## Using funflow to cache a nix based workflow

Posted on September 12, 2018

My latest project has been to plot a map of orienteering maps in the UK. This post explains the technical aspects behind the project and primarily the use of funflow to turn my assortment of scripts into into a resumable workflow.

There was nothing wrong with my ad-hoc python and bash scripts but they downloaded and regenerated the whole output every time. The whole generation takes about 2 hours so it’s desirable to only recompute the necessary portions. This is where funflow comes in, by stringing together these scripts in their DSL, you get caching for free. The workflow is also highly parallelisable so in the future I could distribute the work across multiple machines if necessary.

The code for the project can be found here.

## funflow

There are already two blog posts introducing the concepts of funflow.

The main idea is that you specify your workflow (usually a sequence of external scripts) using a DSL and then funflow will automatically cache and schedule the steps.

My primary motivation for using funflow was the automatic caching. The store is content addressed which means that the location for each file in the store depends on its contents. funflow performs two different types of caching.

1. Input-based caching: A flow will only be executed once for each set of inputs.
2. Output-based caching: If multiple different steps produce the same output then it will only be stored once in the store. Further steps will not be recomputed.

The lack of output-based caching is one of the big missing features of nix which makes it unsuitable for this task. A content-addressed store where the address depends on the contents of the file is sometimes known as an intensional store. Nix’s store model is extensional as the store hash only depends on the inputs to the build.

An intensional store relies on the program producing deterministic output hashes. It can be quite difficult to track down why a step is not being cached when you are relying on the output’s being identified in the store.

## High-level architecture

There are two outputs to the project.

1. A folder of map tiles rendered at different resolutions.
2. A HTML page which contains the javascript to display the map and markers.

This folder is then uploaded to online storage and served as a static site.

The processing pipeline is as follows:

1. Find all the maps with location information from routegadget.co.uk.
3. Convert the maps to a common image format.
4. Reproject the maps to remove any rotation.
5. Merge overlapping maps into groups.
6. Generate tiles at all the different resolutions.
7. Combine all the tiles groups together.
8. Generate the website with the map and location markers.

As can be seen, the workflow is firstly highly parallelisable as much of the processing pipeline happens independently of other steps. However, the main goal is to avoid computing the tiles as much as possible as this is the step which takes by far the longest. At the time of writing there are about 500 maps to process. In general, there are about 5-10 maps added each week. Only recomputing the changed portions of the map saves a lot of time.

## Implementation using funflow

In theory, this is a perfect application for funflow but in order to achieve the perfect caching behaviour I had to rearchitecture several parts of the application.

### Using nix scripts

The recommended way to use funflow is to run each step of the flow in a docker container. I didn’t want to do this was my scripts already declared the correct environment to run in by using the nix-shell shebang.

#! /usr/bin/env nix-shell
#! nix-shell -i bash -p gdal

By placing these two lines at the top of the file, the script will be run using the bash interpreter with the gdal package available. This is more lightweight and flexible than using a docker image as I don’t have to regenerate a new docker image any time I make a change.

However, there is no native support for running these kinds of scripts built into funflow. It was easy enough to define my own function in order to run these kinds of scripts using the external' primitive.

nixScript takes a boolean parameter indicating whether the script is pure and should be cached. The name of the script to run, the names of any files the script depends on and finally a function which supplies any additional arguments to the script.

nixScriptX :: ArrowFlow eff ex arr => Bool
-> Path Rel File
-> [Path Rel File]
-> (a -> [Param])
-> arr (Content Dir, a) CS.Item
nixScriptX impure script scripts params = proc (scriptDir, a) -> do
env <- mergeFiles -< absScripts scriptDir
external' props (\(s, args) -> ExternalTask
{ _etCommand = "perl"
, _etParams = contentParam (s ^</> script) : params args
, _etWriteToStdOut = NoOutputCapture
, _etEnv = [("NIX_PATH", envParam "NIX_PATH")] }) -< (env, a)
where
props = def { ep_impure = impure }
absScripts sd = map (sd ^</>) (script : scripts)

The use of perl as the command relies on the behaviour of perl that it will execute the #! line if it does not contain the word “perl”. Yes, this is dirty.

It would be desirable to set NIX_PATH to a fixed nixpkgs checkout by passing a tarball directly but this worked for now.

All the steps are then defined in terms of nixScriptX indirectly as two helper functions are defined for the two cases of a pure or impure scripts.

nixScript = nixScriptX False
impureNixScript = nixScriptX True

## Step 1 - Finding the map information

Now to the nitty gritty details.

