the storage format.
|
||
|
|
|
||
|
|
## Segment Creation
|
||
|
|
|
||
|
|
Raw data is ingested in `IncrementalIndex.java`, and segments are created in `IndexMerger.java`.
|
||
|
|
|
||
|
|
## Storage Engine
|
||
|
|
|
||
|
|
Druid segments are memory mapped in `IndexIO.java` to be exposed for querying.
|
||
|
|
|
||
|
|
## Query Engine
|
||
|
|
|
||
|
|
Most of the logic related to Druid queries can be found in the Query* classes. Druid leverages query runners to run queries.
|
||
|
|
Query runners often embed other query runners and each query runner adds on a layer of logic. A good starting to point trace
|
||
|
|
the query logic is to start from `QueryResource.java`.
|
||
|
|
|
||
|
|
## Coordination
|
||
|
|
|
||
|
|
Most of the coordination logic for historical nodes is on the Druid coordinator. The starting point here is `DruidCoordinator.java`.
|
||
|
|
Most of the coordination logic for (real-time) ingestion is in the Druid indexing service. The starting point here is `OverlordResource.java`.
|
||
|
|
|
||
|
|
## Real-time Ingestion
|
||
|
|
|
||
|
|
Druid loads data through `FirehoseFactory.java` classes. Firehoses can often wrap other firehoses, where, similar to the
|
||
|
|
query runners, each firehose adds a layer of logic. Much of the core management logic is in `RealtimeManager.java` and the
|
||
|
|
persist logic is in `RealtimePlumber.java`.
|
||
|
|
|
||
|
|
## Hadoop-based Batch Ingestion
|
||
|
|
|
||
|
|
The two main Hadoop indexing classes are `HadoopDruidDetermineConfigurationJob.java` for the job to determine how many Druid
|
||
|
|
segments to create, and `HadoopDruidIndexerJob.java`, which creates Druid segments.
|
||
|
|
|
||
|
|
At some point in the future, we may move the Hadoop ingestion code out of core Druid.
|
||
|
|
|
||
|
|
## Internal UIs
|
||
|
|
|
||
|
|
Druid currently has two internal UIs. One is for the Coordinator and one is for the Overlord.
|
||
|
|
|
||
|
|
At some point in the future, we will likely move the internal UI code out of core Druid.
|
||
|
|
|
||
|
|
## Client Libraries
|
||
|
|
|
||
|
|
We welcome contributions for new client libraries to interact with Druid. See client
|
||
|
|
[libraries](../development/libraries.html) for existing client libraries.
|