Current status from our side: Maxim and I have done research on implementing asynchronous triggers in Cassandra. Based on this, we have provided the patch of this issue. In the discussion that followed, a different implementation of triggers in Cassandra has been suggested. Since then, Maxim and I have not continued to work on our patch. I guess it's up to Jonathan Ellis, T Jake Luciani, and Stu Hood to make a decision on which implementation of triggers to pursue and who does it
Here is a brief summary of our research on asynchronous triggers in the last 1.5 years:
We have published a technical report about the idea of integrating asynchronous triggers into Cassandra . In this technical report we provide three things: (1) a comparison of different approaches to execute triggers on top of Cassandra, (2) a set of new protocols for executing triggers inside Cassandra, and (3) results of performance experiments comparing all of these approaches. The proposed protocols ensure at-least-once semantics of trigger execution. That is, upon a change to a table in Cassandra, a registered trigger is guaranteed to be executed at least once – even in case of node failures. The experimental results show that our "integrated approach" performs just as well as the state-of-the-art approach of executing triggers outside of Cassandra. However, our approach utilizes system resources more efficiently; that is, less machines are needed to scale the execution of triggers.
The patch we have attached to this issue implements the proposed protocols and has been used in our performance experiments. Thus, this patch is fully functional and well-tested. The implementation extends the replication mechanism of Cassandra to execute triggers with at-least-once semantics.
In the discussion above, another approach has been suggested to implement triggers inside Cassandra. The idea is to use the log record to execute triggers. This seems to be a clever way to implement triggers because it would require less extensions to the existing Cassandra infrastructure, I assume. However, this does not increase the guarantees of trigger execution. It won't give you exactly-once semantics.
To guarantee exactly-once semantics of trigger execution, we have build a system that departs significantly from this patch . In short, the system extends the patch with transactions and synchronization mechanisms to execute triggers. It also provides an easier programming model, which is more like MapReduce than a trigger-like programming model.
 Martin Hentschel, Maxim Grinev, Donald Kosssmann: Building Data Flows Using Distributed Key-Value Stores. Technical Report 742, ETH Zurich, ftp://ftp.inf.ethz.ch/pub/publications/tech-reports/7xx/742.pdf
 Maxim Grinev, Maria Grineva, Martin Hentschel, Donald Kossmann: Analytics for the RealTime Web. Demo Paper, VLDB Conference 2011, http://www.vldb.org/pvldb/vol4/p1391-grinev.pdf