This describes the common configuration shared by all Druid nodes. These configurations can be defined in the `common.runtime.properties` file.
## JVM Configuration Best Practices
There are four JVM parameters that we set on all of our processes:
1.`-Duser.timezone=UTC` This sets the default timezone of the JVM to UTC. We always set this and do not test with other default timezones, so local timezones might work, but they also might uncover weird and interesting bugs.
2.`-Dfile.encoding=UTF-8` This is similar to timezone, we test assuming UTF-8. Local encodings might work, but they also might result in weird and interesting bugs.
3.`-Djava.io.tmpdir=<a path>` Various parts of the system that interact with the file system do it via temporary files, and these files can get somewhat large. Many production systems are set up to have small (but fast) `/tmp` directories, which can be problematic with Druid so we recommend pointing the JVM’s tmp directory to something with a little more meat.
4.`-Djava.util.logging.manager=org.apache.logging.log4j.jul.LogManager` This allows log4j2 to handle logs for non-log4j2 components (like jetty) which use standard java logging.
### Extensions
Many of Druid's external dependencies can be plugged in as modules. Extensions can be provided using the following configs:
|`druid.extensions.directory`|The root extension directory where user can put extensions related files. Druid will load extensions stored under this directory.|`extensions` (This is a relative path to Druid's working directory)|
|`druid.extensions.hadoopDependenciesDir`|The root hadoop dependencies directory where user can put hadoop related dependencies files. Druid will load the dependencies based on the hadoop coordinate specified in the hadoop index task.|`hadoop_dependencies` (This is a relative path to Druid's working directory|
|`druid.extensions.loadList`|A JSON array of extensions to load from extension directories by Druid. If it is not specified, its value will be `null` and Druid will load all the extensions under `druid.extensions.directory`. If its value is empty list `[]`, then no extensions will be loaded at all.|null|
|`druid.extensions.searchCurrentClassloader`|This is a boolean flag that determines if Druid will search the main classloader for extensions. It defaults to true but can be turned off if you have reason to not automatically add all modules on the classpath.|true|
### Zookeeper
We recommend just setting the base ZK path and the ZK service host, but all ZK paths that Druid uses can be overwritten to absolute paths.
|`druid.zk.service.host`|The ZooKeeper hosts to connect to. This is a REQUIRED property and therefore a host address must be supplied.|none|
#### Zookeeper Behavior
|Property|Description|Default|
|--------|-----------|-------|
|`druid.zk.service.sessionTimeoutMs`|ZooKeeper session timeout, in milliseconds.|`30000`|
|`druid.curator.compress`|Boolean flag for whether or not created Znodes should be compressed.|`true`|
#### Path Configuration
Druid interacts with ZK through a set of standard path configurations. We recommend just setting the base ZK path, but all ZK paths that Druid uses can be overwritten to absolute paths.
|`druid.zk.paths.indexer.tasksPath`|Used to assign tasks to middle managers.|`${druid.zk.paths.indexer.base}/tasks`|
|`druid.zk.paths.indexer.statusPath`|Parent path for announcement of task statuses.|`${druid.zk.paths.indexer.base}/status`|
|`druid.zk.paths.indexer.leaderLatchPath`|Used for Overlord leader election.|`${druid.zk.paths.indexer.base}/leaderLatchPath`|
If `druid.zk.paths.base` and `druid.zk.paths.indexer.base` are both set, and none of the other `druid.zk.paths.*` or `druid.zk.paths.indexer.*` values are set, then the other properties will be evaluated relative to their respective `base`.
For example, if `druid.zk.paths.base` is set to `/druid1` and `druid.zk.paths.indexer.base` is set to `/druid2` then `druid.zk.paths.announcementsPath` will default to `/druid1/announcements` while `druid.zk.paths.indexer.announcementsPath` will default to `/druid2/announcements`.
|`druid.request.logging.type`|Choices: noop, file, emitter. How to log every query request.|noop|
Note that, you can enable sending all the HTTP requests to log by setting "io.druid.jetty.RequestLog" to DEBUG level. See [Logging](../configuration/logging.html)
|`druid.request.logging.dir`|Historical, Realtime and Broker nodes maintain request logs of all of the requests they get (interacton is via POST, so normal request logs don’t generally capture information about the actual query), this specifies the directory to store the request logs in|none|
#### Emitter Request Logging
Every request is emitted to some external location.
|Property|Description|Default|
|--------|-----------|-------|
|`druid.request.logging.feed`|Feed name for requests.|none|
### Enabling Metrics
Druid nodes periodically emit metrics and different metrics monitors can be included. Each node can overwrite the default list of monitors.
