hi John, Sorry, there's a number of fairly long e-mails in this thread; I'm having a hard time following all of the details.
I suspect the most parsimonious thing would be to have some "sidecar" metadata that tracks the state of your writes into pre-allocated Arrow blocks so that readers know to call "Slice" on the blocks to obtain only the written-so-far portion. I'm not likely to be in favor of making changes to the binary protocol for this use case; if others have opinions I'll let them speak for themselves. - Wes On Mon, May 13, 2019 at 7:50 AM John Muehlhausen <j...@jgm.org> wrote: > > Any thoughts on a RecordBatch distinguishing size from capacity? (To borrow > std::vector terminology) > > Thanks, > John > > On Thu, May 9, 2019 at 2:46 PM John Muehlhausen <j...@jgm.org> wrote: > > > Wes et al, I think my core proposal is that Message.fbs:RecordBatch split > > the "length" parameter into "theoretical max length" and "utilized length" > > (perhaps not those exact names). > > > > "theoretical max length is the same as "length" now ... /// ...The arrays > > in the batch should all have this > > > > "utilized length" are the number of rows (starting from the first one) > > that actually contain interesting data... the rest do not. > > > > The reason we can have a RecordBatch where these numbers are not the same > > is that the RecordBatch space was preallocated (for performance reasons) > > and the number of rows that actually "fit" depends on how correct the > > preallocation was. In any case, it gives the user control of this > > space/time tradeoff... wasted space in order to save time in record batch > > construction. The fact that some space will usually be wasted when there > > are variable-length columns (barring extreme luck) with this batch > > construction paradigm explains the word "theoretical" above. This also > > gives us the ability to look at a partially constructed batch that is still > > being constructed, given appropriate user-supplied concurrency control. > > > > I am not an expert in all of the Arrow variable-length data types, but I > > think this works if they are all similar to variable-length strings where > > we advance through "blob storage" by setting the indexes into that storage > > for the current and next row in order to indicate that we have > > incrementally consumed more blob storage. (Conceptually this storage is > > "unallocated" after the pre-allocation and before rows are populated.) > > > > At a high level I am seeking to shore up the format for event ingress into > > real-time analytics that have some look-back window. If I'm not mistaken I > > think this is the subject of the last multi-sentence paragraph here?: > > https://zd.net/2H0LlBY > > > > Currently we have a less-efficient paradigm where "microbatches" (e.g. of > > length 1 for minimal latency) have to spin the CPU periodically in order to > > be combined into buffers where we get the columnar layout benefit. With > > pre-allocation we can deal with microbatches (a partially populated larger > > RecordBatch) and immediately have the columnar layout benefits for the > > populated section with no additional computation. > > > > For example, consider an event processing system that calculates a "moving > > average" as events roll in. While this is somewhat contrived lets assume > > that the moving average window is 1000 periods and our pre-allocation > > ("theoretical max length") of RecordBatch elements is 100. The algorithm > > would be something like this, for a list of RecordBatch buffers in memory: > > > > initialization(): > > set up configuration of expected variable length storage requirements, > > e.g. the template RecordBatch mentioned below > > > > onIncomingEvent(event): > > obtain lock /// cf. swoopIn() below > > if last RecordBatch theoretical max length is not less than utilized > > length or variable-length components of "event" will not fit in remaining > > blob storage: > > create a new RecordBatch pre-allocation of max utilized length 100 and > > with blob preallocation that is max(expected, event .. in case the single > > event is larger than the expectation for 100 events) > > (note: in the expected case this can be very fast as it is a > > malloc() and a memcpy() from a template!) > > set current RecordBatch to this newly created one > > add event