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Where do my Druid segments end up after ingestion?
Depending on what druid.storage.type is set to, Druid will upload segments to some Deep Storage. Local disk is used as the default deep storage.
My realtime node is not handing segments off
Make sure that the druid.publish.type on your real-time nodes is set to "db". Also make sure that druid.storage.type is set to a deep storage that makes sense. Some example configs:
druid.publish.type=db
druid.db.connector.connectURI=jdbc\:mysql\://localhost\:3306/druid
druid.db.connector.user=druid
druid.db.connector.password=diurd
druid.storage.type=s3
druid.storage.bucket=druid
druid.storage.baseKey=sample
How do I get HDFS to work?
Make sure to include the druid-hdfs-storage module as one of your extensions and set druid.storage.type=hdfs.
I don't see my Druid segments on my historical nodes
You can check the coordinator console located at <COORDINATOR_IP>:<PORT>/cluster.html. Make sure that your segments have actually loaded on historical nodes. If your segments are not present, check the coordinator logs for messages about capacity of replication errors. One reason that segments are not downloaded is because historical nodes have maxSizes that are too small, making them incapable of downloading more data. You can change that with (for example):
-Ddruid.segmentCache.locations=[{"path":"/tmp/druid/storageLocation","maxSize":"500000000000"}]
-Ddruid.server.maxSize=500000000000
My queries are returning empty results
You can check <BROKER_IP>:<PORT>/druid/v2/datasources/<YOUR_DATASOURCE>?interval=0/3000 for the dimensions and metrics that have been created for your datasource. Make sure that the name of the aggregators you use in your query match one of these metrics. Also make sure that the query interval you specify match a valid time range where data exists. Note: the broker endpoint will only return valid results on historical segments.
How can I Reindex existing data in Druid with schema changes?
You can use IngestSegmentFirehose with index task to ingest existing druid segments using a new schema and change the name, dimensions, metrics, rollup, etc. of the segment. See Firehose for more details on IngestSegmentFirehose.
More information
Getting data into Druid can definitely be difficult for first time users. Please don't hesitate to ask questions in our IRC channel or on our google groups page.