|Property|Description|Default|
|--------|-----------|-------|
|`druid.monitoring.emissionPeriod`|How often metrics are emitted.|PT1m|
|`druid.monitoring.monitors`|Sets list of Druid monitors used by a node. See below for names and more information. For example, you can specify monitors for a Broker with `druid.monitoring.monitors=["com.metamx.metrics.SysMonitor","com.metamx.metrics.JvmMonitor"]`.|none (no monitors)|
The following monitors are available:
|Name|Description|
|----|-----------|
|`io.druid.client.cache.CacheMonitor`|Emits metrics (to logs) about the segment results cache for Historical and Broker nodes. Reports typical cache statistics include hits, misses, rates, and size (bytes and number of entries), as well as timeouts and and errors.|
|`com.metamx.metrics.SysMonitor`|This uses the [SIGAR library](http://www.hyperic.com/products/sigar) to report on various system activities and statuses. Make sure to add the [sigar library jar](https://repository.jboss.org/nexus/content/repositories/thirdparty-uploads/org/hyperic/sigar/1.6.5.132/sigar-1.6.5.132.jar) to your classpath if using this monitor.|
|`io.druid.segment.realtime.RealtimeMetricsMonitor`|Reports statistics on Realtime nodes.|
### Emitting Metrics
The Druid servers emit various metrics and alerts via something we call an Emitter. There are three emitter implementations included with the code, a "noop" emitter, one that just logs to log4j ("logging", which is used by default if no emitter is specified) and one that does POSTs of JSON events to a server ("http"). The properties for using the logging emitter are described below.
|`druid.emitter`|Setting this value to "noop", "logging", or "http" will initialize one of the emitter modules. value "composing" can be used to initialize multiple emitter modules. |noop|
|`druid.emitter.logging.loggerClass`|Choices: HttpPostEmitter, LoggingEmitter, NoopServiceEmitter, ServiceEmitter. The class used for logging.|LoggingEmitter|
|`druid.emitter.logging.logLevel`|Choices: debug, info, warn, error. The log level at which message are logged.|info|
#### Http Emitter Module
|Property|Description|Default|
|--------|-----------|-------|
|`druid.emitter.http.timeOut`|The timeout for data reads.|PT5M|
|`druid.emitter.http.flushMillis`|How often to internal message buffer is flushed (data is sent).|60000|
|`druid.emitter.http.flushCount`|How many messages can the internal message buffer hold before flushing (sending).|500|
|`druid.emitter.http.recipientBaseUrl`|The base URL to emit messages to. Druid will POST JSON to be consumed at the HTTP endpoint specified by this property.|none|
These properties specify the jdbc connection and other configuration around the metadata storage. The only processes that connect to the metadata storage with these properties are the [Coordinator](../design/coordinator.html) and [Indexing service](../design/indexing-service.html).
|`druid.cache.sizeInBytes`|Maximum cache size in bytes. You must set this if you enabled populateCache/useCache, or else cache size of zero wouldn't really cache anything.|0|
|`druid.cache.timeout`|Maximum time in milliseconds to wait for a response from Memcached.|500|
|`druid.cache.hosts`|Command separated list of Memcached hosts `<host:port>`.|none|
|`druid.cache.maxObjectSize`|Maximum object size in bytes for a Memcached object.|52428800 (50 MB)|
|`druid.cache.memcachedPrefix`|Key prefix for all keys in Memcached.|druid|
### Indexing Service Discovery
This config is used to find the [Indexing Service](../design/indexing-service.html) using Curator service discovery. Only required if you are actually running an indexing service.
|Property|Description|Default|
|--------|-----------|-------|
|`druid.selectors.indexing.serviceName`|The druid.service name of the indexing service Overlord node. To start the Overlord with a different name, set it with this property. |druid/overlord|
### Announcing Segments
You can optionally configure how to announce and unannounce Znodes in ZooKeeper (using Curator). For normal operations you do not need to override any of these configs.
#### Data Segment Announcer
Data segment announcers are used to announce segments.
|Property|Description|Default|
|--------|-----------|-------|
|`druid.announcer.type`|Choices: legacy or batch. The type of data segment announcer to use.|batch|
##### Single Data Segment Announcer
In legacy Druid, each segment served by a node would be announced as an individual Znode.
##### Batch Data Segment Announcer
In current Druid, multiple data segments may be announced under the same Znode.
|Property|Description|Default|
|--------|-----------|-------|
|`druid.announcer.segmentsPerNode`|Each Znode contains info for up to this many segments.|50|
|`druid.announcer.maxBytesPerNode`|Max byte size for Znode.|524288|
