vivekratnavel commented on a change in pull request #1602: URL: https://github.com/apache/ozone/pull/1602#discussion_r529780480
########## File path: hadoop-hdds/docs/content/concept/Recon.md ########## @@ -0,0 +1,166 @@ +--- +title: "Recon" +date: "2020-10-27" +weight: 8 +menu: + main: + parent: Architecture +summary: Recon serves as a management and monitoring console for Ozone. +--- +<!--- + Licensed to the Apache Software Foundation (ASF) under one or more + contributor license agreements. See the NOTICE file distributed with + this work for additional information regarding copyright ownership. + The ASF licenses this file to You under the Apache License, Version 2.0 + (the "License"); you may not use this file except in compliance with + the License. You may obtain a copy of the License at + + http://www.apache.org/licenses/LICENSE-2.0 + + Unless required by applicable law or agreed to in writing, software + distributed under the License is distributed on an "AS IS" BASIS, + WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. + See the License for the specific language governing permissions and + limitations under the License. +--> + +Recon serves as a management and monitoring console for Ozone. It gives a +bird's-eye view of Ozone and helps users troubleshoot any issues by presenting +the current state of the cluster through REST based APIs and rich web UI. + + +## High Level Design + +{{<figure src="/concept/ReconHighLevelDesign.png" width="800px">}} + +<br/> + +On a high level, Recon collects and aggregates metadata from Ozone Manager (OM), +Storage Container Manager (SCM) and Datanodes (DN) and acts as a central +management and monitoring console. Ozone administrators can use Recon to query +the current state of the system without overloading OM or SCM. + +Recon maintains multiple databases to enable batch processing, faster querying +and to persist aggregate information. It maintains a local copy of OM db and +SCM db along with a SQL database for persisting aggregate information. + +Recon also integrates with Prometheus to provide a HTTP endpoint to query Prometheus +for Ozone metrics and also to display a few crucial point in time metrics in +the web UI. + +## Recon and Ozone Manager + +{{<figure src="/concept/ReconOmDesign.png" width="800px">}} + +<br/> + +Recon gets a full snapshot of OM rocks db initially from the leader OM's HTTP +endpoint, untars the file and initializes RocksDB for querying locally. The +database is kept in sync by periodically requesting delta updates from the leader +OM via RPC calls from the last applied sequence id. If for any reason, the delta +updates could not be retrieved or applied to the local db, a full snapshot is +requested again to keep the local db in sync with OM db. Due to this, Recon can +show stale information since the local db will not always be in sync. + +The db updates retrieved from OM is then converted into a batch of events for +further processing by OM db tasks via [Recon Tasks Framework](#tasks-framework). + + +## Recon and Storage Container Manager + +{{<figure src="/concept/ReconScmDesign.png" width="800px">}} + +<br/> + +Recon also acts as a passive SCM for datanodes. When Recon is configured in the +cluster, all the datanodes register with Recon and send heartbeats, container +reports, incremental container reports etc. to Recon similar to SCM. Recon uses +all the information it gets from datanodes to construct its own copy of SCM rocks db +locally. Recon never sends any command to datanodes in response and just acts as +a passive SCM for faster querying of SCM database. + +The data in local SCM db is consumed by all the SCM db tasks triggered at +defined intervals via [Recon Tasks Framework](#tasks-framework). + +## <a name="tasks-framework"></a> Tasks Framework + +Recon has its own Tasks framework to enable batch processing of data obtained +from OM and SCM. A task can listen to and act upon db events such as `PUT`, `DELETE`, +`UPDATE`, etc. on either OM db or SCM db. Based on this, a task either implements +`org.apache.hadoop.ozone.recon.tasks.ReconOmTask` or extends +`org.apache.hadoop.ozone.recon.scm.ReconScmTask`. + +An example `ReconOmTask` is `ContainerKeyMapperTask` that persists the container -> key +mapping in RocksDB. This is useful to understand which keys were part of the container +when the container is reported missing or is in a bad health state. Another example is +`FileSizeCountTask` which keeps track of count of files within a given file size range in +a SQL database. These tasks have implementations for two scenarios: + + - Full snapshot (reprocess()) + - Delta updates (process()) + +When a full snapshot of OM db is obtained from the leader OM, the reprocess() +is called on all the registered OM tasks. On subsequent delta updates, process() +is called on these OM tasks. + +An example `ReconScmTask` is `ContainerHealthTask` that runs in configurable +intervals to scan the list of all the containers and to persist the state of +unhealthy containers (`MISSING`, `MIS_REPLICATED`, `UNDER_REPLICATED`, `OVER_REPLICATED`) +in a SQL table. This information is used to determine if there are any missing +containers in the cluster. + +## Recon and Prometheus + +Recon can integrate with any Prometheus instance configured to collected metrics +and can display useful information in Recon UI in Datanodes and Pipelines pages. +Recon also exposes a proxy endpoint ([/metrics]({{< ref "interface/ReconApi.md#metrics" >}})) +to query Prometheus. This integration can be enabled by setting this configuration `ozone.recon.prometheus.http.endpoint` +to the Prometheus endpoint like `ozone.recon.prometheus.http.endpoint=localhost:9090`. + +## API Reference + +[Link to complete API Reference]({{< ref "interface/ReconApi.md" >}}) + +## Persisted state + + * A local copy of [OM database]({{< ref "concept/OzoneManager.md#persisted-state" >}}) + * A local copy of [SCM database]({{< ref "concept/StorageContainerManager.md#persisted-state" >}}) + * The following data is persisted in Recon in the specified RocksDB directory: + * ContainerKey table + * Stores the mapping (container, key) -> count + * ContainerKeyCount table + * Stores containerID -> no. of keys count within the container + + * The following data is stored in the configured SQL database (default is Derby): + * GlobalStats table + * A Key -> Value table to store aggregate information like total + number of volumes / buckets / keys present in the cluster + * FileCountBySize table + * Keeps track of the number of files present within a file size range in the cluster + * ReconTaskStatus table + * Keeps track of the status and last run timestamp of the registered OM and SCM + db tasks in the [Recon Tasks Framework](#tasks-framework) + * ContainerHistory table Review comment: Ack. ---------------------------------------------------------------- This is an automated message from the Apache Git Service. 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