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Apache Gobblin is Hadoop ingestion software used at WMF primarily to import data from Kafka into HDFS.
The Data Engineering team maintains a gobblin-wmf repository from which we add our gobblin extensions and build our gobblin fat jar for deployment. The gobblin-wmf-core module mostly contains code to ingest Kafka events to HDFS and to send metrics to Prometheus.
Releasing new Gobblin versions
Releasing is handled by a Jenkins job. Log into Jenkins and run the analytics-gobblin-wmf-maven-release-docker job. This will create a new git tag version, build the gobblin-wmf artifacts and upload them to Archiva.
To deploy, you will need to manually add the gobblin-wmf-core-jar-with-dependencies.jar file to analytics/refinery/artifacts, deploy following Analytics/Systems/Cluster/Deploy/Refinery, and then update the gobblin jar specified in puppet (here for analytics-hadoop, and here for analytics-test-hadoop).
A Gobblin Grafana dashboard exists to visualize some of these metrics.
- If the last successful run timestamp for a gobblin job is older than 2 hours, a critical alert will be fired to the Data-Engineering team
- If a gobblin job run reports any Kafka topic partitions for which the number of actual extracted records is different than the number of expected extracted records, a warning alert will be fired to the Data-Engineering team.
Reading State Store
Gobblin writes state information to a directory configured by its .pull job file. For example, the event_default job writes state to /wmf/gobblin/state_store/event_default. To read this, you can:
java -cp /srv/deployment/analytics/refinery/artifacts/gobblin-wmf-core-jar-with-dependencies.jar:$(hadoop classpath) org.apache.gobblin.runtime.cli.GobblinCli job-state-store --storeurl file:///mnt/hdfs/wmf/gobblin/state_store -n event_default -r
Pulling from Kafka
We experienced regular inconsistencies in volumes of data pulled from Kafka in the first month of usage of Gobblin. More precisely, it happened regularly that some pull-tasks (executed in map-containers) were actually not pulling any data while they were expected to do so. This didn't lead to Gobblin job-errors, just delay in pulling some data. Unfortunately, due to how we trigger computation jobs after data is pulled, the problem led to data-loss alerts when the phenomenon occurred just after the hour. We investigated and found why some tasks were failing to pull data: there is a specific settings making Gobblin time-out (without error, just no data) when fetching it's first batch of data from kafka. Details can be found in that task: https://phabricator.wikimedia.org/T290723.
Gobblin metric details
Gobblin metrics are not easy to grasp. In particular they embed a
Context notion (as tag) that details from how deep in the gobblin-architecture the metrics is generated. In our usage of gobblin, here are some findings:
- The Gobblin CLI process and every Gobblin map-task are different metrics context (the map-tasks have the same job-id tag)
- There are many contexts in which metrics are generated, making it not easy to define which to get (for our use case, it seems that contexts of the form: metricContextName=gobblin.metrics.job_JOBNAME_JOBTS are the ones summarizing all the values of interest).
- Metrics are generated during the task execution with intermediate values, and the last generated metric-event is flagged as