to current RecordBatch (for the non-calculated fields) > > increment utilized length of current RecordBatch > > calculate the calculated fields (in this case, moving average) by > > looking back at previous rows in this and previous RecordBatch objects > > free() any RecordBatch objects that are now before the lookback window > > > > swoopIn(): /// somebody wants to chart the lookback window > > obtain lock > > visit all of the relevant data in the RecordBatches to construct the > > chart /// notice that the last RecordBatch may not yet be "as full as > > possible" > > > > The above analysis (minus the free()) could apply to the IPC file format > > and the lock could be a file lock and the swoopIn() could be a separate > > process. In the case of the file format, while the file is locked, a new > > RecordBatch would overwrite the previous file Footer and a new Footer would > > be written. In order to be able to delete or archive old data multiple > > files could be strung together in a logical series. > > > > -John > > > > On Tue, May 7, 2019 at 2:39 PM Wes McKinney <wesmck...@gmail.com> wrote: > > > >> On Tue, May 7, 2019 at 12:26 PM John Muehlhausen <j...@jgm.org> wrote: > >> > > >> > Wes, are we saying that `pa.ipc.open_file(...).read_pandas()` already > >> reads > >> > the future Feather format? If not, how will the future format differ? I > >> > will work on my access pattern with this format instead of the current > >> > feather format. Sorry I was not clear on that earlier. > >> > > >> > >> Yes, under the hood those will use the same zero-copy binary protocol > >> code paths to read the file. > >> > >> > Micah, thank you! > >> > > >> > On Tue, May 7, 2019 at 11:44 AM Micah Kornfield <emkornfi...@gmail.com> > >> > wrote: > >> > > >> > > Hi John, > >> > > To give a specific pointer [1] describes how the streaming protocol is > >> > > stored to a file. > >> > > > >> > > [1] https://arrow.apache.org/docs/format/IPC.html#file-format > >> > > > >> > > On Tue, May 7, 2019 at 9:40 AM Wes McKinney <wesmck...@gmail.com> > >> wrote: > >> > > > >> > > > hi John, > >> > > > > >> > > > As soon as the R folks can install the Arrow R package consistently, > >> > > > the intent is to replace the Feather internals with the plain Arrow > >> > > > IPC protocol where we have much better platform support all around. > >> > > > > >> > > > If you'd like to experiment with creating an API for pre-allocating > >> > > > fixed-size Arrow protocol blocks and then mutating the data and > >> > > > metadata on disk in-place, please be our guest. We don't have the > >> > > > tools developed yet to do this for you > >> > > > > >> > > > - Wes > >> > > > > >> > > > On Tue, May 7, 2019 at 11:25 AM John Muehlhausen <j...@jgm.org> > >> wrote: > >> > > > > > >> > > > > Thanks Wes: > >> > > > > > >> > > > > "the current Feather format is deprecated" ... yes, but there > >> will be a > >> > > > > future file format that replaces it, correct? And my discussion > >> of > >> > > > > immutable "portions" of Arrow buffers, rather than immutability > >> of the > >> > > > > entire buffer, applies to IPC as well, right? I am only > >> championing > >> > > the > >> > > > > idea that this future file format have the convenience that > >> certain > >> > > > > preallocated rows can be ignored based on a metadata setting. > >> > > > > > >> > > > > "I recommend batching your writes" ... this is extremely > >> inefficient > >> > > and > >> > > > > adds unacceptable latency, relative to the proposed solution. Do > >> you > >> > > > > disagree? Either I have a batch length of 1 to minimize latency, > >> which > >> > > > > eliminates columnar advantages on the read side, or else I add > >> latency. > >> > > > > Neither works, and it seems that a viable alternative is within > >> sight? > >> > > > > > >> > > > > If you don't agree that there is a core issue and opportunity > >> here, I'm > >> > > > not > >> > > > > sure how to better make my case.... > >> > > > > > >> > > > > -John > >> > > > > > >> > > > > On Tue, May 7, 2019 at 11:02 AM Wes McKinney <wesmck...@gmail.com > >> > > >> > > > wrote: > >> > > > > > >> > > > > > hi John, > >> > > > > > > >> > > > > > On Tue, May 7, 2019 at 10:53 AM John Muehlhausen <j...