arkdb
Package rationale
Increasing data sizes create challenges for the fundamental tasks of
publishing, distributing, and preserving data. Despite (or perhaps
because of) the diverse and ever-expanding number of database and file
formats, the humble plain text file such as comma or
tab-separated-values (e.g. .csv or .tsv files)
remains the gold standard for data archiving and distribution. These
files can read on almost any platform or tool and can be efficiently
compressed using long-standing and widely available standard open source
libraries like gzip or bzip2. In contrast,
database storage formats and dumps are usually particular to the
database platform used to generate them, and will likely not be
compatible between different database engines (e.g. PostgreSQL ->
SQLite) or even between different versions of the same engine.
Researchers unfamiliar with these databases will have difficulty
accessing such data, and these dumps may also be in formats that are
less efficient to compress.
Working with tables that are too large for working memory on most
machines by using external relational database stores is now a common R
practice, thanks to ever-rising availability of data and increasing
support and popularity of packages such as DBI,
dplyr, and dbplyr. Working with plain text
files becomes increasingly difficult in this context. Many R users will
not have sufficient RAM to simply read in a 10 GB .tsv file
into R. Similarly, moving a 10 GB database out of a relational data file
and into a plain text file for archiving and distribution is similarly
challenging from R. While most relational database back-ends implement
some form of COPY or IMPORT that allows them
to read in and export out plain text files directly, these methods are
not consistent across database types and not part of the standard SQL
interface. Most importantly for our case, they also cannot be called
directly from R, but require a separate stand-alone installation of the
database client. arkdb provides a simple solution to these
two tasks.
The goal of arkdb is to provide a convenient way to move
data from large compressed text files (e.g. *.tsv.bz2) into
any DBI-compliant database connection (see DBI), and
move tables out of such databases into text files. The key feature of
arkdb is that files are moved between databases and text
files in chunks of a fixed size, allowing the package functions to work
with tables that would be much to large to read into memory all at once.
This will be slower than reading the file into memory at one go, but can
be scaled to larger data and larger data with no additional memory
requirement.
Tutorial
library(arkdb)
# additional libraries just for this demo
library(dbplyr)
library(dplyr)
library(nycflights13)
library(fs)Creating an archive of an existing database
First, we’ll need an example database to work with. Conveniently,
there is a nice example using the NYC flights data built into the
dbplyr package.
tmp <- tempdir() # Or can be your working directory, "."
db <- dbplyr::nycflights13_sqlite(tmp)
#> Caching nycflights db at /tmp/Rtmplomm1j/nycflights13.sqlite
#> Creating table: airlines
#> Creating table: airports
#> Creating table: flights
#> Creating table: planes
#> Creating table: weatherTo create an archive, we just give ark the connection to
the database and tell it where we want the *.tsv.bz2 files
to be archived. We can also set the chunk size as the number of
lines read in a single chunk. More lines per chunk usually
means faster run time at the cost of higher memory requirements.
dir <- fs::dir_create(fs::path(tmp, "nycflights"))
ark(db, dir, lines = 50000)
#> Exporting airlines in 50000 line chunks:
#> ...Done! (in 0.003786325 secs)
#> Exporting airports in 50000 line chunks:
#> ...Done! (in 0.009854555 secs)
#> Exporting flights in 50000 line chunks:
#> ...Done! (in 4.747011 secs)
#> Exporting planes in 50000 line chunks:
#> ...Done! (in 0.01225924 secs)
#> Exporting weather in 50000 line chunks:
#> ...Done! (in 0.3304684 secs)We can take a look and confirm the files have been written. Note that
we can use fs::dir_info to get a nice snapshot of the file
sizes. Compare the compressed sizes to the original database:
fs::dir_info(dir) %>%
select(path, size) %>%
mutate(path = fs::path_file(path))
#> # A tibble: 5 × 2
#> path size
#> <chr> <fs::bytes>
#> 1 airlines.tsv.bz2 260
#> 2 airports.tsv.bz2 28.13K
#> 3 flights.tsv.bz2 4.85M
#> 4 planes.tsv.bz2 11.96K
#> 5 weather.tsv.bz2 278.84K
fs::file_info(fs::path(tmp,"nycflights13.sqlite")) %>%
pull(size)
#> 44.9MUnarchive
Now that we’ve gotten all the database into (compressed) plain text
files, let’s get them back out. We simply need to pass
unark a list of these compressed files and a connection to
the database. Here we create a new local SQLite database. Note that this
design means that it is also easy to use arkdb to move data
between databases.
