Hoard

hoard v. Accumulate (money or valued objects) and hide or store away.

Hoard is an application for collecting data feeds over time. Hoard downloads a configured list of feeds periodically (typically every few seconds), combines the results for each hour into compressed archive files, and then stores these archive files in object storage for later retrieval. The application was originally developed to collect New York City subway data, and is optimized for feeds whose periodicity is between hundreds of milliseconds and minutes.

One of the key features of Hoard is that it can be run with multiple replicas on different machines, each autonomously collecting the same data and contributing the results to the same object storage. This makes the feed collection process resilient to downtime on any machine running Hoard, and ensures the resulting data lake is as complete as possible. Feed downloads from different replicas are actively de-duplicated, so there is no additional object storage cost.

Using Hoard

The Hoard application is compiled by running go build cmd/hoard.go in the repository root. Alternatively the Docker image jamespfennell/hoard:latest can be used. It just takes a bit of tweaking with Docker volumes to enable it to read the configuration.

Hoard works off a configuration file specifying the feeds to collect and the object storage locations in which to store the results. An example configuration file can be printed by running hoard config. The following is a simple configuration for collecting weather forecast data for Central Park, New York every 5 seconds:

workspacePath: /tmp/hoard_workspace  # where to store files locally

feeds:
  - id: us_ny_central_park
    postfix: .json
    url: https://api.weather.gov/gridpoints/OKX/33,37/forecast
    periodicity: 5 seconds

objectStorage:
  - endpoint: nyc3.digitaloceanspaces.com
    accessKey: <access_key>
    secretKey: <secret_key>
    bucketName: weather_bucket
    prefix: forecast

The data collection process is run using:

hoard --config-file path/to/config.yml collector

The Hoard collector typically runs all the time. It can (and generally should) be run simultaneously on multiple machines, so that if one Hoard collector stops running temporarily, data collection still continues. An example of a reason a Hoard collector stops is because the host machine is being restarted to apply OS updates. The collector includes an HTTP server that displays the configuration file and exports metrics in Prometheus format (example of a running Hoard collector).

After collection, data can be retrieved on any computer using the retrieve command:

hoard --config-file path/to/config.yml retrieve \
    --start-hour 2021-02-03-00 --end-hour 2021-02-03-00 .

This downloads all the collected data between the specified times (format YYYY-MM-DD-HH) into the current directory. Hoard always assumes UTC time. Note that there is generally a delay of about an hour from when Hoard collects data to when it can be retrieved - this is because Hoard only stores data for whole hours in object storage.

Prometheus metrics

The collector process exports a number of Prometheus metrics on the /metrics endpoint. These can be used to monitor the collection process. The metrics all begin with the hoard_ prefix, and are documented in the endpoint. The most important ones to track are probably:

• hoard_download_saved_count is a counter that records the absolute number of successful downloads for each feed. Taking the rate gives the number of successful downloads per minute. This can be used to ensure all the data is being collected.

• hoard_remote_storage_objects_size is a gauge that records the total size of all data in object storage, per feed. This can be used to verify the expected amount of data is being stored every day.

Q&A

Why store raw data? Why not process it in realtime?

Hoard is based on a model in which raw unprocessed data is first collected, and processing happens later, perhaps as a periodic batch process. The negative of this approach is that raw data is generally large, and thus incurs a storage cost. However, the model of separating collection from processing has some nice benefits:

• If there is a bug in the post-processing workflow, the raw data is still there and the post-processing can be run again after the bug is fixed.
• If any improvements are made in the post-processing workflow- for example, new data fields are captured - the updated workflow can be run again on old data.
• It is often hard to write realtime processing workflows in a way that can be run concurrently with multiple redundant workers. One of the main benefits of Hoard is that the data collection is tolerant to expected hardware and software downtime.

How should I run Hoard?

Hoard should be run on computers that are always on. Web servers are great places to run Hoard.

There are three costs associated with running Hoard: (1) the machines running the Hoard collectors, (2) the object storage bytes and (3) data transfer between the collectors and object storage. Many cloud providers have a way of eliminating the third cost by running the collectors in the same data center as the object storage. We run Hoard on Droplets in Digital Ocean’s nyc3 data center, and store the data in Spaces in the same location.

Of course this is a classic tradeoff: if there is a systematic problem in the data center, the Hoard collection process will stop entirely. A more resilient approach would be to use virtual machines and object storage from different cloud providers and in different regions as part of a single Hoard setup. It’s a balance between money and resiliency.

How do multiple Hoard replicas work?

Hoard is a distributed system, but doesn't use any fancy protocols like Raft. Instead, all Hoard operations are written to be idempotent and safe to be tried by multiple workers concurrently. The key to this is that the name of each file or object in Hoard (which could be a feed download, or a compressed archive) contains a hash of its contents, like Git and webpack. Each operation takes some files with some hashes, outputs a new file with a new hash, and then deletes the old files. For example, the merge/deduplication operation is responsible for taking archive files in object storage from different replicas, and merging them together into a single archive. It takes two archives with hashes A and B, outputs a combined archive with hash C, and then deletes A and B. Because each file has a unique name, it is not possible for one worker to accidentally delete the results of another worker. If two workers are attempting the same merge, all that will happen is that C will be written twice.

As always, there is some nuance; the design doc goes into more detail.