Posts tagged: hadoop

Session: Hbase – What? Why? Where does it fit?

Big Data Workshop, April 23, 2010
Session 2C
Title: Hbase – What? Why? Where does it fit?

Convener: Jon Gray    Notes-taker: Mason Ng

  • Jon’s background – His startup used Postgres and had problem. He then started to use Hbase. Joined Facebook.
  • List of things to discuss:What is Hbase – distributed, column oriented, scalable database
  • Coming from relational world perspective
  • Hbase does not use local filesystems, only use HDFS
  • Cassandra uses local filesystems and then replicate itself. One read could be pulling from 3 nodes. AP (Availabile, Partition) systemHbase uses HDFS replication. CA (Consistent, Available) system
  • No transaction across row. No ACID.
  • Within a row, the transaction is Atomic.
  • Acid property, row-> column family – c1->[versions…], c2->[versions….]
  • Column family has its own file(s)
  • Relational random reads. Hbase/HDFS access sequential writes/reads.
  • Buffering writes. Inserts buffers in-memory and then batch write (flush) to HDFS. Table breaks into shards. Each shards only on one node.
  • rowA-rowD in memory flushes 64MB to disk
  • Update by versioning. Does not actually delete/update.
  • How bigtable and hbase differ. Hbase is in Java.
  • Bigtable in C++. Lots of crap needs to manage with Java and therefore hbase.Use zookeeper
  • Supports random and sequential access. Supports RDF.
  • Hbase background tasks compaction – takes lots of 64MB shards and compacts to one big chucks (about 3 shards) split – redistribute a-b and b-d to different shards
  • Data model
  • Processing of hadoop. Needs random access. Could not scale relational.
  • Blog / RSS aggregators. Source, stamp, itemid
  • Source a, b, c insert in random. Ends up with large btree. Merge 2000 sources and each source has 10000 items. Relational then delegates to query engine.
  • Wants source A has this list of items – column oriented data. Items are sorted/mapped. Item stamp->id. Source (key) A is a large row across a sequential (3) 64MB chucks.
  • Schema is not fixed but delegates to the application to create/extend the schemas.
  • Time is last. Wants to get latest copies. May not be efficient because needs to skip.
  • FB is not using Hbase but evaluating.
  • @FB Hadoop shop. HDFS committers. Using Hive. Use hbase for the needs of incremental update.
  • Hbase vs. Cassandra – needs eventual consistency then use hbase. Log shipping replication for colo replication. Hbase provides audit trails for slave replication.
  • Hbase slower comparing to Cassandra on random access due to HDFS layer.
  • Hbase and Cassandra should converge over time. Cassandra would get better at range scan. Hbase would get better at random access.
  • 100 nodes per cluster. Use of segmenting. Stumbleupon has a cluster for website. Copy data to another offline cluster for mapreduce processing.
  • Special table lives on one node. If there is a hot row, then the special table could be bottleneck. Use zookeeper for the special table due to read replication with zookeeper to distribute the reads.
  • Hadoop namenode single point of failure. GFS2 uses bigtable to store the meta. GFS1->bigtable->GFS2->biggertable. How to scale namenode and make it highly available.
  • Hbase vs. hypertable. Hypertable is written in C++. Community is supported by strong apache community.
  • Subject to GC performance due to Java dependency on hadoop/hbase. Total throughput depends on GC performance. G1 garbage collector coming, defrag memory.
  • Hbase does not need to run with hadoop. Hbase could run on local filesystem on one node.
  • There is data loss until next Hbase release due to HDFS limitation such as append write to the log file. If writer dies before a close() then data loss could occur.
  • @FB Hive and Hbase integration. Also looking at HBQL. ORM build on top of
  • Hbase. Hadoop + Hbase stack can provide rollup archive.

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