Comments inline. > On 8 Mar 2019, at 11:28, Kevin Yang <yrql...@gmail.com> wrote: > > Hi all, > When I was preparing some work related to this AIP I found something very > concerning. I noticed this JIRA ticket > <https://issues.apache.org/jira/browse/AIRFLOW-3562> is trying to remove the > dependency of dagbag from webserver, which is awesome--we wanted badly but > never got to start work on. However when I looked at some subtasks of it, > which try to remove dagbag dependency from each endpoint, I found the way we > remove the dependency of dagbag is not very ideal. For example this PR > <https://github.com/apache/airflow/pull/4867/files> will require us to parse > the dag file each time we hit the endpoint.
The counter argument: this PR removes the need for the confusing "Refresh" button from the UI, and in general you only pay the cost for the expensive DAGs when you ask about them. (I don't know what/when we call the /pickle_info endpoint of the top of my head) This end point may be one to hold off on (as it can ask for multiple dags) but there are some that def don't need a full dag bag or to even parse the dag file, the current DAG model has enough info. > > > If we go down this path, we indeed can get rid of the dagbag dependency > easily, but we will have to 1. increase the DB load( not too concerning at > the moment ), 2. wait the DAG file to be parsed before getting the page back, > potentially multiple times. DAG file can sometimes take quite a while to > parse, e.g. we have some framework DAG files generating large number of DAGs > from some static config files or even jupyter notebooks and they can take 30+ > seconds to parse. Yes we don't like large DAG files but people do see the > beauty of code as config and sometimes heavily abuseleverage it. Assuming all > users have the same nice small python file that can be parsed fast, I'm still > a bit worried about this approach. Continuing on this path means we've chosen > DagModel to be the serialized representation of DAG and DB columns to hold > different properties--it can be one candidate but I don't know if we should > settle on that now. I would personally prefer a more compact, e.g. JSON5, and > easy to scale representation( such that serializing new fields != DB > upgrade). Do you mean https://json5.org/ or is this a typo? That might be okay for a nicer user front end, but the "canonical" version stored in the DB should be something "plainer" like just JSON. I'm not sure that "serializing new fields != DB upgrade" is that big of a concern, as we don't add fields that often. One possible way of dealing with it if we do is to have a hybrid approach - a few distinct columns, but then a JSON blob. (and if we were only to support postgres we could just use JSONb. But I think our friends at Google may object ;) ) Adding a new column in a DB migration with a default NULL shouldn't be an expensive operation, or difficult to achieve. > > In my imagination we would have to collect the list of dynamic features > depending on unserializable fields of a DAG and start a discussion/vote on > dropping support of them( I'm working on this but if anyone has already done > so please take over), decide on the serialized representation of a DAG and > then replace dagbag with it in webserver. Per previous discussion and some > offline discussions with Dan, one future of DAG serialization that I like > would look similar to this: > > https://imgur.com/ncqqQgc Something I've thought about before for other things was to embed an API server _into_ the scheduler - this would be useful for k8s healthchecks, native Prometheus metrics without needed statsd bridge, and could have endpoints to get information such as this directly. I was thinking it would be _in_ the scheduler process using either threads (ick. Python's still got a GIL doesn't it?) or using async/twisted etc. (not a side-car process like we have with the logs webserver for `airflow worker`). (This is possibly an unrelated discussion, but might be worth talking about?) > We can still discuss/vote which approach we want to take but I don't want the > door to above design to be shut right now or we have to spend a lot effort > switch path later. > > Bas and Peter, I'm very sorry to extend the discussion but I do think this is > tightly related to the AIP and PRs behind it. And my sincere apology for > bringing this up so late( I only pull the open PR list occasionally, if > there's a way to subscribe to new PR event I'd love to know how). It's noisy, but you can subscribe to comm...@airflow.apache.org (but be warned, this also includes all Jira tickets, edits of every comment on github etc.). > > Cheers, > Kevin Y > > On Thu, Feb 28, 2019 at 1:36 PM Peter van t Hof <pjrvant...@gmail.com > <mailto:pjrvant...@gmail.com>> wrote: > Hi all, > > Just some comments one the point Bolke dit give in relation of my PR. > > At first, the main focus is: making the webserver stateless. > > > 1) Make the webserver stateless: needs the graph of the *current* dag > > This is the main goal but for this a lot more PR’s will be coming once my > current is merged. For edges and graph view this is covered in my PR already. > > > 2) Version dags: for consistency mainly and not requiring parsing of the > > dag on every loop > > In my PR the historical graphs will be stored for each DagRun. This means > that you can see if an older DagRun was the same graph structure, even if > some tasks does not exists anymore in the current graph. Especially for > dynamic DAG’s this is very useful. > > > 3) Make the scheduler not require DAG files. This could be done if the > > edges contain all information when to trigger the next task. We can then > > have event driven dag parsing outside of the scheduler loop, ie. by the > > cli. Storage can also be somewhere else (git, artifactory, filesystem, > > whatever). > > The scheduler is almost untouched in this PR. The only thing that is added is > that this edges are saved to the database but the scheduling itself din’t > change. The scheduler depends now still on the DAG object. > > > 4) Fully serialise the dag so it becomes transferable to workers > > It nice to see that people has a lot of idea’s about this. But as Fokko > already mentioned this is out of scope for the issue what we are trying to > solve. I also have some idea’s about this but I like to limit this PR/AIP to > the webserver. > > For now my PR does solve 1 and 2 and the rest of the behaviour (like > scheduling) is untouched. > > Gr, > Peter >
