Zookeeper's traces (i.e., logs in TRACE level) provide some
information that can be helpful to understand what happened. For
instance, they contain information about the clients that are
connected, the operations issued, etc. However, in real deployments
with many clients (say, hundreds), traces are typically turned off
to avoid the high overhead that they cause. Furthermore, the data in
the traces is probably not enough for our purposes because it does
not include, e.g., the replies to operations or the data values.
As far as I've seen, this overhead comes in two forms, CPU and disk.
CPU overhead is mostly due to formatting. Disk obviously because
tracing will fill your disk fairly quickly. Perhaps something could be
done to combat both of these. To fix the formatting problem we could
use a binary log format. I've seen this done in C++ but not in java.
The basic idea is that if you have TRACE("operation %x happened to %s
%p", obj1, obj2, obj3); a preprocessor replaces this with
TRACE(0x1234, obj1, obj2, obj3) where 0x1234 is an identifier for the
trace. Then when the trace occurs a binary blob [0x1234, value of
obj1, value of obj2, value of obj3] is logged. Then when the logs are
pulled of the machine you run a post processor to do all the
formatting and you get your full trace.
Regarding the disk overhead, traces are usually only interesting in
the run up to a failure. We could have a ring buffer in memory that is
constantly traced to, old traces being overwritten when the ring
buffer reaches it's limit. These traces should only be dumped to the
filesystem when an error or fatal level event occurs, thereby giving
you a trace of what was happening before you fell over.