@jgm.org> > >> > > wrote: > >> > > > > > > > >> > > > > > > Wes et al, I completed a preliminary study of populating a > >> Feather > >> > > > file > >> > > > > > > incrementally. Some notes and questions: > >> > > > > > > > >> > > > > > > I wrote the following dataframe to a feather file: > >> > > > > > > a b > >> > > > > > > 0 0123456789 0.0 > >> > > > > > > 1 0123456789 NaN > >> > > > > > > 2 0123456789 NaN > >> > > > > > > 3 0123456789 NaN > >> > > > > > > 4 None NaN > >> > > > > > > > >> > > > > > > In re-writing the flatbuffers metadata (flatc -p doesn't > >> > > > > > > support --gen-mutable! yuck! C++ to the rescue...), it seems > >> that > >> > > > > > > read_feather is not affected by NumRows? It seems to be > >> driven > >> > > > entirely > >> > > > > > by > >> > > > > > > the per-column Length values? > >> > > > > > > > >> > > > > > > Also, it seems as if one of the primary usages of NullCount > >> is to > >> > > > > > determine > >> > > > > > > whether or not a bitfield is present? In the initialization > >> data > >> > > > above I > >> > > > > > > was careful to have a null value in each column in order to > >> > > generate > >> > > > a > >> > > > > > > bitfield. > >> > > > > > > >> > > > > > Per my prior e-mails, the current Feather format is deprecated, > >> so > >> > > I'm > >> > > > > > only willing to engage on a discussion of the official Arrow > >> binary > >> > > > > > protocol that we use for IPC (memory mapping) and RPC (Flight). > >> > > > > > > >> > > > > > > > >> > > > > > > I then wiped the bitfields in the file and set all of the > >> string > >> > > > indices > >> > > > > > to > >> > > > > > > one past the end of the blob buffer (all strings empty): > >> > > > > > > a b > >> > > > > > > 0 None NaN > >> > > > > > > 1 None NaN > >> > > > > > > 2 None NaN > >> > > > > > > 3 None NaN > >> > > > > > > 4 None NaN > >> > > > > > > > >> > > > > > > I then set the first record to some data by consuming some of > >> the > >> > > > string > >> > > > > > > blob and row 0 and 1 indices, also setting the double: > >> > > > > > > > >> > > > > > > a b > >> > > > > > > 0 Hello, world! 5.0 > >> > > > > > > 1 None NaN > >> > > > > > > 2 None NaN > >> > > > > > > 3 None NaN > >> > > > > > > 4 None NaN > >> > > > > > > > >> > > > > > > As mentioned above, NumRows seems to be ignored. I tried > >> adjusting > >> > > > each > >> > > > > > > column Length to mask off higher rows and it worked for 4 > >> (hide > >> > > last > >> > > > row) > >> > > > > > > but not for less than 4. I take this to be due to math used > >> to > >> > > > figure > >> > > > > > out > >> > > > > > > where the buffers are relative to one another since there is > >> only > >> > > one > >> > > > > > > metadata offset for all of: the (optional) bitset, index > >> column and > >> > > > (if > >> > > > > > > string) blobs. > >> > > > > > > > >> > > > > > > Populating subsequent rows would proceed in a similar way > >> until all > >> > > > of > >> > > > > > the > >> > > > > > > blob storage has been consumed, which may come before the > >> > > > pre-allocated > >> > > > > > > rows have been consumed. > >> > > > > > > > >> > > > > > > So what does this mean for my desire to incrementally write > >> these > >> > > > > > > (potentially memory-mapped) pre-allocated files and/or Arrow > >> > > buffers > >> > > > in > >> > > > > > > memory? Some thoughts: > >> > > > > > > > >> > > > > > > - If a single value (such as NumRows) were consulted to > >> determine > >> > > the > >> > > > > > table > >> > > > > > > row dimension then updating this single value would serve to > >> tell a > >> > > > > > reader > >> > > > > > > which rows are relevant. Assuming this value is updated > >> after all > >> > > > other > >> > > > > > > mutations are complete, and assuming that mutations are only > >> > > appends > >> > > > > > > (addition of rows), then concurrency control involves only > >> ensuring > >> > > > that > >> > > > > > > this value is not examined while it is being written. > >> > > > > > > > >> > > > > > > - NullCount presents a concurrency problem if someone reads > >> the > >> > > file > >> > > > > > after > >> > > > > > > this field has been updated, but before NumRows has exposed > >> the new > >> > > > > > record > >> > > > > > > (or vice versa). The idea previously mentioned that there > >> will > >> > > > "likely > >> > > > > > > [be] more statistics in the future" feels like it might be > >> scope > >> > > > creep to > >> > > > > > > me? This is a data representation, not a calculation > >> framework? > >> > > If > >> > > > > > > NullCount had its genesis in the optional nature of the > >> bitfield, I > >> > > > would > >> > > > > > > suggest that perhaps NullCount can be dropped in favor of > >> always > >> > > > > > supplying > >> > > > > > > the bitfield, which in any event is already contemplated by > >> the > >> > > spec: > >> > > > > > > "Implementations may choose to always allocate one anyway as a > >> > > > matter of > >> > > > > > > convenience." If the concern is space savings, Arrow is > >> already an > >> > > > > > > extremely uncompressed format. (Compression is something I > >> would > >> > > > also > >> > > > > > > consider to be scope creep as regards Feather... compressed > >> > > > filesystems > >> > > > > > can > >> > > > > > > be employed and there are other compressed dataframe formats.) > >> > > > However, > >> > > > > > if > >> > > > > > > protecting the 4 bytes required to update NowRows turns out > >> to be > >> > > no > >> > > > > > easier > >> > > > > > > than protecting all of the statistical bytes as well as part > >> of the > >> > > > same > >> > > > > > > "critical section" (locks: yuck!!) then statistics pose no > >> issue. > >> > > I > >> > > > > > have a > >> > > > > > > feeling that the availability of an atomic write of 4 bytes > >> will > >> > > > depend > >> > > > > > on > >> > > > > > > the storage mechanism... memory vs memory map vs write() etc. > >> > > > > > > > >> > > > > > > - The elephant in the room appears to be the presumptive > >> binary > >> > > > yes/no on > >> > > > > > > mutability of Arrow buffers. Perhaps the thought is that > >> certain > >> > > > batch > >> > > > > > > processes will be wrecked if anyone anywhere is mutating > >> buffers in > >> > > > any > >> > > > > > > way. But keep in mind I am not proposing general mutability, > >> only > >> > > > > > > appending of new data. *A huge amount of batch processing > >> that > >> > > will > >> > > > take > >> > > > > > > place with Arrow is on time-series data (whether financial or > >> > > > otherwise). > >> > > > > > > It is only natural that architects will want the minimal > >> impedance > >> > > > > > mismatch > >> > > > > > > when it comes time to grow those time series as the events > >> occur > >> > > > going > >> > > > > > > forward.* So rather than say that I want "mutable" Arrow > >> buffers, > >> > > I > >> > > > > > would > >> > > > > > > pitch this as a call for "immutable populated areas" of Arrow > >> > > buffers > >> > > > > > > combined with the concept that the populated area can grow up > >> to > >> > > > whatever > >> > > > > > > was preallocated. This will not affect anyone who has > >> "memoized" a > >> > > > > > > dimension and wants to continue to consider the then-current > >> data > >> > > as > >> > > > > > > immutable... it will be immutable and will always be immutable > >> > > > according > >> > > > > > to > >> > > > > > > that then-current dimension. > >> > > > > > > > >> > > > > > > Thanks in advance for considering this feedback! I absolutely > >> > > > require > >> > > > > > > efficient row-wise growth of an Arrow-like buffer to deal > >> with time > >> > > > > > series > >> > > > > > > data in near real time. I believe that preallocation is (by > >> far) > >> > > the > >> > > > > > most > >> > > > > > > efficient way to accomplish this. I hope to be able to use > >> Arrow! > >> > > > If I > >> > > > > > > cannot use Arrow than I will be using a home-grown Arrow that > >> is > >> > > > > > identical > >> > > > > > > except for this feature, which would be very sad! Even if > >> Arrow > >> > > > itself > >> > > > > > > could be used in this manner today, I would be hesitant to > >> use it > >> > > if > >> > > > the > >> > > > > > > use-case was not protected on a go-forward basis. > >> > > > > > > > >> > > > > > > >> > > > > > I recommend batching your writes and using the Arrow binary > >> streaming > >> > > > > > protocol so you are only appending to a file rather than > >> mutating > >> > > > > > previously-written bytes. This use case is well defined and > >> supported > >> > > > > > in the software already. > >> > > > > > > >> > > > > > > >> > > > > > > >> > > > > >> > > > >> https://github.com/apache/arrow/blob/master/docs/source/format/IPC.rst#streaming-format > >> > > > > > > >> > > > > > - Wes > >> > > > > > > >> > > > > > > Of course, I am completely open to alternative ideas and > >> > > approaches! > >> > > > > > > > >> > > > > > > -John > >> > > > > > > > >> > > > > > > On Mon, May 6, 2019 at 11:39 AM Wes McKinney < > >> wesmck...