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ZOOKEEPER

what is zookeeper

  • Developed at Yahoo! for internal use
  • Inspired by Chubby (Google's global lock service)
  • Use by Yahoo! YMB and Crawler
  • Open source, as part of Hadoop (MapReduce)
  • Widely-used by companies

Motivation: many applications need a "master" service

  • Example: GFS
    • master has list of chunk servers for each chunk
    • master decides which chunk server is primary
  • Other examples: YMB, Crawler, etc.
    • YMB needs master to shard topics
    • Crawler needs master that commands page fetching (e.g., a bit like the master in mapreduce)
  • Master service typically used for "coordination"
  • Zookeeper: a generic "master" service

Zookeeper API overview

  • replicated state machine
    • several servers implementing the service
    • operations are performed in global order
      • with some exceptions, if consistency isn't important
  • the replicated objects are: znodes
    • hierarchy of znodes
      • named by pathnames
    • znodes contain metadata of application
      • configuration information
        • machines that participate in the application
        • which machine is the primary
      • timestamps
      • version number
    • types of znodes:
      • regular
      • empheral
      • sequential: name + seqno
        • seqno(p) >= max(children)
  • sessions
    • clients sign into zookeeper
    • session allows a client to fail-over to another zookeeper service
      • client know the term and index of last completed operation
      • send it on each request
        • service performs operation only if caught up with what client has seen
    • sessions can timeout
      • client must refresh a session continuously
        • send a heartbeat to the server (like a lease)
      • zookeer considers client "dead" if doesn't hear from a client
      • client may keep doing its thing (e.g., network partition)
        • but cannot perform other zookeeper ops in that session

Operations on znodes

create(path, data, flags)
delete(path, version)
  if znode.version = version, then delete
exists(path, watch)
getData(path, watch)
setData(path, data, version)
	if znode.version = version, then update
getChildren(path, watch)
sync()
  1. above operations are asynchronous
  2. all operations are FIFO-ordered per client
  3. sync waits until all preceding operations have been "propagated"

Ordering guarantees

  • all write operations are totally ordered, if a write is performed by zookeeper, later writes from other clients see it
  • all operations are FIFO-ordered per client
  • implications:
    • a read observes the result of an earlier write from the same client
    • a read observes some prefix of the writes, perhaps not including most recent write -> read can return stale data
    • if a read observes some prefix of writes, a later read observes that prefix too

Example usage 1: slow lock

acquire lock:

retry:
	r = create("app/lock", "", empheral)
 	if r:
   		return
 	else:
   		getData("app/lock", watch=True)

watch_event:
   goto retry

release lock: (voluntarily or session timeout)

delete("app/lock")

Challenge: Read operations

  • Many operations are read operations
    • they don't modify replicated state
  • Must they go through ZAB/Raft or not?
  • Can any replica execute the read op?
  • Performance is slow if read ops go through Raft
    • Adds: 1 PRC to and 1 write at each replica --- ouch!

Problem: read may return stale data if only master performs it

Zookeeper solution: don't promise non-stale data (by default)

  • Reads are allowed to return stale data
  • Reads can be executed by any replica
  • Read throughput increases as number of servers increases
  • Read returns the last xid it has seen
  • Only sync-read() guarantees data is not stale, goes through ZAB layer

References: https://pdos.csail.mit.edu/6.824/notes/l-zookeeper.txt