Jeff Lord created HADOOP-9198:
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Summary: Update Flume Wiki and User Guide to provide clearer
explanation of BatchSize, ChannelCapacity and ChannelTransactionCapacity
properties.
Key: HADOOP-9198
URL: https://issues.apache.org/jira/browse/HADOOP-9198
Project: Hadoop Common
Issue Type: Improvement
Reporter: Jeff Lord
It would be good if we refined our wiki and user guide to help explain the
following in a more clear fashion:
1) Batch Size
1.a) When configured by client code using the flume-core-sdk , to send events
to flume avro source.
The flume client sdk has an appendBatch method. This will take a list of events
and send them to the source as a batch. This is the size of the number of
events to be passed to the source at one time.
1.b) When set as a parameter on HDFS sink (or other sinks which support
BatchSize parameter)
This is the number of events written to file before it is flushed to HDFS
2)
2.a) Channel Capacity
This is the maximum capacity number of events of the channel.
2.b) Channel Transaction Capacity.
This is the max number of events stored in the channel per transaction.
How will setting these parameters to different values, affect throughput,
latency in event flow?
In general you will see better throughput by using memory channel as opposed to
using file channel at the loss of durability.
The channel capacity is going to need to be sized such that it is large enough
to hold as many events as will be added to it by upstream agents. Ideal flow
would see the sink draining events from the channel faster than it is having
events added by its source.
The channel transaction capacity will need to be smaller than the channel
capacity.
e.g. If your Channel capacity is set to 10000 than Channel Transaction Capacity
should be set to something like 100.
Specifically if we have clients with varying frequency of event generation,
i.e. some clients generating thousands of events/sec, while
others at a much slower rate, what effect will different values of these params
have on these clients ?
Transaction Capacity is going to be what throttles or limits how many events
the source can put into the channel. This going to vary depending on how many
tiers of agents/collectors you have setup.
In general though this should probably be equal to whatever you have the batch
size set to in your client.
With regards to the hdfs batch size, the larger your batch size the better
performance will be. However, keep in mind that if a transaction fails the
entire transaction will be replayed which could have the implication of
duplicate events downstream.
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