The python script scraper.py executes a selenium driver to extract the map information. For each map, the metainformation is serialised to its own file in the output directory.

scrape = impureNixScript [relfile|scraper.py|] [[relfile|shell.nix|]]
(\() -> [ outParam ])

This step is marked as impure as we have to run it every time the flow runs to work out if we need to perform any more work.

It is important that the filename of the serialised information is the same if the content of the file is the same. Otherwise, funflow will calculate a different hash for the file. As such, we compute our own hash of the metainformation for the name the serialised file.

In the end the output directory looks like:

9442c7eaa81f82f7e9889f6ee8382e8d047df76db2d5f6a6983d1c82399a2698.pickle
5e7e6994db565126a942d66a9435454d8b55cd7d3023dd37f64eca7bbb46df1f.pickle
...

### Gotcha 1: Using listDirContents defeats caching

Now that we have a directory containing all the metainformation, we want to split it up and then execute the fetching, converting and warping in parallel for all the images. My first attempt was

meta_dir <- step All <<< scrape -< (script_dir, ())
keys <- listDirContents -< meta_dir

but this did not work and even if the keys remained the same, the images would be refetched. The problem was listDirContents does not have the correct caching behaviour.

listDirContents takes a Content Dir and returns a [Content File] as required but the [Content File] are pointers into places into the Content Dir. This means that if the location of Content Dir changes (if there are any changes or new additions to any files in the directory) then the location of all the [Content File] will also be changed. This means the next stage of recompilation will be triggered despite being unnecessary.

Instead, we have to put each file in the directory into its own store location so that the its location depends only on itself rather than the other contents of the directory. I defined the splitDir combinator in order to do this.

splitDir :: ArrowFlow eff ex arr => arr (Content Dir) ([Content File])
splitDir = proc dir -> do
(_, fs) <- listDirContents -< dir
mapA reifyFile -< fs

-- Put a file, which might be a pointer into a dir, into its own store
-- location.
reifyFile :: ArrowFlow eff ex arr => arr (Content File) (Content File)
reifyFile = proc f -> do
file <- getFromStore return -< f
putInStoreAt (\d fn -> copyFile fn d) -< (file, CS.contentFilename f)

It could be improved by using a hardlink rather than copyFile.

Now we have split the metainformation up into individual components we have to download, convert and warp the map files.

We define three flows to do this which correspond to three different scripts.

fetch = nixScript [relfile|fetch.py|] [[relfile|shell.nix|]]

convertToGif = nixScript [relfile|convert_gif|] []
(\dir -> [ pathParam (IPItem dir), outParam ])

warp = nixScript [relfile|do_warp|] []
(\dir -> [ pathParam (IPItem dir), outParam ])

Each script takes an individual input file and produces output in a directory specified by funflow.

fetch.py is a python script whilst convert_gif and do_warp are bash scripts. We can treat them uniformly because of the nix-shell shebang.

These steps are all cached by default because they are external processes.

## Step 3: Merge the images together

In order to get a good looking result, we need to group together the processed images into groups of overlapping images. This time we will use a python script again invoked in a similar manner. The output is a directory of files which specify the groups, remember:

1. We have to be careful naming the files so that the names remain stable across compilation. In my original program the names were supplied by a counter but now they are the hash of the files which were used to create the group.
2. We have to use splitDir after creating the output to put each group file into it’s own store location so the next recompilation step will work.
mergeRasters = nixScript [relfile|merge-rasters.py|] [[relfile|merge-rasters.nix|]]
(\rs -> outParam : map contentParam rs )

This command also relies on merge-rasters.nix which sets up the correct python environment to run the script.

### Gotcha 2: mergeDirs can also defeat caching

The original implementation of this used mergeDirs :: arr [Content File] (Content Dir) in order to group together the files and pass a single directory to merge-rasters.py.

However, this suffers a similar problem to listDirContents as mergeDirs will create a new content store entry which contains all the files in the merge directories. The hash of this store location will then depend on the whole contents of the directory. In this case these file paths ended up in the output so it would cause the next steps to recompile even if nothing had changed.

In this case, we would prefer a “logical” group which groups the files together with a stable filename which wouldn’t affect caching.

The workaround for now was to use splitDir again to put each processed image into its own storage path and then pass each filename individually to merge-rasters.py rather than a directory as before.

