core/src/site/apt/async_ack.apt

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Overview

  Apache Chukwa supports two different reliability strategies.
The first, default strategy, is as follows: collectors write data to HDFS, and
as soon as the HDFS write call returns success, report success to the agent, 
which advances its checkpoint state.

  This is potentially a problem if HDFS (or some other storage tier) has 
non-durable or asynchronous writes. As a result, Apache Chukwa offers a mechanism, 
asynchronous acknowledgement, for coping with this case.

  This mechanism can be enabled by setting option <httpConnector.asyncAcks>.
This option applies to both agents and collectors. On the collector side, it 
tells the collector to return asynchronous acknowledgements. On the agent side,
it tells agents to look for and process them correctly. Agents with the option 
set to false should work OK with collectors where it's set to true. The 
reverse is not generally true: agents will expect a collector to be able to 
answer questions about the state of the filesystem.

Theory

  In this approach, rather than try to build a fault tolerant collector, 
Apache Chukwa agents look <<through>> the collectors to the underlying state of the 
filesystem. This filesystem state is what is used to detect and recover from 
failure. Recovery is handled entirely by the agent, without requiring anything 
at all from the failed collector.

  When an agent sends data to a collector, the collector responds with the name 
of the HDFS file in which the data will be stored and the future location of 
the data within the file. This is very easy to compute -- since each file is 
only written by a single collector, the only requirement is to enqueue the 
data and add up lengths.

  Every few minutes, each agent process polls a collector to find the length of 
each file to which data is being written. The length of the file is then 
compared with the offset at which each chunk was to be written. If the file 
length exceeds this value, then the data has been committed and the agent 
process advances its checkpoint accordingly. (Note that the length returned by 
the filesystem is the amount of data that has been successfully replicated.) 
There is nothing essential about the role of collectors in monitoring the 
written files. Collectors store no per-agent state. The reason to poll 
collectors, rather than the filesystem directly, is to reduce the load on 
the filesystem master and to shield agents from the details of the storage 
system.

  The collector component that handles these requests is 
<datacollection.collector.servlet.CommitCheckServlet>.
This will be started if <httpConnector.asyncAcks> is true in the
collector configuration.

  On error, agents resume from their last checkpoint and pick a new collector. 
In the event of a failure, the total volume of data retransmitted is bounded by 
the period between collector file rotations.

  The solution is end-to-end. Authoritative copies of data can only exist in 
two places: the nodes where data was originally produced, and the HDFS file 
system where it will ultimately be stored. Collectors only hold soft state;  
the only ``hard'' state stored by Apache Chukwa is the agent checkpoints. Below is a 
diagram of the flow of messages in this protocol.

Configuration

  In addition to <httpConnector.asyncAcks> (which enables asynchronous 
acknowledgement) a number of options affect this mode of operation.

  Option <chukwaCollector.asyncAcks.scanperiod> affects how often collectors 
will check the filesystem for commits. It defaults to twice the rotation 
interval.

  Option <chukwaCollector.asyncAcks.scanpaths> determines where in HDFS
collectors will look. It defaults to the data sink dir plus the archive dir.

  In the future, Zookeeper could be used instead to track rotations.