files <- fs::dir_ls(dir, glob = "*.tsv.bz2")
new_db <- DBI::dbConnect(RSQLite::SQLite(), fs::path(tmp, "local.sqlite"))As with ark, we can set the chunk size to control the
memory footprint required:
unark(files, new_db, lines = 50000)
#> Importing /tmp/Rtmplomm1j/nycflights/airlines.tsv.bz2 in 50000 line chunks:
#> ...Done! (in 0.007323742 secs)
#> Importing /tmp/Rtmplomm1j/nycflights/airports.tsv.bz2 in 50000 line chunks:
#> ...Done! (in 0.01327014 secs)
#> Importing /tmp/Rtmplomm1j/nycflights/flights.tsv.bz2 in 50000 line chunks:
#> ...Done! (in 3.658393 secs)
#> Importing /tmp/Rtmplomm1j/nycflights/planes.tsv.bz2 in 50000 line chunks:
#> ...Done! (in 0.02044249 secs)
#> Importing /tmp/Rtmplomm1j/nycflights/weather.tsv.bz2 in 50000 line chunks:
#> ...Done! (in 0.1432383 secs)unark returns a dplyr database connection
that we can use in the usual way:
tbl(new_db, "flights")
#> # Source: table<`flights`> [?? x 19]
#> # Database: sqlite 3.47.0 [/tmp/Rtmplomm1j/local.sqlite]
#> year month day dep_time sched_dep_time dep_delay arr_time sched_arr_time
#> <int> <int> <int> <int> <int> <int> <int> <int>
#> 1 2013 1 1 517 515 2 830 819
#> 2 2013 1 1 533 529 4 850 830
#> 3 2013 1 1 542 540 2 923 850
#> 4 2013 1 1 544 545 -1 1004 1022
#> 5 2013 1 1 554 600 -6 812 837
#> 6 2013 1 1 554 558 -4 740 728
#> 7 2013 1 1 555 600 -5 913 854
#> 8 2013 1 1 557 600 -3 709 723
#> 9 2013 1 1 557 600 -3 838 846
#> 10 2013 1 1 558 600 -2 753 745
#> # ℹ more rows
#> # ℹ 11 more variables: arr_delay <int>, carrier <chr>, flight <int>,
#> # tailnum <chr>, origin <chr>, dest <chr>, air_time <int>, distance <int>,
#> # hour <int>, minute <int>, time_hour <dbl>
# Remove example files we created.
DBI::dbDisconnect(new_db)
unlink(dir, TRUE)
unlink(fs::path(tmp, "local.sqlite"))Pluggable text formats
By default, arkdb uses tsv format,
implemented in base tools, as the text-based serialization. The
tsv standard is particularly attractive because it
side-steps some of the ambiguities present in the CSV format due to
string quoting. The IANA
Standard for TSV neatly avoids this for tab-separated values by
insisting that a tab can only ever be a separator.