## Step 4: Making the tiles

Making the tiles is another straightforward step which takes each of the groups and makes the necessary tiles for that group.

makeTiles = nixScript [relfile|make_tiles|] [] (\dir -> [ contentParam dir, outParam, textParam "16" ])

### Gotcha 3: mergeDirs doesn’t merge duplicate files

Once we have made all the tiles we need to merge them all together. This is safe as we already ensured that they didn’t overlap each other. The problem is that mergeDirs will not merge duplicate files. The make_tiles step creates some unnecessary files which we don’t need but would cause mergeDirs to fail as they are contained in the output of each directory.

The solution was to write my own version of mergeDirs which checks to see whether a file already exists before trying to merge it. It would be more hygienic to ensure that the directories I was trying to merge were properly distinct but this worked well for this use case.

## Step 5: Creating the static site

Our final script is a python script which creates the static site displaying all the markers and the map tiles. It takes the output of processing all the images and the metainformation to produce a single html file.

leaflet <- step All <<< makeLeaflet -< ( script_dir, (merge_dir, meta_dir))

The final step then merges together the static page and all the tiles. This is a nice bundle we can directly upload and serve our static site.

mergeDirs -< [leaflet, tiles]

## Putting it all together

The complete flow is shown below:

mainFlow :: SimpleFlow () (Content Dir)
mainFlow = proc () -> do
cwd <- stepIO (const getCurrentDir) -< ()
script_dir <- copyDirToStore -< (DirectoryContent (cwd </> [reldir|scripts/|]), Nothing)

# Step 1
meta_dir <- step All <<< scrape -< (script_dir, ())
keys <- splitDir -< meta_dir
# Step 2
maps <- mapA (fetch) -< [( script_dir, event) | event <- keys]
mapJpgs <- mapA convertToGif -< [(script_dir, m) | m <- maps]
merge_dir <- mergeDirs' <<< mapA (step All) <<< mapA warp -< [(script_dir, jpg) | jpg <- mapJpgs ]
toMerge <- splitDir -< merge_dir
# Step 3
vrt_dir <- step All <<< mergeRasters -< (script_dir, toMerge)
merged_vrts <- splitDir -< vrt_dir
# Step 4
tiles <- mergeDirs' <<< mapA (step All) <<< mapA makeTiles -< [(script_dir, vrt) | vrt <- merged_vrts]

# Step 5
leaflet <- step All <<< makeLeaflet -< ( script_dir, (merge_dir, meta_dir))

mergeDirs -< [leaflet, tiles]

Once all the kinks are ironed out – it’s quite short but a very powerful specification which avoids a lot of redundant work being carried out.

### Gotcha 4: copyDirToStore can defeat caching

Using copyDirToStore seems much more convenient than copying each script into the store manually but it can again have confusing caching behaviour. The hash of the store location for script_dir depends on the whole script_dir directory. If you change any file in the directory then the hash of it will change. This means that all steps will recompile if you modify any script!

This is the reason for the mergeFiles call in nixScriptX. mergeFiles will take the necessary files from script_dir and put them into their own store directory. The hash of this directory will only depend on the files necessary for that step.

## Running the flow

The flow is run with the simple local runner. We pass in a location for the local store to the runner which is just a local directory in this case. The library has support for more complicated runners but I haven’t explored using those yet.

main :: IO ()
main = do
cwd <- getCurrentDir
r <- withSimpleLocalRunner (cwd </> [reldir|funflow-example/store|]) $\run -> run (mainFlow >>> storePath) () case r of Left err -> putStrLn$ "FAILED: " ++ displayException err
Right out -> do
putStrLn $"SUCCESS" putStrLn$ toFilePath out

### Displaying the outpath

A nice feature of nix-build is that it displays the path of the final output in the nix store once the build has finished. This is possible to replicate using funflow after defining your own combinator. It would be good to put this in the standard library.

storePath :: ArrowFlow eff ex arr => arr (Content Dir) (Path Abs Dir)
storePath = getFromStore return

It means that we can run our flow and deploy the site in a single command given we have a script which performs the deployment given an output path.

Mine looks a bit like:

#! /usr/bin/env nix-shell
#! nix-shell -i bash -p awscli
if [[ $# -eq 0 ]] ; then echo 'Must pass output directory' exit 1 fi aws s3 sync$1 s3://<bucket-name>

Putting them together:

cabal new-run | ./upload-s3

## Conclusion

Once everything is set up properly, funflow is a joy to use. It abstracts beautifully away from the annoying problems of scheduling and caching leaving the core logic visible. An unfortunate consequence of the intensional store model is that debugging why a build step is not being cached can be very time consuming and fiddly. When I explain the problems I faced, they are obvious but each one required careful thought and reading the source code to understand the intricacies of each of the different operations.

It was also very pleasant to combine using nix and funflow rather than the suggested docker support.