I believe Vitalii is actively looking at a more robust metadata store strategy for Drill and in the long term we would want to move all metadata to the new store. I don't think we would want to maintain a TTL for the metadata store so introducing one now would mean that we might break backward compatibility down the road. Not saying we shouldn't do this, it's just something to keep in mind.
On Thu, Jul 12, 2018 at 8:07 AM, Joel Pfaff <joel.pf...@gmail.com> wrote: > Hello, > > Thanks for the feedback. > > The logic I had in mind was to add the TTL, as a refresh_interval field in > the root metadata file. > > At each query, the current time would be compared to the addition of the > modification time of the root metadata file and the refresh_interval. > If the current time is greater, it would mean the metadata may be invalid, > so the regular process would apply: recursively going through the file to > check for updates, and trig a full metadata cache refresh any change is > detected, or just touch the metadata file to align its modification time > with the current time if no change is detected. > If the current time is smaller, the root metadata would be trusted (without > additional checks) and the planning would continue. > > So in most of the cases, only the timestamp of the root metadata file would > be checked. In the worst case (at most once per TTL), all the timestamps > would be checked. > > Regards, Joel > > On Thu, Jul 12, 2018 at 4:47 PM, Vitalii Diravka < > vitalii.dira...@gmail.com> > wrote: > > > Hi Joel, > > > > Sounds reasonable. > > But if Drill checks this TTL property from metadata cache file for every > > query and for every file instead of file timestamp, it will not give the > > benefit. > > I suppose we can add this TTL property to only root metadata cache file > and > > check it only once per query. > > > > Could you clarify the details, what is the TTL time? > > How TTL info could be used to determine whether refresh is needed for the > > query? > > > > Kind regards > > Vitalii > > > > > > On Thu, Jul 12, 2018 at 4:40 PM Joel Pfaff <joel.pf...@gmail.com> wrote: > > > > > Hello, > > > > > > Today, on a table for which we have created statistics (through the > > REFRESH > > > TABLE METADATA <path to table> command), Drill validates the timestamp > of > > > every files or directory involved in the scan. > > > > > > If the timestamps of the files are greater than the one of the metadata > > > file, then a re-regeneration of the meta-data file is triggered. > > > In the case the timestamp of the metadata file is the greatest, then > the > > > planning continues without regenerating the metadata. > > > > > > When the number of files to be queried increases, this operation can > > take a > > > significant amount of time. > > > We have seen cases where this validation step alone is taking 3 to 5 > > > seconds (just checking the timestamps), meaning the planning time was > > > taking way more time than the querying time. > > > And this can be problematic in some usecases where the response time is > > > favored compared to the `accuracy` of the data. > > > > > > What would you think about adding an option to the metadata generation, > > so > > > that the metadata is trusted for a configurable time period > > > Example : REFRESH TABLE METADATA <path to table> WITH TTL='15m' > > > The exact syntax, of course, needs to be thought through. > > > > > > This TTL would be stored in the metadata file, and used to determine > if a > > > refresh is needed at each query. And this would significantly decrease > > the > > > planning time when the number of files represented in the metadata file > > is > > > important. > > > > > > Of course, this means that there could be cases where the metadata > would > > be > > > wrong, so cases like the one below would need to be solved (since they > > may > > > happen much more frequently): > > > https://issues.apache.org/jira/browse/DRILL-6194 > > > But my feeling is that since we already do have a kind of race > condition > > > between the view of the file system at the planning time, and the state > > > that will be found during the execution, we could gracefully accept > that > > > some files may have disappeared between the planning and the execution. > > > > > > In the case the TTL would need to be changed, or be removed completely, > > > this could be done by re-issuing a REFRESH TABLE METADATA, either with > a > > > new TTL, or without TTL at all. > > > > > > What do you think? > > > > > > Regards, Joel > > > > > >
