There is also the now standard in Storm implementation of this pattern: https://github.com/apache/storm/commit/9e898247845f99392fe0b714a6df39fe2cfc5fe7 https://github.com/apache/storm/blob/master/storm-client/src/jvm/org/apache/storm/bolt/JoinBolt.java
( sorry, I will look for a better link ). On May 16, 2017 at 10:58:34, Casey Stella (ceste...@gmail.com) wrote: Hi All, Last week, I encountered some weirdness in the Enrichment topology. Doing some somewhat high-latency enrichment work, I noticed that at some point, data stopped flowing through the enrichment topology. I tracked down the problem to the join bolt. For those who aren't aware, we do a split/join pattern so that enrichments can be done in parallel. It works as follows: - A split bolt sends the appropriate subset of the message to each enrichment bolt as well as the whole message to the join bolt - The join bolt will receive each of the pieces of the message and then, when fully joined, it will send the message on. What is happening under load or high velocity, however, is that the cache is evicting the partially joined message before it can be fully joined due to the volume of traffic. This is obviously not ideal. As such, it is clear that adjusting the size of the cache and the characteristics of eviction is likely a good idea and a necessary part to tuning enrichments. The cache size is sensitive to: - The latency of the *slowest* enrichment - The number of tuples in flight at once As such, the knobs you have to tune are either the parallelism of the join bolt or the size of the cache. As it stands, I see a couple of things wrong here that we can correct with minimal issue: - We have no message of warning indicating that this is happening - Changing cache sizes means changing flux. We should promote this to the properties file. - We should document the knobs mentioned above clearly in the enrichment topology README Those small changes, I think, are table stakes, but what I wanted to discuss more in depth is the lingering questions: - Is this an architectural pattern that we can use as-is? - Should we consider a persistent cache a la HBase or Apache Ignite as a pluggable component to Metron? - Should we consider taking the performance hit and doing the enrichments serially? - When an eviction happens, what should we do? - Fail the tuple, thereby making congestion worse - Pass through the partially enriched results, thereby making enrichments "best effort" Anyway, I wanted to talk this through and inform of some of the things I'm seeing. Sorry for the novel. ;) Casey
