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Google BigTable

Google BigTable

  • Bigtable: A Distributed Storage System for Structured Data (https://research.google/pubs/pub27898/)
  • hosted on GCP: Cloud Bigtable
  • Inspired Apache Cassandra (from Facebook)
  • Spanner is layered on top

Logical data model

  • Key (row:string, column:string, time:int64) → Value: string
  • Row:
    • data stored in lexicographic order by row key
    • row range = tablet
    • when storing URLs, use reversed URL as row ID so URLs from the same domain are close to each other: com.google.maps com.google.images
  • Column Families
    • Colume key: family:qualifier
  • Timestamps
    • multiple versions of data
    • garbage-collect rules:
      • # of versions to keep
      • discard versions older than x days

API

  • Write - atomic operation on rows - supports single-row transactions 

    Table *T = OpenOrDie("/bigtable/web/webtable");
// Write a new anchor and delete an old anchor
RowMutation r1(T, "com.cnn.www"); 
r1.Set("anchor:www.c-span.org", "CNN"); 
r1.Delete("anchor:www.abc.com");
Operation op;
Apply(&op, &r1);

  • Read - Clients can iterate over multiple column families for a row 

    Scanner scanner(T);
ScanStream *stream;
stream = scanner.FetchColumnFamily("anchor"); 
stream->SetReturnAllVersions(); 
scanner.Lookup("com.cnn.www");
for (; !stream->Done(); stream->Next()) {
    printf("%s %s %lld %s\n", scanner.RowName(), stream->ColumnName(),stream->MicroTimestamp(), stream->Value());

Implementation

  • master:
    • assign tablets to tablet servers
    • detecting the addition and expiration of tablet servers
    • balancing tablet-server load
    • GFS garbage collection
  • tablet servers:
    • 100-200 MB each by default
    • tablet server holds an exclusive lock in Chubby
    • master detect by asking each tablet server for lock status
      • if fails, master attempts to aquire the server lock. if success, delete the server file, and move assgiment to other tablets
      • master failure does not change assigment
    • master at startup:
      • takes a master lock
      • scan server directory in Chubby
      • contact tablet to learn current assigments
      • reads METADATA to find tablets
      • assign un-assigned tablets