@gmail.com> > >> > > > > > wrote: > >> > > > > > > > >> > > > > > > > hi John -- again, I would caution you against using Feather > >> files > >> > > > for > >> > > > > > > > issues of longevity -- the internal memory layout of those > >> files > >> > > > is a > >> > > > > > > > "dead man walking" so to speak. > >> > > > > > > > > >> > > > > > > > I would advise against forking the project, IMHO that is a > >> dark > >> > > > path > >> > > > > > > > that leads nowhere good. We have a large community here and > >> we > >> > > > accept > >> > > > > > > > pull requests -- I think the challenge is going to be > >> defining > >> > > the > >> > > > use > >> > > > > > > > case to suitable clarity that a general purpose solution > >> can be > >> > > > > > > > developed. > >> > > > > > > > > >> > > > > > > > - Wes > >> > > > > > > > > >> > > > > > > > > >> > > > > > > > On Mon, May 6, 2019 at 11:16 AM John Muehlhausen < > >> j...@jgm.org> > >> > > > wrote: > >> > > > > > > > > > >> > > > > > > > > François, Wes, > >> > > > > > > > > > >> > > > > > > > > Thanks for the feedback. I think the most practical > >> thing for > >> > > > me to > >> > > > > > do > >> > > > > > > > is > >> > > > > > > > > 1- write a Feather file that is structured to > >> pre-allocate the > >> > > > space > >> > > > > > I > >> > > > > > > > need > >> > > > > > > > > (e.g. initial variable-length strings are of average size) > >> > > > > > > > > 2- come up with code to monkey around with the values > >> contained > >> > > > in > >> > > > > > the > >> > > > > > > > > vectors so that before and after each manipulation the > >> file is > >> > > > valid > >> > > > > > as I > >> > > > > > > > > walk the rows ... this is a writer that uses memory > >> mapping > >> > > > > > > > > 3- check back in here once that works, assuming that it > >> does, > >> > > to > >> > > > see > >> > > > > > if > >> > > > > > > > we > >> > > > > > > > > can bless certain mutation paths > >> > > > > > > > > 4- if we can't bless certain mutation paths, fork the > >> project > >> > > for > >> > > > > > those > >> > > > > > > > who > >> > > > > > > > > care more about stream processing? That would not seem > >> to be > >> > > > ideal > >> > > > > > as I > >> > > > > > > > > think mutation in row-order across the data set is > >> relatively > >> > > low > >> > > > > > impact > >> > > > > > > > on > >> > > > > > > > > the overall design? > >> > > > > > > > > > >> > > > > > > > > Thanks again for engaging the topic! > >> > > > > > > > > -John > >> > > > > > > > > > >> > > > > > > > > On Mon, May 6, 2019 at 10:26 AM Francois Saint-Jacques < > >> > > > > > > > > fsaintjacq...@gmail.com> wrote: > >> > > > > > > > > > >> > > > > > > > > > Hello John, > >> > > > > > > > > > > >> > > > > > > > > > Arrow is not yet suited for partial writes. The > >> specification > >> > > > only > >> > > > > > > > > > talks about fully frozen/immutable objects, you're in > >> > > > > > implementation > >> > > > > > > > > > defined territory here. For example, the C++ library > >> assumes > >> > > > the > >> > > > > > Array > >> > > > > > > > > > object is immutable; it memoize the null count, and > >> likely > >> > > more > >> > > > > > > > > > statistics in the future. > >> > > > > > > > > > > >> > > > > > > > > > If you want to use pre-allocated buffers and array, you > >> can > >> > > > use the > >> > > > > > > > > > column validity bitmap for this purpose, e.g. set all > >> null by > >> > > > > > default > >> > > > > > > > > > and flip once the row is written. It suffers from > >> concurrency > >> > > > > > issues > >> > > > > > > > > > (+ invalidation issues as pointed) when dealing with > >> multiple > >> > > > > > columns. > >> > > > > > > > > > You'll have to use a barrier of some kind, e.g. a > >> per-batch > >> > > > global > >> > > > > > > > > > atomic (if append-only), or dedicated column(s) à-la > >> MVCC. > >> > > But > >> > > > > > then, > >> > > > > > > > > > the reader needs to be aware of this and compute a mask > >> each > >> > > > time > >> > > > > > it > >> > > > > > > > > > needs to query the partial batch. > >> > > > > > > > > > > >> > > > > > > > > > This is a common columnar database problem, see [1] for > >> a > >> > > > recent > >> > > > > > paper > >> > > > > > > > > > on the subject. The usual technique is to store the > >> recent > >> > > data > >> > > > > > > > > > row-wise, and transform it in column-wise when a > >> threshold is > >> > > > met > >> > > > > > akin > >> > > > > > > > > > to a compaction phase. There was a somewhat related > >> thread > >> > > [2] > >> > > > > > lately > >> > > > > > > > > > about streaming vs batching. In the end, I think your > >> > > solution > >> > > > > > will be > >> > > > > > > > > > very application specific. > >> > > > > > > > > > > >> > > > > > > > > > François > >> > > > > > > > > > > >> > > > > > > > > > [1] > >> > > https://db.in.tum.de/downloads/publications/datablocks.pdf > >> > > > > > > > > > [2] > >> > > > > > > > > > > >> > > > > > > > > >> > > > > > > >> > > > > >> > > > >> https://lists.apache.org/thread.html/27945533db782361143586fd77ca08e15e96e2f2a5250ff084b462d6@%3Cdev.arrow.apache.org%3E > >> > > > > > > > > > > >> > > > > > > > > > > >> > > > > > > > > > > >> > > > > > > > > > > >> > > > > > > > > > > >> > > > > > > > > > > >> > > > > > > > > > > >> > > > > > > > > > On Mon, May 6, 2019 at 10:39 AM John Muehlhausen < > >> > > j...@jgm.org> > >> > > > > > wrote: > >> > > > > > > > > > > > >> > > > > > > > > > > Wes, > >> > > > > > > > > > > > >> > > > > > > > > > > I’m not afraid of writing my own C++ code to deal > >> with all > >> > > of > >> > > > > > this > >> > > > > > > > on the > >> > > > > > > > > > > writer side. I just need a way to “append” > >> (incrementally > >> > > > > > populate) > >> > > > > > > > e.g. > >> > > > > > > > > > > feather files so that a person using e.g. pyarrow > >> doesn’t > >> > > > suffer > >> > > > > > some > >> > > > > > > > > > > catastrophic failure... and “on the side” I tell them > >> which > >> > > > rows > >> > > > > > are > >> > > > > > > > junk > >> > > > > > > > > > > and deal with any concurrency issues that can’t be > >> solved > >> > > in > >> > > > the > >> > > > > > > > arena of > >> > > > > > > > > > > atomicity and ordering of ops. For now I care about > >> basic > >> > > > types > >> > > > > > but > >> > > > > > > > > > > including variable-width strings. > >> > > > > > > > > > > > >> > > > > > > > > > > For event-processing, I think Arrow has to have the > >> concept > >> > > > of a > >> > > > > > > > > > partially > >> > > > > > > > > > > full record set. Some alternatives are: > >> > > > > > > > > > > - have a batch size of one, thus littering the > >> landscape > >> > > with > >> > > > > > > > trivially > >> > > > > > > > > > > small Arrow buffers > >> > > > > > > > > > > - artificially increase latency with a batch size > >> larger > >> > > than > >> > > > > > one, > >> > > > > > > > but > >> > > > > > > > > > not > >> > > > > > > > > > > processing any data until a batch is complete > >> > > > > > > > > > > - continuously re-write the (entire!) “main” buffer as > >> > > > batches of > >> > > > > > > > length > >> > > > > > > > > > 1 > >> > > > > > > > > > > roll in > >> > > > > > > > > > > - instead of one main buffer, several, and at some > >> > > threshold > >> > > > > > combine > >> > > > > > > > the > >> > > > > > > > > > > last N length-1 batches into a length N buffer ... > >> still an > >> > > > > > > > inefficiency > >> > > > > > > > > > > > >> > > > > > > > > > > Consider the case of QAbstractTableModel as the > >> underlying > >> > > > data > >> > > > > > for a > >> > > > > > > > > > table > >> > > > > > > > > > > or a chart. This visualization shows all of the data > >> for > >> > > the > >> > > > > > recent > >> > > > > > > > past > >> > > > > > > > > > > as well as events rolling in. If this model > >> interface is > >> > > > > > > > implemented as > >> > > > > > > > > > a > >> > > > > > > > > > > view onto “many thousands” of individual event > >> buffers then > >> > > > we > >> > > > > > gain > >> > > > > > > > > > nothing > >> > > > > > > > > > > from columnar layout. (Suppose there are tons of > >> columns > >> > > and > >> > > > > > most of > >> > > > > > > > > > them > >> > > > > > > > > > > are scrolled out of the view.). Likewise we cannot > >> re-write > >> > > > the > >> > > > > > > > entire > >> > > > > > > > > > > model on each event... time complexity blows up. > >> What we > >> > > > want > >> > > > > > is to > >> > > > > > > > > > have a > >> > > > > > > > > > > large pre-allocated chunk and just change rowCount() > >> as > >> > > data > >> > > > is > >> > > > > > > > > > “appended.” > >> > > > > > > > > > > Sure, we may run out of space and have another and > >> another > >> > > > > > chunk for > >> > > > > > > > > > > future row ranges, but a handful of chunks chained > >> together > >> > > > is > >> > > > > > better > >> > > > > > > > > > than > >> > > > > > > > > > > as many chunks as there were events! > >> > > > > > > > > > > > >> > > > > > > > > > > And again, having a batch size >1 and delaying the > >> data > >> > > > until a > >> > > > > > > > batch is > >> > > > > > > > > > > full is a non-starter. > >> > > > > > > > > > > > >> > > > > > > > > > > I am really hoping to see partially-filled buffers as > >> > > > something > >> > > > > > we > >> > > > > > > > keep > >> > > > > > > > > > our > >> > > > > > > > > > > finger on moving forward! Or else, what am I missing? > >> > > > > > > > > > > > >> > > > > > > > > > > -John > >> > > > > > > > > > > > >> > > > > > > > > > > On Mon, May 6, 2019 at 8:24 AM Wes McKinney < > >> > > > wesmck...@gmail.com > >> > > > > > > > >> > > > > > > > wrote: > >> > > > > > > > > > > > >> > > > > > > > > > > > hi John, > >> > > > > > > > > > > > > >> > > > > > > > > > > > In C++ the builder classes don't yet support > >> writing into > >> > > > > > > > preallocated > >> > > > > > > > > > > > memory. It would be tricky for applications to > >> determine > >> > > a > >> > > > > > priori > >> > > > > > > > > > > > which segments of memory to pass to the builder. It > >> seems > >> > > > only > >> > > > > > > > > > > > feasible for primitive / fixed-size types so my > >> guess > >> > > > would be > >> > > > > > > > that a > >> > > > > > > > > > > > separate set of interfaces would need to be > >> developed for > >> > > > this > >> > > > > > > > task. > >> > > > > > > > > > > > > >> > > > > > > > > > > > - Wes > >> > > > > > > > > > > > > >> > > > > > > > > > > > On Mon, May 6, 2019 at 8:18 AM Jacques Nadeau < > >> > > > > > jacq...@apache.org> > >> > > > > > > > > > wrote: > >> > > > > > > > > > > > > > >> > > > > > > > > > > > > This is more of a question of implementation > >> versus > >> > > > > > > > specification. An > >> > > > > > > > > > > > arrow > >> > > > > > > > > > > > > buffer is generally built and then sealed. In > >> different > >> > > > > > > > languages, > >> > > > > > > > > > this > >> > > > > > > > > > > > > building process works differently (a concern of > >> the > >> > > > language > >> > > > > > > > rather > >> > > > > > > > > > than > >> > > > > > > > > > > > > the memory specification). We don't currently > >> allow a > >> > > > half > >> > > > > > built > >> > > > > > > > > > vector > >> > > > > > > > > > > > to > >> > > > > > > > > > > > > be moved to another language and then be further > >> built. > >> > > > So > >> > > > > > the > >> > > > > > > > > > question > >> > > > > > > > > > > > is > >> > > > > > > > > > > > > really more concrete: what language are you > >> looking at > >> > > > and > >> > > > > > what > >> > > > > > > > is > >> > > > > > > > > > the > >> > > > > > > > > > > > > specific pattern you're trying to undertake for > >> > > building. > >> > > > > > > > > > > > > > >> > > > > > > > > > > > > If you're trying to go across independent > >> processes > >> > > > (whether > >> > > > > > the > >> > > > > > > > same > >> > > > > > > > > > > > > process restarted or two separate processes active > >> > > > > > > > simultaneously) > >> > > > > > > > > > you'll > >> > > > > > > > > > > > > need to build up your own data structures to help > >> with > >> > > > this. > >> > > > > > > > > > > > > > >> > > > > > > > > > > > > On Mon, May 6, 2019 at 6:28 PM John Muehlhausen < > >> > > > j...