arkdb provides a pluggable mechanism for changing the
back end utility used to write text files. For instance, if we need to
read in or export in .csv format, we can simply swap in a
csv based reader in both ark() and
unark() methods, as illustrated here:
dir <- fs::dir_create(fs::path(tmp, "nycflights"))
ark(db, dir,
streamable_table = streamable_base_csv())
#> Exporting airlines in 50000 line chunks:
#> ...Done! (in 0.001661062 secs)
#> Exporting airports in 50000 line chunks:
#> ...Done! (in 0.008907795 secs)
#> Exporting flights in 50000 line chunks:
#> ...Done! (in 4.982001 secs)
#> Exporting planes in 50000 line chunks:
#> ...Done! (in 0.01419973 secs)
#> Exporting weather in 50000 line chunks:
#> ...Done! (in 0.3363523 secs)
files <- fs::dir_ls(dir, glob = "*.csv.bz2")
new_db <- DBI::dbConnect(RSQLite::SQLite(), fs::path(tmp, "local.sqlite"))
unark(files, new_db,
streamable_table = streamable_base_csv())
#> Importing /tmp/Rtmplomm1j/nycflights/airlines.csv.bz2 in 50000 line chunks:
#> ...Done! (in 0.007359505 secs)
#> Importing /tmp/Rtmplomm1j/nycflights/airports.csv.bz2 in 50000 line chunks:
#> ...Done! (in 0.0136323 secs)
#> Importing /tmp/Rtmplomm1j/nycflights/flights.csv.bz2 in 50000 line chunks:
#> ...Done! (in 3.812304 secs)
#> Importing /tmp/Rtmplomm1j/nycflights/planes.csv.bz2 in 50000 line chunks:
#> ...Done! (in 0.01930547 secs)
#> Importing /tmp/Rtmplomm1j/nycflights/weather.csv.bz2 in 50000 line chunks:
#> ...Done! (in 0.1416924 secs)arkdb also provides the function
streamable_table() to facilitate users creating their own
streaming table interfaces. For instance, if you would prefer to use
readr methods to read and write tsv files, we
could construct the table as follows
(streamable_readr_tsv() and
streamable_readr_csv() are also shipped inside
arkdb for convenience):
stream <-
streamable_table(
function(file, ...) readr::read_tsv(file, ...),
function(x, path, omit_header)
readr::write_tsv(x = x, path = path, append = omit_header),
"tsv")and we can then pass such a streamable table directly to
ark() and unark(), like so:
ark(db, dir,
streamable_table = stream)
#> Exporting airlines in 50000 line chunks:
#> Warning: The `path` argument of `write_tsv()` is deprecated as of readr 1.4.0.
#> ℹ Please use the `file` argument instead.
#> This warning is displayed once every 8 hours.
#> Call `lifecycle::last_lifecycle_warnings()` to see where this warning was
#> generated.
#> ...Done! (in 0.121258 secs)
#> Exporting airports in 50000 line chunks:
#> ...Done! (in 0.04229593 secs)
#> Exporting flights in 50000 line chunks:
#> ...Done! (in 2.900887 secs)
#> Exporting planes in 50000 line chunks:
#> ...Done! (in 0.04106331 secs)
#> Exporting weather in 50000 line chunks:
#> ...Done! (in 0.2192285 secs)Note several constraints on this design. The write method must be
able to take a generic R connection object (which will
allow it to handle the compression methods used, if any), and the read
method must be able to take a textConnection object.
readr functions handle these cases out of the box, so the
above method is easy to write. Also note that the write method must be
able to append, i.e. it should use a header if
append=TRUE, but omit when it is FALSE. See
the built-in methods for more examples.
A note on compression
unark can read from a variety of compression formats
recognized by base R: bzip2, gzip,
zip, and xz, and ark can choose
any of these as the compression algorithm. Note that there is some
trade-off between speed of compression and efficiency (i.e. the final
file size). ark uses the bz2 compression
algorithm by default, supported in base R, to compress tsv
files. The bz2 offers excellent compression levels, but is
considerably slower to compress than gzip or
zip. It is comparably fast to uncompress. For faster
archiving when maximum file size reduction is not critical,
gzip will give nearly as effective compression in
significantly less time. Compression can also be turned off, e.g. by
using ark() with compress="none" and
unark() with files that have no compression suffix
(e.g. *.tsv instead of *.tsv.gz).
Distributing data
Once you have archived your database files with ark,
consider sharing them privately or publicly as part of your project
GitHub repo using the piggyback R
package. For more permanent, versioned, and citable data archiving,
upload your *.tsv.bz2 files to a data repository like Zenodo.org.