@jgm.org > >> > > > > > > > >> > > > > > > > wrote: > >> > > > > > > > > > > > > > >> > > > > > > > > > > > > > Hello, > >> > > > > > > > > > > > > > > >> > > > > > > > > > > > > > Glad to learn of this project— good work! > >> > > > > > > > > > > > > > > >> > > > > > > > > > > > > > If I allocate a single chunk of memory and start > >> > > > building > >> > > > > > Arrow > >> > > > > > > > > > format > >> > > > > > > > > > > > > > within it, does this chunk save any state > >> regarding > >> > > my > >> > > > > > > > progress? > >> > > > > > > > > > > > > > > >> > > > > > > > > > > > > > For example, suppose I allocate a column for > >> floating > >> > > > point > >> > > > > > > > (fixed > >> > > > > > > > > > > > width) > >> > > > > > > > > > > > > > and a column for string (variable width). > >> Suppose I > >> > > > start > >> > > > > > > > > > building the > >> > > > > > > > > > > > > > floating point column at offset X into my single > >> > > > buffer, > >> > > > > > and > >> > > > > > > > the > >> > > > > > > > > > string > >> > > > > > > > > > > > > > “pointer” column at offset Y into the same > >> single > >> > > > buffer, > >> > > > > > and > >> > > > > > > > the > >> > > > > > > > > > > > string > >> > > > > > > > > > > > > > data elements at offset Z. > >> > > > > > > > > > > > > > > >> > > > > > > > > > > > > > I write one floating point number and one > >> string, > >> > > then > >> > > > go > >> > > > > > away. > >> > > > > > > > > > When I > >> > > > > > > > > > > > > > come back to this buffer to append another > >> value, > >> > > does > >> > > > the > >> > > > > > > > buffer > >> > > > > > > > > > > > itself > >> > > > > > > > > > > > > > know where I would begin? I.e. is there a > >> > > > differentiation > >> > > > > > in > >> > > > > > > > the > >> > > > > > > > > > > > column > >> > > > > > > > > > > > > > (or blob) data itself between the available > >> space and > >> > > > the > >> > > > > > used > >> > > > > > > > > > space? > >> > > > > > > > > > > > > > > >> > > > > > > > > > > > > > Suppose I write a lot of large variable width > >> strings > >> > > > and > >> > > > > > “run > >> > > > > > > > > > out” of > >> > > > > > > > > > > > > > space for them before running out of space for > >> > > floating > >> > > > > > point > >> > > > > > > > > > numbers > >> > > > > > > > > > > > or > >> > > > > > > > > > > > > > string pointers. (I guessed badly when doing > >> the > >> > > > original > >> > > > > > > > > > > > allocation.). I > >> > > > > > > > > > > > > > consider this to be Ok since I can always > >> “copy” the > >> > > > data > >> > > > > > to > >> > > > > > > > > > “compress > >> > > > > > > > > > > > out” > >> > > > > > > > > > > > > > the unused fp/pointer buckets... the choice is > >> up to > >> > > > me. > >> > > > > > > > > > > > > > > >> > > > > > > > > > > > > > The above applied to a (feather?) file is how I > >> > > > anticipate > >> > > > > > > > > > appending > >> > > > > > > > > > > > data > >> > > > > > > > > > > > > > to disk... pre-allocate a mem-mapped file and > >> > > gradually > >> > > > > > fill > >> > > > > > > > it up. > >> > > > > > > > > > > > The > >> > > > > > > > > > > > > > efficiency of file utilization will depend on my > >> > > > > > projections > >> > > > > > > > > > regarding > >> > > > > > > > > > > > > > variable-width data types, but as I said above, > >> I can > >> > > > > > always > >> > > > > > > > > > re-write > >> > > > > > > > > > > > the > >> > > > > > > > > > > > > > file if/when this bothers me. > >> > > > > > > > > > > > > > > >> > > > > > > > > > > > > > Is this the recommended and supported approach > >> for > >> > > > > > incremental > >> > > > > > > > > > appends? > >> > > > > > > > > > > > > > I’m really hoping to use Arrow instead of > >> rolling my > >> > > > own, > >> > > > > > but > >> > > > > > > > > > > > functionality > >> > > > > > > > > > > > > > like this is absolutely key! Hoping not to use > >> a > >> > > > side-car > >> > > > > > > > file (or > >> > > > > > > > > > > > memory > >> > > > > > > > > > > > > > chunk) to store “append progress” information. > >> > > > > > > > > > > > > > > >> > > > > > > > > > > > > > I am brand new to this project so please > >> forgive me > >> > > if > >> > > > I > >> > > > > > have > >> > > > > > > > > > > > overlooked > >> > > > > > > > > > > > > > something obvious. And again, looks like great > >> work > >> > > so > >> > > > > > far! > >> > > > > > > > > > > > > > > >> > > > > > > > > > > > > > Thanks! > >> > > > > > > > > > > > > > -John > >> > > > > > > > > > > > > > > >> > > > > > > > > > > > > >> > > > > > > > > > > >> > > > > > > > > >> > > > > > > >> > > > > >> > > > >> > >
